[mlir] Require folders to produce Values of same type (#75887)

This commit adds extra assertions to `OperationFolder` and `OpBuilder`
to ensure that the types of the folded SSA values match with the result
types of the op. There used to be checks that discard the folded results
if the types do not match. This commit makes these checks stricter and
turns them into assertions.

Discarding folded results with the wrong type (without failing
explicitly) can hide bugs in op folders. Two such bugs became apparent
in MLIR (and some more in downstream projects) and are fixed with this
change.

Note: The existing type checks were introduced in
https://reviews.llvm.org/D95991.

Migration guide: If you see failing assertions (`folder produced value
of incorrect type`; make sure to run with assertions enabled!), run with
`-debug` or dump the operation right before the failing assertion. This
will point you to the op that has the broken folder. A common mistake is
a mismatch between static/dynamic dimensions (e.g., input has a static
dimension but folded result has a dynamic dimension).

Author: matthias-springer

flang/lib/Optimizer/Dialect/FIROps.cpp

@@ -625,11 +625,13 @@ void fir::BoxAddrOp::build(mlir::OpBuilder &builder,
 mlir::OpFoldResult fir::BoxAddrOp::fold(FoldAdaptor adaptor) {
   if (auto *v = getVal().getDefiningOp()) {
     if (auto box = mlir::dyn_cast<fir::EmboxOp>(v)) {
-      if (!box.getSlice()) // Fold only if not sliced
+      // Fold only if not sliced
+      if (!box.getSlice() && box.getMemref().getType() == getType())
         return box.getMemref();
     }
     if (auto box = mlir::dyn_cast<fir::EmboxCharOp>(v))
-      return box.getMemref();
+      if (box.getMemref().getType() == getType())
+        return box.getMemref();
   }
   return {};
 }

mlir/lib/Dialect/Arith/IR/ArithOps.cpp

@@ -1352,9 +1352,11 @@ OpFoldResult arith::TruncIOp::fold(FoldAdaptor adaptor) {
       setOperand(src);
       return getResult();
     }
+
     // trunci(zexti(a)) -> a
     // trunci(sexti(a)) -> a
-    return src;
+    if (srcType == dstType)
+      return src;
   }
 
   // trunci(trunci(a)) -> trunci(a))

mlir/lib/Dialect/Tosa/IR/TosaCanonicalizations.cpp

@@ -771,6 +771,8 @@ OpFoldResult ConstOp::fold(FoldAdaptor adaptor) { return getValueAttr(); }
     ShapedType inputTy = llvm::cast<ShapedType>(getInput().getType());         \
     if (!inputTy.hasRank())                                                    \
       return {};                                                               \
+    if (inputTy != getType())                                                  \
+      return {};                                                               \
     if (inputTy.getRank() == 0 || inputTy.getDimSize(getAxis()) == 1)          \
       return getInput();                                                       \
     return {};                                                                 \

mlir/lib/Dialect/Vector/IR/VectorOps.cpp

@@ -1602,9 +1602,10 @@ static Value foldExtractFromBroadcast(ExtractOp extractOp) {
     return llvm::isa<VectorType>(type) ? llvm::cast<VectorType>(type).getRank()
                                        : 0;
   };
+
   // If splat or broadcast from a scalar, just return the source scalar.
   unsigned broadcastSrcRank = getRank(source.getType());
-  if (broadcastSrcRank == 0)
+  if (broadcastSrcRank == 0 && source.getType() == extractOp.getType())
     return source;
 
   unsigned extractResultRank = getRank(extractOp.getType());

mlir/lib/IR/Builders.cpp

@@ -486,14 +486,11 @@ LogicalResult OpBuilder::tryFold(Operation *op,
 
   // Populate the results with the folded results.
   Dialect *dialect = op->getDialect();
-  for (auto it : llvm::zip(foldResults, opResults.getTypes())) {
+  for (auto it : llvm::zip_equal(foldResults, opResults.getTypes())) {
     Type expectedType = std::get<1>(it);
 
     // Normal values get pushed back directly.
     if (auto value = llvm::dyn_cast_if_present<Value>(std::get<0>(it))) {
-      if (value.getType() != expectedType)
-        return cleanupFailure();
-
       results.push_back(value);
       continue;
     }

mlir/lib/IR/Operation.cpp

@@ -606,13 +606,30 @@ void Operation::setSuccessor(Block *block, unsigned index) {
   getBlockOperands()[index].set(block);
 }
 
