gas: Emit name, comp_dir and producer strings in .debug_str.

Putting the name, comp_dir and producer strings in the .debug_str section
makes it possible to share them across CUs. This saves a small amount of
space (about ~20K on a glibc libc.so.6 build with debuginfo). And makes
it easier for tools like rpm debugedit to adjust the source paths when
generating separate debuginfo files.

gas/
       * dwarf2dbg.c (out_debug_abbrev): Use DW_FORM_strp instead of
       DW_FORM_string for DW_AT_name, DW_AT_comp_dir and DW_AT_producer.
       (out_debug_info): Accept symbols to name, comp_dir and producer
       in the .debug_str section and emit those offsets not full strings.
       (out_debug_str): New function that outputs the strings for name,
       comp_dir and producer in .debug_str and generates symbols to
       those strings.
       (out_debug_line): Create a .debug_str section if necessary and
       call out_debug_str before calling out_debug_info.
       * testsuite/gas/aarch64/dwarf.d: Add extra section symbol to
       expected output.

gas/ChangeLog

@@ -1,3 +1,17 @@
+2017-03-05  Mark Wielaard  <mark@klomp.org>
+
+	* dwarf2dbg.c (out_debug_abbrev): Use DW_FORM_strp instead of
+	DW_FORM_string for DW_AT_name, DW_AT_comp_dir and DW_AT_producer.
+	(out_debug_info): Accept symbols to name, comp_dir and producer in
+	the .debug_str section and emit those offsets not full strings.
+	(out_debug_str): New function that outputs the strings for name,
+	comp_dir and producer in .debug_str and generates symbols to those
+	strings.
+	(out_debug_line): Create a .debug_str section if necessary and call
+	out_debug_str before calling out_debug_info.
+	* testsuite/gas/aarch64/dwarf.d: Add extra section symbol to expected
+	output.
+
 2017-03-02  Maciej W. Rozycki  <macro@imgtec.com>
 
 	* write.c (relax_segment) <rs_org>: Only bail out if the fixed

gas/dwarf2dbg.c

@@ -1726,9 +1726,9 @@ out_debug_abbrev (segT abbrev_seg,
       else
 	out_abbrev (DW_AT_ranges, DW_FORM_data8);
     }
-  out_abbrev (DW_AT_name, DW_FORM_string);
+  out_abbrev (DW_AT_name, DW_FORM_strp);
-  out_abbrev (DW_AT_comp_dir, DW_FORM_string);
+  out_abbrev (DW_AT_comp_dir, DW_FORM_strp);
-  out_abbrev (DW_AT_producer, DW_FORM_string);
+  out_abbrev (DW_AT_producer, DW_FORM_strp);
   out_abbrev (DW_AT_language, DW_FORM_data2);
   out_abbrev (0, 0);
 
@@ -1739,15 +1739,11 @@ out_debug_abbrev (segT abbrev_seg,
 /* Emit a description of this compilation unit for .debug_info.  */
 
 static void
-out_debug_info (segT info_seg, segT abbrev_seg, segT line_seg, segT ranges_seg)
+out_debug_info (segT info_seg, segT abbrev_seg, segT line_seg, segT ranges_seg,
+		symbolS *name_sym, symbolS *comp_dir_sym, symbolS *producer_sym)
 {
-  char producer[128];
-  const char *comp_dir;
-  const char *dirname;
   expressionS exp;
   symbolS *info_end;
-  char *p;
-  int len;
   int sizeof_offset;
 
   sizeof_offset = out_header (info_seg, &exp);
@@ -1798,10 +1794,38 @@ out_debug_info (segT info_seg, segT abbrev_seg, segT line_seg, segT ranges_seg)
       TC_DWARF2_EMIT_OFFSET (section_symbol (ranges_seg), sizeof_offset);
     }
 
+  /* DW_AT_name, DW_AT_comp_dir and DW_AT_producer.  Symbols in .debug_str
+     setup in out_debug_str below.  */
+  TC_DWARF2_EMIT_OFFSET (name_sym, sizeof_offset);
+  TC_DWARF2_EMIT_OFFSET (comp_dir_sym, sizeof_offset);
+  TC_DWARF2_EMIT_OFFSET (producer_sym, sizeof_offset);
+
+  /* DW_AT_language.  Yes, this is probably not really MIPS, but the
+     dwarf2 draft has no standard code for assembler.  */
+  out_two (DW_LANG_Mips_Assembler);
+
+  symbol_set_value_now (info_end);
+}
+
+/* Emit the three debug strings needed in .debug_str and setup symbols
+   to them for use in out_debug_info.  */
+static void
+out_debug_str (segT str_seg, symbolS **name_sym, symbolS **comp_dir_sym,
+	       symbolS **producer_sym)
+{
+  char producer[128];
+  const char *comp_dir;
+  const char *dirname;
+  char *p;
+  int len;
+
+  subseg_set (str_seg, 0);
+
   /* DW_AT_name.  We don't have the actual file name that was present
      on the command line, so assume files[1] is the main input file.
      We're not supposed to get called unless at least one line number
      entry was emitted, so this should always be defined.  */
+  *name_sym = symbol_temp_new_now ();
   if (files_in_use == 0)
     abort ();
   if (files[1].dir)
@@ -1823,22 +1847,18 @@ out_debug_info (segT info_seg, segT abbrev_seg, segT line_seg, segT ranges_seg)
   memcpy (p, files[1].filename, len);
 
