https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57359
--- Comment #25 from Pascal Cuoq <pascal_cuoq at hotmail dot com> --- Would it be reasonable to have three settings for -fstrict-aliasing, rather then the current two? - off - strict - assume-no-reuse (I would let you find a better name for the third one.) It seems to me that the wrong transformation corresponds to code patterns that a C developers familiar with the compiled code would be able to tell are used or not in the compiled code: repurposing of dynamically allocated memory and access to a union through pointers. (Note: in https://trust-in-soft.com/wp-content/uploads/2017/01/vmcai.pdf we remarked that GCC is particularly user-friendly in both documenting what uses of unions are compatible with -fstrict-aliasing and in sticking to the documented behavior. The place in the GCC documentation where this is documented would already explain a lot of the context for explaining the difference between the strict and assume-no-reuse settings.) Even if you set -fstrict-aliasing=assume-no-reuse as implied by -O2, this would still be a much welcome improvement compared to the current situation. GCC would continue to be compared fairly in benchmarks to other compilers that have the same approximation, most programs would continue to work fine because they do not rely on the dangerous patterns, and those that rely on the dangerous pattern would have a way out. It would be vaguely comparable to -ffast-math. One possible psychological drawback may be that if the “strict” option exists, some users may ask why GCC does not stick to it in the -On settings.
