Change .gdb_index de-duplication implementation

While investigating PR symtab/29179, I found that one Ada test failed
because, although a certain symbol was present in the index, with the
new DWARF reader it pointed to a different CU than was chosen by
earlier versions of gdb.

This patch changes how symbol de-duplication is done, deferring the
process until the entire symbol table has been constructed.  This way,
it's possible to always choose the lower-numbered CU among duplicates,
which is how gdb (implicitly) previously worked.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29179

gdb/dwarf2/index-write.c

@@ -176,6 +176,10 @@ struct symtab_index_entry
   /* A sorted vector of the indices of all the CUs that hold an object
      of this name.  */
   std::vector<offset_type> cu_indices;
+
+  /* Minimize CU_INDICES, sorting them and removing duplicates as
+     appropriate.  */
+  void minimize ();
 };
 
 /* The symbol table.  This is a power-of-2-sized hash table.  */
@@ -186,6 +190,13 @@ struct mapped_symtab
     data.resize (1024);
   }
 
+  /* Minimize each entry in the symbol table, removing duplicates.  */
+  void minimize ()
+  {
+    for (symtab_index_entry &item : data)
+      item.minimize ();
+  }
+
   offset_type n_elements = 0;
   std::vector<symtab_index_entry> data;
 
@@ -271,21 +282,36 @@ add_index_entry (struct mapped_symtab *symtab, const char *name,
   slot.cu_indices.push_back (cu_index_and_attrs);
 }
 
-/* Sort and remove duplicates of all symbols' cu_indices lists.  */
+/* See symtab_index_entry.  */
 
-static void
-uniquify_cu_indices (struct mapped_symtab *symtab)
+void
+symtab_index_entry::minimize ()
 {
-  for (auto &entry : symtab->data)
+  if (name == nullptr || cu_indices.empty ())
+    return;
+
+  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 ());
+
+  /* We don't want to enter a variable or type more than once, so
+     remove any such duplicates from the list as well.  When doing
+     this, we want to keep the entry from the first CU -- but this is
+     implicit due to the sort.  This choice is done because it's
+     similar to what gdb historically did for partial symbols.  */
+  std::unordered_set<offset_type> seen;
+  from = std::remove_if (cu_indices.begin (), cu_indices.end (),
+			 [&] (offset_type val)
     {
-      if (entry.name != NULL && !entry.cu_indices.empty ())
-	{
-	  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 ());
-	}
-    }
+      gdb_index_symbol_kind kind = GDB_INDEX_SYMBOL_KIND_VALUE (val);
+      if (kind != GDB_INDEX_SYMBOL_KIND_TYPE
+	  && kind != GDB_INDEX_SYMBOL_KIND_VARIABLE)
+	return false;
+
+      val &= ~GDB_INDEX_CU_MASK;
+      return !seen.insert (val).second;
+    });
+  cu_indices.erase (from, cu_indices.end ());
 }
 
 /* A form of 'const char *' suitable for container keys.  Only the
@@ -1100,15 +1126,6 @@ write_cooked_index (cooked_index_vector *table,
 		    const cu_index_map &cu_index_htab,
 		    struct mapped_symtab *symtab)
 {
-  /* We track type names and only enter a given type once.  */
-  htab_up type_names (htab_create_alloc (10, htab_hash_string, htab_eq_string,
-					 nullptr, xcalloc, xfree));
-  /* Same with variable names.  However, if a type and variable share
-     a name, we want both, which is why there are two hash tables
-     here.  */
-  htab_up var_names (htab_create_alloc (10, htab_hash_string, htab_eq_string,
-					nullptr, xcalloc, xfree));
-
   const char *main_for_ada = main_name ();
 
   for (const cooked_index_entry *entry : table->all_entries ())
@@ -1157,24 +1174,12 @@ write_cooked_index (cooked_index_vector *table,
       else if (entry->tag == DW_TAG_variable
 	       || entry->tag == DW_TAG_constant
 	       || entry->tag == DW_TAG_enumerator)
-	{
-	  kind = GDB_INDEX_SYMBOL_KIND_VARIABLE;
-	  void **slot = htab_find_slot (var_names.get (), name, INSERT);
-	  if (*slot != nullptr)
-	    continue;
-	  *slot = (void *) name;
-	}
+	kind = GDB_INDEX_SYMBOL_KIND_VARIABLE;
       else if (entry->tag == DW_TAG_module
 	       || entry->tag == DW_TAG_common_block)
 	kind = GDB_INDEX_SYMBOL_KIND_OTHER;
       else
-	{
-	  kind = GDB_INDEX_SYMBOL_KIND_TYPE;
-	  void **slot = htab_find_slot (type_names.get (), name, INSERT);
-	  if (*slot != nullptr)
-	    continue;
-	  *slot = (void *) name;
-	}
+	kind = GDB_INDEX_SYMBOL_KIND_TYPE;
 
       add_index_entry (symtab, name, (entry->flags & IS_STATIC) != 0,
 		       kind, it->second);
@@ -1252,7 +1257,7 @@ write_gdbindex (dwarf2_per_objfile *per_objfile,
 
   /* Now that we've processed all symbols we can shrink their cu_indices
      lists.  */
-  uniquify_cu_indices (&symtab);
+  symtab.minimize ();
 
   data_buf symtab_vec, constant_pool;
   if (symtab.n_elements == 0)