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

[Luke Lau 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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lukel97 wrote:

Thanks for the explanation, that makes sense to me. From the sounds of things if the ExtractFromEnd is going to be binned anyway I don't see the need to include this transform. Archiving it in another branch sounds good to me, in case we ever do want to return to it!

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