[Bug d/101127] d: Compile-time reflection for supported built-ins

[ibuclaw at gdcproject dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101127

--- Comment #1 from Iain Buclaw <ibuclaw at gdcproject dot org> ---
There's the language hook LANG_HOOKS_BUILTIN_FUNCTION_EXT_SCOPE, which seems to
do what we want on the surface, but then there's a question over whether this
is to be correct.

---
// static condition is false
static if (__traits(compiles, __builtin_ia32_pmulhrsw128))
    pragma(msg, "Yes SSSE3 support");
else
{
    short8 truc(short8 a, short8 b) @target("ssse3") // adds SSSE3 built-ins.
    {
        return __builtin_ia32_pmulhrsw128(a, b);
    }
}

// static condition could be true, depending on order of evaluation.
static if (__traits(compiles, __builtin_ia32_pmaddubsw128))
    pragma(msg, "Yes SSSE3 support");