Merge branch 'nd/untracked-cache'

Teach the index to optionally remember already seen untracked files
to speed up "git status" in a working tree with tons of cruft.

* nd/untracked-cache: (24 commits)
  git-status.txt: advertisement for untracked cache
  untracked cache: guard and disable on system changes
  mingw32: add uname()
  t7063: tests for untracked cache
  update-index: test the system before enabling untracked cache
  update-index: manually enable or disable untracked cache
  status: enable untracked cache
  untracked-cache: temporarily disable with $GIT_DISABLE_UNTRACKED_CACHE
  untracked cache: mark index dirty if untracked cache is updated
  untracked cache: print stats with $GIT_TRACE_UNTRACKED_STATS
  untracked cache: avoid racy timestamps
  read-cache.c: split racy stat test to a separate function
  untracked cache: invalidate at index addition or removal
  untracked cache: load from UNTR index extension
  untracked cache: save to an index extension
  ewah: add convenient wrapper ewah_serialize_strbuf()
  untracked cache: don't open non-existent .gitignore
  untracked cache: mark what dirs should be recursed/saved
  untracked cache: record/validate dir mtime and reuse cached output
  untracked cache: make a wrapper around {open,read,close}dir()
  ...

21 files changed, 1822 insertions, 58 deletions

diff --git a/.gitignore b/.gitignore
index a05241916c..422c5382c1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -184,6 +184,7 @@
 /test-delta
 /test-dump-cache-tree
 /test-dump-split-index
+/test-dump-untracked-cache
 /test-scrap-cache-tree
 /test-genrandom
 /test-hashmap
diff --git a/Documentation/git-status.txt b/Documentation/git-status.txt
index 5221f950ce..335f312335 100644
--- a/Documentation/git-status.txt
+++ b/Documentation/git-status.txt
@@ -66,7 +66,10 @@ When `-u` option is not used, untracked files and directories are
 shown (i.e. the same as specifying `normal`), to help you avoid
 forgetting to add newly created files.  Because it takes extra work
 to find untracked files in the filesystem, this mode may take some
-time in a large working tree.  You can use `no` to have `git status`
+time in a large working tree.
+Consider enabling untracked cache and split index if supported (see
+`git update-index --untracked-cache` and `git update-index
+--split-index`), Otherwise you can use `no` to have `git status`
 return more quickly without showing untracked files.
 +
 The default can be changed using the status.showUntrackedFiles
diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index aff01798cd..1a296bc29a 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -170,6 +170,20 @@ may not support it yet.
 	the shared index file. This mode is designed for very large
 	indexes that take a significant amount of time to read or write.
 
+--untracked-cache::
+--no-untracked-cache::
+	Enable or disable untracked cache extension. This could speed
+	up for commands that involve determining untracked files such
+	as `git status`. The underlying operating system and file
+	system must change `st_mtime` field of a directory if files
+	are added or deleted in that directory.
+
+--force-untracked-cache::
+	For safety, `--untracked-cache` performs tests on the working
+	directory to make sure untracked cache can be used. These
+	tests can take a few seconds. `--force-untracked-cache` can be
+	used to skip the tests.
+
 \--::
 	Do not interpret any more arguments as options.
 
diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
index 35112e4966..b7093af8b2 100644
--- a/Documentation/technical/index-format.txt
+++ b/Documentation/technical/index-format.txt
@@ -233,3 +233,65 @@ Git index format
   The remaining index entries after replaced ones will be added to the
   final index. These added entries are also sorted by entry name then
   stage.
+
+== Untracked cache
+
+  Untracked cache saves the untracked file list and necessary data to
+  verify the cache. The signature for this extension is { 'U', 'N',
+  'T', 'R' }.
+
+  The extension starts with
+
+  - A sequence of NUL-terminated strings, preceded by the size of the
+    sequence in variable width encoding. Each string describes the
+    environment where the cache can be used.
+
+  - Stat data of $GIT_DIR/info/exclude. See "Index entry" section from
+    ctime field until "file size".
+
+  - Stat data of core.excludesfile
+
+  - 32-bit dir_flags (see struct dir_struct)
+
+  - 160-bit SHA-1 of $GIT_DIR/info/exclude. Null SHA-1 means the file
+    does not exist.
+
+  - 160-bit SHA-1 of core.excludesfile. Null SHA-1 means the file does
+    not exist.
+
+  - NUL-terminated string of per-dir exclude file name. This usually
+    is ".gitignore".
+
+  - The number of following directory blocks, variable width
+    encoding. If this number is zero, the extension ends here with a
+    following NUL.
+
+  - A number of directory blocks in depth-first-search order, each
+    consists of
+
+    - The number of untracked entries, variable width encoding.
+
+    - The number of sub-directory blocks, variable width encoding.
+
+    - The directory name terminated by NUL.
+
+    - A number of untrached file/dir names terminated by NUL.
+
+The remaining data of each directory block is grouped by type:
+
+  - An ewah bitmap, the n-th bit marks whether the n-th directory has
+    valid untracked cache entries.
+
+  - An ewah bitmap, the n-th bit records "check-only" bit of
+    read_directory_recursive() for the n-th directory.
+
+  - An ewah bitmap, the n-th bit indicates whether SHA-1 and stat data
+    is valid for the n-th directory and exists in the next data.
+
+  - An array of stat data. The n-th data corresponds with the n-th
+    "one" bit in the previous ewah bitmap.
+
+  - An array of SHA-1. The n-th SHA-1 corresponds with the n-th "one" bit
+    in the previous ewah bitmap.
+
+  - One NUL.
diff --git a/Makefile b/Makefile
index 25a453bf2b..323c401e96 100644
--- a/Makefile
+++ b/Makefile
@@ -574,6 +574,7 @@ TEST_PROGRAMS_NEED_X += test-date
 TEST_PROGRAMS_NEED_X += test-delta
 TEST_PROGRAMS_NEED_X += test-dump-cache-tree
 TEST_PROGRAMS_NEED_X += test-dump-split-index
+TEST_PROGRAMS_NEED_X += test-dump-untracked-cache
 TEST_PROGRAMS_NEED_X += test-genrandom
 TEST_PROGRAMS_NEED_X += test-hashmap
 TEST_PROGRAMS_NEED_X += test-index-version
diff --git a/builtin/commit.c b/builtin/commit.c
index d6515a2a50..254477fd1d 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1366,13 +1366,14 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 	refresh_index(&the_index, REFRESH_QUIET|REFRESH_UNMERGED, &s.pathspec, NULL, NULL);
 
 	fd = hold_locked_index(&index_lock, 0);
-	if (0 <= fd)
-		update_index_if_able(&the_index, &index_lock);
 
 	s.is_initial = get_sha1(s.reference, sha1) ? 1 : 0;
 	s.ignore_submodule_arg = ignore_submodule_arg;
 	wt_status_collect(&s);
 
+	if (0 <= fd)
+		update_index_if_able(&the_index, &index_lock);
+
 	if (s.relative_paths)
 		s.prefix = prefix;
 
diff --git a/builtin/update-index.c b/builtin/update-index.c
index 0665b31ea1..7431938fa6 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -33,6 +33,7 @@ static int mark_valid_only;
 static int mark_skip_worktree_only;
 #define MARK_FLAG 1
 #define UNMARK_FLAG 2
+static struct strbuf mtime_dir = STRBUF_INIT;
 
 __attribute__((format (printf, 1, 2)))
 static void report(const char *fmt, ...)
@@ -48,6 +49,166 @@ static void report(const char *fmt, ...)
 	va_end(vp);
 }
 
+static void remove_test_directory(void)
+{
+	if (mtime_dir.len)
+		remove_dir_recursively(&mtime_dir, 0);
+}
+
+static const char *get_mtime_path(const char *path)
+{
+	static struct strbuf sb = STRBUF_INIT;
+	strbuf_reset(&sb);
+	strbuf_addf(&sb, "%s/%s", mtime_dir.buf, path);
+	return sb.buf;
+}
+
+static void xmkdir(const char *path)
+{
+	path = get_mtime_path(path);
+	if (mkdir(path, 0700))
+		die_errno(_("failed to create directory %s"), path);
+}
+
+static int xstat_mtime_dir(struct stat *st)
+{
+	if (stat(mtime_dir.buf, st))
+		die_errno(_("failed to stat %s"), mtime_dir.buf);
+	return 0;
+}
+
+static int create_file(const char *path)
+{
+	int fd;
+	path = get_mtime_path(path);
+	fd = open(path, O_CREAT | O_RDWR, 0644);
+	if (fd < 0)
+		die_errno(_("failed to create file %s"), path);
+	return fd;
+}
+
+static void xunlink(const char *path)
+{
+	path = get_mtime_path(path);
+	if (unlink(path))
+		die_errno(_("failed to delete file %s"), path);
+}
+
+static void xrmdir(const char *path)
+{
+	path = get_mtime_path(path);
+	if (rmdir(path))
+		die_errno(_("failed to delete directory %s"), path);
+}
+
+static void avoid_racy(void)
+{
+	/*
+	 * not use if we could usleep(10) if USE_NSEC is defined. The
+	 * field nsec could be there, but the OS could choose to
+	 * ignore it?
+	 */
+	sleep(1);
+}
+
+static int test_if_untracked_cache_is_supported(void)
+{
+	struct stat st;
+	struct stat_data base;
+	int fd, ret = 0;
+
+	strbuf_addstr(&mtime_dir, "mtime-test-XXXXXX");
+	if (!mkdtemp(mtime_dir.buf))
+		die_errno("Could not make temporary directory");
+
+	fprintf(stderr, _("Testing "));
+	atexit(remove_test_directory);
+	xstat_mtime_dir(&st);
+	fill_stat_data(&base, &st);
+	fputc('.', stderr);
+
+	avoid_racy();
+	fd = create_file("newfile");
+	xstat_mtime_dir(&st);
+	if (!match_stat_data(&base, &st)) {
+		close(fd);
+		fputc('\n', stderr);
+		fprintf_ln(stderr,_("directory stat info does not "
+				    "change after adding a new file"));
+		goto done;
+	}
+	fill_stat_data(&base, &st);
+	fputc('.', stderr);
+
+	avoid_racy();
+	xmkdir("new-dir");
+	xstat_mtime_dir(&st);
+	if (!match_stat_data(&base, &st)) {
+		close(fd);
+		fputc('\n', stderr);
+		fprintf_ln(stderr, _("directory stat info does not change "
+				     "after adding a new directory"));
+		goto done;
+	}
+	fill_stat_data(&base, &st);
+	fputc('.', stderr);
+
+	avoid_racy();
+	write_or_die(fd, "data", 4);
+	close(fd);
+	xstat_mtime_dir(&st);
+	if (match_stat_data(&base, &st)) {
+		fputc('\n', stderr);
+		fprintf_ln(stderr, _("directory stat info changes "
+				     "after updating a file"));
+		goto done;
+	}
+	fputc('.', stderr);
+
+	avoid_racy();
+	close(create_file("new-dir/new"));
+	xstat_mtime_dir(&st);
+	if (match_stat_data(&base, &st)) {
+		fputc('\n', stderr);
+		fprintf_ln(stderr, _("directory stat info changes after "
+				     "adding a file inside subdirectory"));
+		goto done;
+	}
+	fputc('.', stderr);
+
+	avoid_racy();
+	xunlink("newfile");
+	xstat_mtime_dir(&st);
+	if (!match_stat_data(&base, &st)) {
+		fputc('\n', stderr);
+		fprintf_ln(stderr, _("directory stat info does not "
+				     "change after deleting a file"));
+		goto done;
+	}
+	fill_stat_data(&base, &st);
+	fputc('.', stderr);
+
+	avoid_racy();
+	xunlink("new-dir/new");
+	xrmdir("new-dir");
+	xstat_mtime_dir(&st);
+	if (!match_stat_data(&base, &st)) {
+		fputc('\n', stderr);
+		fprintf_ln(stderr, _("directory stat info does not "
+				     "change after deleting a directory"));
+		goto done;
+	}
+
+	if (rmdir(mtime_dir.buf))
+		die_errno(_("failed to delete directory %s"), mtime_dir.buf);
+	fprintf_ln(stderr, _(" OK"));
+	ret = 1;
+
+done:
+	strbuf_release(&mtime_dir);
+	return ret;
+}
+
 static int mark_ce_flags(const char *path, int flag, int mark)
 {
 	int namelen = strlen(path);
@@ -741,6 +902,7 @@ static int reupdate_callback(struct parse_opt_ctx_t *ctx,
 int cmd_update_index(int argc, const char **argv, const char *prefix)
 {
 	int newfd, entries, has_errors = 0, line_termination = '\n';
+	int untracked_cache = -1;
 	int read_from_stdin = 0;
 	int prefix_length = prefix ? strlen(prefix) : 0;
 	int preferred_index_format = 0;
@@ -832,6 +994,10 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 			N_("write index in this format")),
 		OPT_BOOL(0, "split-index", &split_index,
 			N_("enable or disable split index")),
+		OPT_BOOL(0, "untracked-cache", &untracked_cache,
+			N_("enable/disable untracked cache")),
+		OPT_SET_INT(0, "force-untracked-cache", &untracked_cache,
+			    N_("enable untracked cache without testing the filesystem"), 2),
 		OPT_END()
 	};
 
