[compiler-rt][builtins] Switch to using __builtin_memcpy_inline

[alexander-shaposhnikov]

Switch to using __builtin_memcpy_inline.
This code has existed for quite a while, to the best of my knowledge compilers
try to be nice not to exploit this UB (writing to one field of a union and reading from another),
yet strictly speaking this UB.

Test plan: ninja && ninja check-compiler-rt

[frederick-vs-ja]

Would it be simpler to use __builtin_bit_cast?

[alexander-shaposhnikov]

__builtin_bit_cast seems to be available only in C++ (https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html), not in C

[compnerd]

I think that the functional change here is fine and the right thing to do. I worry about compatibility with compilers. Could you please do a follow up (or merge into this change) a change to ensure that the compiler supports this builtin and if not, report a proper error.

[alexander-shaposhnikov]

@compnerd - yeah, MSVC doesn't seem to support it https://godbolt.org/z/EbPhMKqdz

[compnerd]

@alexander-shaposhnikov, right. Older clang will also not support this (it was added in 2020?). I think that we should double check with @rnk and @HansW about the need for building the builtins with MSVC support.

[alexander-shaposhnikov]

I'm under impression that i've seen build bots failures (related to compiler-rt/builtins) on Windows, so unfortunately I suspect I won't be able to land this change for now.

[frederick-vs-ja]

__builtin_bit_cast seems to be available only in C++ (https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html), not in C

Oh, I'm sorry for the wrong suggestion. It seems that only clang supports __builtin_bit_cast in C (demo).

Do these builtins even need to be compiled in C++? I think the existing approach is UB in C++ but not in C.