Module simd::x86_64

std::simd, x86_64 convenience layer

Pure naming/documentation layer over std/simd/simd.h's portable vec<T,N>-based types and functions — there is no separate x86_64 implementation here; LLVM's X86 backend already lowers every operation in simd.sc to real SSE/SSE2 (128-bit) or AVX/AVX2 (256-bit) instructions (vaddps, vmulps, xmm/ymm registers, ...) directly from the portable source. This header just documents the native-width mapping and re-exports the types under names that read naturally for x86_64 code:

- 128-bit (SSE/SSE2, always available in x86-64's baseline ISA):
    f32x4, i32x4, i8x16, u8x16, f64x2  ≈ __m128/__m128i/__m128d
- 256-bit (AVX/AVX2 — requires the target CPU to actually support it,
  e.g. compile with '-mattr=+avx2' when lowering the emitted IR):
    f32x8, i32x8, i8x32, f64x4  ≈ __m256/__m256i/__m256d

If AVX2 isn't available on the target CPU, LLVM legalizes the 256-bit vector types by splitting them into a pair of 128-bit SSE operations — still correct, just not a single instruction.