AVX2 backend not enabled by default

[dkostic]

Describe the bug
Since this commit the AVX2 backend doesn't compile by default. My platform is a c7i EC2 instance with Intel(R) Xeon(R) Platinum 8488C, running Ubuntu 22.04 with gcc 11.4.0.

Platform: x86_64 Linux

Compiler: gcc 11.4.0

How to reproduce
Steps to reproduce the behavior:

make clean; make -j

One of the easiest ways to confirm that the AVX2 backend is not enabled is to run any of the benchmarks and compare the numbers against the expected ones (https://pq-code-package.github.io/mlkem-native/dev/bench/). Otherwise, you can insert #error fail_the_build_if_this_is_compiled for example here and try to build the library which will succeed, meaning that the AVX2 backend is not compiled.

Expected behavior
A clear and concise description of what you expected to happen.

Additional context
Add any other context about the problem here.

[hanno-becker]

Thank you @dkostic for reporting this. We should have caught this in CI which even tests with this machine, but is likely not checking the backend. Interestingly, the benchmarks did not show a regression, so the AVX2 backend must still work in the bench tests on c7i.

Can you confirm whether the build fails with FORCE_X86_64?

[hanno-becker]

@dkostic can you confirm that #578 fixed the issue for you?

[dkostic]

Thanks for quick reaction! The fix in #578 works on my machine as well.