ASan run twice under fat lto

[PiJoules]

Clang gives the following warning when compiling with ASan and fat LTO:

$ ~/llvm-projects/llvm-project-build-1/bin/clang++ -flto -ffat-lto-objects -c -fsanitize=address  /tmp/test.cc
warning: Redundant instrumentation detected, with module flag: nosanitize_address [-Wbackend-plugin]
1 warning generated.

The warning indicates the pass is being run twice, perhaps one for the LTO'd bitcode and another for the normal IR that would be codegen'd. The warning doesn't appear if -ffat-lto-objects is omitted.

Functionally this doesn't affect ASan instrumentation since the pass will exit early if the module was already instrumented, but I wonder if perhaps this would affect other passes which don't do redundant checks.

[PiJoules]

cc @ilovepi

[llvmbot]

@llvm/issue-subscribers-bug

Author: None (PiJoules)

Clang gives the following warning when compiling with ASan and fat LTO:

$ ~/llvm-projects/llvm-project-build-1/bin/clang++ -flto -ffat-lto-objects -c -fsanitize=address  /tmp/test.cc
warning: Redundant instrumentation detected, with module flag: nosanitize_address [-Wbackend-plugin]
1 warning generated.

The warning indicates the pass is being run twice, perhaps one for the LTO'd bitcode and another for the normal IR that would be codegen'd. The warning doesn't appear if -ffat-lto-objects is omitted.

Functionally this doesn't affect ASan instrumentation since the pass will exit early if the module was already instrumented, but I wonder if perhaps this would affect other passes which don't do redundant checks.

[ilovepi]

Hmm, FatLTO pipeline has the normal sanitizer early/late hooks run AFAIK. I can look into this for you. I assume this would show up for any source file.

[ilovepi]

Looking into this, it seems that yes, because we share the callbacks, we try to run the instrumentation twice. once on the prelink pipeline to generate the bitcode section, and then again when we run default module optimization to generate the ELF section.

Thankfully this doesn't seem to create a correctness problem, since it bails out, but I'd like to fix it anyway. One thought is to only add the pass for the prelink pipeline. Another is to tell the instr pass if its FatLTO, and then just bail w/o the diagnostic, since its kind of expected. I'd still prefer to avoid trying to run it at all if its already ran before though.