[llvm] [LV] Support strided memory accesses with a stride of -1 (PR #128718)

[Luke Lau via llvm-commits]

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@@ -6132,6 +6174,16 @@ void LoopVectorizationCostModel::setCostBasedWideningDecision(ElementCount VF) {
                "Expected consecutive stride.");
         InstWidening Decision =
             ConsecutiveStride == 1 ? CM_Widen : CM_Widen_Reverse;
+        // Consider using strided load/store for consecutive reverse accesses to
+        // achieve more efficient memory operations.
+        if (ConsecutiveStride == -1 && stridedAccessCanBeWidened(&I, VF)) {
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lukel97 wrote:

I can't imagine any target that would want to emit a non-decreasing consecutive strided access instead of a regular unit-stride access. So I think `Legal->isConsecutivePtr(ScalarTy, Ptr) != -1` makes more sense. 

In any case, aren't `stridedAccessCanBeWidened` and `setCostBasedWideningDecision` both checking `Legal->isConsecutivePtr(ScalarTy, Ptr) != -1` currently?

https://github.com/llvm/llvm-project/pull/128718