[mlir][scf] Implement getSingle... of LoopLikeOpinterface for scf::ParallelOp

This adds implementations for `getSingleIterationVar`, `getSingleLowerBound`,
`getSingleUpperBound`, `getSingleStep` of `LoopLikeOpInterface` to
`scf::ParallelOp`. Until now, the implementations for these methods defaulted
to returning `std::nullopt`, even in the special case where the parallel
Op only has one dimension.

Related: llvm#67883

Author: ubfx

mlir/include/mlir/Dialect/SCF/IR/SCFOps.td

@@ -791,7 +791,8 @@ def IfOp : SCF_Op<"if", [DeclareOpInterfaceMethods<RegionBranchOpInterface, [
 def ParallelOp : SCF_Op<"parallel",
     [AutomaticAllocationScope,
      AttrSizedOperandSegments,
-     DeclareOpInterfaceMethods<LoopLikeOpInterface>,
+     DeclareOpInterfaceMethods<LoopLikeOpInterface, ["getSingleInductionVar",
+          "getSingleLowerBound", "getSingleUpperBound", "getSingleStep"]>,
      RecursiveMemoryEffects,
      DeclareOpInterfaceMethods<RegionBranchOpInterface>,
      SingleBlockImplicitTerminator<"scf::YieldOp">]> {

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

@@ -2936,6 +2936,30 @@ void ParallelOp::print(OpAsmPrinter &p) {
 
 SmallVector<Region *> ParallelOp::getLoopRegions() { return {&getRegion()}; }
 
+std::optional<Value> ParallelOp::getSingleInductionVar() {
+  if (getNumLoops() != 1)
+    return std::nullopt;
+  return getBody()->getArgument(0);
+}
+
+std::optional<OpFoldResult> ParallelOp::getSingleLowerBound() {
+  if (getNumLoops() != 1)
+    return std::nullopt;
+  return getLowerBound()[0];
+}
+
+std::optional<OpFoldResult> ParallelOp::getSingleUpperBound() {
+  if (getNumLoops() != 1)
+    return std::nullopt;
+  return getUpperBound()[0];
+}
+
+std::optional<OpFoldResult> ParallelOp::getSingleStep() {
+  if (getNumLoops() != 1)
+    return std::nullopt;
+  return getStep()[0];
+}
+
 ParallelOp mlir::scf::getParallelForInductionVarOwner(Value val) {
   auto ivArg = llvm::dyn_cast<BlockArgument>(val);
   if (!ivArg)