@@ -938,6 +1104,28 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 		the_index.split_index = NULL;
 		the_index.cache_changed |= SOMETHING_CHANGED;
 	}
+	if (untracked_cache > 0) {
+		struct untracked_cache *uc;
+
+		if (untracked_cache < 2) {
+			setup_work_tree();
+			if (!test_if_untracked_cache_is_supported())
+				return 1;
+		}
+		if (!the_index.untracked) {
+			uc = xcalloc(1, sizeof(*uc));
+			strbuf_init(&uc->ident, 100);
+			uc->exclude_per_dir = ".gitignore";
+			/* should be the same flags used by git-status */
+			uc->dir_flags = DIR_SHOW_OTHER_DIRECTORIES | DIR_HIDE_EMPTY_DIRECTORIES;
+			the_index.untracked = uc;
+		}
+		add_untracked_ident(the_index.untracked);
+		the_index.cache_changed |= UNTRACKED_CHANGED;
+	} else if (!untracked_cache && the_index.untracked) {
+		the_index.untracked = NULL;
+		the_index.cache_changed |= UNTRACKED_CHANGED;
+	}
 
 	if (active_cache_changed) {
 		if (newfd < 0) {
diff --git a/cache.h b/cache.h
index 1f4226be15..9da9784824 100644
--- a/cache.h
+++ b/cache.h
@@ -297,8 +297,11 @@ static inline unsigned int canon_mode(unsigned int mode)
 #define RESOLVE_UNDO_CHANGED	(1 << 4)
 #define CACHE_TREE_CHANGED	(1 << 5)
 #define SPLIT_INDEX_ORDERED	(1 << 6)
+#define UNTRACKED_CHANGED	(1 << 7)
 
 struct split_index;
+struct untracked_cache;
+
 struct index_state {
 	struct cache_entry **cache;
 	unsigned int version;
@@ -312,6 +315,7 @@ struct index_state {
 	struct hashmap name_hash;
 	struct hashmap dir_hash;
 	unsigned char sha1[20];
+	struct untracked_cache *untracked;
 };
 
 extern struct index_state the_index;
@@ -563,6 +567,8 @@ extern void fill_stat_data(struct stat_data *sd, struct stat *st);
  * INODE_CHANGED, and DATA_CHANGED.
  */
 extern int match_stat_data(const struct stat_data *sd, struct stat *st);
+extern int match_stat_data_racy(const struct index_state *istate,
+				const struct stat_data *sd, struct stat *st);
 
 extern void fill_stat_cache_info(struct cache_entry *ce, struct stat *st);
 
diff --git a/compat/mingw.c b/compat/mingw.c
index 70f3191a4f..496e6f8bb0 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -2128,3 +2128,14 @@ void mingw_startup()
 	/* initialize Unicode console */
 	winansi_init();
 }
+
+int uname(struct utsname *buf)
+{
+	DWORD v = GetVersion();
+	memset(buf, 0, sizeof(*buf));
+	strcpy(buf->sysname, "Windows");
+	sprintf(buf->release, "%u.%u", v & 0xff, (v >> 8) & 0xff);
+	/* assuming NT variants only.. */
+	sprintf(buf->version, "%u", (v >> 16) & 0x7fff);
+	return 0;
+}
diff --git a/compat/mingw.h b/compat/mingw.h
index 98c5e44294..738865c6c0 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -76,6 +76,14 @@ struct itimerval {
 };
 #define ITIMER_REAL 0
 
+struct utsname {
+	char sysname[16];
+	char nodename[1];
+	char release[16];
+	char version[16];
+	char machine[1];
+};
+
 /*
  * sanitize preprocessor namespace polluted by Windows headers defining
  * macros which collide with git local versions
@@ -175,6 +183,7 @@ struct passwd *getpwuid(uid_t uid);
 int setitimer(int type, struct itimerval *in, struct itimerval *out);
 int sigaction(int sig, struct sigaction *in, struct sigaction *out);
 int link(const char *oldpath, const char *newpath);
+int uname(struct utsname *buf);
 
 /*
  * replacements of existing functions
diff --git a/dir.c b/dir.c
index 4183acc082..d318ffcb2a 100644
--- a/dir.c
+++ b/dir.c
@@ -13,6 +13,8 @@
 #include "wildmatch.h"
 #include "pathspec.h"
 #include "utf8.h"
+#include "varint.h"
+#include "ewah/ewok.h"
 
 struct path_simplify {
 	int len;
@@ -32,8 +34,22 @@ enum path_treatment {
 	path_untracked
 };
 
+/*
+ * Support data structure for our opendir/readdir/closedir wrappers
+ */
+struct cached_dir {
+	DIR *fdir;
+	struct untracked_cache_dir *untracked;
+	int nr_files;
+	int nr_dirs;
+
+	struct dirent *de;
+	const char *file;
+	struct untracked_cache_dir *ucd;
+};
+
 static enum path_treatment read_directory_recursive(struct dir_struct *dir,
-	const char *path, int len,
+	const char *path, int len, struct untracked_cache_dir *untracked,
 	int check_only, const struct path_simplify *simplify);
 static int get_dtype(struct dirent *de, const char *path, int len);
 
@@ -510,7 +526,8 @@ void add_exclude(const char *string, const char *base,
 	x->el = el;
 }
 
-static void *read_skip_worktree_file_from_index(const char *path, size_t *size)
+static void *read_skip_worktree_file_from_index(const char *path, size_t *size,
+						struct sha1_stat *sha1_stat)
 {
 	int pos, len;
 	unsigned long sz;
@@ -529,6 +546,10 @@ static void *read_skip_worktree_file_from_index(const char *path, size_t *size)
 		return NULL;
 	}
 	*size = xsize_t(sz);
+	if (sha1_stat) {
+		memset(&sha1_stat->stat, 0, sizeof(sha1_stat->stat));
+		hashcpy(sha1_stat->sha1, active_cache[pos]->sha1);
+	}
 	return data;
 }
 
@@ -573,11 +594,93 @@ static void trim_trailing_spaces(char *buf)
 		*last_space = '\0';
 }
 
-int add_excludes_from_file_to_list(const char *fname,
-				   const char *base,
-				   int baselen,
-				   struct exclude_list *el,
-				   int check_index)
+/*
+ * Given a subdirectory name and "dir" of the current directory,
+ * search the subdir in "dir" and return it, or create a new one if it
+ * does not exist in "dir".
+ *
+ * If "name" has the trailing slash, it'll be excluded in the search.
+ */
+static struct untracked_cache_dir *lookup_untracked(struct untracked_cache *uc,
+						    struct untracked_cache_dir *dir,
+						    const char *name, int len)
+{
+	int first, last;
+	struct untracked_cache_dir *d;
+	if (!dir)
+		return NULL;
+	if (len && name[len - 1] == '/')
+		len--;
+	first = 0;
+	last = dir->dirs_nr;
+	while (last > first) {
+		int cmp, next = (last + first) >> 1;
+		d = dir->dirs[next];
+		cmp = strncmp(name, d->name, len);
+		if (!cmp && strlen(d->name) > len)
+			cmp = -1;
+		if (!cmp)
+			return d;
+		if (cmp < 0) {
+			last = next;
+			continue;
+		}
+		first = next+1;
+	}
+
+	uc->dir_created++;
+	d = xmalloc(sizeof(*d) + len + 1);
+	memset(d, 0, sizeof(*d));
+	memcpy(d->name, name, len);
+	d->name[len] = '\0';
+
+	ALLOC_GROW(dir->dirs, dir->dirs_nr + 1, dir->dirs_alloc);
+	memmove(dir->dirs + first + 1, dir->dirs + first,
+		(dir->dirs_nr - first) * sizeof(*dir->dirs));
+	dir->dirs_nr++;
+	dir->dirs[first] = d;
+	return d;
+}
+
+static void do_invalidate_gitignore(struct untracked_cache_dir *dir)
+{
+	int i;
+	dir->valid = 0;
+	dir->untracked_nr = 0;
+	for (i = 0; i < dir->dirs_nr; i++)
+		do_invalidate_gitignore(dir->dirs[i]);
+}
+
+static void invalidate_gitignore(struct untracked_cache *uc,
+				 struct untracked_cache_dir *dir)
+{
+	uc->gitignore_invalidated++;
+	do_invalidate_gitignore(dir);
+}
+
+static void invalidate_directory(struct untracked_cache *uc,
+				 struct untracked_cache_dir *dir)
+{
+	int i;
+	uc->dir_invalidated++;
+	dir->valid = 0;
+	dir->untracked_nr = 0;
+	for (i = 0; i < dir->dirs_nr; i++)
+		dir->dirs[i]->recurse = 0;
+}
+
+/*
+ * Given a file with name "fname", read it (either from disk, or from
+ * the index if "check_index" is non-zero), parse it and store the
+ * exclude rules in "el".
+ *
+ * If "ss" is not NULL, compute SHA-1 of the exclude file and fill
+ * stat data from disk (only valid if add_excludes returns zero). If
+ * ss_valid is non-zero, "ss" must contain good value as input.
+ */
+static int add_excludes(const char *fname, const char *base, int baselen,
+			struct exclude_list *el, int check_index,
+			struct sha1_stat *sha1_stat)
 {
 	struct stat st;
 	int fd, i, lineno = 1;
@@ -591,7 +694,7 @@ int add_excludes_from_file_to_list(const char *fname,
 		if (0 <= fd)
 			close(fd);
 		if (!check_index ||
-		    (buf = read_skip_worktree_file_from_index(fname, &size)) == NULL)
+		    (buf = read_skip_worktree_file_from_index(fname, &size, sha1_stat)) == NULL)
 			return -1;
 		if (size == 0) {
 			free(buf);
@@ -604,6 +707,11 @@ int add_excludes_from_file_to_list(const char *fname,
 	} else {
 		size = xsize_t(st.st_size);
 		if (size == 0) {
+			if (sha1_stat) {
+				fill_stat_data(&sha1_stat->stat, &st);
+				hashcpy(sha1_stat->sha1, EMPTY_BLOB_SHA1_BIN);
+				sha1_stat->valid = 1;
+			}
 			close(fd);
 			return 0;
 		}
@@ -615,6 +723,22 @@ int add_excludes_from_file_to_list(const char *fname,
 		}
 		buf[size++] = '\n';
 		close(fd);
+		if (sha1_stat) {
+			int pos;
+			if (sha1_stat->valid &&
+			    !match_stat_data_racy(&the_index, &sha1_stat->stat, &st))
+				; /* no content change, ss->sha1 still good */
+			else if (check_index &&
+				 (pos = cache_name_pos(fname, strlen(fname))) >= 0 &&
+				 !ce_stage(active_cache[pos]) &&
+				 ce_uptodate(active_cache[pos]) &&
+				 !would_convert_to_git(fname))
+				hashcpy(sha1_stat->sha1, active_cache[pos]->sha1);
+			else
+				hash_sha1_file(buf, size, "blob", sha1_stat->sha1);
+			fill_stat_data(&sha1_stat->stat, &st);
+			sha1_stat->valid = 1;
+		}
 	}
 
 	el->filebuf = buf;
@@ -638,6 +762,13 @@ int add_excludes_from_file_to_list(const char *fname,
 	return 0;
 }
 
