[Bug]: Constexpr expansion compilation error with GCC's UndefinedBehaviorSanitizer

[kaxub]

Describe the issue

When building abseil with GCC with -fsanitize=undefined, the following compilation error occurs:

/abseil/absl/container/internal/raw_hash_set.h:3364:56:   in 'constexpr' expansion of 'absl::container_internal::hash_policy_traits<absl::container_internal::FlatHashMapPolicy<std::basic_string_view<char>, absl::CommandLineFlag*>, void>::get_hash_slot_fn<absl::container_internal::StringHash>()'
/abseil/absl/container/internal/hash_policy_traits.h:163:54: error: '(absl::container_internal::TypeErasedApplyToSlotFn<absl::container_internal::StringHash, std::basic_string_view<char> > == 0)' is not a constant expression
  163 |     return Policy::template get_hash_slot_fn<Hash>() == nullptr
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
make-4.3[4]: *** [absl/flags/CMakeFiles/flags_reflection.dir/build.make:66: absl/flags/CMakeFiles/flags_reflection.dir/reflection.cc.o] Error 1   

This code producing the error has been introduced with commit 4c7e7c7.

The error does not occur when building with no sanitizer or when building with -fsanitize=address or -fsanitize=thread.

This seems to be a known GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71962

Steps to reproduce the problem

git clone https://github.com/abseil/abseil-cpp.git
cd abseil-cpp
mkdir build
cd build
cmake -DCMAKE_CXX_FLAGS="-fsanitize=undefined" ..
make

Isolated expression from the method where the error occurs (absl::container_internal::hash_policy_traits::get_hash_slot_fn<Hash>()) with -std=gnu++17 -fsanitize=undefined: https://godbolt.org/z/8bEazf6e5

What version of Abseil are you using?

7bd9ff9

What operating system and version are you using?

Ubuntu 20.04.6 LTS

What compiler and version are you using?

GCC 9.4.0
Also tested with GCC 13.2 on linked godbolt.

What build system are you using?

cmake 3.16.3

Additional context

No response

[jnooree]

Same issue here.

[ptrmtrx]

@derekmauro can somebody from abseil look at it please, it blocks our development. We use google quiche that uses abseil, but we build it by gcc with undefined behavior sanitizer and it results into this bug.

[jnooree]

For a temporary workaround, I used clang for sanitizer builds.

[ptrmtrx]

Unfortunately, t's not an option in our environment.

[derekmauro]

Sorry, this issue is not a priority for me at the moment.

[BotellaA]

Is there any update on this? Is it an abseil issue or a GCC issue?

[jnooree]

This seems to be a known GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71962

[min-tian-thomas]

A bug of gcc yet abseil may need some workaround to avoid considering it is such a fundamental library

[BotellaA]

I had to disable sanitizer checks, which is a bad idea, but I'm not sure what else we can do...

[ptrmtrx]

We needed the ubsan in gcc, so I created a patch to fix this issue. If anybody is interested, I am posting it here. The patch is based on the commit 60d1ce9 (current master).
It just replaces raw pointers with contexpr std::optional and workarounds a gcc ubsan bug with it. std::optional is used because absl::optional is not very constexpr due to support for c++14 (which is now dropped, but I did the fix it in December).

It works for the abseil itself, built by cmake and for abseil tests. Abseil benchmarks build is not tested as they are not in cmake scripts, or I didn't find them there. So maybe an "auto" return value will need to be specified in raw_hash_set_probe_benchmark.cc.

@derekmauro feel free to use it, modify it or just be inspired by it and integrate it into master if you want to (some kind of fix in the master would make my life easier :) )

0001-Gcc-undefined-sanitizer-compilation-fix.patch