[llvm] [X86] Fold VPERMV3(X, M, Y) -> VPERMV(CONCAT(X, Y), WIDEN(M)) iff the CONCAT is free (PR #122485)

[Phoebe Wang via llvm-commits]

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@@ -65,10 +65,9 @@ define void @shuffle_v16i32_to_v8i32_1(ptr %L, ptr %S) nounwind {
 ;
 ; AVX512BWVL-FAST-ALL-LABEL: shuffle_v16i32_to_v8i32_1:
 ; AVX512BWVL-FAST-ALL:       # %bb.0:
-; AVX512BWVL-FAST-ALL-NEXT:    vmovdqa (%rdi), %ymm0
-; AVX512BWVL-FAST-ALL-NEXT:    vpmovsxbd {{.*#+}} ymm1 = [1,3,5,7,9,11,13,15]
-; AVX512BWVL-FAST-ALL-NEXT:    vpermi2d 32(%rdi), %ymm0, %ymm1
-; AVX512BWVL-FAST-ALL-NEXT:    vmovdqa %ymm1, (%rsi)
+; AVX512BWVL-FAST-ALL-NEXT:    vmovaps {{.*#+}} ymm0 = [1,3,5,7,9,11,13,15]
+; AVX512BWVL-FAST-ALL-NEXT:    vpermps (%rdi), %zmm0, %zmm0
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phoebewang wrote:

But we have the equivalent integer instruction VPERMD, which can solve domain stalls, isn't it?

Though the 64-bit load vs. 256-bit load is a problem, my initial question was just about
`vpermi2d 32(%rdi), %ymm0, %ymm1 ; 256-bit load`
vs
`vpermps (%rdi), %zmm0, %zmm0 ; 512-bit load`

So put them together, it looks like a negative benefit to me.

https://github.com/llvm/llvm-project/pull/122485