+int add_excludes_from_file_to_list(const char *fname, const char *base,
+				   int baselen, struct exclude_list *el,
+				   int check_index)
+{
+	return add_excludes(fname, base, baselen, el, check_index, NULL);
+}
+
 struct exclude_list *add_exclude_list(struct dir_struct *dir,
 				      int group_type, const char *src)
 {
@@ -655,14 +786,28 @@ struct exclude_list *add_exclude_list(struct dir_struct *dir,
 /*
  * Used to set up core.excludesfile and .git/info/exclude lists.
  */
-void add_excludes_from_file(struct dir_struct *dir, const char *fname)
+static void add_excludes_from_file_1(struct dir_struct *dir, const char *fname,
+				     struct sha1_stat *sha1_stat)
 {
 	struct exclude_list *el;
+	/*
+	 * catch setup_standard_excludes() that's called before
+	 * dir->untracked is assigned. That function behaves
+	 * differently when dir->untracked is non-NULL.
+	 */
+	if (!dir->untracked)
+		dir->unmanaged_exclude_files++;
 	el = add_exclude_list(dir, EXC_FILE, fname);
-	if (add_excludes_from_file_to_list(fname, "", 0, el, 0) < 0)
+	if (add_excludes(fname, "", 0, el, 0, sha1_stat) < 0)
 		die("cannot use %s as an exclude file", fname);
 }
 
+void add_excludes_from_file(struct dir_struct *dir, const char *fname)
+{
+	dir->unmanaged_exclude_files++; /* see validate_untracked_cache() */
+	add_excludes_from_file_1(dir, fname, NULL);
+}
+
 int match_basename(const char *basename, int basenamelen,
 		   const char *pattern, int prefix, int patternlen,
 		   int flags)
@@ -837,6 +982,7 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 	struct exclude_list_group *group;
 	struct exclude_list *el;
 	struct exclude_stack *stk = NULL;
+	struct untracked_cache_dir *untracked;
 	int current;
 
 	group = &dir->exclude_list_group[EXC_DIRS];
@@ -874,8 +1020,14 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 	/* Read from the parent directories and push them down. */
 	current = stk ? stk->baselen : -1;
 	strbuf_setlen(&dir->basebuf, current < 0 ? 0 : current);
+	if (dir->untracked)
+		untracked = stk ? stk->ucd : dir->untracked->root;
+	else
+		untracked = NULL;
+
 	while (current < baselen) {
 		const char *cp;
+		struct sha1_stat sha1_stat;
 
 		stk = xcalloc(1, sizeof(*stk));
 		if (current < 0) {
@@ -886,10 +1038,15 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 			if (!cp)
 				die("oops in prep_exclude");
 			cp++;
+			untracked =
+				lookup_untracked(dir->untracked, untracked,
+						 base + current,
+						 cp - base - current);
 		}
 		stk->prev = dir->exclude_stack;
 		stk->baselen = cp - base;
 		stk->exclude_ix = group->nr;
+		stk->ucd = untracked;
 		el = add_exclude_list(dir, EXC_DIRS, NULL);
 		strbuf_add(&dir->basebuf, base + current, stk->baselen - current);
 		assert(stk->baselen == dir->basebuf.len);
@@ -912,7 +1069,23 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 		}
 
 		/* Try to read per-directory file */
-		if (dir->exclude_per_dir) {
+		hashclr(sha1_stat.sha1);
+		sha1_stat.valid = 0;
+		if (dir->exclude_per_dir &&
+		    /*
+		     * If we know that no files have been added in
+		     * this directory (i.e. valid_cached_dir() has
+		     * been executed and set untracked->valid) ..
+		     */
+		    (!untracked || !untracked->valid ||
+		     /*
+		      * .. and .gitignore does not exist before
+		      * (i.e. null exclude_sha1 and skip_worktree is
+		      * not set). Then we can skip loading .gitignore,
+		      * which would result in ENOENT anyway.
+		      * skip_worktree is taken care in read_directory()
+		      */
+		     !is_null_sha1(untracked->exclude_sha1))) {
 			/*
 			 * dir->basebuf gets reused by the traversal, but we
 			 * need fname to remain unchanged to ensure the src
@@ -925,8 +1098,27 @@ static void prep_exclude(struct dir_struct *dir, const char *base, int baselen)
 			strbuf_addbuf(&sb, &dir->basebuf);
 			strbuf_addstr(&sb, dir->exclude_per_dir);
 			el->src = strbuf_detach(&sb, NULL);
-			add_excludes_from_file_to_list(el->src, el->src,
-						       stk->baselen, el, 1);
+			add_excludes(el->src, el->src, stk->baselen, el, 1,
+				     untracked ? &sha1_stat : NULL);
+		}
+		/*
+		 * NEEDSWORK: when untracked cache is enabled, prep_exclude()
+		 * will first be called in valid_cached_dir() then maybe many
+		 * times more in last_exclude_matching(). When the cache is
+		 * used, last_exclude_matching() will not be called and
+		 * reading .gitignore content will be a waste.
+		 *
+		 * So when it's called by valid_cached_dir() and we can get
+		 * .gitignore SHA-1 from the index (i.e. .gitignore is not
+		 * modified on work tree), we could delay reading the
+		 * .gitignore content until we absolutely need it in
+		 * last_exclude_matching(). Be careful about ignore rule
+		 * order, though, if you do that.
+		 */
+		if (untracked &&
+		    hashcmp(sha1_stat.sha1, untracked->exclude_sha1)) {
+			invalidate_gitignore(dir->untracked, untracked);
+			hashcpy(untracked->exclude_sha1, sha1_stat.sha1);
 		}
 		dir->exclude_stack = stk;
 		current = stk->baselen;
@@ -1107,6 +1299,7 @@ static enum exist_status directory_exists_in_index(const char *dirname, int len)
  *  (c) otherwise, we recurse into it.
  */
 static enum path_treatment treat_directory(struct dir_struct *dir,
+	struct untracked_cache_dir *untracked,
 	const char *dirname, int len, int exclude,
 	const struct path_simplify *simplify)
 {
@@ -1134,7 +1327,9 @@ static enum path_treatment treat_directory(struct dir_struct *dir,
 	if (!(dir->flags & DIR_HIDE_EMPTY_DIRECTORIES))
 		return exclude ? path_excluded : path_untracked;
 
-	return read_directory_recursive(dir, dirname, len, 1, simplify);
+	untracked = lookup_untracked(dir->untracked, untracked, dirname, len);
+	return read_directory_recursive(dir, dirname, len,
+					untracked, 1, simplify);
 }
 
 /*
@@ -1250,6 +1445,7 @@ static int get_dtype(struct dirent *de, const char *path, int len)
 }
 
 static enum path_treatment treat_one_path(struct dir_struct *dir,
+					  struct untracked_cache_dir *untracked,
 					  struct strbuf *path,
 					  const struct path_simplify *simplify,
 					  int dtype, struct dirent *de)
@@ -1302,7 +1498,7 @@ static enum path_treatment treat_one_path(struct dir_struct *dir,
 		return path_none;
 	case DT_DIR:
 		strbuf_addch(path, '/');
-		return treat_directory(dir, path->buf, path->len, exclude,
+		return treat_directory(dir, untracked, path->buf, path->len, exclude,
 			simplify);
 	case DT_REG:
 	case DT_LNK:
@@ -1310,14 +1506,52 @@ static enum path_treatment treat_one_path(struct dir_struct *dir,
 	}
 }
 
+static enum path_treatment treat_path_fast(struct dir_struct *dir,
+					   struct untracked_cache_dir *untracked,
+					   struct cached_dir *cdir,
+					   struct strbuf *path,
+					   int baselen,
+					   const struct path_simplify *simplify)
+{
+	strbuf_setlen(path, baselen);
+	if (!cdir->ucd) {
+		strbuf_addstr(path, cdir->file);
+		return path_untracked;
+	}
+	strbuf_addstr(path, cdir->ucd->name);
+	/* treat_one_path() does this before it calls treat_directory() */
+	if (path->buf[path->len - 1] != '/')
+		strbuf_addch(path, '/');
+	if (cdir->ucd->check_only)
+		/*
+		 * check_only is set as a result of treat_directory() getting
+		 * to its bottom. Verify again the same set of directories
+		 * with check_only set.
+		 */
+		return read_directory_recursive(dir, path->buf, path->len,
+						cdir->ucd, 1, simplify);
+	/*
+	 * We get path_recurse in the first run when
+	 * directory_exists_in_index() returns index_nonexistent. We
+	 * are sure that new changes in the index does not impact the
+	 * outcome. Return now.
+	 */
+	return path_recurse;
+}
+
 static enum path_treatment treat_path(struct dir_struct *dir,
-				      struct dirent *de,
+				      struct untracked_cache_dir *untracked,
+				      struct cached_dir *cdir,
 				      struct strbuf *path,
 				      int baselen,
 				      const struct path_simplify *simplify)
 {
 	int dtype;
+	struct dirent *de = cdir->de;
 
