[llvm] [RISCV] Rewrite vrgather.vx undef, (vmv.s.x), 0, v0 as vmv.v.x (PR #136010)

[Philip Reames via llvm-commits]

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@@ -96,13 +96,11 @@ define <8 x float> @vmerge_vxm(<8 x float> %v, float %s) {
 ; CHECK-LABEL: vmerge_vxm:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    li a0, 25
-; CHECK-NEXT:    vsetivli zero, 8, e32, m1, tu, ma
-; CHECK-NEXT:    vfmv.s.f v8, fa0
+; CHECK-NEXT:    vsetivli zero, 1, e32, m4, tu, ma
 ; CHECK-NEXT:    vmv.s.x v0, a0
-; CHECK-NEXT:    vmv2r.v v10, v8
-; CHECK-NEXT:    vsetivli zero, 8, e32, m2, ta, mu
-; CHECK-NEXT:    vrgather.vi v10, v8, 0, v0.t
-; CHECK-NEXT:    vmv.v.v v8, v10
+; CHECK-NEXT:    vfmv.s.f v8, fa0
----------------
preames wrote:

Note that this vfmv.s.f could be eliminated if we rewrote the mask on the vmerge.  I don't plan to do this, just noting it's vaguely possible.  I'm mildly of the opinion that this approach (the post lowering DAG) has been pushed as far as we should, and that if we want to further improve, we should instead starting canonicalizing shuffles before lowering.  I may change my mind based on what future cases I stumble into.  :)

https://github.com/llvm/llvm-project/pull/136010