[llvm] [RISCV] Support postRA vsetvl insertion pass (PR #70549)

[Philip Reames via llvm-commits]

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@@ -10,19 +10,19 @@ define void @test(ptr %ref_array, ptr %sad_array) {
 ; RV32-NEXT:    th.lwd a2, a3, (a0), 0, 3
 ; RV32-NEXT:    vsetivli zero, 4, e8, mf4, ta, ma
 ; RV32-NEXT:    vle8.v v8, (a2)
-; RV32-NEXT:    vmv.v.i v9, 0
 ; RV32-NEXT:    vsetivli zero, 16, e32, m4, ta, ma
 ; RV32-NEXT:    vzext.vf4 v12, v8
 ; RV32-NEXT:    vmv.s.x v8, zero
-; RV32-NEXT:    vredsum.vs v10, v12, v8
-; RV32-NEXT:    vmv.x.s a0, v10
+; RV32-NEXT:    vredsum.vs v9, v12, v8
+; RV32-NEXT:    vmv.x.s a0, v9
 ; RV32-NEXT:    th.swia a0, (a1), 4, 0
 ; RV32-NEXT:    vsetivli zero, 4, e8, mf4, ta, ma
-; RV32-NEXT:    vle8.v v10, (a3)
+; RV32-NEXT:    vle8.v v9, (a3)
+; RV32-NEXT:    vmv.v.i v10, 0
 ; RV32-NEXT:    vsetivli zero, 8, e8, mf2, ta, ma
-; RV32-NEXT:    vslideup.vi v10, v9, 4
+; RV32-NEXT:    vslideup.vi v9, v10, 4
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preames wrote:

Another (unrelated) opportunity.  We're inserting a subvec into a zero vector just to compute a sum reduction where the zero elements are nops.  We could have computed the sum on the sub-vec directly.  

https://github.com/llvm/llvm-project/pull/70549