[llvm] [VPlan] Try to hoist Previous (and operands), if sinking fails for FORs. (PR #108945)

[via llvm-commits]

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@@ -278,3 +278,67 @@ exit:
   store double %.lcssa, ptr %C
   ret i64 %.in.lcssa
 }
+
+; Test for https://github.com/llvm/llvm-project/issues/106523.
+define void @for_iv_trunc_optimized(ptr %dst) {
+; CHECK-LABEL: @for_iv_trunc_optimized(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
+; CHECK:       vector.ph:
+; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
+; CHECK:       vector.body:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VECTOR_RECUR:%.*]] = phi <4 x i32> [ <i32 poison, i32 poison, i32 poison, i32 1>, [[VECTOR_PH]] ], [ [[STEP_ADD:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VECTOR_RECUR1:%.*]] = phi <4 x i32> [ <i32 poison, i32 poison, i32 poison, i32 0>, [[VECTOR_PH]] ], [ [[TMP3:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VEC_IND:%.*]] = phi <4 x i32> [ <i32 1, i32 2, i32 3, i32 4>, [[VECTOR_PH]] ], [ [[VEC_IND_NEXT:%.*]], [[VECTOR_BODY]] ]
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ayalz wrote:

The loop seems to be vectorized by VF=4 and unrolled by UF=2 as VEC_IND and INDEX are bumped by 8's, but there's only a single copy of <4 x i32> vectors, presumably due to dce.

https://github.com/llvm/llvm-project/pull/108945