[MLIR][SCF] Verify number of regions in scf.reduce (#171450)

This patch adds `ReduceOp::verifyRegions` to ensure that the number of
reduction regions equals the number of operands (`getReductions().size()
== getOperands().size()`).

Additionally, `ParallelOp::verify` is updated to gracefully handle cases
where the number of reduce operands differs from the initial values,
preventing verification logic crashes and relying on `ReduceOp` to
report structural inconsistencies.

Fixes: #118768

mlir/lib/Dialect/SCF/IR/SCF.cpp

@@ -3152,6 +3152,9 @@ LogicalResult ParallelOp::verify() {
     return emitOpError() << "expects number of results: " << resultsSize
                          << " to be the same as number of initial values: "
                          << initValsSize;
+  if (reduceOp.getNumOperands() != initValsSize)
+    // Delegate error reporting to ReduceOp
+    return success();
 
   // Check that the types of the results and reductions are the same.
   for (int64_t i = 0; i < static_cast<int64_t>(reductionsSize); ++i) {
@@ -3454,6 +3457,11 @@ void ReduceOp::build(OpBuilder &builder, OperationState &result,
 }
 
 LogicalResult ReduceOp::verifyRegions() {
+  if (getReductions().size() != getOperands().size())
+    return emitOpError() << "expects number of reduction regions: "
+                         << getReductions().size()
+                         << " to be the same as number of reduction operands: "
+                         << getOperands().size();
   // The region of a ReduceOp has two arguments of the same type as its
   // corresponding operand.
   for (int64_t i = 0, e = getReductions().size(); i < e; ++i) {

mlir/test/Dialect/SCF/invalid.mlir

@@ -274,6 +274,37 @@ func.func @parallel_different_types_of_results_and_reduces(
 
 // -----
 
+// The scf.parallel operation requires the number of operands in the terminator
+// (scf.reduce) to match the number of initial values provided to the loop.
+func.func @invalid_reduce_too_few_regions() {
+  %c0 = arith.constant 0 : index
+  %c1 = arith.constant 1 : index
+  scf.parallel (%arg1) = (%c0) to (%c1) step (%c1) {
+    // expected-error @+1 {{expects number of reduction regions: 0 to be the same as number of reduction operands: 1}}
+    scf.reduce(%c1 : index)
+  }
+  return
+}
+
+// -----
+
+// The scf.parallel operation requires the number of operands in the terminator
+// (scf.reduce) to match the number of initial values provided to the loop.
+func.func @invalid_reduce_too_many_regions() {
+  %c0 = arith.constant 0 : index
+  %c1 = arith.constant 1 : index
+  %0 = scf.parallel (%i0) = (%c0) to (%c1) step (%c1) init (%c0) -> (index) {
+    // expected-error @+1 {{expects number of reduction regions: 1 to be the same as number of reduction operands: 0}}
+    scf.reduce {
+      ^bb0(%lhs : index, %rhs : index):
+        scf.reduce.return %lhs : index
+    }
+  }
+  return
+}
+
+// -----
+
 func.func @top_level_reduce(%arg0 : f32) {
   // expected-error@+1 {{expects parent op 'scf.parallel'}}
   scf.reduce(%arg0 : f32) {