[MLIR][SCF] Loop pipelining fails on failed predication (no assert) (llvm#107442)

The SCFLoopPipelining allows predication on peeled or loop ops. When the
predicationFn returns a nullptr this signifies the op type is
unsupported and the pipeliner fails except in `emitPrologue` where it
asserts.

This patch fixes handling in the prologue to gracefully fail.

Author: sjw36

mlir/lib/Dialect/SCF/Transforms/LoopPipelining.cpp

@@ -77,7 +77,7 @@ struct LoopPipelinerInternal {
   bool initializeLoopInfo(ForOp op, const PipeliningOption &options);
   /// Emits the prologue, this creates `maxStage - 1` part which will contain
   /// operations from stages [0; i], where i is the part index.
-  void emitPrologue(RewriterBase &rewriter);
+  LogicalResult emitPrologue(RewriterBase &rewriter);
   /// Gather liverange information for Values that are used in a different stage
   /// than its definition.
   llvm::MapVector<Value, LiverangeInfo> analyzeCrossStageValues();
@@ -263,7 +263,7 @@ cloneAndUpdateOperands(RewriterBase &rewriter, Operation *op,
   return clone;
 }
 
-void LoopPipelinerInternal::emitPrologue(RewriterBase &rewriter) {
+LogicalResult LoopPipelinerInternal::emitPrologue(RewriterBase &rewriter) {
   // Initialize the iteration argument to the loop initial values.
   for (auto [arg, operand] :
        llvm::zip(forOp.getRegionIterArgs(), forOp.getInitsMutable())) {
@@ -311,7 +311,8 @@ void LoopPipelinerInternal::emitPrologue(RewriterBase &rewriter) {
       if (predicates[predicateIdx]) {
         OpBuilder::InsertionGuard insertGuard(rewriter);
         newOp = predicateFn(rewriter, newOp, predicates[predicateIdx]);
-        assert(newOp && "failed to predicate op.");
+        if (newOp == nullptr)
+          return failure();
       }
       if (annotateFn)
         annotateFn(newOp, PipeliningOption::PipelinerPart::Prologue, i);
@@ -339,6 +340,7 @@ void LoopPipelinerInternal::emitPrologue(RewriterBase &rewriter) {
       }
     }
   }
+  return success();
 }
 
 llvm::MapVector<Value, LoopPipelinerInternal::LiverangeInfo>
@@ -772,7 +774,8 @@ FailureOr<ForOp> mlir::scf::pipelineForLoop(RewriterBase &rewriter, ForOp forOp,
     *modifiedIR = true;
 
   // 1. Emit prologue.
-  pipeliner.emitPrologue(rewriter);
+  if (failed(pipeliner.emitPrologue(rewriter)))
+    return failure();
 
   // 2. Track values used across stages. When a value cross stages it will
   // need to be passed as loop iteration arguments.