+	if (!de)
+		return treat_path_fast(dir, untracked, cdir, path,
+				       baselen, simplify);
 	if (is_dot_or_dotdot(de->d_name) || !strcmp(de->d_name, ".git"))
 		return path_none;
 	strbuf_setlen(path, baselen);
@@ -1326,7 +1560,121 @@ static enum path_treatment treat_path(struct dir_struct *dir,
 		return path_none;
 
 	dtype = DTYPE(de);
-	return treat_one_path(dir, path, simplify, dtype, de);
+	return treat_one_path(dir, untracked, path, simplify, dtype, de);
+}
+
+static void add_untracked(struct untracked_cache_dir *dir, const char *name)
+{
+	if (!dir)
+		return;
+	ALLOC_GROW(dir->untracked, dir->untracked_nr + 1,
+		   dir->untracked_alloc);
+	dir->untracked[dir->untracked_nr++] = xstrdup(name);
+}
+
+static int valid_cached_dir(struct dir_struct *dir,
+			    struct untracked_cache_dir *untracked,
+			    struct strbuf *path,
+			    int check_only)
+{
+	struct stat st;
+
+	if (!untracked)
+		return 0;
+
+	if (stat(path->len ? path->buf : ".", &st)) {
+		invalidate_directory(dir->untracked, untracked);
+		memset(&untracked->stat_data, 0, sizeof(untracked->stat_data));
+		return 0;
+	}
+	if (!untracked->valid ||
+	    match_stat_data_racy(&the_index, &untracked->stat_data, &st)) {
+		if (untracked->valid)
+			invalidate_directory(dir->untracked, untracked);
+		fill_stat_data(&untracked->stat_data, &st);
+		return 0;
+	}
+
+	if (untracked->check_only != !!check_only) {
+		invalidate_directory(dir->untracked, untracked);
+		return 0;
+	}
+
+	/*
+	 * prep_exclude will be called eventually on this directory,
+	 * but it's called much later in last_exclude_matching(). We
+	 * need it now to determine the validity of the cache for this
+	 * path. The next calls will be nearly no-op, the way
+	 * prep_exclude() is designed.
+	 */
+	if (path->len && path->buf[path->len - 1] != '/') {
+		strbuf_addch(path, '/');
+		prep_exclude(dir, path->buf, path->len);
+		strbuf_setlen(path, path->len - 1);
+	} else
+		prep_exclude(dir, path->buf, path->len);
+
+	/* hopefully prep_exclude() haven't invalidated this entry... */
+	return untracked->valid;
+}
+
+static int open_cached_dir(struct cached_dir *cdir,
+			   struct dir_struct *dir,
+			   struct untracked_cache_dir *untracked,
+			   struct strbuf *path,
+			   int check_only)
+{
+	memset(cdir, 0, sizeof(*cdir));
+	cdir->untracked = untracked;
+	if (valid_cached_dir(dir, untracked, path, check_only))
+		return 0;
+	cdir->fdir = opendir(path->len ? path->buf : ".");
+	if (dir->untracked)
+		dir->untracked->dir_opened++;
+	if (!cdir->fdir)
+		return -1;
+	return 0;
+}
+
+static int read_cached_dir(struct cached_dir *cdir)
+{
+	if (cdir->fdir) {
+		cdir->de = readdir(cdir->fdir);
+		if (!cdir->de)
+			return -1;
+		return 0;
+	}
+	while (cdir->nr_dirs < cdir->untracked->dirs_nr) {
+		struct untracked_cache_dir *d = cdir->untracked->dirs[cdir->nr_dirs];
+		if (!d->recurse) {
+			cdir->nr_dirs++;
+			continue;
+		}
+		cdir->ucd = d;
+		cdir->nr_dirs++;
+		return 0;
+	}
+	cdir->ucd = NULL;
+	if (cdir->nr_files < cdir->untracked->untracked_nr) {
+		struct untracked_cache_dir *d = cdir->untracked;
+		cdir->file = d->untracked[cdir->nr_files++];
+		return 0;
+	}
+	return -1;
+}
+
+static void close_cached_dir(struct cached_dir *cdir)
+{
+	if (cdir->fdir)
+		closedir(cdir->fdir);
+	/*
+	 * We have gone through this directory and found no untracked
+	 * entries. Mark it valid.
+	 */
+	if (cdir->untracked) {
+		cdir->untracked->valid = 1;
+		cdir->untracked->recurse = 1;
+	}
 }
 
 /*
@@ -1342,38 +1690,48 @@ static enum path_treatment treat_path(struct dir_struct *dir,
  */
 static enum path_treatment read_directory_recursive(struct dir_struct *dir,
 				    const char *base, int baselen,
-				    int check_only,
+				    struct untracked_cache_dir *untracked, int check_only,
 				    const struct path_simplify *simplify)
 {
-	DIR *fdir;
+	struct cached_dir cdir;
 	enum path_treatment state, subdir_state, dir_state = path_none;
-	struct dirent *de;
 	struct strbuf path = STRBUF_INIT;
 
 	strbuf_add(&path, base, baselen);
 
-	fdir = opendir(path.len ? path.buf : ".");
-	if (!fdir)
+	if (open_cached_dir(&cdir, dir, untracked, &path, check_only))
 		goto out;
 
-	while ((de = readdir(fdir)) != NULL) {
+	if (untracked)
+		untracked->check_only = !!check_only;
+
+	while (!read_cached_dir(&cdir)) {
 		/* check how the file or directory should be treated */
-		state = treat_path(dir, de, &path, baselen, simplify);
+		state = treat_path(dir, untracked, &cdir, &path, baselen, simplify);
+
 		if (state > dir_state)
 			dir_state = state;
 
 		/* recurse into subdir if instructed by treat_path */
 		if (state == path_recurse) {
-			subdir_state = read_directory_recursive(dir, path.buf,
-				path.len, check_only, simplify);
+			struct untracked_cache_dir *ud;
+			ud = lookup_untracked(dir->untracked, untracked,
+					      path.buf + baselen,
+					      path.len - baselen);
+			subdir_state =
+				read_directory_recursive(dir, path.buf, path.len,
+							 ud, check_only, simplify);
 			if (subdir_state > dir_state)
 				dir_state = subdir_state;
 		}
 
 		if (check_only) {
 			/* abort early if maximum state has been reached */
-			if (dir_state == path_untracked)
+			if (dir_state == path_untracked) {
+				if (cdir.fdir)
+					add_untracked(untracked, path.buf + baselen);
 				break;
+			}
 			/* skip the dir_add_* part */
 			continue;
 		}
@@ -1391,15 +1749,18 @@ static enum path_treatment read_directory_recursive(struct dir_struct *dir,
 			break;
 
 		case path_untracked:
-			if (!(dir->flags & DIR_SHOW_IGNORED))
-				dir_add_name(dir, path.buf, path.len);
+			if (dir->flags & DIR_SHOW_IGNORED)
+				break;
+			dir_add_name(dir, path.buf, path.len);
+			if (cdir.fdir)
+				add_untracked(untracked, path.buf + baselen);
 			break;
 
 		default:
 			break;
 		}
 	}
-	closedir(fdir);
+	close_cached_dir(&cdir);
  out:
 	strbuf_release(&path);
 
@@ -1469,7 +1830,7 @@ static int treat_leading_path(struct dir_struct *dir,
 			break;
 		if (simplify_away(sb.buf, sb.len, simplify))
 			break;
-		if (treat_one_path(dir, &sb, simplify,
+		if (treat_one_path(dir, NULL, &sb, simplify,
 				   DT_DIR, NULL) == path_none)
 			break; /* do not recurse into it */
 		if (len <= baselen) {
@@ -1482,9 +1843,139 @@ static int treat_leading_path(struct dir_struct *dir,
 	return rc;
 }
 
+static const char *get_ident_string(void)
+{
+	static struct strbuf sb = STRBUF_INIT;
+	struct utsname uts;
+
+	if (sb.len)
+		return sb.buf;
+	if (uname(&uts))
+		die_errno(_("failed to get kernel name and information"));
+	strbuf_addf(&sb, "Location %s, system %s %s %s", get_git_work_tree(),
+		    uts.sysname, uts.release, uts.version);
+	return sb.buf;
+}
+
+static int ident_in_untracked(const struct untracked_cache *uc)
+{
+	const char *end = uc->ident.buf + uc->ident.len;
+	const char *p   = uc->ident.buf;
+
+	for (p = uc->ident.buf; p < end; p += strlen(p) + 1)
+		if (!strcmp(p, get_ident_string()))
+			return 1;
+	return 0;
+}
+
+void add_untracked_ident(struct untracked_cache *uc)
+{
+	if (ident_in_untracked(uc))
+		return;
+	strbuf_addstr(&uc->ident, get_ident_string());
+	/* this strbuf contains a list of strings, save NUL too */
+	strbuf_addch(&uc->ident, 0);
+}
+
+static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *dir,
+						      int base_len,
+						      const struct pathspec *pathspec)
+{
+	struct untracked_cache_dir *root;
+	int i;
+
+	if (!dir->untracked || getenv("GIT_DISABLE_UNTRACKED_CACHE"))
+		return NULL;
+
+	/*
+	 * We only support $GIT_DIR/info/exclude and core.excludesfile
+	 * as the global ignore rule files. Any other additions
+	 * (e.g. from command line) invalidate the cache. This
+	 * condition also catches running setup_standard_excludes()
+	 * before setting dir->untracked!
+	 */
+	if (dir->unmanaged_exclude_files)
+		return NULL;
+
+	/*
+	 * Optimize for the main use case only: whole-tree git
+	 * status. More work involved in treat_leading_path() if we
+	 * use cache on just a subset of the worktree. pathspec
+	 * support could make the matter even worse.
+	 */
+	if (base_len || (pathspec && pathspec->nr))
+		return NULL;
+
+	/* Different set of flags may produce different results */
+	if (dir->flags != dir->untracked->dir_flags ||
+	    /*
+	     * See treat_directory(), case index_nonexistent. Without
+	     * this flag, we may need to also cache .git file content
+	     * for the resolve_gitlink_ref() call, which we don't.
+	     */
+	    !(dir->flags & DIR_SHOW_OTHER_DIRECTORIES) ||
+	    /* We don't support collecting ignore files */
+	    (dir->flags & (DIR_SHOW_IGNORED | DIR_SHOW_IGNORED_TOO |
+			   DIR_COLLECT_IGNORED)))
+		return NULL;
+
+	/*
+	 * If we use .gitignore in the cache and now you change it to
+	 * .gitexclude, everything will go wrong.
+	 */
+	if (dir->exclude_per_dir != dir->untracked->exclude_per_dir &&
+	    strcmp(dir->exclude_per_dir, dir->untracked->exclude_per_dir))
+		return NULL;
+
+	/*
+	 * EXC_CMDL is not considered in the cache. If people set it,
+	 * skip the cache.
+	 */
+	if (dir->exclude_list_group[EXC_CMDL].nr)
+		return NULL;
+
+	/*
+	 * An optimization in prep_exclude() does not play well with
+	 * CE_SKIP_WORKTREE. It's a rare case anyway, if a single
+	 * entry has that bit set, disable the whole untracked cache.
+	 */
+	for (i = 0; i < active_nr; i++)
+		if (ce_skip_worktree(active_cache[i]))
+			return NULL;
+
+	if (!ident_in_untracked(dir->untracked)) {
+		warning(_("Untracked cache is disabled on this system."));
+		return NULL;
+	}
+
+	if (!dir->untracked->root) {
+		const int len = sizeof(*dir->untracked->root);
+		dir->untracked->root = xmalloc(len);
+		memset(dir->untracked->root, 0, len);
+	}
+
+	/* Validate $GIT_DIR/info/exclude and core.excludesfile */
+	root = dir->untracked->root;
+	if (hashcmp(dir->ss_info_exclude.sha1,
+		    dir->untracked->ss_info_exclude.sha1)) {
+		invalidate_gitignore(dir->untracked, root);
+		dir->untracked->ss_info_exclude = dir->ss_info_exclude;
+	}
+	if (hashcmp(dir->ss_excludes_file.sha1,
+		    dir->untracked->ss_excludes_file.sha1)) {
+		invalidate_gitignore(dir->untracked, root);
+		dir->untracked->ss_excludes_file = dir->ss_excludes_file;
+	}
+
+	/* Make sure this directory is not dropped out at saving phase */
+	root->recurse = 1;
+	return root;
+}
+
 int read_directory(struct dir_struct *dir, const char *path, int len, const struct pathspec *pathspec)
 {
 	struct path_simplify *simplify;
+	struct untracked_cache_dir *untracked;
 