   /* DW_AT_comp_dir */
+  *comp_dir_sym = symbol_temp_new_now ();
   comp_dir = remap_debug_filename (getpwd ());
   len = strlen (comp_dir) + 1;
   p = frag_more (len);
   memcpy (p, comp_dir, len);
 
   /* DW_AT_producer */
+  *producer_sym = symbol_temp_new_now ();
   sprintf (producer, "GNU AS %s", VERSION);
   len = strlen (producer) + 1;
   p = frag_more (len);
   memcpy (p, producer, len);
-
-  /* DW_AT_language.  Yes, this is probably not really MIPS, but the
-     dwarf2 draft has no standard code for assembler.  */
-  out_two (DW_LANG_Mips_Assembler);
-
-  symbol_set_value_now (info_end);
 }
 
 void
@@ -1907,19 +1927,22 @@ dwarf2_finish (void)
   out_debug_line (line_seg);
 
   /* If this is assembler generated line info, and there is no
-     debug_info already, we need .debug_info and .debug_abbrev
+     debug_info already, we need .debug_info, .debug_abbrev and
-     sections as well.  */
+     .debug_str sections as well.  */
   if (emit_other_sections)
     {
       segT abbrev_seg;
       segT aranges_seg;
       segT ranges_seg;
+      segT str_seg;
+      symbolS *name_sym, *comp_dir_sym, *producer_sym;
 
       gas_assert (all_segs);
 
       info_seg = subseg_new (".debug_info", 0);
       abbrev_seg = subseg_new (".debug_abbrev", 0);
       aranges_seg = subseg_new (".debug_aranges", 0);
+      str_seg = subseg_new (".debug_str", 0);
 
       bfd_set_section_flags (stdoutput, info_seg,
 			     SEC_READONLY | SEC_DEBUGGING);
@@ -1927,6 +1950,10 @@ dwarf2_finish (void)
 			     SEC_READONLY | SEC_DEBUGGING);
       bfd_set_section_flags (stdoutput, aranges_seg,
 			     SEC_READONLY | SEC_DEBUGGING);
+      bfd_set_section_flags (stdoutput, str_seg,
+			     (SEC_READONLY | SEC_DEBUGGING
+			      | SEC_MERGE | SEC_STRINGS));
+      str_seg->entsize = 1;
 
       record_alignment (aranges_seg, ffs (2 * sizeof_address) - 1);
 
@@ -1943,6 +1970,8 @@ dwarf2_finish (void)
 
       out_debug_aranges (aranges_seg, info_seg);
       out_debug_abbrev (abbrev_seg, info_seg, line_seg);
-      out_debug_info (info_seg, abbrev_seg, line_seg, ranges_seg);
+      out_debug_str (str_seg, &name_sym, &comp_dir_sym, &producer_sym);
+      out_debug_info (info_seg, abbrev_seg, line_seg, ranges_seg,
+		      name_sym, comp_dir_sym, producer_sym);
     }
 }

gas/testsuite/gas/aarch64/dwarf.d

@@ -1,7 +1,7 @@
 #readelf: -s --debug-dump=aranges
 #as: -g
 
-Symbol table '.symtab' contains 10 entries:
+Symbol table '.symtab' contains 11 entries:
    Num:[ ]+Value[ ]+Size[ ]+Type[ ]+Bind[ ]+Vis[ ]+Ndx[ ]+Name
      0: 0+     0 NOTYPE  LOCAL  DEFAULT  UND[ ]+
      1: 0+     0 SECTION LOCAL  DEFAULT    1[ ]+
@@ -11,8 +11,9 @@ Symbol table '.symtab' contains 10 entries:
      5: 0+     0 SECTION LOCAL  DEFAULT    6[ ]+
      6: 0+     0 SECTION LOCAL  DEFAULT    8[ ]+
      7: 0+     0 SECTION LOCAL  DEFAULT    4[ ]+
-     8: 0+     0 SECTION LOCAL  DEFAULT    9[ ]+
+     8: 0+     0 SECTION LOCAL  DEFAULT   11[ ]+
-     9: 0+     8 FUNC    GLOBAL DEFAULT    1 testfunc
+     9: 0+     0 SECTION LOCAL  DEFAULT    9[ ]+
+    10: 0+     8 FUNC    GLOBAL DEFAULT    1 testfunc
 Contents of the .debug_aranges section:
 
   Length:                   (44|28)