[llvm] [LV][EVL] Support fixed-order recurrence idiom with EVL tail folding. (PR #124093)

[Mel Chen via llvm-commits]

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@@ -1635,18 +1635,62 @@ void VPlanTransforms::addActiveLaneMask(
     HeaderMask->replaceAllUsesWith(LaneMask);
 }
 
+/// Adjust the way the resume value is obtained when using tail folding by EVL.
+/// Expanding ExtractFromEnd since the penultimate EVL could not equals to
+/// VFxUF. Expand
+///   %resume = ExtractFromEnd %vec, 1
+/// to
+///   %last.active.idx = sub %EVL, 1
+///   %resume = extractelement %vec, %last.active.idx
+static void adjustResumePhisForEVL(VPlan &Plan, VPValue &EVL) {
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Mel-Chen wrote:

Archive ExtractFromEnd transformation to [another branch](https://github.com/Mel-Chen/llvm-project/tree/evl-extract-for-resumephi).

https://github.com/llvm/llvm-project/pull/124093