[llvm] [VPlan] Replace EVL branch condition with (branch-on-count AVLNext, 0) (PR #152167)

[Ramkumar Ramachandra via llvm-commits]

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@@ -2447,19 +2447,43 @@ void VPlanTransforms::canonicalizeEVLLoops(VPlan &Plan) {
   // Find EVL loop entries by locating VPEVLBasedIVPHIRecipe.
   // There should be only one EVL PHI in the entire plan.
   VPEVLBasedIVPHIRecipe *EVLPhi = nullptr;
+  VPValue *AVLNext = nullptr;
 
   for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
            vp_depth_first_shallow(Plan.getEntry())))
-    for (VPRecipeBase &R : VPBB->phis())
-      if (auto *PhiR = dyn_cast<VPEVLBasedIVPHIRecipe>(&R)) {
+    for (VPRecipeBase &R : VPBB->phis()) {
+      auto *PhiR = dyn_cast<VPSingleDefRecipe>(&R);
+      if (!PhiR)
+        continue;
+      VPValue *Backedge;
+      if (auto *EVL = dyn_cast<VPEVLBasedIVPHIRecipe>(PhiR)) {
         assert(!EVLPhi && "Found multiple EVL PHIs. Only one expected");
-        EVLPhi = PhiR;
+        EVLPhi = EVL;
+        continue;
+      }
+      if (match(PhiR,
+                m_VPInstruction<Instruction::PHI>(
+                    m_Specific(Plan.getTripCount()), m_VPValue(Backedge))) &&
+          match(Backedge,
+                m_VPInstruction<Instruction::Sub>(
+                    m_Specific(PhiR),
+                    m_ZExtOrSelf(
+                        m_VPInstruction<VPInstruction::ExplicitVectorLength>(
+                            m_CombineOr(
+                                m_Specific(PhiR),
+                                // The AVL may be capped to a safe distance.
+                                m_Select(m_VPValue(), m_Specific(PhiR),
+                                         m_VPValue()))))))) {
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artagnon wrote:

Hm, this seems like a very specific pattern, and I wonder if this is fragile?

https://github.com/llvm/llvm-project/pull/152167