Getting rid of duplicate .debug_ranges

[Mark Wielaard <mjw at redhat dot com>]

Hi,

I noticed that when you generate dwarf for an inlined function it often
comes with duplicate range lists for both the DW_TAG_inlined_subroutine
and the child DW_TAG_lexical_block DIE. For example:

static int k;

static int foo (int i)
{
  int j = i + 42;
  return k + (j > 14 ? j : i);
}

int main (int argc, char **argv)
{
  int c = argc;
  k = 2 * c;
  c = foo (c);
  return c;
}

Generates with -O2 -gdwarf-4:

DWARF section [27] '.debug_info' at offset 0x895:
 [Offset]
 Compilation unit at offset 0:
 Version: 4, Abbreviation section offset: 0, Address size: 8, Offset size: 4
[...]
 [    a8]      inlined_subroutine
               abstract_origin      (ref4) [    31]
               entry_pc             (addr) 0x0000000000400360 <main>
               ranges               (data4) range list [     0]
               call_file            (data1) 1
               call_line            (data1) 13
 [    bb]        formal_parameter
                 abstract_origin      (ref4) [    42]
                 location             (block1) 
                  [   0] reg5
 [    c2]        lexical_block
                 ranges               (data4) range list [    40]
 [    c7]          variable
                   abstract_origin      (ref4) [    4b]
                   location             (data4) location list [    23]
[...]
DWARF section [32] '.debug_ranges' at offset 0xb4e:
 [     0]  0x0000000000400360 <main>..0x0000000000400363 <main+0x3>
           0x0000000000400366 <main+0x6>..0x0000000000400369 <main+0x9>
           0x000000000040036f <main+0xf>..0x0000000000400374 <main+0x14>
 [    40]  0x0000000000400360 <main>..0x0000000000400363 <main+0x3>
           0x0000000000400366 <main+0x6>..0x0000000000400369 <main+0x9>
           0x000000000040036f <main+0xf>..0x0000000000400374 <main+0x14>
 [    80]  0x0000000000400360 <main>..0x0000000000400375

So range list 0 for the inlined_subroutine DIE a8 is the same as range
list 40 for the lexical_block DIE c2.

I had hoped I could detect and then reduce the duplication of the range
lists in this case with the attached patch where I just check that the
basic block numbers for the ranges are the same as the ranges of its
context DIE. But that doesn't actually work since by looking at the
generated assembly the basic block asm labels are in the same place,
the block numbers are actually different (and so there are actually
two differently named/numbered labels generated at the same place). e.g:

.LVL0:
.LBB4:
.LBB5:
	.loc 1 5 0
	leal	42(%rdi), %eax
.LBE5:
.LBE4:
	.loc 1 12 0
	leal	(%rdi,%rdi), %edx
.LBB8:
.LBB6:
	.loc 1 6 0
	cmpl	$14, %eax
.LBE6:
.LBE8:
	.loc 1 12 0
	movl	%edx, k(%rip)
.LVL1:
.LBB9:
.LBB7:
	.loc 1 6 0
	cmovle	%edi, %eax
.LVL2:
	addl	%edx, %eax
.LBE7:
.LBE9:
	.loc 1 15 0
	ret
	.cfi_endproc
[...]
	.section	.debug_ranges,"",@progbits
.Ldebug_ranges0:
	.quad	.LBB4
	.quad	.LBE4
	.quad	.LBB8
	.quad	.LBE8
	.quad	.LBB9
	.quad	.LBE9
	.quad	0
	.quad	0
	.quad	.LBB5
	.quad	.LBE5
	.quad	.LBB6
	.quad	.LBE6
	.quad	.LBB7
	.quad	.LBE7
	.quad	0
	.quad	0
	.quad	.LFB1
	.quad	.LFE1
	.quad	0
	.quad	0

Is there a way to detect that basic blocks have the same range even
though they have different block numbers? Or am I not looking/thinking
about this issue correctly?

Thanks,

Mark

Attachment: dup_ranges.patch
Description: Text document