[llvm] [VPlan] Introduce explicit ExtractFromEnd recipes for live-outs. (PR #100658)

[via llvm-commits]

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@@ -8578,21 +8616,31 @@ static void addLiveOutsForFirstOrderRecurrences(VPlan &Plan) {
   // TODO: Should be replaced by
   // Plan->getScalarLoopRegion()->getSinglePredecessor() in the future once the
   // scalar region is modeled as well.
-  VPBasicBlock *ScalarPHVPBB = nullptr;
   auto *MiddleVPBB = cast<VPBasicBlock>(VectorRegion->getSingleSuccessor());
-  for (VPBlockBase *Succ : MiddleVPBB->getSuccessors()) {
-    if (isa<VPIRBasicBlock>(Succ))
-      continue;
-    assert(!ScalarPHVPBB && "Two candidates for ScalarPHVPBB?");
-    ScalarPHVPBB = cast<VPBasicBlock>(Succ);
+  BasicBlock *ExitBB = nullptr;
+  VPBasicBlock *ScalarPHVPBB = nullptr;
+  if (MiddleVPBB->getNumSuccessors() == 2) {
+    // Order is strict: first is the exit block, second is the scalar preheader.
+    ExitBB =
+        cast<VPIRBasicBlock>(MiddleVPBB->getSuccessors()[0])->getIRBasicBlock();
+    ScalarPHVPBB = cast<VPBasicBlock>(MiddleVPBB->getSuccessors()[1]);
+  } else if (ExitingValuesToFix.empty()) {
+    ScalarPHVPBB = cast<VPBasicBlock>(MiddleVPBB->getSingleSuccessor());
+  } else {
+    ExitBB = cast<VPIRBasicBlock>(MiddleVPBB->getSingleSuccessor())
+                 ->getIRBasicBlock();
   }
-  if (!ScalarPHVPBB)
+  if (!ScalarPHVPBB) {
+    assert(ExitingValuesToFix.empty() &&
+           "missed inserting extracts for exiting values");
     return;
+  }
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ayalz wrote:

Can ScalarPHVPBB be null?

https://github.com/llvm/llvm-project/pull/100658