[llvm] [RISCV] Lower VP_SELECT constant false to use vmerge.vxm/vmerge.vim (PR #144461)

[Liao Chunyu via llvm-commits]

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@@ -34,10 +34,10 @@ define <vscale x 1 x i8> @masked_load_passthru_nxv1i8(ptr %a, <vscale x 1 x i1>
 ; ZVE32:       # %bb.0:
 ; ZVE32-NEXT:    csrr a1, vlenb
 ; ZVE32-NEXT:    srli a1, a1, 3
-; ZVE32-NEXT:    vsetvli a2, zero, e8, mf4, ta, ma
-; ZVE32-NEXT:    vmv.v.i v8, 0
-; ZVE32-NEXT:    vsetvli zero, a1, e8, mf4, ta, mu
+; ZVE32-NEXT:    vsetvli zero, a1, e8, mf4, ta, ma
 ; ZVE32-NEXT:    vle8.v v8, (a0), v0.t
+; ZVE32-NEXT:    vmnot.m v0, v0
+; ZVE32-NEXT:    vmerge.vim v8, v8, 0, v0
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ChunyuLiao wrote:

> I don't think using `vmerge` is better. Usually `vmerge` is slower.

Can you help explain why vmerge is slower? According to this example, https://github.com/llvm/llvm-project/blob/main/llvm/test/tools/llvm-mca/RISCV/SiFiveX280/vector-integer-arithmetic.s#L1477
vmv, 4 cycle
vmerge, 4 cycles
vmnot.m, 4 cycles
vsetvl, 3 cycles 

https://github.com/llvm/llvm-project/pull/144461