Re: [PATCH v2 0/4] MR375: ntdll: Fixes for runtime 64-bit shift functions.

[Jinoh Kang (＠iamahuman)]

Jinoh Kang (@iamahuman) commented about dlls/ntdll/large_int.c:
>  return udivmod(a, b, NULL);
>
> }
> -
>
> +LONGLONG __regs__allshl( LONGLONG a, unsigned char b )
> +{
> - const LARGE_INTEGER x = { .QuadPart = a };
> - LARGE_INTEGER ret;
> -
> - if (b >= 64)
> -
>    return 0;

It appears that GCC's optimizer is having a hard time dealing with mixing full (64-bit) and partial (32-bit) writes.

Compare:

- https://godbolt.org/z/KvdGfr4bY (original)

With:

- https://godbolt.org/z/vG5TGrhaY (64-bit return ellided)

I'd suggest folding the special case into the if statement below.
We do already lose performance by thunking, but it's still a good idea not to slow down (or, rather, amplify the I-cache usage of) a builtin too much.

Meanwhile, clang does not appear to suffer from this problem.