[llvm] [VPlan] Convert EVL loops to variable-length stepping after dissolution (PR #147222)

[Luke Lau via llvm-commits]

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@@ -2357,6 +2357,58 @@ bool VPlanTransforms::tryAddExplicitVectorLength(
   return true;
 }
 
+void VPlanTransforms::simplifyEVLIVs(VPlan &Plan) {
+  auto ConvertEVLPhi = [](VPlan &Plan, VPBasicBlock *Entry,
+                          VPEVLBasedIVPHIRecipe *EVLPhi) {
+    using namespace llvm::VPlanPatternMatch;
+    VPValue *EVLIncrement = EVLPhi->getBackedgeValue();
+
+    // Convert EVLPhi to concrete recipe.
+    auto *ScalarR = VPBuilder(EVLPhi).createScalarPhi(
+        {EVLPhi->getStartValue(), EVLIncrement}, EVLPhi->getDebugLoc(),
+        "evl.based.iv");
+    EVLPhi->replaceAllUsesWith(ScalarR);
+    EVLPhi->eraseFromParent();
+
+    // Find the latch-exiting block and convert to variable-length stepping.
+    // Before: (branch-on-count CanonicalIVInc, VectorTripCount)
+    // After: (branch-on-count EVLIVInc, TripCount)
+    auto FindLatchExiting = [](VPBasicBlock *Entry) {
+      auto Range =
+          VPBlockUtils::blocksOnly<VPBasicBlock>(vp_depth_first_shallow(Entry));
+      auto It = find_if(Range, [&](VPBasicBlock *VPBB) {
+        return any_of(VPBB->successors(),
+                      [&](VPBlockBase *Succ) { return Succ == Entry; });
+      });
+      return It != Range.end() ? *It : nullptr;
+    };
+    VPBasicBlock *LatchExiting = FindLatchExiting(Entry);
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lukel97 wrote:

Do we need a lambda if it's only called in one place? I.e. we could just inline the body and then `assert(It != Range.end() && "LatchExisting is not found")`

https://github.com/llvm/llvm-project/pull/147222