[Mlir-commits] [mlir] [mlir] Dialect Conversion: Fix expensive pattern check in no-rollback mode (PR #166576)

[Tim Noack]

https://github.com/timnoack created https://github.com/llvm/llvm-project/pull/166576

Fixes a bug causing every conversion to fail fatally with "expected pattern to replace the root operation or modify it in place" when `MLIR_ENABLE_EXPENSIVE_PATTERN_API_CHECKS` is enabled and pattern rollback is disabled. 

When `allowPatternRollback` is disabled, the rewriter does not keep track of the rewrites it performs and can therefore not use that list to check whether the root op was replaced or updated in place.

>From f6ba4c909bf225c52fe05e3da6b52672800599c3 Mon Sep 17 00:00:00 2001
From: tn <noack at esa.tu-darmstadt.de>
Date: Wed, 5 Nov 2025 16:50:12 +0100
Subject: [PATCH] [mlir] Dialect Conversion: Fix expensive pattern check in
 no-rollback mode

---
 .../Transforms/Utils/DialectConversion.cpp    | 24 ++++++++++---------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/mlir/lib/Transforms/Utils/DialectConversion.cpp b/mlir/lib/Transforms/Utils/DialectConversion.cpp
index 2fe06970eb568..424349b555228 100644
--- a/mlir/lib/Transforms/Utils/DialectConversion.cpp
+++ b/mlir/lib/Transforms/Utils/DialectConversion.cpp
@@ -2804,17 +2804,19 @@ LogicalResult OperationLegalizer::legalizePatternResult(
   assert(impl.pendingRootUpdates.empty() && "dangling root updates");
 
 #if MLIR_ENABLE_EXPENSIVE_PATTERN_API_CHECKS
-  // Check that the root was either replaced or updated in place.
-  auto newRewrites = llvm::drop_begin(impl.rewrites, curState.numRewrites);
-  auto replacedRoot = [&] {
-    return hasRewrite<ReplaceOperationRewrite>(newRewrites, op);
-  };
-  auto updatedRootInPlace = [&] {
-    return hasRewrite<ModifyOperationRewrite>(newRewrites, op);
-  };
-  if (!replacedRoot() && !updatedRootInPlace())
-    llvm::report_fatal_error(
-        "expected pattern to replace the root operation or modify it in place");
+  if (impl.config.allowPatternRollback) {
+    // Check that the root was either replaced or updated in place.
+    auto newRewrites = llvm::drop_begin(impl.rewrites, curState.numRewrites);
+    auto replacedRoot = [&] {
+      return hasRewrite<ReplaceOperationRewrite>(newRewrites, op);
+    };
+    auto updatedRootInPlace = [&] {
+      return hasRewrite<ModifyOperationRewrite>(newRewrites, op);
+    };
+    if (!replacedRoot() && !updatedRootInPlace())
+      llvm::report_fatal_error("expected pattern to replace the root operation "
+                               "or modify it in place");
+  }
 #endif // MLIR_ENABLE_EXPENSIVE_PATTERN_API_CHECKS
 
   // Legalize each of the actions registered during application.