[PATCH 6/6] .gdb_index prod perf regression: mapped_symtab now vector of values

[Pedro Alves <palves@redhat.com>]

... instead of vector of pointers

There's no real reason for having mapped_symtab::data be a vector of
heap-allocated symtab_index_entries.  symtab_index_entries is not that
large, it's movable, and it's cheap to move.  Making the vector hold
values instead improves cache locality and eliminates many roundtrips
to the heap.

Using the same test as in the previous patch, against the same gdb
inferior, timing improves ~13% further:

  ~6.0s => ~5.2s (average of 5 runs).

Note that before the .gdb_index C++ifycation patch, we were at ~5.7s.
We're now consistenly better than before.

gdb/ChangeLog
2017-06-12  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (mapped_symtab::data): Now a vector of
	symtab_index_entry instead of vector of
	std::unique_ptr<symtab_index_entry>.  All users adjusted to check
	whether an element's name is NULL instead of checking whether the
	element itself is NULL.
	(find_slot): Change return type.  Adjust.
	(hash_expand, , add_index_entry, uniquify_cu_indices)
	(write_hash_table): Adjust.
---
 gdb/ChangeLog    | 11 +++++++++++
 gdb/dwarf2read.c | 56 ++++++++++++++++++++++++++++----------------------------
 2 files changed, 39 insertions(+), 28 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 9dbc059..82f972a 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,16 @@
 2017-06-12  Pedro Alves  <palves@redhat.com>
 
+	* dwarf2read.c (mapped_symtab::data): Now a vector of
+	symtab_index_entry instead of vector of
+	std::unique_ptr<symtab_index_entry>.  All users adjusted to check
+	whether an element's name is NULL instead of checking whether the
+	element itself is NULL.
+	(find_slot): Change return type.  Adjust.
+	(hash_expand, , add_index_entry, uniquify_cu_indices)
+	(write_hash_table): Adjust.
+
+2017-06-12  Pedro Alves  <palves@redhat.com>
+
 	* dwarf2read.c (recursively_count_psymbols): New function.
 	(write_psymtabs_to_index): Call it to compute number of psyms and
 	pass estimate size of psyms_seen to unordered_set's ctor.
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index bff2fcb..3f872b7 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -23269,7 +23269,7 @@ struct mapped_symtab
   }
 
   offset_type n_elements = 0;
-  std::vector<std::unique_ptr<symtab_index_entry>> data;
+  std::vector<symtab_index_entry> data;
 };
 
 /* Find a slot in SYMTAB for the symbol NAME.  Returns a reference to
@@ -23278,7 +23278,7 @@ struct mapped_symtab
    Function is used only during write_hash_table so no index format backward
    compatibility is needed.  */
 
-static std::unique_ptr<symtab_index_entry> &
+static symtab_index_entry &
 find_slot (struct mapped_symtab *symtab, const char *name)
 {
   offset_type index, step, hash = mapped_index_string_hash (INT_MAX, name);
@@ -23288,7 +23288,8 @@ find_slot (struct mapped_symtab *symtab, const char *name)
 
   for (;;)
     {
-      if (!symtab->data[index] || !strcmp (name, symtab->data[index]->name))
+      if (symtab->data[index].name == NULL
+	  || strcmp (name, symtab->data[index].name) == 0)
 	return symtab->data[index];
       index = (index + step) & (symtab->data.size () - 1);
     }
@@ -23305,9 +23306,9 @@ hash_expand (struct mapped_symtab *symtab)
   symtab->data.resize (old_entries.size () * 2);
 
   for (auto &it : old_entries)
-    if (it != NULL)
+    if (it.name != NULL)
       {
-	auto &ref = find_slot (symtab, it->name);
+	auto &ref = find_slot (symtab, it.name);
 	ref = std::move (it);
       }
 }
@@ -23327,11 +23328,10 @@ add_index_entry (struct mapped_symtab *symtab, const char *name,
   if (4 * symtab->n_elements / 3 >= symtab->data.size ())
     hash_expand (symtab);
 
