magpie09 (u/magpie09)

Edit: Someone has explained the issue below

I don't understand how that broke anything?

The C++ codegen for && and & is the same:

    bool good(bool a, bool b) { return a && b; }

    good(bool, bool):                              
        mov     eax, edi
        and     eax, esi
        ret


    bool bad(bool a, bool b) { return a & b; }

    bad(bool, bool):
        mov     eax, edi
        and     eax, esi
        ret

[0] https://godbolt.org/z/84MMqTehs

magpie09 · 5 years ago

I made what I think is a minimal example here: https://godbolt.org/z/dcnzonjar

Interestingly I couldn't get it to fail on GCC (even with -fnoexceptions and -Ofast), only on Clang. Maybe someone else has some info as to why this is?

There is a clear difference in the assembly output for the correct and incorrect implementations, with a testb instruction run much earlier in the correct version. You can see this at line 22 of the output, if you highlight it the && in the source is highlighted in bold.