[Bug debug/101575] New: [gcc-11, -gdwarf-4] Missing .file <n> directive causes invalid line info

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101575

            Bug ID: 101575
           Summary: [gcc-11, -gdwarf-4] Missing .file <n> directive causes
                    invalid line info
           Product: gcc
           Version: 11.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Consider test-case defs.adb/defs.ads from a gdb testcase (
https://sourceware.org/git/?p=binutils-gdb.git;a=tree;f=gdb/testsuite/gdb.ada/formatted_ref
).

If we compile with dwarf 5, we have only one address at defs.adb:20, 0x2a:
...
$ gcc -c -gdwarf-5 -g defs.adb
$ readelf -wL defs.o
Contents of the .debug_line section:

CU: ./defs.adb:
File name                            Line number    Starting address    View   
Stmt
defs.adb                                      18                0x22           
   x
defs.adb                                      20                0x2a           
   x
defs.adb                                      21                0x30           
   x

./defs.adb:[++]
defs.adb                                      16                0x32           
   x
defs.adb                                      25                0x36           
   x
defs.adb                                      27                0x45           
   x
defs.adb                                       -                0x48
...

With dwarf 4, we have two addresses:
...
$ gcc -c -gdwarf-4 -g defs.adb 
$ readelf -wL defs.o          
Contents of the .debug_line section:

CU: ./defs.adb:
File name                            Line number    Starting address    View   
Stmt
defs.adb                                      10                   0           
   x
defs.adb                                      13                 0x1           
   x
defs.adb                                      15                 0x4           
   x
defs.adb                                      16                 0x8           
   x
defs.adb                                      17                 0xb           
   x
defs.adb                                      18                 0xf           
   x
defs.adb                                      19                0x15           
   x
defs.adb                                      20                0x19           
   x
defs.adb                                      21                0x20           
   x
defs.adb                                      23                0x21           
   x
defs.adb                                      18                0x22           
   x
defs.adb                                      20                0x2a           
   x
defs.adb                                      21                0x30           
   x

./defs.ads:[++]
defs.ads                                      16                0x32           
   x
defs.ads                                      25                0x36           
   x
defs.ads                                      27                0x45           
   x
defs.ads                                       -                0x48
...

The additional breakpoint at address 0x19 falls into:
...
0000000000000000 <defs__struct1IP>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 89 7d f8             mov    %rdi,-0x8(%rbp)
   8:   89 75 f4                mov    %esi,-0xc(%rbp)
   b:   48 8b 45 f8             mov    -0x8(%rbp),%rax
   f:   c7 00 0d 00 00 00       movl   $0xd,(%rax)
  15:   48 8b 45 f8             mov    -0x8(%rbp),%rax
  19:   c7 40 04 13 00 00 00    movl   $0x13,0x4(%rax)
  20:   5d                      pop    %rbp
  21:   c3                      ret    
...
which is an initialization function corresponding to this code in defs.ads:
...
   type Struct1 is limited record
      X : Integer := 13;
      Y : Integer := 19;
   end record;
...

The dwarf 4 assembly starts like this:
...
$ cat -n defs.s
     1          .file   "defs.adb"
     2          .text
     3  .Ltext0:
     4          .align 2
     5          .globl  defs__struct1IP
     6          .type   defs__struct1IP, @function
     7  defs__struct1IP:
     8  .LFB2:
     9          .cfi_startproc
    10          pushq   %rbp
    11          .cfi_def_cfa_offset 16
    12          .cfi_offset 6, -16
    13          movq    %rsp, %rbp
    14          .cfi_def_cfa_register 6
    15          movq    %rdi, -8(%rbp)
    16          movl    %esi, -12(%rbp)
    17          movq    -8(%rbp), %rax
    18          movl    $13, (%rax)
    19          movq    -8(%rbp), %rax
    20          movl    $19, 4(%rax)
    21          popq    %rbp
    22          .cfi_def_cfa 7, 8
    23          ret
    24          .cfi_endproc
    25  .LFE2:
    26          .size   defs__struct1IP, .-defs__struct1IP
    27          .align 2
    28          .globl  defs__f1
    29          .type   defs__f1, @function
    30  defs__f1:
    31  .LFB3:
    32          .file 1 "defs.adb"
    33          .loc 1 18 4
...
and gas generates line-info for this assembly because:
- we pass --gdwarf-4 to gas
- of the absence of a .file  <n> <file> directive in defs__struct1IP

The dwarf 5 assembly starts like:
...
        .file   "defs.adb"
        .text
.Ltext0:
        .file 0 "/home/vries/gdb_versions/devel" "defs.adb"
        .align 2
        .globl  defs__struct1IP
        .type   defs__struct1IP, @function
defs__struct1IP:
.LFB2:
        .cfi_startproc
        pushq   %rbp
...
and due to the .file 0 directive we hit this in gas/dwarf2dbg.c:
...
  /* A .file directive implies compiler generated debug information is
     being supplied.  Turn off gas generated debug info.  */
  debug_type = DEBUG_NONE;
...
and no line info is generated for the assembly.

AFAIU, this can be fixed by having gcc issue a .file <n> directive in
defs__struct1IP.