[Bug c/24570] New: unit-at-a-time: debug info not emitted for unused global variables

["tomas dot vanek at fbl dot cz" <gcc-bugzilla at gcc dot gnu dot org>]

Both

Target: i686-pc-linux-gnu
Configured with: ../configure --disable-nls --with-dwarf2
Thread model: posix
gcc version 4.0.2

and

Target: avr
Configured with: ../configure --prefix=/usr/local/avr --target=avr
--disable-nls --with-dwarf2 --enable-languages=c 
Thread model: single
gcc version 4.0.2

do not emit stabs or emit incomplete dwarf debug info for an unused global
variable when compiled unit-at-a-time (-O2 or -Os).

var.c:
int used_variable;
int unused_variable __attribute__((unused));

int use_one(void)
{
    return used_variable;
}

gcc -S var.c -gstabs
and avr-gcc -S -mmcu=atmega8 var.c -gstabs 

works good and generates stabs for both variables

.Lscope0:
        .comm   used_variable,4,4
        .comm   unused_variable,4,4
        .stabs  "used_variable:G(0,1)",32,0,0,0
        .stabs  "unused_variable:G(0,1)",32,0,0,0
        .stabs  "",100,0,0,.Letext0
.Letext0:


gcc -S var.c -gstabs -funit-at-a-time
or avr-gcc -S -mmcu=atmega8 var.c -gstabs -funit-at-a-time

As soon as unit-at-a-time is active, stabs for unused var is gone

.Lscope0:
        .stabs  "used_variable:G(0,1)",32,0,0,0
        .comm   unused_variable,4,4
        .comm   used_variable,4,4
        .stabs  "",100,0,0,.Letext0
.Letext0:



gcc -S var.c -gdwarf-2 -dA -funit-at-a-time
or avr-gcc -S -mmcu=atmega8 var.c -gdwarf-2 -dA -funit-at-a-time

With dwarf, the unused variable is emited without location as if it was
external.

        .uleb128 0x4    # (DIE (0x63) DW_TAG_variable)
        .ascii "used_variable\0"        # DW_AT_name
        .byte   0x1     # DW_AT_decl_file
        .byte   0x1     # DW_AT_decl_line
        .long   0x5c    # DW_AT_type
        .byte   0x1     # DW_AT_external
        .byte   0x5     # DW_AT_location
        .byte   0x3     # DW_OP_addr
        .long   used_variable
        .uleb128 0x5    # (DIE (0x7f) DW_TAG_variable)
        .ascii "unused_variable\0"      # DW_AT_name
        .byte   0x1     # DW_AT_decl_file
        .byte   0x2     # DW_AT_decl_line
        .long   0x5c    # DW_AT_type
        .byte   0x1     # DW_AT_external
        .byte   0x0     # end of children of DIE 0xb
        .section        .debug_abbrev


The problem is caused by calling c_write_global_declarations_2 () before
cgraph_varpool_assemble_pending_decls (), which generates rtx for unused
variables.

I tried a dirty hack: insert an extra call of
cgraph_varpool_assemble_pending_decls () into
procedure c_write_global_declarations() in c-decl.c just before the last block
     if - for - c_write_global_declarations_2
and it solved the problem. Of course I don't know what it breaks...


-- 
           Summary: unit-at-a-time: debug info not emitted for unused global
                    variables
           Product: gcc
           Version: 4.0.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tomas dot vanek at fbl dot cz


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24570