[Bug target/82005] [8 regression] early lto debug creates invalid assembly on Darwin

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82005

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
To recap the situation we have sth like

earlyDebug.o

section .debug_info:
sym1:
  CU1
...
  CU2 at coff1


lateDebug.o

section .debug_info:
...
 DW_TAG_imported_unit
   DW_AT_import $sym1 - .debug_info + coff1


I suppose we could somehow rely on .debug_info being relocated to 0.  Changing
the assembler that way works (but we're missing the provider of the early debug
so I can't double-check the final dwarf).

I suppose we'd need a new dw2_asm_output primitive for this (offset
within zero-relocated section, sym + offset) plus the associated target
macro.  Or put that knowledge (section is relocated to zero) into
darwin_asm_output_dwarf_offset somehow.  Like with

Index: config/darwin.c
===================================================================
--- config/darwin.c     (revision 251449)
+++ config/darwin.c     (working copy)
@@ -2859,7 +2859,8 @@ darwin_asm_output_dwarf_delta (FILE *fil
                               const char *lab1, const char *lab2,
                               HOST_WIDE_INT offset)
 {
-  int islocaldiff = (lab1[0] == '*' && lab1[1] == 'L'
+  int islocaldiff = (lab2
+                    && lab1[0] == '*' && lab1[1] == 'L'
                     && lab2[0] == '*' && lab2[1] == 'L');
   const char *directive = (size == 8 ? ".quad" : ".long");

@@ -2869,8 +2870,11 @@ darwin_asm_output_dwarf_delta (FILE *fil
     fprintf (file, "\t%s\t", directive);

   assemble_name_raw (file, lab1);
-  fprintf (file, "-");
-  assemble_name_raw (file, lab2);
+  if (lab2)
+    {
+      fprintf (file, "-");
+      assemble_name_raw (file, lab2);
+    }
   if (offset != 0)
     fprintf (file, "+" HOST_WIDE_INT_PRINT_DEC, offset);
   if (islocaldiff)
@@ -2905,7 +2909,9 @@ darwin_asm_output_dwarf_offset (FILE *fi
       extra = 4;
     }
   snprintf (sname, 64, "*Lsection%.*s%.*s", namelen, name, extra, lto_add);
-  darwin_asm_output_dwarf_delta (file, size, lab, sname, offset);
+  darwin_asm_output_dwarf_delta (file, size, lab,
+                                strncmp (name, "__debug_info", namelen)
+                                ? sname : NULL, offset);
 }

 /* Called from the within the TARGET_ASM_FILE_START for each target.  */


that seems to work with and without -flto up to the extent that UNDEFs
(the early debug is not extracted and thus not linked) seem to be
resolved to zero by the link editor.