[llvm] [LV] Only require a scalar epilogue for interleave groups actually widened as interleave accesses (PR #213904)

[Florian Hahn via llvm-commits]

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@@ -6739,6 +6724,20 @@ VPlanPtr LoopVectorizationPlanner::tryToBuildVPlan(VPlanPtr Plan,
     InterleaveGroups.insert(IG);
   }
 
+  bool RequiresScalarEpilogue =
+      CM.isEpilogueAllowed() && any_of(InterleaveGroups, [](auto *IG) {
+        return isa<LoadInst>(IG->getInsertPos()) &&
+               IG->requiresScalarEpilogue();
+      });
+  // Update the branch in the middle block if a scalar epilogue is required.
+  VPBasicBlock *MiddleVPBB = Plan->getMiddleBlock();
+  if (RequiresScalarEpilogue && MiddleVPBB->getNumSuccessors() == 2) {
+    auto *BranchOnCond = cast<VPInstruction>(MiddleVPBB->getTerminator());
+    assert(MiddleVPBB->getSuccessors()[1] == Plan->getScalarPreheader() &&
+           "second successor must be scalar preheader");
+    BranchOnCond->setOperand(0, Plan->getFalse());
+  }
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fhahn wrote:

would it be possible to only se the condition once we formed interleave groups inthe VPlan, and check the IG recipes in the plan?

Otherwise we may unnecessarily require a scalar epilogue if we fail to form interleave groups for some reason.

https://github.com/llvm/llvm-project/pull/213904