[gdb/symtab] Enable ada .gdb_index

Currently the .gdb_index is not enabled for ada executables (PR24713).

Fix this by adding the required support in write_psymbols, similar to how that
is done for .debug_names in debug_names::insert.

Tested on x86_64-linux, with native and target board cc-with-gdb-index.

gdb/ChangeLog:

2020-06-10  Tom de Vries  <tdevries@suse.de>

	PR ada/24713
	* dwarf2/index-write.c (struct mapped_symtab): Add m_string_obstack.
	(write_psymbols): Enable .gdb_index for ada.
	* dwarf2/read.c: Remove comment stating .gdb_index is unsupported for
	ada.

gdb/testsuite/ChangeLog:

2020-06-10  Tom de Vries  <tdevries@suse.de>

	* gdb.ada/ptype_union.exp: Remove PR24713 workaround.

2 files changed, 36 insertions, 7 deletions

diff --git a/gdb/dwarf2/index-write.c b/gdb/dwarf2/index-write.c
index eabb67f..97b2310 100644
--- a/gdb/dwarf2/index-write.c
+++ b/gdb/dwarf2/index-write.c
@@ -184,6 +184,9 @@ struct mapped_symtab
 
   offset_type n_elements = 0;
   std::vector<symtab_index_entry> data;
+
+  /* Temporary storage for Ada names.  */
+  auto_obstack m_string_obstack;
 };
 
 /* Find a slot in SYMTAB for the symbol NAME.  Returns a reference to
@@ -543,18 +546,47 @@ write_psymbols (struct mapped_symtab *symtab,
   for (; count-- > 0; ++psymp)
     {
       struct partial_symbol *psym = *psymp;
+      const char *name = psym->ginfo.search_name ();
 
       if (psym->ginfo.language () == language_ada)
-	error (_("Ada is not currently supported by the index; "
-		 "use the DWARF 5 index instead"));
+	{
+	  /* We want to ensure that the Ada main function's name appears
+	     verbatim in the index.  However, this name will be of the
+	     form "_ada_mumble", and will be rewritten by ada_decode.
+	     So, recognize it specially here and add it to the index by
+	     hand.  */
+	  if (strcmp (main_name (), name) == 0)
+	    {
+	      gdb_index_symbol_kind kind = symbol_kind (psym);
+
+	      add_index_entry (symtab, name, is_static, kind, cu_index);
+	    }
+
+	  /* In order for the index to work when read back into gdb, it
+	     has to supply a funny form of the name: it should be the
+	     encoded name, with any suffixes stripped.  Using the
+	     ordinary encoded name will not work properly with the
+	     searching logic in find_name_components_bounds; nor will
+	     using the decoded name.  Furthermore, an Ada "verbatim"
+	     name (of the form "<MumBle>") must be entered without the
+	     angle brackets.  Note that the current index is unusual,
+	     see PR symtab/24820 for details.  */
+	  std::string decoded = ada_decode (name);
+	  if (decoded[0] == '<')
+	    name = (char *) obstack_copy0 (&symtab->m_string_obstack,
+					   decoded.c_str () + 1,
+					   decoded.length () - 2);
+	  else
+	    name = obstack_strdup (&symtab->m_string_obstack,
+				   ada_encode (decoded.c_str ()));
+	}
 
       /* Only add a given psymbol once.  */
       if (psyms_seen.insert (psym).second)
 	{
 	  gdb_index_symbol_kind kind = symbol_kind (psym);
 
-	  add_index_entry (symtab, psym->ginfo.search_name (),
-			   is_static, kind, cu_index);
+	  add_index_entry (symtab, name, is_static, kind, cu_index);
 	}
     }
 }
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index c33f0a1..e3073fe 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -3778,9 +3778,6 @@ dw2_map_matching_symbols
 
   if (per_objfile->per_bfd->index_table != nullptr)
     {
-      /* Ada currently doesn't support .gdb_index (see PR24713).  We can get
-	 here though if the current language is Ada for a non-Ada objfile
-	 using GNU index.  */
       mapped_index &index = *per_objfile->per_bfd->index_table;
 
       const char *match_name = name.ada ().lookup_name ().c_str ();