From 5e3202749ac19f95737dc15ff5db739c77c0b992 Mon Sep 17 00:00:00 2001
From: Mehdi Amini <joker.eph@gmail.com>
Date: Wed, 18 Mar 2026 16:01:49 +0100
Subject: [PATCH] [MLIR] Fix crash in FrozenRewritePatternSet when PDL lowering
 is skipped by debug counter (#186159)

When using --mlir-debug-counter=pass-execution-skip=N, the MLIR debug
counter can skip the internal PDL-to-PDLInterp lowering pass that runs
inside FrozenRewritePatternSet's constructor. This caused an assertion
failure in PDLByteCode::Generator::generate() because the PDL module
wasn't properly converted to the interpreter dialect.

The fix adds a check after the PDL lowering pipeline runs to verify that
the expected matcher function symbol was produced. If the symbol is
absent (e.g., because the lowering was skipped by a debug counter),
bytecode generation is skipped entirely and PDL patterns are not
applied. This allows debug counter bisection to work without crashing.

Fixes #131441
Fixes #128342

Assisted-by: Claude Code
---
 mlir/lib/Rewrite/FrozenRewritePatternSet.cpp | 10 ++++++++++
 mlir/test/mlir-opt/debugcounter.mlir         |  6 ++++++
 2 files changed, 16 insertions(+)

diff --git a/mlir/lib/Rewrite/FrozenRewritePatternSet.cpp b/mlir/lib/Rewrite/FrozenRewritePatternSet.cpp
index 34f1442ece7e..d4cd24372efb 100644
--- a/mlir/lib/Rewrite/FrozenRewritePatternSet.cpp
+++ b/mlir/lib/Rewrite/FrozenRewritePatternSet.cpp
@@ -18,6 +18,7 @@ using namespace mlir;
 #if MLIR_ENABLE_PDL_IN_PATTERNMATCH
 #include "mlir/Conversion/PDLToPDLInterp/PDLToPDLInterp.h"
 #include "mlir/Dialect/PDL/IR/PDLOps.h"
+#include "mlir/Dialect/PDLInterp/IR/PDLInterp.h"
 
 static LogicalResult
 convertPDLToPDLInterp(ModuleOp pdlModule,
@@ -134,6 +135,15 @@ FrozenRewritePatternSet::FrozenRewritePatternSet(
     llvm::report_fatal_error(
         "failed to lower PDL pattern module to the PDL Interpreter");
 
+  // Verify that the PDL module was actually lowered to the interpreter
+  // dialect. If the lowering pass was skipped (e.g., by a debug counter
+  // via --mlir-debug-counter), the matcher function will not be present and
+  // we skip bytecode construction. PDL patterns will not be applied in this
+  // case.
+  if (!pdlModule.lookupSymbol(
+          pdl_interp::PDLInterpDialect::getMatcherFunctionName()))
+    return;
+
   // Generate the pdl bytecode.
   impl->pdlByteCode = std::make_unique<detail::PDLByteCode>(
       pdlModule, pdlPatterns.takeConfigs(), configMap,
diff --git a/mlir/test/mlir-opt/debugcounter.mlir b/mlir/test/mlir-opt/debugcounter.mlir
index 594a2183558b..e724699e705b 100644
--- a/mlir/test/mlir-opt/debugcounter.mlir
+++ b/mlir/test/mlir-opt/debugcounter.mlir
@@ -3,6 +3,7 @@
 
 // RUN: mlir-opt %s -mlir-debug-counter=unique-tag-for-my-action-skip=-1 -mlir-print-debug-counter --pass-pipeline="builtin.module(func.func(canonicalize))" --mlir-disable-threading 2>&1 | FileCheck %s --check-prefix=CHECK-UNKNOWN-TAG
 // RUN: mlir-opt %s -mlir-debug-counter=pass-execution-skip=1 -mlir-print-debug-counter --pass-pipeline="builtin.module(func.func(canonicalize))" --mlir-disable-threading 2>&1 | FileCheck %s --check-prefix=CHECK-PASS
+// RUN: mlir-opt %s -mlir-debug-counter=pass-execution-skip=1 -test-dialect-conversion-pdll 2>&1 | FileCheck %s --check-prefix=CHECK-PDL-SKIP
 
 func.func @foo() {
     return
@@ -13,3 +14,8 @@ func.func @foo() {
 
 // CHECK-PASS: DebugCounter counters:
 // CHECK-PASS: pass-execution                  : {1,1,-1}
+
+// Regression test for https://github.com/llvm/llvm-project/issues/131441
+// When --mlir-debug-counter skips the internal PDL lowering pass, the
+// FrozenRewritePatternSet should handle it gracefully (no crash).
+// CHECK-PDL-SKIP-LABEL: func @foo