[llvm] [VPlan] Verify dominance for incoming values of phi-like recipes. (PR #124838)

[Florian Hahn via llvm-commits]

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@@ -207,22 +212,65 @@ bool VPlanVerifier::verifyVPBasicBlock(const VPBasicBlock *VPBB) {
 
       for (const VPUser *U : V->users()) {
         auto *UI = cast<VPRecipeBase>(U);
-        // TODO: check dominance of incoming values for phis properly.
-        if (!UI ||
-            isa<VPHeaderPHIRecipe, VPWidenPHIRecipe, VPPredInstPHIRecipe>(UI))
+        const VPBlockBase *UserVPBB = UI->getParent();
+
+        // Verify incoming values of VPIRInstructions wrapping phis. V most
+        // dominate the end of the incoming block. The operand index of the
+        // incoming value matches the predecessor block index of the
+        // corresponding incoming block.
+        if (isVPIRInstructionPhi(*UI)) {
+          for (const auto &[Idx, Op] : enumerate(UI->operands())) {
+            if (V != Op)
+              continue;
+            const VPBlockBase *Incoming = UserVPBB->getPredecessors()[Idx];
+            if (Incoming != VPBB && !VPDT.dominates(VPBB, Incoming)) {
+              errs() << "Use before def!\n";
+              return false;
+            }
+          }
           continue;
+        }
+        // Verify incoming value of various phi-like recipes.
+        if (isa<VPWidenPHIRecipe, VPHeaderPHIRecipe, VPPredInstPHIRecipe>(UI)) {
+          const VPBlockBase *Incoming = nullptr;
+          // Get the incoming block based on the number of predecessors.
+          if (UserVPBB->getNumPredecessors() == 0) {
+            assert((isa<VPWidenPHIRecipe>(UI) || isa<VPHeaderPHIRecipe>(UI)) &&
+                   "Unexpected recipe with 0 predecessors");
+            const VPRegionBlock *UserRegion = UserVPBB->getParent();
+            Incoming = V == UI->getOperand(0)
+                           ? UserRegion->getSinglePredecessor()
+                           : UserRegion->getExiting();
+          } else if (UserVPBB->getNumPredecessors() == 1) {
+            assert(isa<VPWidenPHIRecipe>(UI) &&
+                   "Unexpected recipe with 1 predecessors");
+            Incoming = UserVPBB->getSinglePredecessor();
+          } else {
+            assert(UserVPBB->getNumPredecessors() == 2 &&
+                   isa<VPPredInstPHIRecipe>(UI) &&
+                   "Unexpected recipe with 2 or more predecessors");
+            Incoming = UserVPBB->getPredecessors()[1];
+          }
+          if (auto *R = dyn_cast<VPRegionBlock>(Incoming))
----------------
fhahn wrote:

The only case at the moment is in VPWidenPHIRecipes added in the  VPlan-native path. I added an assert; it cannot happen in the inner-loop vectorizer path.

https://github.com/llvm/llvm-project/pull/124838