+#ifndef NDEBUG
+/// Assert that the folded results (in case of values) have the same type as
+/// the results of the given op.
+static void checkFoldResultTypes(Operation *op,
+                                 SmallVectorImpl<OpFoldResult> &results) {
+  if (!results.empty())
+    for (auto [ofr, opResult] : llvm::zip_equal(results, op->getResults()))
+      if (auto value = ofr.dyn_cast<Value>())
+        assert(value.getType() == opResult.getType() &&
+               "folder produced value of incorrect type");
+}
+#endif // NDEBUG
+
 /// Attempt to fold this operation using the Op's registered foldHook.
 LogicalResult Operation::fold(ArrayRef<Attribute> operands,
                               SmallVectorImpl<OpFoldResult> &results) {
   // If we have a registered operation definition matching this one, use it to
   // try to constant fold the operation.
-  if (succeeded(name.foldHook(this, operands, results)))
+  if (succeeded(name.foldHook(this, operands, results))) {
+#ifndef NDEBUG
+    checkFoldResultTypes(this, results);
+#endif // NDEBUG
     return success();
+  }
 
   // Otherwise, fall back on the dialect hook to handle it.
   Dialect *dialect = getDialect();
@@ -623,7 +640,12 @@ LogicalResult Operation::fold(ArrayRef<Attribute> operands,
   if (!interface)
     return failure();
 
-  return interface->fold(this, operands, results);
+  LogicalResult status = interface->fold(this, operands, results);
+#ifndef NDEBUG
+  if (succeeded(status))
+    checkFoldResultTypes(this, results);
+#endif // NDEBUG
+  return status;
 }
 
 LogicalResult Operation::fold(SmallVectorImpl<OpFoldResult> &results) {

mlir/lib/Transforms/Utils/FoldUtils.cpp

@@ -247,10 +247,6 @@ OperationFolder::processFoldResults(Operation *op,
 
     // Check if the result was an SSA value.
     if (auto repl = llvm::dyn_cast_if_present<Value>(foldResults[i])) {
-      if (repl.getType() != op->getResult(i).getType()) {
-        results.clear();
-        return failure();
-      }
       results.emplace_back(repl);
       continue;
     }

mlir/test/Transforms/test-canonicalize.mlir

@@ -70,19 +70,6 @@ func.func @test_commutative_multi_cst(%arg0: i32, %arg1: i32) -> (i32, i32) {
   return %y, %z: i32, i32
 }
 
-// CHECK-LABEL: func @typemismatch
-
-func.func @typemismatch() -> i32 {
-  %c42 = arith.constant 42.0 : f32
-
-  // The "passthrough_fold" folder will naively return its operand, but we don't
-  // want to fold here because of the type mismatch.
-
-  // CHECK: "test.passthrough_fold"
-  %0 = "test.passthrough_fold"(%c42) : (f32) -> (i32)
-  return %0 : i32
-}
-
 // CHECK-LABEL: test_dialect_canonicalizer
 func.func @test_dialect_canonicalizer() -> (i32) {
   %0 = "test.dialect_canonicalizable"() : () -> (i32)

mlir/test/Transforms/test-legalizer.mlir

@@ -310,16 +310,6 @@ builtin.module {
 
 // -----
 
-// The "passthrough_fold" folder will naively return its operand, but we don't
-// want to fold here because of the type mismatch.
-func.func @typemismatch(%arg: f32) -> i32 {
-  // expected-remark@+1 {{op 'test.passthrough_fold' is not legalizable}}
-  %0 = "test.passthrough_fold"(%arg) : (f32) -> (i32)
-  "test.return"(%0) : (i32) -> ()
-}
-
-// -----
-
 // expected-remark @below {{applyPartialConversion failed}}
 module {
   func.func private @callee(%0 : f32) -> f32

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

@@ -542,10 +542,6 @@ OpFoldResult TestOpInPlaceFold::fold(FoldAdaptor adaptor) {
   return {};
 }
 
-OpFoldResult TestPassthroughFold::fold(FoldAdaptor adaptor) {
-  return getOperand();
-}
-
 OpFoldResult TestOpFoldWithFoldAdaptor::fold(FoldAdaptor adaptor) {
   int64_t sum = 0;
   if (auto value = dyn_cast_or_null<IntegerAttr>(adaptor.getOp()))

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

@@ -1363,13 +1363,6 @@ def TestOpFoldWithFoldAdaptor
   let hasFolder = 1;
 }
 
-// An op that always fold itself.
-def TestPassthroughFold : TEST_Op<"passthrough_fold"> {
-  let arguments = (ins AnyType:$op);
-  let results = (outs AnyType);
-  let hasFolder = 1;
-}
-
 def TestDialectCanonicalizerOp : TEST_Op<"dialect_canonicalizable"> {
   let arguments = (ins);
   let results = (outs I32);