 	/*
 	 * Check out create_simplify()
@@ -1508,11 +1999,39 @@ int read_directory(struct dir_struct *dir, const char *path, int len, const stru
 	 * create_simplify().
 	 */
 	simplify = create_simplify(pathspec ? pathspec->_raw : NULL);
+	untracked = validate_untracked_cache(dir, len, pathspec);
+	if (!untracked)
+		/*
+		 * make sure untracked cache code path is disabled,
+		 * e.g. prep_exclude()
+		 */
+		dir->untracked = NULL;
 	if (!len || treat_leading_path(dir, path, len, simplify))
-		read_directory_recursive(dir, path, len, 0, simplify);
+		read_directory_recursive(dir, path, len, untracked, 0, simplify);
 	free_simplify(simplify);
 	qsort(dir->entries, dir->nr, sizeof(struct dir_entry *), cmp_name);
 	qsort(dir->ignored, dir->ignored_nr, sizeof(struct dir_entry *), cmp_name);
+	if (dir->untracked) {
+		static struct trace_key trace_untracked_stats = TRACE_KEY_INIT(UNTRACKED_STATS);
+		trace_printf_key(&trace_untracked_stats,
+				 "node creation: %u\n"
+				 "gitignore invalidation: %u\n"
+				 "directory invalidation: %u\n"
+				 "opendir: %u\n",
+				 dir->untracked->dir_created,
+				 dir->untracked->gitignore_invalidated,
+				 dir->untracked->dir_invalidated,
+				 dir->untracked->dir_opened);
+		if (dir->untracked == the_index.untracked &&
+		    (dir->untracked->dir_opened ||
+		     dir->untracked->gitignore_invalidated ||
+		     dir->untracked->dir_invalidated))
+			the_index.cache_changed |= UNTRACKED_CHANGED;
+		if (dir->untracked != the_index.untracked) {
+			free(dir->untracked);
+			dir->untracked = NULL;
+		}
+	}
 	return dir->nr;
 }
 
@@ -1678,12 +2197,14 @@ void setup_standard_excludes(struct dir_struct *dir)
 	if (!excludes_file)
 		excludes_file = xdg_config_home("ignore");
 	if (excludes_file && !access_or_warn(excludes_file, R_OK, 0))
-		add_excludes_from_file(dir, excludes_file);
+		add_excludes_from_file_1(dir, excludes_file,
+					 dir->untracked ? &dir->ss_excludes_file : NULL);
 
 	/* per repository user preference */
 	path = git_path("info/exclude");
 	if (!access_or_warn(path, R_OK, 0))
-		add_excludes_from_file(dir, path);
+		add_excludes_from_file_1(dir, path,
+					 dir->untracked ? &dir->ss_info_exclude : NULL);
 }
 
 int remove_path(const char *name)
@@ -1735,3 +2256,404 @@ void clear_directory(struct dir_struct *dir)
 	}
 	strbuf_release(&dir->basebuf);
 }
+
+struct ondisk_untracked_cache {
+	struct stat_data info_exclude_stat;
+	struct stat_data excludes_file_stat;
+	uint32_t dir_flags;
+	unsigned char info_exclude_sha1[20];
+	unsigned char excludes_file_sha1[20];
+	char exclude_per_dir[FLEX_ARRAY];
+};
+
+#define ouc_size(len) (offsetof(struct ondisk_untracked_cache, exclude_per_dir) + len + 1)
+
+struct write_data {
+	int index;	   /* number of written untracked_cache_dir */
+	struct ewah_bitmap *check_only; /* from untracked_cache_dir */
+	struct ewah_bitmap *valid;	/* from untracked_cache_dir */
+	struct ewah_bitmap *sha1_valid; /* set if exclude_sha1 is not null */
+	struct strbuf out;
+	struct strbuf sb_stat;
+	struct strbuf sb_sha1;
+};
+
+static void stat_data_to_disk(struct stat_data *to, const struct stat_data *from)
+{
+	to->sd_ctime.sec  = htonl(from->sd_ctime.sec);
+	to->sd_ctime.nsec = htonl(from->sd_ctime.nsec);
+	to->sd_mtime.sec  = htonl(from->sd_mtime.sec);
+	to->sd_mtime.nsec = htonl(from->sd_mtime.nsec);
+	to->sd_dev	  = htonl(from->sd_dev);
+	to->sd_ino	  = htonl(from->sd_ino);
+	to->sd_uid	  = htonl(from->sd_uid);
+	to->sd_gid	  = htonl(from->sd_gid);
+	to->sd_size	  = htonl(from->sd_size);
+}
+
+static void write_one_dir(struct untracked_cache_dir *untracked,
+			  struct write_data *wd)
+{
+	struct stat_data stat_data;
+	struct strbuf *out = &wd->out;
+	unsigned char intbuf[16];
+	unsigned int intlen, value;
+	int i = wd->index++;
+
+	/*
+	 * untracked_nr should be reset whenever valid is clear, but
+	 * for safety..
+	 */
+	if (!untracked->valid) {
+		untracked->untracked_nr = 0;
+		untracked->check_only = 0;
+	}
+
+	if (untracked->check_only)
+		ewah_set(wd->check_only, i);
+	if (untracked->valid) {
+		ewah_set(wd->valid, i);
+		stat_data_to_disk(&stat_data, &untracked->stat_data);
+		strbuf_add(&wd->sb_stat, &stat_data, sizeof(stat_data));
+	}
+	if (!is_null_sha1(untracked->exclude_sha1)) {
+		ewah_set(wd->sha1_valid, i);
+		strbuf_add(&wd->sb_sha1, untracked->exclude_sha1, 20);
+	}
+
+	intlen = encode_varint(untracked->untracked_nr, intbuf);
+	strbuf_add(out, intbuf, intlen);
+
+	/* skip non-recurse directories */
+	for (i = 0, value = 0; i < untracked->dirs_nr; i++)
+		if (untracked->dirs[i]->recurse)
+			value++;
+	intlen = encode_varint(value, intbuf);
+	strbuf_add(out, intbuf, intlen);
+
+	strbuf_add(out, untracked->name, strlen(untracked->name) + 1);
+
+	for (i = 0; i < untracked->untracked_nr; i++)
+		strbuf_add(out, untracked->untracked[i],
+			   strlen(untracked->untracked[i]) + 1);
+
+	for (i = 0; i < untracked->dirs_nr; i++)
+		if (untracked->dirs[i]->recurse)
+			write_one_dir(untracked->dirs[i], wd);
+}
+
+void write_untracked_extension(struct strbuf *out, struct untracked_cache *untracked)
+{
+	struct ondisk_untracked_cache *ouc;
+	struct write_data wd;
+	unsigned char varbuf[16];
+	int len = 0, varint_len;
+	if (untracked->exclude_per_dir)
+		len = strlen(untracked->exclude_per_dir);
+	ouc = xmalloc(sizeof(*ouc) + len + 1);
+	stat_data_to_disk(&ouc->info_exclude_stat, &untracked->ss_info_exclude.stat);
+	stat_data_to_disk(&ouc->excludes_file_stat, &untracked->ss_excludes_file.stat);
+	hashcpy(ouc->info_exclude_sha1, untracked->ss_info_exclude.sha1);
+	hashcpy(ouc->excludes_file_sha1, untracked->ss_excludes_file.sha1);
+	ouc->dir_flags = htonl(untracked->dir_flags);
+	memcpy(ouc->exclude_per_dir, untracked->exclude_per_dir, len + 1);
+
+	varint_len = encode_varint(untracked->ident.len, varbuf);
+	strbuf_add(out, varbuf, varint_len);
+	strbuf_add(out, untracked->ident.buf, untracked->ident.len);
+
+	strbuf_add(out, ouc, ouc_size(len));
+	free(ouc);
+	ouc = NULL;
+
+	if (!untracked->root) {
+		varint_len = encode_varint(0, varbuf);
+		strbuf_add(out, varbuf, varint_len);
+		return;
+	}
+
+	wd.index      = 0;
+	wd.check_only = ewah_new();
+	wd.valid      = ewah_new();
+	wd.sha1_valid = ewah_new();
+	strbuf_init(&wd.out, 1024);
+	strbuf_init(&wd.sb_stat, 1024);
+	strbuf_init(&wd.sb_sha1, 1024);
+	write_one_dir(untracked->root, &wd);
+
+	varint_len = encode_varint(wd.index, varbuf);
+	strbuf_add(out, varbuf, varint_len);
+	strbuf_addbuf(out, &wd.out);
+	ewah_serialize_strbuf(wd.valid, out);
+	ewah_serialize_strbuf(wd.check_only, out);
+	ewah_serialize_strbuf(wd.sha1_valid, out);
+	strbuf_addbuf(out, &wd.sb_stat);
+	strbuf_addbuf(out, &wd.sb_sha1);
+	strbuf_addch(out, '\0'); /* safe guard for string lists */
+
+	ewah_free(wd.valid);
+	ewah_free(wd.check_only);
+	ewah_free(wd.sha1_valid);
+	strbuf_release(&wd.out);
+	strbuf_release(&wd.sb_stat);
+	strbuf_release(&wd.sb_sha1);
+}
+
+static void free_untracked(struct untracked_cache_dir *ucd)
+{
+	int i;
+	if (!ucd)
+		return;
+	for (i = 0; i < ucd->dirs_nr; i++)
+		free_untracked(ucd->dirs[i]);
+	for (i = 0; i < ucd->untracked_nr; i++)
+		free(ucd->untracked[i]);
+	free(ucd->untracked);
+	free(ucd->dirs);
+	free(ucd);
+}
+
+void free_untracked_cache(struct untracked_cache *uc)
+{
+	if (uc)
+		free_untracked(uc->root);
+	free(uc);
+}
+
+struct read_data {
+	int index;
+	struct untracked_cache_dir **ucd;
+	struct ewah_bitmap *check_only;
+	struct ewah_bitmap *valid;
+	struct ewah_bitmap *sha1_valid;
+	const unsigned char *data;
+	const unsigned char *end;
+};
+
+static void stat_data_from_disk(struct stat_data *to, const struct stat_data *from)
+{
+	to->sd_ctime.sec  = get_be32(&from->sd_ctime.sec);
+	to->sd_ctime.nsec = get_be32(&from->sd_ctime.nsec);
+	to->sd_mtime.sec  = get_be32(&from->sd_mtime.sec);
+	to->sd_mtime.nsec = get_be32(&from->sd_mtime.nsec);
+	to->sd_dev	  = get_be32(&from->sd_dev);
+	to->sd_ino	  = get_be32(&from->sd_ino);
+	to->sd_uid	  = get_be32(&from->sd_uid);
+	to->sd_gid	  = get_be32(&from->sd_gid);
+	to->sd_size	  = get_be32(&from->sd_size);
+}
+
+static int read_one_dir(struct untracked_cache_dir **untracked_,
+			struct read_data *rd)
+{
+	struct untracked_cache_dir ud, *untracked;
+	const unsigned char *next, *data = rd->data, *end = rd->end;
+	unsigned int value;
+	int i, len;
+
+	memset(&ud, 0, sizeof(ud));
+
+	next = data;
+	value = decode_varint(&next);
+	if (next > end)
+		return -1;
+	ud.recurse	   = 1;
+	ud.untracked_alloc = value;
+	ud.untracked_nr	   = value;
+	if (ud.untracked_nr)
+		ud.untracked = xmalloc(sizeof(*ud.untracked) * ud.untracked_nr);
+	data = next;
+
+	next = data;
+	ud.dirs_alloc = ud.dirs_nr = decode_varint(&next);
+	if (next > end)
+		return -1;
+	ud.dirs = xmalloc(sizeof(*ud.dirs) * ud.dirs_nr);
+	data = next;
+
+	len = strlen((const char *)data);
+	next = data + len + 1;
+	if (next > rd->end)
+		return -1;
+	*untracked_ = untracked = xmalloc(sizeof(*untracked) + len);
+	memcpy(untracked, &ud, sizeof(ud));
+	memcpy(untracked->name, data, len + 1);
+	data = next;
+
+	for (i = 0; i < untracked->untracked_nr; i++) {
+		len = strlen((const char *)data);
+		next = data + len + 1;
+		if (next > rd->end)
+			return -1;
+		untracked->untracked[i] = xstrdup((const char*)data);
+		data = next;
+	}
+
+	rd->ucd[rd->index++] = untracked;
+	rd->data = data;
+
+	for (i = 0; i < untracked->dirs_nr; i++) {
+		len = read_one_dir(untracked->dirs + i, rd);
+		if (len < 0)
+			return -1;
+	}
+	return 0;
+}
+
+static void set_check_only(size_t pos, void *cb)
+{
+	struct read_data *rd = cb;
+	struct untracked_cache_dir *ud = rd->ucd[pos];
+	ud->check_only = 1;
+}
+
+static void read_stat(size_t pos, void *cb)
+{
+	struct read_data *rd = cb;
+	struct untracked_cache_dir *ud = rd->ucd[pos];
+	if (rd->data + sizeof(struct stat_data) > rd->end) {
+		rd->data = rd->end + 1;
+		return;
+	}
+	stat_data_from_disk(&ud->stat_data, (struct stat_data *)rd->data);
+	rd->data += sizeof(struct stat_data);
+	ud->valid = 1;
+}
+
+static void read_sha1(size_t pos, void *cb)
+{
+	struct read_data *rd = cb;
+	struct untracked_cache_dir *ud = rd->ucd[pos];
+	if (rd->data + 20 > rd->end) {
+		rd->data = rd->end + 1;
+		return;
+	}
+	hashcpy(ud->exclude_sha1, rd->data);
+	rd->data += 20;
+}
+
+static void load_sha1_stat(struct sha1_stat *sha1_stat,
+			   const struct stat_data *stat,
+			   const unsigned char *sha1)
+{
+	stat_data_from_disk(&sha1_stat->stat, stat);
+	hashcpy(sha1_stat->sha1, sha1);
+	sha1_stat->valid = 1;
+}
+
+struct untracked_cache *read_untracked_extension(const void *data, unsigned long sz)
+{
+	const struct ondisk_untracked_cache *ouc;
+	struct untracked_cache *uc;
+	struct read_data rd;
+	const unsigned char *next = data, *end = (const unsigned char *)data + sz;
+	const char *ident;
+	int ident_len, len;
+
+	if (sz <= 1 || end[-1] != '\0')
+		return NULL;
+	end--;
+
+	ident_len = decode_varint(&next);
+	if (next + ident_len > end)
+		return NULL;
+	ident = (const char *)next;
+	next += ident_len;
+
+	ouc = (const struct ondisk_untracked_cache *)next;
+	if (next + ouc_size(0) > end)
+		return NULL;
+
+	uc = xcalloc(1, sizeof(*uc));
+	strbuf_init(&uc->ident, ident_len);
+	strbuf_add(&uc->ident, ident, ident_len);
+	load_sha1_stat(&uc->ss_info_exclude, &ouc->info_exclude_stat,
+		       ouc->info_exclude_sha1);
+	load_sha1_stat(&uc->ss_excludes_file, &ouc->excludes_file_stat,
+		       ouc->excludes_file_sha1);
+	uc->dir_flags = get_be32(&ouc->dir_flags);
+	uc->exclude_per_dir = xstrdup(ouc->exclude_per_dir);
+	/* NUL after exclude_per_dir is covered by sizeof(*ouc) */
+	next += ouc_size(strlen(ouc->exclude_per_dir));
+	if (next >= end)
+		goto done2;
+
+	len = decode_varint(&next);
+	if (next > end || len == 0)
+		goto done2;
+
+	rd.valid      = ewah_new();
+	rd.check_only = ewah_new();
+	rd.sha1_valid = ewah_new();
+	rd.data	      = next;
+	rd.end	      = end;
+	rd.index      = 0;
+	rd.ucd        = xmalloc(sizeof(*rd.ucd) * len);
+
+	if (read_one_dir(&uc->root, &rd) || rd.index != len)
+		goto done;
+
+	next = rd.data;
+	len = ewah_read_mmap(rd.valid, next, end - next);
+	if (len < 0)
+		goto done;
+
+	next += len;
+	len = ewah_read_mmap(rd.check_only, next, end - next);
+	if (len < 0)
+		goto done;
+
+	next += len;
+	len = ewah_read_mmap(rd.sha1_valid, next, end - next);
+	if (len < 0)
+		goto done;
+
+	ewah_each_bit(rd.check_only, set_check_only, &rd);
+	rd.data = next + len;
+	ewah_each_bit(rd.valid, read_stat, &rd);
+	ewah_each_bit(rd.sha1_valid, read_sha1, &rd);
+	next = rd.data;
+
+done:
+	free(rd.ucd);
+	ewah_free(rd.valid);
+	ewah_free(rd.check_only);
+	ewah_free(rd.sha1_valid);
+done2:
+	if (next != end) {
+		free_untracked_cache(uc);
+		uc = NULL;
+	}
+	return uc;
+}
+
+void untracked_cache_invalidate_path(struct index_state *istate,
+				     const char *path)
+{
+	const char *sep;
+	struct untracked_cache_dir *d;
+	if (!istate->untracked || !istate->untracked->root)
+		return;
+	sep = strrchr(path, '/');
+	if (sep)
+		d = lookup_untracked(istate->untracked,
+				     istate->untracked->root,
+				     path, sep - path);
+	else
+		d = istate->untracked->root;
+	istate->untracked->dir_invalidated++;
+	d->valid = 0;
+	d->untracked_nr = 0;
+}
+
+void untracked_cache_remove_from_index(struct index_state *istate,
+				       const char *path)
+{
+	untracked_cache_invalidate_path(istate, path);
+}
+
+void untracked_cache_add_to_index(struct index_state *istate,
+				  const char *path)
+{
+	untracked_cache_invalidate_path(istate, path);
+}
diff --git a/dir.h b/dir.h
index 72b73c65dc..7b5855dd80 100644
--- a/dir.h
+++ b/dir.h
@@ -66,6 +66,7 @@ struct exclude_stack {
 	struct exclude_stack *prev; /* the struct exclude_stack for the parent directory */
 	int baselen;
 	int exclude_ix; /* index of exclude_list within EXC_DIRS exclude_list_group */
+	struct untracked_cache_dir *ucd;
 };
 
