[llvm] [LV] Use frozen start value for FindLastIV if needed. (PR #132691)

[Mel Chen via llvm-commits]

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@@ -6,6 +6,7 @@ define i64 @select_icmp_nuw_nsw(ptr %a, ptr %b, i64 %ii, i64 %n) {
 ; CHECK-SAME: ptr [[A:%.*]], ptr [[B:%.*]], i64 [[II:%.*]], i64 [[N:%.*]]) {
 ; CHECK-NEXT:  [[ENTRY:.*]]:
 ; CHECK-NEXT:    [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[N]], 4
+; CHECK-NEXT:    [[FR:%.*]] = freeze i64 [[II]]
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Mel-Chen wrote:

Although the incoming value of FindLastIV PHI in epilogue vectorization can be directly broadcasted with the sentinel value to avoid generating this icmp, I'm not sure if AnyOf has a similar approach to bypass the icmp that propagates undef/poison. Therefore, I think continuing with the approach of freezing the start value is reasonable, as it allows both FindLastIV and AnyOf to share the same approach. :)

https://github.com/llvm/llvm-project/pull/132691