[Bug lto/65015] LTO produces randomly ordered debug information

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65015

--- Comment #19 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to H.J. Lu from comment #18)
> Created attachment 34753 [details]
> A patch

even for -flto-partition=none we produce

    45: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS ccPyi2gu.o

In theory we can also emit a .file directive before each variable/function
we output (from location info).  I suppose the debugger consumes them
for backtraces?

Thus produce

        .file   "test.c"
        .text
        .globl  main
        .type   main, @function
main:
.LFB0:  
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        call    helper
        popq    %rbp
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE0:  
        .size   main, .-main
        .file   "dependency.c"
        .type   helper, @function
helper:
.LFB1:  
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        movl    $1, %eax
        popq    %rbp
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE1:  
        .size   helper, .-helper
        .ident  "GCC: (GNU) 5.0.0 20150213 (experimental)"
        .section        .note.GNU-stack,"",@progbits

but appearantly as/ld doesn't handle multiple global .file directives well?
I get

    45: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS dependency.c
    46: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS test.c
    47: 00000000004005b1    11 FUNC    LOCAL  DEFAULT   13 helper
...
    71: 00000000004005a6    11 FUNC    GLOBAL DEFAULT   13 main

so 'helper' is associated with test.c wrongly(?)

Patch I was playing with (for functions only):

Index: gcc/varasm.c
===================================================================
--- gcc/varasm.c        (revision 220677)
+++ gcc/varasm.c        (working copy)
@@ -1713,6 +1713,10 @@ assemble_start_function (tree decl, cons
   char tmp_label[100];
   bool hot_label_written = false;

+  if (targetm.asm_file_start_file_directive
+      && in_lto_p)
+    output_file_directive (asm_out_file, DECL_SOURCE_FILE (decl));
+
   if (flag_reorder_blocks_and_partition)
     {
       ASM_GENERATE_INTERNAL_LABEL (tmp_label, "LHOTB", const_labelno);
@@ -7042,7 +7046,9 @@ default_file_start (void)
       && !(flag_verbose_asm || flag_debug_asm || flag_dump_rtl_in_asm))
     fputs (ASM_APP_OFF, asm_out_file);

-  if (targetm.asm_file_start_file_directive)
+  if (targetm.asm_file_start_file_directive
+      /* LTO produced units have no meaningful main_input_filename.  */
+      && !in_lto_p)
     output_file_directive (asm_out_file, main_input_filename);
 }


which shows an alternative to <artificial> by picking a random source
file name via

  output_file_directive (asm_out_file, DEC_SOURCE_FILE
(symtab->first_defined_symbol ()->decl));


That said, I think we can go with <artificial> for now and try to improve
that later.  I'm going to test an adjusted patch using in_lto_p again.