Accelerate lookup_symbol_aux_objfile 85x

During debugging I get 10-30 seconds for a response to simple commands like:
	(gdb) print vectorvar.size()
With this patch the performance gets to 1-2 seconds which is somehow
acceptable.  The problem is that dwarf2_gdb_index_functions.lookup_symbol
(quick_symbol_functions::lookup_symbol) may return (and returns) NULL even for
symbols which are present in .gdb_index but which can be found in already
expanded symtab.  But searching in the already expanded symtabs is just too
slow when there are 400000+ expanded symtabs.  There would be needed some
single global hash table for each objfile so that one does not have to iterate
all symtabs.  Which .gdb_index could perfectly serve for, just its
lookup_symbol() would need to return authoritative yes/no answers.

Even after such fix these two simple patches are useful for example for
non-.gdb_index files.

One can reproduce the slugging interactive GDB performance with:
	#include <string>
	using namespace std;
	string var;
	class C {
	public:
	  void m() {}
	};
	int main() {
	  C c;
	  c.m();
	  return 0;
	}
g++ -o slow slow.C -Wall -g $(pkg-config --libs gtkmm-3.0)
gdb ./slow -ex 'b C::m' -ex 'maintenance set per-command space' -ex 'maintenance set per-command symtab' -ex 'maintenance set per-command
time' -ex r
[...]
(gdb) p <tab><tab>
Display all 183904 possibilities? (y or n) n
(gdb) p/r var
$1 = {static npos = <optimized out>, _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No
data fields>}, _M_p = 0x3a4db073d8 <std::string::_Rep::_S_empty_rep_storage+24> ""}}
Command execution time: 20.023000 (cpu), 20.118665 (wall)
                        ^^^^^^^^^
Space used: 927997952 (+0 for this command)

Without DWZ there are X global blocks for X primary symtabs for X CUs of
objfile.  With DWZ there are X+Y global blocks for X+Y primary symtabs for
X+Y CUs where Y are 'DW_TAG_partial_unit's.

For 'DW_TAG_partial_unit's (Ys) their blockvector is usually empty.  But not
always, I have found there typedef symbols, there can IMO be optimized-out
static variables etc.

Neither of the patches should cause any visible behavior change.

gdb/ChangeLog
2014-12-04  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* block.c (block_lookup_symbol_primary): New function.
	* block.h (block_lookup_symbol_primary): New declaration.
	* symtab.c (lookup_symbol_in_objfile_symtabs): Assert BLOCK_INDEX.
	Call block_lookup_symbol_primary.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2014-12-04  Jan Kratochvil  <jan.kratochvil@redhat.com>
+
+	* block.c (block_lookup_symbol_primary): New function.
+	* block.h (block_lookup_symbol_primary): New declaration.
+	* symtab.c (lookup_symbol_in_objfile_symtabs): Assert BLOCK_INDEX.
+	Call block_lookup_symbol_primary.
+
 2014-12-03  Maciej W. Rozycki  <macro@codesourcery.com>
 
 	* tramp-frame.h (tramp_frame): Add `validate' member.

gdb/block.c

@@ -746,3 +746,28 @@ block_lookup_symbol (const struct block *block, const char *name,
       return (sym_found);	/* Will be NULL if not found.  */
     }
 }
+
+/* See block.h.  */
+
+struct symbol *
+block_lookup_symbol_primary (const struct block *block, const char *name,
+			     const domain_enum domain)
+{
+  struct symbol *sym;
+  struct dict_iterator dict_iter;
+
+  /* Verify BLOCK is STATIC_BLOCK or GLOBAL_BLOCK.  */
+  gdb_assert (BLOCK_SUPERBLOCK (block) == NULL
+	      || BLOCK_SUPERBLOCK (BLOCK_SUPERBLOCK (block)) == NULL);
+
+  for (sym = dict_iter_name_first (block->dict, name, &dict_iter);
+       sym != NULL;
+       sym = dict_iter_name_next (name, &dict_iter))
+    {
+      if (symbol_matches_domain (SYMBOL_LANGUAGE (sym),
+				 SYMBOL_DOMAIN (sym), domain))
+	return sym;
+    }
+
+  return NULL;
+}

gdb/block.h

@@ -276,6 +276,14 @@ extern struct symbol *block_lookup_symbol (const struct block *block,
 					   const char *name,
 					   const domain_enum domain);
 
+/* Search BLOCK for symbol NAME in DOMAIN but only in primary symbol table of
+   BLOCK.  BLOCK must be STATIC_BLOCK or GLOBAL_BLOCK.  Function is useful if
+   one iterates all global/static blocks of an objfile.  */
+
+extern struct symbol *block_lookup_symbol_primary (const struct block *block,
+						   const char *name,
+						   const domain_enum domain);
+
 /* Macro to loop through all symbols in BLOCK, in no particular
    order.  ITER helps keep track of the iteration, and must be a
    struct block_iterator.  SYM points to the current symbol.  */

gdb/symtab.c

@@ -1585,6 +1585,8 @@ lookup_symbol_in_objfile_symtabs (struct objfile *objfile, int block_index,
 {
   struct compunit_symtab *cust;
 
+  gdb_assert (block_index == GLOBAL_BLOCK || block_index == STATIC_BLOCK);
+
   ALL_OBJFILE_COMPUNITS (objfile, cust)
     {
       const struct blockvector *bv;
@@ -1593,7 +1595,7 @@ lookup_symbol_in_objfile_symtabs (struct objfile *objfile, int block_index,
 
       bv = COMPUNIT_BLOCKVECTOR (cust);
       block = BLOCKVECTOR_BLOCK (bv, block_index);
-      sym = block_lookup_symbol (block, name, domain);
+      sym = block_lookup_symbol_primary (block, name, domain);
       if (sym)
 	{
 	  block_found = block;