[llvm] [VPlan] Invert condition if needed when creating inner regions. (PR #132292)

[via llvm-commits]

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@@ -386,33 +386,53 @@ std::unique_ptr<VPlan> VPlanTransforms::buildPlainCFG(
 /// Checks if \p HeaderVPB is a loop header block in the plain CFG; that is, it
 /// has exactly 2 predecessors (preheader and latch), where the block
 /// dominates the latch and the preheader dominates the block. If it is a
-/// header block return true, making sure the preheader appears first and
-/// the latch second. Otherwise return false.
-static bool canonicalHeader(VPBlockBase *HeaderVPB,
-                            const VPDominatorTree &VPDT) {
+/// header block return true and canonicalize the predecessors of the header
+/// (making sure the preheader appears first and the latch second) and the
+/// successors of the latch (making sure the loop exit comes first). Otherwise
+/// return false.
+static bool canonicalHeaderAndLatch(VPBlockBase *HeaderVPB,
+                                    const VPDominatorTree &VPDT) {
   ArrayRef<VPBlockBase *> Preds = HeaderVPB->getPredecessors();
   if (Preds.size() != 2)
     return false;
 
   auto *PreheaderVPBB = Preds[0];
   auto *LatchVPBB = Preds[1];
-  if (VPDT.dominates(PreheaderVPBB, HeaderVPB) &&
-      VPDT.dominates(HeaderVPB, LatchVPBB))
-    return true;
+  if (!VPDT.dominates(PreheaderVPBB, HeaderVPB) ||
+      !VPDT.dominates(HeaderVPB, LatchVPBB)) {
+    std::swap(PreheaderVPBB, LatchVPBB);
 
-  std::swap(PreheaderVPBB, LatchVPBB);
+    if (!VPDT.dominates(PreheaderVPBB, HeaderVPB) ||
+        !VPDT.dominates(HeaderVPB, LatchVPBB)) {
+      return false;
+    } else {
+      // Canonicalize predecessors of header so that preheader is first and
+      // latch second.
+      HeaderVPB->swapPredecessors();
+      for (VPRecipeBase &R : cast<VPBasicBlock>(HeaderVPB)->phis())
+        R.swapOperands();
+    }
+  }
 
-  if (VPDT.dominates(PreheaderVPBB, HeaderVPB) &&
-      VPDT.dominates(HeaderVPB, LatchVPBB)) {
-    // Canonicalize predecessors of header so that preheader is first and latch
-    // second.
-    HeaderVPB->swapPredecessors();
-    for (VPRecipeBase &R : cast<VPBasicBlock>(HeaderVPB)->phis())
-      R.swapOperands();
-    return true;
+  // The two successors of conditional branch match the condition, with the
+  // first successor corresponding to true and the second to false. We
+  // canonicalize the successors of the latch when introducing the region, such
+  // that the latch exits the region when its condition is true; invert the
+  // original condition if the original CFG branches to the header on true.
+  if (!LatchVPBB->getSingleSuccessor() &&
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ayalz wrote:

nit: consider reversing (the above conditional branch which is also terminating ;-) to early-return.

https://github.com/llvm/llvm-project/pull/132292