-  std::unique_ptr<symtab_index_entry> &slot = find_slot (symtab, name);
-  if (slot == NULL)
+  symtab_index_entry &slot = find_slot (symtab, name);
+  if (slot.name == NULL)
     {
-      slot.reset (new symtab_index_entry ());
-      slot->name = name;
+      slot.name = name;
       /* index_offset is set later.  */
     }
 
@@ -23347,7 +23347,7 @@ add_index_entry (struct mapped_symtab *symtab, const char *name,
      the last entry pushed), but a symbol could have multiple kinds in one CU.
      To keep things simple we don't worry about the duplication here and
      sort and uniqufy the list after we've processed all symbols.  */
-  slot->cu_indices.push_back (cu_index_and_attrs);
+  slot.cu_indices.push_back (cu_index_and_attrs);
 }
 
 /* Sort and remove duplicates of all symbols' cu_indices lists.  */
@@ -23355,11 +23355,11 @@ add_index_entry (struct mapped_symtab *symtab, const char *name,
 static void
 uniquify_cu_indices (struct mapped_symtab *symtab)
 {
-  for (const auto &entry : symtab->data)
+  for (auto &entry : symtab->data)
     {
-      if (entry && !entry->cu_indices.empty ())
+      if (entry.name != NULL && !entry.cu_indices.empty ())
 	{
-	  auto &cu_indices = entry->cu_indices;
+	  auto &cu_indices = entry.cu_indices;
 	  std::sort (cu_indices.begin (), cu_indices.end ());
 	  auto from = std::unique (cu_indices.begin (), cu_indices.end ());
 	  cu_indices.erase (from, cu_indices.end ());
@@ -23425,11 +23425,11 @@ write_hash_table (mapped_symtab *symtab, data_buf &output, data_buf &cpool)
 
     /* We add all the index vectors to the constant pool first, to
        ensure alignment is ok.  */
-    for (const std::unique_ptr<symtab_index_entry> &it : symtab->data)
+    for (symtab_index_entry &entry : symtab->data)
       {
-	if (it == NULL)
+	if (entry.name == NULL)
 	  continue;
-	gdb_assert (it->index_offset == 0);
+	gdb_assert (entry.index_offset == 0);
 
 	/* Finding before inserting is faster than always trying to
 	   insert, because inserting always allocates a node, does the
@@ -23437,34 +23437,34 @@ write_hash_table (mapped_symtab *symtab, data_buf &output, data_buf &cpool)
 	   already had the same key.  C++17 try_emplace will avoid
 	   this.  */
 	const auto found
-	  = symbol_hash_table.find (it->cu_indices);
+	  = symbol_hash_table.find (entry.cu_indices);
 	if (found != symbol_hash_table.end ())
 	  {
-	    it->index_offset = found->second;
+	    entry.index_offset = found->second;
 	    continue;
 	  }
 
-	symbol_hash_table.emplace (it->cu_indices, cpool.size ());
-	it->index_offset = cpool.size ();
-	cpool.append_data (MAYBE_SWAP (it->cu_indices.size ()));
-	for (const auto iter : it->cu_indices)
-	  cpool.append_data (MAYBE_SWAP (iter));
+	symbol_hash_table.emplace (entry.cu_indices, cpool.size ());
+	entry.index_offset = cpool.size ();
+	cpool.append_data (MAYBE_SWAP (entry.cu_indices.size ()));
+	for (const auto index : entry.cu_indices)
+	  cpool.append_data (MAYBE_SWAP (index));
       }
   }
 
   /* Now write out the hash table.  */
   std::unordered_map<c_str_view, offset_type, c_str_view_hasher> str_table;
-  for (const auto &it : symtab->data)
+  for (const auto &entry : symtab->data)
     {
       offset_type str_off, vec_off;
 
-      if (it != NULL)
+      if (entry.name != NULL)
 	{
-	  const auto insertpair = str_table.emplace (it->name, cpool.size ());
+	  const auto insertpair = str_table.emplace (entry.name, cpool.size ());
 	  if (insertpair.second)
-	    cpool.append_cstr0 (it->name);
+	    cpool.append_cstr0 (entry.name);
 	  str_off = insertpair.first->second;
-	  vec_off = it->index_offset;
+	  vec_off = entry.index_offset;
 	}
       else
 	{
-- 
2.5.5