Re: [RFC][debug] Add -fadd-debug-nops

[Tom de Vries <tdevries@suse.de>]

On 07/16/2018 03:50 PM, Richard Biener wrote:
> On Mon, 16 Jul 2018, Tom de Vries wrote:
> 
>> Hi,
>>
>> this is an idea that I'm currently playing around with: adding nops in
>> an optimized application with debug info can improve the debug info.
>>
>>
>> Consider f.i. this gdb session in foo of pr54200-2.c (using -Os):
>> ...
>> (gdb) n
>> 26            return a; /* { dg-final { gdb-test . "(int)a" "6" } } */
>> (gdb) p a
>> 'a' has unknown type; cast it to its declared type
>> (gdb) n
>> main () at pr54200-2.c:34
>> 34        return 0;
>> ...
>>
>> The problem is that the scope in which a is declared ends at .LBE7, and the
>> statement .loc for line 26 ends up attached to the ret insn:
>> ...
>>         .loc 1 24 11
>>         addl    %edx, %eax
>> .LVL1:
>>         # DEBUG a => ax
>>         .loc 1 26 7 is_stmt 1
>> .LBE7:
>>         .loc 1 28 1 is_stmt 0
>>         ret
>>         .cfi_endproc
>> ...
>>
>> This patch fixes the problem (for Og and Os, the 'DEBUG a => ax' is missing
>> for O1 and higher) by adding a nop before the ret insn:
>> ...
>>         .loc 1 24 11
>>         addl    %edx, %eax
>>  .LVL1:
>>         # DEBUG a => ax
>>         .loc 1 26 7 is_stmt 1
>> +       nop
>>  .LBE7:
>>         .loc 1 28 1 is_stmt 0
>>         ret
>>         .cfi_endproc
>> ...
>>
>> and instead we have:
>> ...
>> (gdb) n
>> 26            return a; /* { dg-final { gdb-test . "(int)a" "6" } } */
>> (gdb) p a
>> $1 = 6
>> (gdb) n
>> main () at pr54200-2.c:34
>> 34        return 0;
>> ...
>>
>> Any comments?
> 
> Interesting idea.  I wonder if that should be generalized
> to other places

I kept the option name general, to allow for that.

And indeed, this is a point-fix patch. I've been playing around with a
more generic patch that adds nops such that each is_stmt .loc is
associated with a unique insn, but that was embedded in an
fkeep-vars-live branch, so this patch is minimally addressing the first
problem I managed to reproduce on trunk.

> and how we can avoid compare-debug failures
> (var-tracking usually doesn't change code-generation).
> 

I could remove the cfun->debug_nonbind_markers test and move the
nop-insertion to a separate pass or some such.

Thanks,
- Tom