Clang does not recognize hand-written buffer aliasing checks

[davidben]

Clang (and GCC) don't seem to recognize the __fh_overlap macro in https://godbolt.org/z/EqjKe716r as a buffer aliasing check.

The sample was adapted from https://github.com/jvoisin/fortify-headers/blob/master/include/string.h, which is reportedly used in Musl-based Linux distributions like Alpine.

In this case, since r was just allocated, the compiler should be able to reason that r cannot overlap with a and delete the __fh_overlap check. It's also important to be able to optimize this because memcpy is the usual pattern to load memory without tripping strict aliasing issues.

CC @andres-erbsen

[thesamesam]

Filed https://gcc.gnu.org/PR120903 for GCC.

[andres-erbsen]

Re "TBAA": while the motivation for writing code where the missed optimization is observed comes from avoiding TBAA, I think the optimization itself would be valid just on the basis that r is freshly allocated, even with -fno-strict-aliasing.

[thesamesam]

(FWIW, I just added that label because there isn't another relevant label, but feel free to drop it.)