gdb: Restructure the completion_tracker class

In this commit I rewrite how the completion tracker tracks the
completions, and builds its lowest common denominator (LCD) string.
The LCD string is now built lazily when required, and we only track
the completions in one place, the hash table, rather than maintaining
a separate vector of completions.

The motivation for these changes is that the next commit will add the
ability to remove completions from the list, removing a completion
will invalidate the LCD string, so we need to keep hold of enough
information to recompute the LCD string as needed.

Additionally, keeping the completions in a vector makes removing a
completion expensive, so better to only keep the completions in the
hash table.

This commit doesn't add any new functionality itself, and there should
be no user visible changes after this commit.

For testing, I ran the testsuite as usual, but I also ran some manual
completion tests under valgrind, and didn't get any reports about
leaked memory.

gdb/ChangeLog:

	* completer.c (completion_tracker::completion_hash_entry): Define
	new class.
	(advance_to_filename_complete_word_point): Call
	recompute_lowest_common_denominator.
	(completion_tracker::completion_tracker): Call discard_completions
	to setup the hash table.
	(completion_tracker::discard_completions): Allow for being called
	from the constructor, pass new equal function, and element deleter
	when constructing the hash table.  Initialise new class member
	variables.
	(completion_tracker::maybe_add_completion): Remove use of
	m_entries_vec, and store more information into m_entries_hash.
	(completion_tracker::recompute_lcd_visitor): New function, most
	content taken from...
	(completion_tracker::recompute_lowest_common_denominator):
	...here, this now just visits each item in the hash calling the
	above visitor.
	(completion_tracker::build_completion_result): Remove use of
	m_entries_vec, call recompute_lowest_common_denominator.
	* completer.h (completion_tracker::have_completions): Remove use
	of m_entries_vec.
	(completion_tracker::completion_hash_entry): Declare new class.
	(completion_tracker::recompute_lowest_common_denominator): Change
	function signature.
	(completion_tracker::recompute_lcd_visitor): Declare new function.
	(completion_tracker::m_entries_vec): Delete.
	(completion_tracker::m_entries_hash): Initialize to NULL.
	(completion_tracker::m_lowest_common_denominator_valid): New
	member variable.
	(completion_tracker::m_lowest_common_denominator_max_length): New
	member variable.

Change-Id: I9d1db52c489ca0041b8959ca0d53b7d3af8aea72

1 files changed, 164 insertions, 31 deletions

diff --git a/gdb/completer.c b/gdb/completer.c
index 619fb8a0285..14c7a579970 100644
--- a/gdb/completer.c
+++ b/gdb/completer.c
@@ -45,6 +45,60 @@
 
 #include "completer.h"
 
+/* See completer.h.  */
+
+class completion_tracker::completion_hash_entry
+{
+public:
+  /* Constructor.  */
+  completion_hash_entry (gdb::unique_xmalloc_ptr<char> name,
+			 gdb::unique_xmalloc_ptr<char> lcd)
+    : m_name (std::move (name)),
+      m_lcd (std::move (lcd))
+  {
+    /* Nothing.  */
+  }
+
+  /* Returns a pointer to the lowest common denominator string.  This
+     string will only be valid while this hash entry is still valid as the
+     string continues to be owned by this hash entry and will be released
+     when this entry is deleted.  */
+  char *get_lcd () const
+  {
+    return m_lcd.get ();
+  }
+
+  /* Get, and release the name field from this hash entry.  This can only
+     be called once, after which the name field is no longer valid.  This
+     should be used to pass ownership of the name to someone else.  */
+  char *release_name ()
+  {
+    return m_name.release ();
+  }
+
+  /* Return true of the name in this hash entry is STR.  */
+  bool is_name_eq (const char *str) const
+  {
+    return strcmp (m_name.get (), str) == 0;
+  }
+
+  /* A static function that can be passed to the htab hash system to be
+     used as a callback that deletes an item from the hash.  */
+  static void deleter (void *arg)
+  {
+    completion_hash_entry *entry = (completion_hash_entry *) arg;
+    delete entry;
+  }
+
+private:
+
+  /* The symbol name stored in this hash entry.  */
+  gdb::unique_xmalloc_ptr<char> m_name;
+
+  /* The lowest common denominator string computed for this hash entry.  */
+  gdb::unique_xmalloc_ptr<char> m_lcd;
+};
+
 /* Misc state that needs to be tracked across several different
    readline completer entry point calls, all related to a single
    completion invocation.  */
@@ -407,6 +461,7 @@ advance_to_filename_complete_word_point (completion_tracker &tracker,
 bool
 completion_tracker::completes_to_completion_word (const char *word)
 {
+  recompute_lowest_common_denominator ();
   if (m_lowest_common_denominator_unique)
     {
       const char *lcd = m_lowest_common_denominator;
@@ -1512,9 +1567,7 @@ int max_completions = 200;
 
