JIT: Improve codegen for xarch vector byte multiply

[saucecontrol]

Resolves #109775

In cases where we can widen to the next vector size up and multiply once, the current codegen is already optimal. When that's not possible, the current codegen falls back to a version that splits into two vectors and runs the same basic algorithm, which is not optimal.

This implements the suggestion made by @MineCake147E on #109775, which still requires two multiplications, but avoids double widening and narrowing. The result is a ~2x perf improvement.

Typical diff:

        vmovups  zmm0, zmmword ptr [r8]
        vpbroadcastb zmm1, byte  ptr [rcx]
        vpsubb   zmm0, zmm0, zmm1
-       vmovaps  zmm1, zmm0
-       vpmovzxbw zmm1, zmm1
-       vpmullw  zmm1, zmm1, zmm1
-       vpmovwb  zmm1, zmm1
-       vextracti32x8 ymm0, zmm0, 1
-       vpmovzxbw zmm0, zmm0
+       vpsrlw   zmm1, zmm0, 8
+       vpandd   zmm2, zmm0, dword ptr [reloc @RWD00] {1to16}
+       vpmullw  zmm1, zmm2, zmm1
        vpmullw  zmm0, zmm0, zmm0
-       vpmovwb  zmm0, zmm0
-       vinserti32x8 zmm0, zmm1, ymm0, 1
+       vpternlogd zmm0, zmm1, dword ptr [reloc @RWD04] {1to16}, -20
        vmovups  zmmword ptr [rdx], zmm0
        mov      rax, rdx
        vzeroupper 
        ret      

+RWD00  	dd	FF00FF00h
+RWD04  	dd	00FF00FFh
 
-; Total bytes of code 87
+; Total bytes of code 71

Full diffs

NB: codegen could actually be better, but currently JIT imports (and morphs) AND_NOT(x, y) as AND(x, NOT(y)), which in the case of constant y that is re-used, creates two constants where one would have sufficed.

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[Copilot]

Pull request overview

Improves xarch JIT codegen for SIMD byte multiplication by using a more efficient two-multiply “odd/even byte” strategy when widening to the next vector size isn’t possible, reducing unnecessary widen/narrow work compared to the prior fallback.

Changes:

• Adds a fast-path that widens to the next vector size (AVX2 for SIMD16, AVX512 for SIMD32) to perform a single multiply and then narrow.
• Replaces the previous fallback (split/widen/mul/narrow twice) with an odd/even byte approach that uses two 16-bit multiplies and recombines bytes with masks/shifts.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 3 comments.

[xtqqczze]

vpmovzxbw zmm1, zmm1

the current codegen has an invalid operand for the instruction?

should be VPMOVZXBW zmm1 {k1}{z}, ymm2/m256?

and a similar issue for vpmovwb?

[saucecontrol]

the current codegen has an invalid operand for the instruction?

Ha, I didn't notice that. Looks like that's just a bug in the JIT disasm. Running the code bytes through another disassembler shows it correctly.

[saucecontrol]

cc @dotnet/jit-contrib

Diffs