Timing attacks are a very specialized problem. If you don't care about performance, why not wrap the critical section in:

  #pragma GCC push_options #pragma GCC optimize ("O0")

Exploiting UB in the optimizer can be annoying, but most projects with bad practices from the 1990s have figured it out by now. UBsan helps of course.

I'm pretty grateful for aggressive optimizations. I would not want to compile a large C++ codebase with g++ that has itself been compiled with -O0. Even a 20% speedup helps.

The only annoying issue with C/C++ compilers is the growing list of false positive warnings (usually 100% false positives in well written projects).