[mlir] Enable disabling folding in dialect conversion (#152890)

Previously this only happened post checking if the op is legal, but was
done unconditionally post (and before other legalization patterns). Add
option to not attempt folding and one to do so as last resort.

Did consider but did not add a always attempt to fold option (which
would have folded whether or not legal), but removed TODO about it.

mlir/include/mlir/Transforms/DialectConversion.h

@@ -1161,6 +1161,16 @@ public:
 // ConversionConfig
 //===----------------------------------------------------------------------===//
 
+/// An enum to control folding behavior during dialect conversion.
+enum class DialectConversionFoldingMode {
+  /// Never attempt to fold.
+  Never,
+  /// Only attempt to fold not legal operations before applying patterns.
+  BeforePatterns,
+  /// Only attempt to fold not legal operations after applying patterns.
+  AfterPatterns,
+};
+
 /// Dialect conversion configuration.
 struct ConversionConfig {
   /// An optional callback used to notify about match failure diagnostics during
@@ -1243,6 +1253,10 @@ struct ConversionConfig {
   /// your patterns do not trigger any IR rollbacks. For details, see
   /// https://discourse.llvm.org/t/rfc-a-new-one-shot-dialect-conversion-driver/79083.
   bool allowPatternRollback = true;
+
+  /// The folding mode to use during conversion.
+  DialectConversionFoldingMode foldingMode =
+      DialectConversionFoldingMode::BeforePatterns;
 };
 
 //===----------------------------------------------------------------------===//

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -2257,15 +2257,17 @@ OperationLegalizer::legalize(Operation *op,
     return success();
   }
 
-  // If the operation isn't legal, try to fold it in-place.
-  // TODO: Should we always try to do this, even if the op is
-  // already legal?
-  if (succeeded(legalizeWithFold(op, rewriter))) {
-    LLVM_DEBUG({
-      logSuccess(logger, "operation was folded");
-      logger.startLine() << logLineComment;
-    });
-    return success();
+  // If the operation is not legal, try to fold it in-place if the folding mode
+  // is 'BeforePatterns'. 'Never' will skip this.
+  const ConversionConfig &config = rewriter.getConfig();
+  if (config.foldingMode == DialectConversionFoldingMode::BeforePatterns) {
+    if (succeeded(legalizeWithFold(op, rewriter))) {
+      LLVM_DEBUG({
+        logSuccess(logger, "operation was folded");
+        logger.startLine() << logLineComment;
+      });
+      return success();
+    }
   }
 
   // Otherwise, we need to apply a legalization pattern to this operation.
@@ -2277,6 +2279,18 @@ OperationLegalizer::legalize(Operation *op,
     return success();
   }
 
+  // If the operation can't be legalized via patterns, try to fold it in-place
+  // if the folding mode is 'AfterPatterns'.
+  if (config.foldingMode == DialectConversionFoldingMode::AfterPatterns) {
+    if (succeeded(legalizeWithFold(op, rewriter))) {
+      LLVM_DEBUG({
+        logSuccess(logger, "operation was folded");
+        logger.startLine() << logLineComment;
+      });
+      return success();
+    }
+  }
+
   LLVM_DEBUG({
     logFailure(logger, "no matched legalization pattern");
     logger.startLine() << logLineComment;

mlir/test/Transforms/test-legalizer-fold-after.mlir

@@ -0,0 +1,9 @@
+// RUN: mlir-opt %s -test-legalize-patterns="test-legalize-folding-mode=after-patterns" | FileCheck %s
+
+// CHECK-LABEL: @fold_legalization
+func.func @fold_legalization() -> i32 {
+  // CHECK-NOT: op_in_place_self_fold
+  // CHECK: 97
+  %1 = "test.op_in_place_self_fold"() : () -> (i32)
+  "test.return"(%1) : (i32) -> ()
+}

mlir/test/Transforms/test-legalizer-fold-before.mlir

@@ -0,0 +1,9 @@
+// RUN: mlir-opt %s -test-legalize-patterns="test-legalize-folding-mode=before-patterns" | FileCheck %s
+
+// CHECK-LABEL: @fold_legalization
+func.func @fold_legalization() -> i32 {
+  // CHECK: op_in_place_self_fold
+  // CHECK-SAME: folded
+  %1 = "test.op_in_place_self_fold"() : () -> (i32)
+  "test.return"(%1) : (i32) -> ()
+}

