[MLIR][Affine] Remove assert that prevents 0-dim affine.parallel loops

[flaub]

Summary

It's legal to have affine.parallel ops that don't have any IVs specified.

[bondhugula]

Sounds good, but a test case is missing here. (You could set up one with -canonicalize which would set the lower/upper bounds in an attempt to simplify?). Separately, are there test cases with 0-d affine.parallel ops especially with -lower-affine to ensure they correctly lower to a similar scf.parallel or just the body?

[flaub]

@bondhugula Sorry I haven't touched this in a long time. I've failed to come up with a test case that will exercise this code path with the existing passes/canonicalizations that exist in MLIR itself. This patch however is important for many of our downstream passes that we do plan to submit to MLIR once it becomes more mature and we've done a review on the forum.

Do you think it's worth the trouble to write unit tests for this case?

[flaub]

I believe that when I first wrote the setLowerBounds and setUpperBounds methods, I copy/pasted them largely from the affine.for op. This is why I think those assertions exist currently, but they were never intended in our original design of the affine.parallel op.

mlir/lib/Dialect/Affine/IR/AffineOps.cpp

 
 OpBuilder AffineParallelOp::getBodyBuilder() {
   return OpBuilder(getBody(), std::prev(getBody()->end()));
 }
 
 void AffineParallelOp::setLowerBounds(ValueRange lbOperands, AffineMap map) {
   assert(lbOperands.size() == map.getNumInputs() &&
          "operands to map must match number of inputs");
-  assert(map.getNumResults() >= 1 && "bounds map has at least one result");
 
   auto ubOperands = getUpperBoundsOperands();
 
   SmallVector<Value, 4> newOperands(lbOperands);
   newOperands.append(ubOperands.begin(), ubOperands.end());
   (*this)->setOperands(newOperands);
 
   lowerBoundsMapAttr(AffineMapAttr::get(map));
 }
 
 void AffineParallelOp::setUpperBounds(ValueRange ubOperands, AffineMap map) {
   assert(ubOperands.size() == map.getNumInputs() &&
          "operands to map must match number of inputs");
-  assert(map.getNumResults() >= 1 && "bounds map has at least one result");
 
   SmallVector<Value, 4> newOperands(getLowerBoundsOperands());
   newOperands.append(ubOperands.begin(), ubOperands.end());
   (*this)->setOperands(newOperands);
 
   upperBoundsMapAttr(AffineMapAttr::get(map));
 }