X86-specific C support library functions

[nwf]

I recognize that this is surely not a huge priority, but:

lib/int128/sail.c and lib/sail.c each make use of X86-specific built-ins and __attribute__((target ("bmi2"))). This means that the resulting C translations are necessarily post-Haswell X86 only, which is not great (though one assumes the performance gain from BMI2 for bit twiddling is considerable).

[Alasdair]

I think the only one I really use is bzhi_u64. I remember the performance boost was nice (but not earth shattering) on my Intel desktop in the Lab, but now I'm using an AMD ryzen on my home computer and I think the bmi instructions are actually slower than the shift and mask version:

bzhi_u64(bits,len) = bits & (UINT64_MAX >> (64 - len))

So it might be worth making it optional anyway

[MattWindsor91]

I just ran into this, or something similar, on trying to compile sail-riscv on an Apple M1 computer (ie, AArch64) - the inclusion of x86intrin.h in sail.c produced the following (rather strange) error:

In file included from sail.c:10:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/12.0.0/include/x86intrin.h:15:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/12.0.0/include/immintrin.h:14:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/12.0.0/include/mmintrin.h:50:12: error: invalid conversion between vector type
      '__m64' (vector of 1 'long long' value) and integer type 'int' of different size
    return (__m64)__builtin_ia32_vec_init_v2si(__i, 0);
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and many other similar errors.

On my copy there are instances of _pdep_u64 and _bzhi_u64 in sail.c, both of which can presumably be stubbed out with less efficient C equivalents on things without x86 BMI2 - I'm not sure if there are any other x86 specific things in sail? Apologies if what I'm trying to do is explicitly unsupported/will cause issues elsewhere.

[Alasdair]

I think it's probably best to just remove these intrinsics for portability now that arm based macs are something we will want to support

[Alasdair]

Removed in 55a91e5