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

[Dehao Chen via llvm-dev]

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?

Any comments?

Thanks,
Dehao
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