 struct exclude_list_group {
@@ -73,6 +74,73 @@ struct exclude_list_group {
 	struct exclude_list *el;
 };
 
+struct sha1_stat {
+	struct stat_data stat;
+	unsigned char sha1[20];
+	int valid;
+};
+
+/*
+ *  Untracked cache
+ *
+ *  The following inputs are sufficient to determine what files in a
+ *  directory are excluded:
+ *
+ *   - The list of files and directories of the directory in question
+ *   - The $GIT_DIR/index
+ *   - dir_struct flags
+ *   - The content of $GIT_DIR/info/exclude
+ *   - The content of core.excludesfile
+ *   - The content (or the lack) of .gitignore of all parent directories
+ *     from $GIT_WORK_TREE
+ *   - The check_only flag in read_directory_recursive (for
+ *     DIR_HIDE_EMPTY_DIRECTORIES)
+ *
+ *  The first input can be checked using directory mtime. In many
+ *  filesystems, directory mtime (stat_data field) is updated when its
+ *  files or direct subdirs are added or removed.
+ *
+ *  The second one can be hooked from cache_tree_invalidate_path().
+ *  Whenever a file (or a submodule) is added or removed from a
+ *  directory, we invalidate that directory.
+ *
+ *  The remaining inputs are easy, their SHA-1 could be used to verify
+ *  their contents (exclude_sha1[], info_exclude_sha1[] and
+ *  excludes_file_sha1[])
+ */
+struct untracked_cache_dir {
+	struct untracked_cache_dir **dirs;
+	char **untracked;
+	struct stat_data stat_data;
+	unsigned int untracked_alloc, dirs_nr, dirs_alloc;
+	unsigned int untracked_nr;
+	unsigned int check_only : 1;
+	/* all data except 'dirs' in this struct are good */
+	unsigned int valid : 1;
+	unsigned int recurse : 1;
+	/* null SHA-1 means this directory does not have .gitignore */
+	unsigned char exclude_sha1[20];
+	char name[FLEX_ARRAY];
+};
+
+struct untracked_cache {
+	struct sha1_stat ss_info_exclude;
+	struct sha1_stat ss_excludes_file;
+	const char *exclude_per_dir;
+	struct strbuf ident;
+	/*
+	 * dir_struct#flags must match dir_flags or the untracked
+	 * cache is ignored.
+	 */
+	unsigned dir_flags;
+	struct untracked_cache_dir *root;
+	/* Statistics */
+	int dir_created;
+	int gitignore_invalidated;
+	int dir_invalidated;
+	int dir_opened;
+};
+
 struct dir_struct {
 	int nr, alloc;
 	int ignored_nr, ignored_alloc;
@@ -120,6 +188,12 @@ struct dir_struct {
 	struct exclude_stack *exclude_stack;
 	struct exclude *exclude;
 	struct strbuf basebuf;
+
+	/* Enable untracked file cache if set */
+	struct untracked_cache *untracked;
+	struct sha1_stat ss_info_exclude;
+	struct sha1_stat ss_excludes_file;
+	unsigned unmanaged_exclude_files;
 };
 
 /*
@@ -226,4 +300,12 @@ static inline int dir_path_match(const struct dir_entry *ent,
 			      has_trailing_dir);
 }
 
+void untracked_cache_invalidate_path(struct index_state *, const char *);
+void untracked_cache_remove_from_index(struct index_state *, const char *);
+void untracked_cache_add_to_index(struct index_state *, const char *);
+
+void free_untracked_cache(struct untracked_cache *);
+struct untracked_cache *read_untracked_extension(const void *data, unsigned long sz);
+void write_untracked_extension(struct strbuf *out, struct untracked_cache *untracked);
+void add_untracked_ident(struct untracked_cache *);
 #endif
diff --git a/ewah/ewah_io.c b/ewah/ewah_io.c
index 1c2d7afd4c..43481b9c60 100644
--- a/ewah/ewah_io.c
+++ b/ewah/ewah_io.c
@@ -19,6 +19,7 @@
  */
 #include "git-compat-util.h"
 #include "ewok.h"
+#include "strbuf.h"
 
 int ewah_serialize_native(struct ewah_bitmap *self, int fd)
 {
@@ -110,6 +111,18 @@ int ewah_serialize(struct ewah_bitmap *self, int fd)
 	return ewah_serialize_to(self, write_helper, (void *)(intptr_t)fd);
 }
 
+static int write_strbuf(void *user_data, const void *data, size_t len)
+{
+	struct strbuf *sb = user_data;
+	strbuf_add(sb, data, len);
+	return len;
+}
+
+int ewah_serialize_strbuf(struct ewah_bitmap *self, struct strbuf *sb)
+{
+	return ewah_serialize_to(self, write_strbuf, sb);
+}
+
 int ewah_read_mmap(struct ewah_bitmap *self, const void *map, size_t len)
 {
 	const uint8_t *ptr = map;
diff --git a/ewah/ewok.h b/ewah/ewok.h
index 13c6e20412..e732525367 100644
--- a/ewah/ewok.h
+++ b/ewah/ewok.h
@@ -30,6 +30,7 @@
 #	define ewah_calloc xcalloc
 #endif
 
+struct strbuf;
 typedef uint64_t eword_t;
 #define BITS_IN_WORD (sizeof(eword_t) * 8)
 