 completion_tracker::completion_tracker ()
 {
-  m_entries_hash = htab_create_alloc (INITIAL_COMPLETION_HTAB_SIZE,
-				      htab_hash_string, streq_hash,
-				      NULL, xcalloc, xfree);
+  discard_completions ();
 }
 
 /* See completer.h.  */
@@ -1526,13 +1579,33 @@ completion_tracker::discard_completions ()
   m_lowest_common_denominator = NULL;
 
   m_lowest_common_denominator_unique = false;
+  m_lowest_common_denominator_valid = false;
+
+  /* A null check here allows this function to be used from the
+     constructor.  */
+  if (m_entries_hash != NULL)
+    htab_delete (m_entries_hash);
+
+  /* A callback used by the hash table to compare new entries with existing
+     entries.  We can't use the standard streq_hash function here as the
+     key to our hash is just a single string, while the values we store in
+     the hash are a struct containing multiple strings.  */
+  static auto entry_eq_func
+    = [] (const void *first, const void *second) -> int
+      {
+	/* The FIRST argument is the entry already in the hash table, and
+	   the SECOND argument is the new item being inserted.  */
+	const completion_hash_entry *entry
+	  = (const completion_hash_entry *) first;
+	const char *name_str = (const char *) second;
 
-  m_entries_vec.clear ();
+	return entry->is_name_eq (name_str);
+      };
 
-  htab_delete (m_entries_hash);
   m_entries_hash = htab_create_alloc (INITIAL_COMPLETION_HTAB_SIZE,
-				      htab_hash_string, streq_hash,
-				      NULL, xcalloc, xfree);
+				      htab_hash_string, entry_eq_func,
+				      completion_hash_entry::deleter,
+				      xcalloc, xfree);
 }
 
 /* See completer.h.  */
@@ -1559,7 +1632,8 @@ completion_tracker::maybe_add_completion
   if (htab_elements (m_entries_hash) >= max_completions)
     return false;
 
-  slot = htab_find_slot (m_entries_hash, name.get (), INSERT);
+  hashval_t hash = htab_hash_string (name.get ());
+  slot = htab_find_slot_with_hash (m_entries_hash, name.get (), hash, INSERT);
   if (*slot == HTAB_EMPTY_ENTRY)
     {
       const char *match_for_lcd_str = NULL;
@@ -1573,10 +1647,12 @@ completion_tracker::maybe_add_completion
       gdb::unique_xmalloc_ptr<char> lcd
 	= make_completion_match_str (match_for_lcd_str, text, word);
 
-      recompute_lowest_common_denominator (std::move (lcd));
+      size_t lcd_len = strlen (lcd.get ());
+      *slot = new completion_hash_entry (std::move (name), std::move (lcd));
 
-      *slot = name.get ();
-      m_entries_vec.push_back (std::move (name));
+      m_lowest_common_denominator_valid = false;
+      m_lowest_common_denominator_max_length
+	= std::max (m_lowest_common_denominator_max_length, lcd_len);
     }
 
   return true;
@@ -1982,23 +2058,23 @@ completion_find_completion_word (completion_tracker &tracker, const char *text,
 /* See completer.h.  */
 
