move some content around to make README more focused

llvm-svn: 39130

1 files changed, 78 insertions, 6 deletions

diff --git a/clang/NOTES.txt b/clang/NOTES.txt
index cbb0bf314c21..baa2edd8d860 100644
--- a/clang/NOTES.txt
+++ b/clang/NOTES.txt
@@ -31,13 +31,85 @@ This is similar to -Eonly.
 
 //===---------------------------------------------------------------------===//
 
-Interesting fact:
-  clang -Eonly INPUTS/carbon-header-C-E.c
+TODO: File Manager Speedup:
+
+ We currently do a lot of stat'ing for files that don't exist, particularly
+ when lots of -I paths exist (e.g. see the <iostream> example, check for
+ failures in stat in FileManager::getFile).  It would be far better to make
+ the following changes:
+   1. FileEntry contains a sys::Path instead of a std::string for Name.
+   2. sys::Path contains timestamp and size, lazily computed.  Eliminate from
+      FileEntry.
+   3. File UIDs are created on request, not when files are opened.
+ These changes make it possible to efficiently have FileEntry objects for
+ files that exist on the file system, but have not been used yet.
+ 
+ Once this is done:
+   1. DirectoryEntry gets a boolean value "has read entries".  When false, not
+      all entries in the directory are in the file mgr, when true, they are.
+   2. Instead of stat'ing the file in FileManager::getFile, check to see if 
+      the dir has been read.  If so, fail immediately, if not, read the dir,
+      then retry.
+   3. Reading the dir uses the getdirentries syscall, creating an FileEntry
+      for all files found.
+
+//===---------------------------------------------------------------------===//
+
+TODO: Fast #Import:
+
+ * Get frameworks that don't use #import to do so, e.g. 
+   DirectoryService, AudioToolbox, CoreFoundation, etc.  Why not using #import?
+   Because they work in C mode? C has #import.
+ * Have the lexer return a token for #import instead of handling it itself.
+   - Create a new preprocessor object with no external state (no -D/U options
+     from the command line, etc).  Alternatively, keep track of exactly which
+     external state is used by a #import: declare it somehow.
+ * When having reading a #import file, keep track of whether we have (and/or
+   which) seen any "configuration" macros.  Various cases:
+   - Uses of target args (__POWERPC__, __i386): Header has to be parsed 
+     multiple times, per-target.  What about #ifndef checks?  How do we know?
+   - "Configuration" preprocessor macros not defined: POWERPC, etc.  What about
+     things like __STDC__ etc?  What is and what isn't allowed.
+ * Special handling for "umbrella" headers, which just contain #import stmts:
+   - Cocoa.h/AppKit.h - Contain pointers to digests instead of entire digests
+     themselves?  Foundation.h isn't pure umbrella!
+ * Frameworks digests:
+   - Can put "digest" of a framework-worth of headers into the framework
+     itself.  To open AppKit, just mmap
+     /System/Library/Frameworks/AppKit.framework/"digest", which provides a
+     symbol table in a well defined format.  Lazily unstream stuff that is
+     needed.  Contains declarations, macros, and debug information.
+   - System frameworks ship with digests.  How do we handle configuration
+     information?  How do we handle stuff like:
+       #if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_2
+     which guards a bunch of decls?  Should there be a couple of default
+     configs, then have the UI fall back to building/caching its own?
+   - GUI automatically builds digests when UI is idle, both of system
+     frameworks if they aren't not available in the right config, and of app
+     frameworks.
+   - GUI builds dependence graph of frameworks/digests based on #imports.  If a
+     digest is out date, dependent digests are automatically invalidated.
+
+ * New constraints on #import for objc-v3:
+   - #imported file must not define non-inline function bodies.
+     - Alternatively, they can, and these bodies get compiled/linked *once*
+       per app into a dylib.  What about building user dylibs?
+   - Restrictions on ObjC grammar: can't #import the body of a for stmt or fn.
+   - Compiler must detect and reject these cases.
+   - #defines defined within a #import have two behaviors:
+     - By default, they escape the header.  These macros *cannot* be #undef'd
+       by other code: this is enforced by the front-end.
+     - Optionally, user can specify what macros escape (whitelist) or can use
+       #undef.
+
+//===---------------------------------------------------------------------===//
+
+TODO: New language feature: Configuration queries:
+  - Instead of #ifdef __POWERPC__, use "if (strcmp(`cpu`, __POWERPC__))", or
+    some other, better, syntax.
+  - Use it to increase the number of "architecture-clean" #import'd files,
+    allowing a single index to be used for all fat slices.
 
-is much faster than:
-  wc -w INPUTS/carbon-header-C-E.c
-!!
-   
 //===---------------------------------------------------------------------===//
 
 The 'portability' model in clang is sufficient to catch translation units (or