[llvm] [VPlan] Only exempt MaskedCond from cross-block dominance in verifier. (PR #226512)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/226512

Previously the verifier skipped all use-before-def checks for MaskedCond,
including uses in the same block before the definition. Restrict the
exemption to users in blocks not dominated by the MaskedCond's block, so
same-block use-before-def is still diagnosed.

>From 54920b7ed3167c05b0f454c21f85249e7c8278ad Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 25 Sep 2026 14:21:23 +0100
Subject: [PATCH 1/2] Add verifier test

---
 .../Transforms/Vectorize/VPlanVerifierTest.cpp | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/llvm/unittests/Transforms/Vectorize/VPlanVerifierTest.cpp b/llvm/unittests/Transforms/Vectorize/VPlanVerifierTest.cpp
index 7ce87be841c2d..86dc6da2aa514 100644
--- a/llvm/unittests/Transforms/Vectorize/VPlanVerifierTest.cpp
+++ b/llvm/unittests/Transforms/Vectorize/VPlanVerifierTest.cpp
@@ -66,6 +66,24 @@ TEST_F(VPVerifierTest, VPInstructionUseBeforeDefSameBB) {
 #endif
 }
 
+TEST_F(VPVerifierTest, MaskedCondUseBeforeDefSameBB) {
+  VPlan &Plan = getPlan();
+  auto *DefI = new VPInstruction(VPInstruction::MaskedCond, {Plan.getFalse()});
+  VPBasicBlock *Entry = Plan.getEntry();
+  Entry->appendRecipe(new VPInstruction(Instruction::Freeze, {DefI}));
+  Entry->appendRecipe(DefI);
+  VPBlockUtils::connectBlocks(Entry, Plan.getScalarHeader());
+
+#if GTEST_HAS_STREAM_REDIRECTION
+  ::testing::internal::CaptureStderr();
+#endif
+  EXPECT_FALSE(verifyVPlanIsValid(Plan));
+#if GTEST_HAS_STREAM_REDIRECTION
+  EXPECT_TRUE(StringRef(::testing::internal::GetCapturedStderr())
+                  .starts_with("Use before def!\n"));
+#endif
+}
+
 TEST_F(VPVerifierTest, VPInstructionUseBeforeDefDifferentBB) {
   VPlan &Plan = getPlan();
   VPIRValue *Zero = Plan.getConstantInt(32, 0);

>From 5cbad1d9abeacbee29774ac389246dadc425c3c8 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 25 Sep 2026 09:38:03 +0100
Subject: [PATCH 2/2] [VPlan] Only exempt MaskedCond from cross-block dominance
 in verifier.

Previously the verifier skipped all use-before-def checks for MaskedCond,
including uses in the same block before the definition. Restrict the
exemption to users in blocks not dominated by the MaskedCond's block, so
same-block use-before-def is still diagnosed.
---
 llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp b/llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp
index e36ad81cfae2a..6fbd53d529380 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp
@@ -248,11 +248,6 @@ bool VPlanVerifier::verifyVPBasicBlock(const VPBasicBlock *VPBB) {
         return false;
       }
 
-      // MaskedCond may be used from blocks it don't dominate; the block will be
-      // linearized and it will dominate its users after linearization.
-      if (match(&R, m_VPInstruction<VPInstruction::MaskedCond>()))
-        continue;
-
       for (const VPUser *U : V->users()) {
         auto *UI = cast<VPRecipeBase>(U);
         if (isa<VPIRPhi>(UI) &&
@@ -293,7 +288,11 @@ bool VPlanVerifier::verifyVPBasicBlock(const VPBasicBlock *VPBB) {
           if (RecipeNumbering[UI] >= RecipeNumbering[&R])
             continue;
         } else {
-          if (VPDT.dominates(VPBB, UI->getParent()))
+          // MaskedCond may be used from blocks it don't dominate; the block
+          // will be linearized and it will dominate its users after
+          // linearization.
+          if (match(&R, m_VPInstruction<VPInstruction::MaskedCond>()) ||
+              VPDT.dominates(VPBB, UI->getParent()))
             continue;
         }