[Bug debug/42454] New: debug_ranges table contains empty range for unused .text section with -ffunction-sections

[ccoutant at gcc dot gnu dot org]

When -ffunction-sections is turned on with -g2, gcc emits an entry into the
.debug_ranges section for the DW_TAG_compile_unit for the range (.Ltext,
.Letext) unconditionally. With -ffunction-sections, the .text section remains
unused, and this entry in .debug_ranges refers to a zero-length range.

That's not normally a problem, but when --gc-sections is turned on in the
linker, the empty .text section can be removed, and the relocations for that
range apply zeroes to that entry, and it now looks like the end of the range
list. Because that's actually the first range entry that gcc emits for the
compile unit, DWARF consumers see an empty range list. This causes significant
slowdowns for objdump and addr2line.

Example:

$ cat foo.c
int foo(int i)
{       
  return i + 1; 
}       
int bar(int i)  
{       
  return i + 2; 
}       
$ gcc -S -dA -g -ffunction-sections foo.c

The resulting assembly code has this:

        .text
.Ltext0:
        .section        .text.foo,"ax",@progbits
.globl foo
        .type   foo, @function
foo:
.LFB0:
...
        .text
.Letext0:
...
        .section        .debug_ranges,"",@progbits
.Ldebug_ranges0:
        .quad   .Ltext0 # Offset 0x0
        .quad   .Letext0
        .quad   .LFB0   # Offset 0x10
        .quad   .LFE0
        .quad   .LFB1   # Offset 0x20
        .quad   .LFE1
        .quad   0x0
        .quad   0x0


-- 
           Summary: debug_ranges table contains empty range for unused .text
                    section with -ffunction-sections
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
        AssignedTo: ccoutant at gcc dot gnu dot org
        ReportedBy: ccoutant at gcc dot gnu dot org
 GCC build triplet: x86_64-unknown-linux
  GCC host triplet: x86_64-unknown-linux
GCC target triplet: x86_64-unknown-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42454