 void
-completion_tracker::recompute_lowest_common_denominator
-  (gdb::unique_xmalloc_ptr<char> &&new_match_up)
+completion_tracker::recompute_lcd_visitor (completion_hash_entry *entry)
 {
-  if (m_lowest_common_denominator == NULL)
+  if (!m_lowest_common_denominator_valid)
     {
-      /* We don't have a lowest common denominator yet, so simply take
-	 the whole NEW_MATCH_UP as being it.  */
-      m_lowest_common_denominator = new_match_up.release ();
+      /* This is the first lowest common denominator that we are
+	 considering, just copy it in.  */
+      strcpy (m_lowest_common_denominator, entry->get_lcd ());
       m_lowest_common_denominator_unique = true;
+      m_lowest_common_denominator_valid = true;
     }
   else
     {
-      /* Find the common denominator between the currently-known
-	 lowest common denominator and NEW_MATCH_UP.  That becomes the
-	 new lowest common denominator.  */
+      /* Find the common denominator between the currently-known lowest
+	 common denominator and NEW_MATCH_UP.  That becomes the new lowest
+	 common denominator.  */
       size_t i;
-      const char *new_match = new_match_up.get ();
+      const char *new_match = entry->get_lcd ();
 
       for (i = 0;
 	   (new_match[i] != '\0'
@@ -2016,6 +2092,35 @@ completion_tracker::recompute_lowest_common_denominator
 /* See completer.h.  */
 
 void
+completion_tracker::recompute_lowest_common_denominator ()
+{
+  /* We've already done this.  */
+  if (m_lowest_common_denominator_valid)
+    return;
+
+  /* Resize the storage to ensure we have enough space, the plus one gives
+     us space for the trailing null terminator we will include.  */
+  m_lowest_common_denominator
+    = (char *) xrealloc (m_lowest_common_denominator,
+			 m_lowest_common_denominator_max_length + 1);
+
+  /* Callback used to visit each entry in the m_entries_hash.  */
+  auto visitor_func
+    = [] (void **slot, void *info) -> int
+      {
+	completion_tracker *obj = (completion_tracker *) info;
+	completion_hash_entry *entry = (completion_hash_entry *) *slot;
+	obj->recompute_lcd_visitor (entry);
+	return 1;
+      };
+
+  htab_traverse (m_entries_hash, visitor_func, this);
+  m_lowest_common_denominator_valid = true;
+}
+
+/* See completer.h.  */
+
+void
 completion_tracker::advance_custom_word_point_by (int len)
 {
   m_custom_word_point += len;
@@ -2092,16 +2197,17 @@ completion_result
 completion_tracker::build_completion_result (const char *text,
 					     int start, int end)
 {
-  completion_list &list = m_entries_vec;	/* The completions.  */
+  size_t element_count = htab_elements (m_entries_hash);
 
-  if (list.empty ())
+  if (element_count == 0)
     return {};
 
   /* +1 for the LCD, and +1 for NULL termination.  */
-  char **match_list = XNEWVEC (char *, 1 + list.size () + 1);
+  char **match_list = XNEWVEC (char *, 1 + element_count + 1);
 
   /* Build replacement word, based on the LCD.  */
 
+  recompute_lowest_common_denominator ();
   match_list[0]
     = expand_preserving_ws (text, end - start,
 			    m_lowest_common_denominator);
@@ -2128,13 +2234,40 @@ completion_tracker::build_completion_result (const char *text,
     }
   else
     {
-      int ix;
-
-      for (ix = 0; ix < list.size (); ++ix)
-	match_list[ix + 1] = list[ix].release ();
-      match_list[ix + 1] = NULL;
-
-      return completion_result (match_list, list.size (), false);
+      /* State object used while building the completion list.  */
+      struct list_builder
+      {
+	list_builder (char **ml)
+	  : match_list (ml),
+	    index (1)
+	{ /* Nothing.  */ }
+
+	/* The list we are filling.  */
+	char **match_list;
+
+	/* The next index in the list to write to.  */
+	int index;
+      };
+      list_builder builder (match_list);
+
+      /* Visit each entry in m_entries_hash and add it to the completion
+	 list, updating the builder state object.  */
+      auto func
+	= [] (void **slot, void *info) -> int
+	  {
+	    completion_hash_entry *entry = (completion_hash_entry *) *slot;
+	    list_builder *state = (list_builder *) info;
+
+	    state->match_list[state->index] = entry->release_name ();
+	    state->index++;
+	    return 1;
+	  };
+
+      /* Build the completion list and add a null at the end.  */
+      htab_traverse_noresize (m_entries_hash, func, &builder);
+      match_list[builder.index] = NULL;
+
+      return completion_result (match_list, builder.index - 1, false);
     }
 }