mlir/test/Transforms/test-legalizer-no-fold.mlir

@@ -0,0 +1,12 @@
+// RUN: mlir-opt %s -allow-unregistered-dialect -test-legalize-patterns="test-legalize-folding-mode=never" | FileCheck %s
+
+// CHECK-LABEL: @remove_foldable_op(
+func.func @remove_foldable_op(%arg0 : i32) -> (i32) {
+  // Check that op was not folded.
+  // CHECK: "test.op_with_region_fold"
+  %0 = "test.op_with_region_fold"(%arg0) ({
+    "foo.op_with_region_terminator"() : () -> ()
+  }) : (i32) -> (i32)
+  "test.return"(%0) : (i32) -> ()
+}
+

mlir/test/lib/Dialect/Test/TestOps.td

@@ -1498,6 +1498,8 @@ def TestOpInPlaceSelfFold : TEST_Op<"op_in_place_self_fold"> {
   let results = (outs I32);
   let hasFolder = 1;
 }
+def : Pat<(TestOpInPlaceSelfFold:$op $_),
+          (TestOpConstant ConstantAttr<I32Attr, "97">)>;
 
 // Test op that simply returns success.
 def TestOpInPlaceFoldSuccess : TEST_Op<"op_in_place_fold_success"> {

mlir/test/lib/Dialect/Test/TestPatterns.cpp

@@ -1507,8 +1507,8 @@ struct TestLegalizePatternDriver
     ConversionTarget target(getContext());
     target.addLegalOp<ModuleOp>();
     target.addLegalOp<LegalOpA, LegalOpB, LegalOpC, TestCastOp, TestValidOp,
-                      TerminatorOp, OneRegionOp, TestValidProducerOp,
-                      TestValidConsumerOp>();
+                      TerminatorOp, TestOpConstant, OneRegionOp,
+                      TestValidProducerOp, TestValidConsumerOp>();
     target.addLegalOp(OperationName("test.legal_op", &getContext()));
     target
         .addIllegalOp<ILLegalOpF, TestRegionBuilderOp, TestOpWithRegionFold>();
@@ -1563,6 +1563,7 @@ struct TestLegalizePatternDriver
       DumpNotifications dumpNotifications;
       config.listener = &dumpNotifications;
       config.unlegalizedOps = &unlegalizedOps;
+      config.foldingMode = foldingMode;
       if (failed(applyPartialConversion(getOperation(), target,
                                         std::move(patterns), config))) {
         getOperation()->emitRemark() << "applyPartialConversion failed";
@@ -1582,6 +1583,7 @@ struct TestLegalizePatternDriver
 
       ConversionConfig config;
       DumpNotifications dumpNotifications;
+      config.foldingMode = foldingMode;
       config.listener = &dumpNotifications;
       if (failed(applyFullConversion(getOperation(), target,
                                      std::move(patterns), config))) {
@@ -1596,6 +1598,7 @@ struct TestLegalizePatternDriver
     // Analyze the convertible operations.
     DenseSet<Operation *> legalizedOps;
     ConversionConfig config;
+    config.foldingMode = foldingMode;
     config.legalizableOps = &legalizedOps;
     if (failed(applyAnalysisConversion(getOperation(), target,
                                        std::move(patterns), config)))
@@ -1616,6 +1619,21 @@ struct TestLegalizePatternDriver
           clEnumValN(ConversionMode::Full, "full", "Perform a full conversion"),
           clEnumValN(ConversionMode::Partial, "partial",
                      "Perform a partial conversion"))};
+
+  Option<DialectConversionFoldingMode> foldingMode{
+      *this, "test-legalize-folding-mode",
+      llvm::cl::desc("The folding mode to use with the test driver"),
+      llvm::cl::init(DialectConversionFoldingMode::BeforePatterns),
+      llvm::cl::values(clEnumValN(DialectConversionFoldingMode::Never, "never",
+                                  "Never attempt to fold"),
+                       clEnumValN(DialectConversionFoldingMode::BeforePatterns,
+                                  "before-patterns",
+                                  "Only attempt to fold not legal operations "
+                                  "before applying patterns"),
+                       clEnumValN(DialectConversionFoldingMode::AfterPatterns,
+                                  "after-patterns",
+                                  "Only attempt to fold not legal operations "
+                                  "after applying patterns"))};
 };
 } // namespace