@@ -98,6 +99,7 @@ int ewah_serialize_to(struct ewah_bitmap *self,
 		      void *out);
 int ewah_serialize(struct ewah_bitmap *self, int fd);
 int ewah_serialize_native(struct ewah_bitmap *self, int fd);
+int ewah_serialize_strbuf(struct ewah_bitmap *self, struct strbuf *);
 
 int ewah_deserialize(struct ewah_bitmap *self, int fd);
 int ewah_read_mmap(struct ewah_bitmap *self, const void *map, size_t len);
diff --git a/git-compat-util.h b/git-compat-util.h
index b7a97fbe83..17584adbd0 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -189,6 +189,7 @@
 #elif defined(_MSC_VER)
 #include "compat/msvc.h"
 #else
+#include <sys/utsname.h>
 #include <sys/wait.h>
 #include <sys/resource.h>
 #include <sys/socket.h>
diff --git a/read-cache.c b/read-cache.c
index 36ff89f29e..723d48dddf 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -39,11 +39,12 @@ static struct cache_entry *refresh_cache_entry(struct cache_entry *ce,
 #define CACHE_EXT_TREE 0x54524545	/* "TREE" */
 #define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
 #define CACHE_EXT_LINK 0x6c696e6b	  /* "link" */
+#define CACHE_EXT_UNTRACKED 0x554E5452	  /* "UNTR" */
 
 /* changes that can be kept in $GIT_DIR/index (basically all extensions) */
 #define EXTMASK (RESOLVE_UNDO_CHANGED | CACHE_TREE_CHANGED | \
 		 CE_ENTRY_ADDED | CE_ENTRY_REMOVED | CE_ENTRY_CHANGED | \
-		 SPLIT_INDEX_ORDERED)
+		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED)
 
 struct index_state the_index;
 static const char *alternate_index_output;
@@ -79,6 +80,7 @@ void rename_index_entry_at(struct index_state *istate, int nr, const char *new_n
 	memcpy(new->name, new_name, namelen + 1);
 
 	cache_tree_invalidate_path(istate, old->name);
+	untracked_cache_remove_from_index(istate, old->name);
 	remove_index_entry_at(istate, nr);
 	add_index_entry(istate, new, ADD_CACHE_OK_TO_ADD|ADD_CACHE_OK_TO_REPLACE);
 }
@@ -270,20 +272,34 @@ static int ce_match_stat_basic(const struct cache_entry *ce, struct stat *st)
 	return changed;
 }
 
-static int is_racy_timestamp(const struct index_state *istate,
-			     const struct cache_entry *ce)
+static int is_racy_stat(const struct index_state *istate,
+			const struct stat_data *sd)
 {
-	return (!S_ISGITLINK(ce->ce_mode) &&
-		istate->timestamp.sec &&
+	return (istate->timestamp.sec &&
 #ifdef USE_NSEC
 		 /* nanosecond timestamped files can also be racy! */
-		(istate->timestamp.sec < ce->ce_stat_data.sd_mtime.sec ||
-		 (istate->timestamp.sec == ce->ce_stat_data.sd_mtime.sec &&
-		  istate->timestamp.nsec <= ce->ce_stat_data.sd_mtime.nsec))
+		(istate->timestamp.sec < sd->sd_mtime.sec ||
+		 (istate->timestamp.sec == sd->sd_mtime.sec &&
+		  istate->timestamp.nsec <= sd->sd_mtime.nsec))
 #else
-		istate->timestamp.sec <= ce->ce_stat_data.sd_mtime.sec
+		istate->timestamp.sec <= sd->sd_mtime.sec
 #endif
-		 );
+		);
+}
+
+static int is_racy_timestamp(const struct index_state *istate,
+			     const struct cache_entry *ce)
+{
+	return (!S_ISGITLINK(ce->ce_mode) &&
+		is_racy_stat(istate, &ce->ce_stat_data));
+}
+
+int match_stat_data_racy(const struct index_state *istate,
+			 const struct stat_data *sd, struct stat *st)
+{
+	if (is_racy_stat(istate, sd))
+		return MTIME_CHANGED;
+	return match_stat_data(sd, st);
 }
 
 int ie_match_stat(const struct index_state *istate,
@@ -538,6 +554,7 @@ int remove_file_from_index(struct index_state *istate, const char *path)
 	if (pos < 0)
 		pos = -pos-1;
 	cache_tree_invalidate_path(istate, path);
+	untracked_cache_remove_from_index(istate, path);
 	while (pos < istate->cache_nr && !strcmp(istate->cache[pos]->name, path))
 		remove_index_entry_at(istate, pos);
 	return 0;
@@ -982,6 +999,8 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e
 	}
 	pos = -pos-1;
 
+	untracked_cache_add_to_index(istate, ce->name);
+
 	/*
 	 * Inserting a merged entry ("stage 0") into the index
 	 * will always replace all non-merged entries..
@@ -1372,6 +1391,9 @@ static int read_index_extension(struct index_state *istate,
 		if (read_link_extension(istate, data, sz))
 			return -1;
 		break;
+	case CACHE_EXT_UNTRACKED:
+		istate->untracked = read_untracked_extension(data, sz);
+		break;
 	default:
 		if (*ext < 'A' || 'Z' < *ext)
 			return error("index uses %.4s extension, which we do not understand",
@@ -1667,6 +1689,8 @@ int discard_index(struct index_state *istate)
 	istate->cache = NULL;
 	istate->cache_alloc = 0;
 	discard_split_index(istate);
+	free_untracked_cache(istate->untracked);
+	istate->untracked = NULL;
 	return 0;
 }
 
@@ -2053,6 +2077,17 @@ static int do_write_index(struct index_state *istate, int newfd,
 		if (err)
 			return -1;
 	}
+	if (!strip_extensions && istate->untracked) {
+		struct strbuf sb = STRBUF_INIT;
+
+		write_untracked_extension(&sb, istate->untracked);
+		err = write_index_ext_header(&c, newfd, CACHE_EXT_UNTRACKED,
+					     sb.len) < 0 ||
+			ce_write(&c, newfd, sb.buf, sb.len) < 0;
+		strbuf_release(&sb);
+		if (err)
+			return -1;
+	}
 
 	if (ce_flush(&c, newfd, istate->sha1) || fstat(newfd, &st))
 		return -1;
diff --git a/split-index.c b/split-index.c
index 21485e2066..968b780a06 100644
--- a/split-index.c
+++ b/split-index.c
@@ -41,13 +41,6 @@ int read_link_extension(struct index_state *istate,
 	return 0;
 }
 
-static int write_strbuf(void *user_data, const void *data, size_t len)
-{
-	struct strbuf *sb = user_data;
-	strbuf_add(sb, data, len);
-	return len;
-}
-
 int write_link_extension(struct strbuf *sb,
 			 struct index_state *istate)
 {
@@ -55,8 +48,8 @@ int write_link_extension(struct strbuf *sb,
 	strbuf_add(sb, si->base_sha1, 20);
 	if (!si->delete_bitmap && !si->replace_bitmap)
 		return 0;
-	ewah_serialize_to(si->delete_bitmap, write_strbuf, sb);
-	ewah_serialize_to(si->replace_bitmap, write_strbuf, sb);
+	ewah_serialize_strbuf(si->delete_bitmap, sb);
+	ewah_serialize_strbuf(si->replace_bitmap, sb);
 	return 0;
 }
 
