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

[Adrian Prantl via llvm-dev]

> On Oct 7, 2016, at 1:27 PM, Dehao Chen via llvm-dev <llvm-dev at lists.llvm.org> 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?

We have in the past always treated situations where the presence of debug info caused different code to be emitted as pretty serious bugs. Typically these bugs came from code that didn't properly skip over debug intrinsics when doing peephole-style transformations.

> * 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)

I certainly don't mind getting CC'ed on any PRs that we find :-)

-- adrian

> * How to setup a regression test to ensure future changes does not break codegen consistency?
> 
> Any comments?
> 
> Thanks,
> Dehao
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