[llvm-dev] Debug info interacting with optimization and code generation

[Xinliang David Li via llvm-dev]

On Fri, Oct 7, 2016 at 1:35 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Fri, Oct 7, 2016 at 1:27 PM Dehao Chen <dehao at google.com> wrote:
>
>> In theory, compiler should generate bit-identical code with and without
>> debug info. I.e.
>> # clang -c -O2 -g a.cc -o a.g.o
>> # clang -c -O2 -g0 a.cc -o a.g0.o
>> # strip a.g.o a.g0.o
>> # diff a.g.o a.g0.o
>> The diff should find two binaries identical. For brevity, in the rest of
>> the mail, I'll refer to this requirement as "codegen consistency" (any
>> better name?)
>>
>> Unfortunately, LLVM does not guarantee codegen consistency. Recently,
>> I've spent quite some time try to fix related issues (e.g.
>> https://reviews.llvm.org/D25286 and https://reviews.llvm.org/D25098).
>> The most recent issue I'm looking at is that during isel, the IROrder is
>> used by both debug info and the actual codegen, which is relative harder to
>> fix.
>>
>> I initially thought that it's just a couple of careless bugs to fix. But
>> looks like there are much more issues than I expected. So I'm calling the
>> community for help:
>>
>> * Is there anyone else who also cares about codegen consistency?
>> * Any volunteers to help fix codegen consistency issues? (It is easy to
>> find issues, just build speccpu with -g and -g0, then compare the "objdump
>> -d" output)
>> * How to setup a regression test to ensure future changes does not break
>> codegen consistency?
>>
>
> Specific test cases would be checked in as usual - beyond that, probably a
> self-host that checks for consistency (like a 3 stage bootstrap checks that
> stage 2 and 3 are identical). Potentially other workloads could be added if
> a selfhost didn't offer enough certainty for common cases.
>
> It's an abstract good/intended goal, for sure - but it's not been a
> priority for anyone (as you've seen), so just hasn't been pushed very
> hard/far.
>

I agree with you that this is a good/intended goal, but it is not
'abstract' good goal :)

David


>
> - Dave
>
>
>>
>> Any comments?
>>
>> Thanks,
>> Dehao
>>
>
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