diff --git a/t/t7063-status-untracked-cache.sh b/t/t7063-status-untracked-cache.sh
new file mode 100755
index 0000000000..2b2ffd7d6b
--- /dev/null
+++ b/t/t7063-status-untracked-cache.sh
@@ -0,0 +1,353 @@
+#!/bin/sh
+
+test_description='test untracked cache'
+
+. ./test-lib.sh
+
+avoid_racy() {
+	sleep 1
+}
+
+git update-index --untracked-cache
+# It's fine if git update-index returns an error code other than one,
+# it'll be caught in the first test.
+if test $? -eq 1; then
+	skip_all='This system does not support untracked cache'
+	test_done
+fi
+
+test_expect_success 'setup' '
+	git init worktree &&
+	cd worktree &&
+	mkdir done dtwo dthree &&
+	touch one two three done/one dtwo/two dthree/three &&
+	git add one two done/one &&
+	: >.git/info/exclude &&
+	git update-index --untracked-cache
+'
+
+test_expect_success 'untracked cache is empty' '
+	test-dump-untracked-cache >../actual &&
+	cat >../expect <<EOF &&
+info/exclude 0000000000000000000000000000000000000000
+core.excludesfile 0000000000000000000000000000000000000000
+exclude_per_dir .gitignore
+flags 00000006
+EOF
+	test_cmp ../expect ../actual
+'
+
+cat >../status.expect <<EOF &&
+A  done/one
+A  one
+A  two
+?? dthree/
+?? dtwo/
+?? three
+EOF
+
+cat >../dump.expect <<EOF &&
+info/exclude e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
+core.excludesfile 0000000000000000000000000000000000000000
+exclude_per_dir .gitignore
+flags 00000006
+/ 0000000000000000000000000000000000000000 recurse valid
+dthree/
+dtwo/
+three
+/done/ 0000000000000000000000000000000000000000 recurse valid
+/dthree/ 0000000000000000000000000000000000000000 recurse check_only valid
+three
+/dtwo/ 0000000000000000000000000000000000000000 recurse check_only valid
+two
+EOF
+
+test_expect_success 'status first time (empty cache)' '
+	avoid_racy &&
+	: >../trace &&
+	GIT_TRACE_UNTRACKED_STATS="$TRASH_DIRECTORY/trace" \
+	git status --porcelain >../actual &&
+	test_cmp ../status.expect ../actual &&
+	cat >../trace.expect <<EOF &&
+node creation: 3
+gitignore invalidation: 1
+directory invalidation: 0
+opendir: 4
+EOF
+	test_cmp ../trace.expect ../trace
+'
+
+test_expect_success 'untracked cache after first status' '
+	test-dump-untracked-cache >../actual &&
+	test_cmp ../dump.expect ../actual
+'
+
+test_expect_success 'status second time (fully populated cache)' '
+	avoid_racy &&
+	: >../trace &&
+	GIT_TRACE_UNTRACKED_STATS="$TRASH_DIRECTORY/trace" \
+	git status --porcelain >../actual &&
+	test_cmp ../status.expect ../actual &&
+	cat >../trace.expect <<EOF &&
+node creation: 0
+gitignore invalidation: 0
+directory invalidation: 0
+opendir: 0
+EOF
+	test_cmp ../trace.expect ../trace
+'
+
+test_expect_success 'untracked cache after second status' '
+	test-dump-untracked-cache >../actual &&
+	test_cmp ../dump.expect ../actual
+'
+
+test_expect_success 'modify in root directory, one dir invalidation' '
+	avoid_racy &&
+	: >four &&
+	: >../trace &&
+	GIT_TRACE_UNTRACKED_STATS="$TRASH_DIRECTORY/trace" \
+	git status --porcelain >../actual &&
+	cat >../status.expect <<EOF &&
+A  done/one
+A  one
+A  two
+?? dthree/
+?? dtwo/
+?? four
+?? three
+EOF
+	test_cmp ../status.expect ../actual &&
+	cat >../trace.expect <<EOF &&
+node creation: 0
+gitignore invalidation: 0
+directory invalidation: 1
+opendir: 1
+EOF
+	test_cmp ../trace.expect ../trace
+
+'
+
+test_expect_success 'verify untracked cache dump' '
+	test-dump-untracked-cache >../actual &&
+	cat >../expect <<EOF &&
+info/exclude e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
+core.excludesfile 0000000000000000000000000000000000000000
+exclude_per_dir .gitignore
+flags 00000006
+/ 0000000000000000000000000000000000000000 recurse valid
+dthree/
+dtwo/
+four
+three
+/done/ 0000000000000000000000000000000000000000 recurse valid
+/dthree/ 0000000000000000000000000000000000000000 recurse check_only valid
+three
+/dtwo/ 0000000000000000000000000000000000000000 recurse check_only valid
+two
+EOF
+	test_cmp ../expect ../actual
+'
+
+test_expect_success 'new .gitignore invalidates recursively' '
+	avoid_racy &&
+	echo four >.gitignore &&
+	: >../trace &&
+	GIT_TRACE_UNTRACKED_STATS="$TRASH_DIRECTORY/trace" \
+	git status --porcelain >../actual &&
+	cat >../status.expect <<EOF &&
+A  done/one
+A  one
+A  two
+?? .gitignore
+?? dthree/
+?? dtwo/
+?? three
+EOF
+	test_cmp ../status.expect ../actual &&
+	cat >../trace.expect <<EOF &&
+node creation: 0
+gitignore invalidation: 1
+directory invalidation: 1
+opendir: 4
+EOF
+	test_cmp ../trace.expect ../trace
+
+'
+
+test_expect_success 'verify untracked cache dump' '
+	test-dump-untracked-cache >../actual &&
+	cat >../expect <<EOF &&
+info/exclude e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
+core.excludesfile 0000000000000000000000000000000000000000
+exclude_per_dir .gitignore
+flags 00000006
+/ e6fcc8f2ee31bae321d66afd183fcb7237afae6e recurse valid
+.gitignore
+dthree/
+dtwo/
+three
+/done/ 0000000000000000000000000000000000000000 recurse valid
+/dthree/ 0000000000000000000000000000000000000000 recurse check_only valid
+three
+/dtwo/ 0000000000000000000000000000000000000000 recurse check_only valid
+two
+EOF
+	test_cmp ../expect ../actual
+'
+
+test_expect_success 'new info/exclude invalidates everything' '
+	avoid_racy &&
+	echo three >>.git/info/exclude &&
+	: >../trace &&
+	GIT_TRACE_UNTRACKED_STATS="$TRASH_DIRECTORY/trace" \
+	git status --porcelain >../actual &&
+	cat >../status.expect <<EOF &&
+A  done/one
+A  one
+A  two
+?? .gitignore
+?? dtwo/
+EOF
+	test_cmp ../status.expect ../actual &&
+	cat >../trace.expect <<EOF &&
+node creation: 0
+gitignore invalidation: 1
+directory invalidation: 0
+opendir: 4
+EOF
+	test_cmp ../trace.expect ../trace
+'
+
+test_expect_success 'verify untracked cache dump' '
+	test-dump-untracked-cache >../actual &&
+	cat >../expect <<EOF &&
+info/exclude 13263c0978fb9fad16b2d580fb800b6d811c3ff0
+core.excludesfile 0000000000000000000000000000000000000000
+exclude_per_dir .gitignore
+flags 00000006
+/ e6fcc8f2ee31bae321d66afd183fcb7237afae6e recurse valid
+.gitignore
+dtwo/
+/done/ 0000000000000000000000000000000000000000 recurse valid
+/dthree/ 0000000000000000000000000000000000000000 recurse check_only valid
+/dtwo/ 0000000000000000000000000000000000000000 recurse check_only valid
+two
+EOF
+	test_cmp ../expect ../actual
+'
+
+test_expect_success 'move two from tracked to untracked' '
+	git rm --cached two &&
+	test-dump-untracked-cache >../actual &&
+	cat >../expect <<EOF &&
+info/exclude 13263c0978fb9fad16b2d580fb800b6d811c3ff0
+core.excludesfile 0000000000000000000000000000000000000000
+exclude_per_dir .gitignore
+flags 00000006
+/ e6fcc8f2ee31bae321d66afd183fcb7237afae6e recurse
+/done/ 0000000000000000000000000000000000000000 recurse valid
+/dthree/ 0000000000000000000000000000000000000000 recurse check_only valid
+/dtwo/ 0000000000000000000000000000000000000000 recurse check_only valid
+two
+EOF
+	test_cmp ../expect ../actual
+'
+
+test_expect_success 'status after the move' '
+	: >../trace &&
+	GIT_TRACE_UNTRACKED_STATS="$TRASH_DIRECTORY/trace" \
+	git status --porcelain >../actual &&
+	cat >../status.expect <<EOF &&
+A  done/one
+A  one
+?? .gitignore
+?? dtwo/
+?? two
+EOF
+	test_cmp ../status.expect ../actual &&
+	cat >../trace.expect <<EOF &&
+node creation: 0
+gitignore invalidation: 0
+directory invalidation: 0
+opendir: 1
+EOF
+	test_cmp ../trace.expect ../trace
+'
+
+test_expect_success 'verify untracked cache dump' '
+	test-dump-untracked-cache >../actual &&
+	cat >../expect <<EOF &&
+info/exclude 13263c0978fb9fad16b2d580fb800b6d811c3ff0
+core.excludesfile 0000000000000000000000000000000000000000
+exclude_per_dir .gitignore
+flags 00000006
+/ e6fcc8f2ee31bae321d66afd183fcb7237afae6e recurse valid
+.gitignore
+dtwo/
+two
+/done/ 0000000000000000000000000000000000000000 recurse valid
+/dthree/ 0000000000000000000000000000000000000000 recurse check_only valid
+/dtwo/ 0000000000000000000000000000000000000000 recurse check_only valid
+two
+EOF
+	test_cmp ../expect ../actual
+'
+
+test_expect_success 'move two from untracked to tracked' '
+	git add two &&
+	test-dump-untracked-cache >../actual &&
+	cat >../expect <<EOF &&
+info/exclude 13263c0978fb9fad16b2d580fb800b6d811c3ff0
+core.excludesfile 0000000000000000000000000000000000000000
+exclude_per_dir .gitignore
+flags 00000006
+/ e6fcc8f2ee31bae321d66afd183fcb7237afae6e recurse
+/done/ 0000000000000000000000000000000000000000 recurse valid
+/dthree/ 0000000000000000000000000000000000000000 recurse check_only valid
+/dtwo/ 0000000000000000000000000000000000000000 recurse check_only valid
+two
+EOF
+	test_cmp ../expect ../actual
+'
+
+test_expect_success 'status after the move' '
+	: >../trace &&
+	GIT_TRACE_UNTRACKED_STATS="$TRASH_DIRECTORY/trace" \
+	git status --porcelain >../actual &&
+	cat >../status.expect <<EOF &&
+A  done/one
+A  one
+A  two
+?? .gitignore
+?? dtwo/
+EOF
+	test_cmp ../status.expect ../actual &&
+	cat >../trace.expect <<EOF &&
+node creation: 0
+gitignore invalidation: 0
+directory invalidation: 0
+opendir: 1
+EOF
+	test_cmp ../trace.expect ../trace
+'
+
+test_expect_success 'verify untracked cache dump' '
+	test-dump-untracked-cache >../actual &&
+	cat >../expect <<EOF &&
+info/exclude 13263c0978fb9fad16b2d580fb800b6d811c3ff0
+core.excludesfile 0000000000000000000000000000000000000000
+exclude_per_dir .gitignore
+flags 00000006
+/ e6fcc8f2ee31bae321d66afd183fcb7237afae6e recurse valid
+.gitignore
+dtwo/
+/done/ 0000000000000000000000000000000000000000 recurse valid
+/dthree/ 0000000000000000000000000000000000000000 recurse check_only valid
+/dtwo/ 0000000000000000000000000000000000000000 recurse check_only valid
+two
+EOF
+	test_cmp ../expect ../actual
+'
+
+test_done
diff --git a/test-dump-untracked-cache.c b/test-dump-untracked-cache.c
new file mode 100644
index 0000000000..25d855d98b
--- /dev/null
+++ b/test-dump-untracked-cache.c
@@ -0,0 +1,62 @@
+#include "cache.h"
+#include "dir.h"
+
+static int compare_untracked(const void *a_, const void *b_)
+{
+	const char *const *a = a_;
+	const char *const *b = b_;
+	return strcmp(*a, *b);
+}
+
+static int compare_dir(const void *a_, const void *b_)
+{
+	const struct untracked_cache_dir *const *a = a_;
+	const struct untracked_cache_dir *const *b = b_;
+	return strcmp((*a)->name, (*b)->name);
+}
+
+static void dump(struct untracked_cache_dir *ucd, struct strbuf *base)
+{
+	int i, len;
+	qsort(ucd->untracked, ucd->untracked_nr, sizeof(*ucd->untracked),
+	      compare_untracked);
+	qsort(ucd->dirs, ucd->dirs_nr, sizeof(*ucd->dirs),
+	      compare_dir);
+	len = base->len;
+	strbuf_addf(base, "%s/", ucd->name);
+	printf("%s %s", base->buf,
+	       sha1_to_hex(ucd->exclude_sha1));
+	if (ucd->recurse)
+		fputs(" recurse", stdout);
+	if (ucd->check_only)
+		fputs(" check_only", stdout);
+	if (ucd->valid)
+		fputs(" valid", stdout);
+	printf("\n");
+	for (i = 0; i < ucd->untracked_nr; i++)
+		printf("%s\n", ucd->untracked[i]);
+	for (i = 0; i < ucd->dirs_nr; i++)
+		dump(ucd->dirs[i], base);
+	strbuf_setlen(base, len);
+}
+
+int main(int ac, char **av)
+{
+	struct untracked_cache *uc;
+	struct strbuf base = STRBUF_INIT;
+	setup_git_directory();
+	if (read_cache() < 0)
+		die("unable to read index file");
+	uc = the_index.untracked;
+	if (!uc) {
+		printf("no untracked cache\n");
+		return 0;
+	}
+	printf("info/exclude %s\n", sha1_to_hex(uc->ss_info_exclude.sha1));
+	printf("core.excludesfile %s\n", sha1_to_hex(uc->ss_excludes_file.sha1));
+	printf("exclude_per_dir %s\n", uc->exclude_per_dir);
+	printf("flags %08x\n", uc->dir_flags);
+	if (uc->root)
+		dump(uc->root, &base);
+	return 0;
+}
diff --git a/unpack-trees.c b/unpack-trees.c
index be84ba2607..2927660d92 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -9,6 +9,7 @@
 #include "refs.h"
 #include "attr.h"
 #include "split-index.h"
+#include "dir.h"
 
 /*
  * Error messages expected by scripts out of plumbing commands such as
@@ -1259,8 +1260,10 @@ static int verify_uptodate_sparse(const struct cache_entry *ce,
 static void invalidate_ce_path(const struct cache_entry *ce,
 			       struct unpack_trees_options *o)
 {
-	if (ce)
-		cache_tree_invalidate_path(o->src_index, ce->name);
+	if (!ce)
+		return;
+	cache_tree_invalidate_path(o->src_index, ce->name);
+	untracked_cache_invalidate_path(o->src_index, ce->name);
 }
 
 /*
diff --git a/wt-status.c b/wt-status.c
index 38cb165f12..33452f169d 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -585,6 +585,8 @@ static void wt_status_collect_untracked(struct wt_status *s)
 			DIR_SHOW_OTHER_DIRECTORIES | DIR_HIDE_EMPTY_DIRECTORIES;
 	if (s->show_ignored_files)
 		dir.flags |= DIR_SHOW_IGNORED_TOO;
+	else
+		dir.untracked = the_index.untracked;
 	setup_standard_excludes(&dir);
 
 	fill_directory(&dir, &s->pathspec);