[mlir] Migrate away from ArrayRef(std::nullopt) (NFC) (#144989)

ArrayRef has a constructor that accepts std::nullopt.  This
constructor dates back to the days when we still had llvm::Optional.

Since the use of std::nullopt outside the context of std::optional is
kind of abuse and not intuitive to new comers, I would like to move
away from the constructor and eventually remove it.

This patch takes care of the mlir side of the migration, starting with
straightforward places where I see ArrayRef or ValueRange nearby.
Note that ValueRange has a constructor that forwards arguments to an
ArrayRef constructor.

Author: kazutakahirata

mlir/include/mlir/AsmParser/AsmParserState.h

@@ -195,7 +195,7 @@ class AsmParserState {
   /// Finalize the most recently started operation definition.
   void finalizeOperationDefinition(
       Operation *op, SMRange nameLoc, SMLoc endLoc,
-      ArrayRef<std::pair<unsigned, SMLoc>> resultGroups = std::nullopt);
+      ArrayRef<std::pair<unsigned, SMLoc>> resultGroups = {});
 
   /// Start a definition for a region nested under the current operation.
   void startRegionDefinition();

mlir/include/mlir/Dialect/Linalg/Utils/Utils.h

@@ -94,7 +94,7 @@ bool isReductionIterator(utils::IteratorType iteratorType);
 /// ```
 Value makeComposedPadHighOp(OpBuilder &b, Location loc, RankedTensorType type,
                             Value source, Value padding, bool nofold,
-                            ValueRange typeDynDims = std::nullopt);
+                            ValueRange typeDynDims = {});
 
 /// Returns GenericOp that copies an n-D memref. Unlike the current
 /// implementation of memref::CopyOp, this op can further tile, lower to loops

mlir/include/mlir/Dialect/Tensor/Utils/Utils.h

@@ -30,7 +30,7 @@ namespace tensor {
 // for _static_ dimensions.
 PadOp createPadHighOp(RankedTensorType resType, Value source, Value pad,
                       bool nofold, Location loc, OpBuilder &builder,
-                      ValueRange dynOutDims = std::nullopt);
+                      ValueRange dynOutDims = {});
 
 // Creates dim ops for each dynamic dimension of the ranked tensor argument and
 // returns these as values.

mlir/include/mlir/ExecutionEngine/ExecutionEngine.h

@@ -157,8 +157,7 @@ class ExecutionEngine {
 
   /// Invokes the function with the given name passing it the list of opaque
   /// pointers to the actual arguments.
-  llvm::Error invokePacked(StringRef name,
-                           MutableArrayRef<void *> args = std::nullopt);
+  llvm::Error invokePacked(StringRef name, MutableArrayRef<void *> args = {});
 
   /// Trait that defines how a given type is passed to the JIT code. This
   /// defaults to passing the address but can be specialized.

mlir/include/mlir/IR/BlockSupport.h

@@ -106,7 +106,7 @@ class BlockRange final
           Block *, Block *, Block *> {
 public:
   using RangeBaseT::RangeBaseT;
-  BlockRange(ArrayRef<Block *> blocks = std::nullopt);
+  BlockRange(ArrayRef<Block *> blocks = {});
   BlockRange(SuccessorRange successors);
   template <typename Arg, typename = std::enable_if_t<std::is_constructible<
                               ArrayRef<Block *>, Arg>::value>>

mlir/include/mlir/IR/Builders.h

@@ -454,15 +454,14 @@ class OpBuilder : public Builder {
   /// 'parent'. `locs` contains the locations of the inserted arguments, and
   /// should match the size of `argTypes`.
   Block *createBlock(Region *parent, Region::iterator insertPt = {},
-                     TypeRange argTypes = std::nullopt,
-                     ArrayRef<Location> locs = std::nullopt);
+                     TypeRange argTypes = {}, ArrayRef<Location> locs = {});
 
   /// Add new block with 'argTypes' arguments and set the insertion point to the
   /// end of it. The block is placed before 'insertBefore'. `locs` contains the
   /// locations of the inserted arguments, and should match the size of
   /// `argTypes`.
-  Block *createBlock(Block *insertBefore, TypeRange argTypes = std::nullopt,
-                     ArrayRef<Location> locs = std::nullopt);
+  Block *createBlock(Block *insertBefore, TypeRange argTypes = {},
+                     ArrayRef<Location> locs = {});
 
   //===--------------------------------------------------------------------===//
   // Operation Creation

mlir/include/mlir/IR/BuiltinTypes.td

@@ -1200,7 +1200,7 @@ def Builtin_UnrankedMemRef : Builtin_Type<"UnrankedMemRef", "unranked_memref", [
     using ShapedType::Trait<UnrankedMemRefType>::getDimSize;
     using ShapedType::Trait<UnrankedMemRefType>::getDynamicDimIndex;
 
-    ArrayRef<int64_t> getShape() const { return std::nullopt; }
+    ArrayRef<int64_t> getShape() const { return {}; }
 
     /// [deprecated] Returns the memory space in old raw integer representation.
     /// New `Attribute getMemorySpace()` method should be used instead.
@@ -1259,7 +1259,7 @@ def Builtin_UnrankedTensor : Builtin_Type<"UnrankedTensor", "unranked_tensor", [
     using ShapedType::Trait<UnrankedTensorType>::getDimSize;
     using ShapedType::Trait<UnrankedTensorType>::getDynamicDimIndex;
 
-    ArrayRef<int64_t> getShape() const { return std::nullopt; }
+    ArrayRef<int64_t> getShape() const { return {}; }
   }];
   let skipDefaultBuilders = 1;
   let genVerifyDecl = 1;

mlir/include/mlir/IR/PatternMatch.h

@@ -520,7 +520,7 @@ class RewriterBase : public OpBuilder {
   /// unreachable operations.
   virtual void inlineBlockBefore(Block *source, Block *dest,
                                  Block::iterator before,
-                                 ValueRange argValues = std::nullopt);
+                                 ValueRange argValues = {});
 
   /// Inline the operations of block 'source' before the operation 'op'. The
   /// source block will be deleted and must have no uses. 'argValues' is used to
@@ -529,16 +529,15 @@ class RewriterBase : public OpBuilder {
   /// The source block must have no successors. Otherwise, the resulting IR
   /// would have unreachable operations.
   void inlineBlockBefore(Block *source, Operation *op,
-                         ValueRange argValues = std::nullopt);
+                         ValueRange argValues = {});
 
   /// Inline the operations of block 'source' into the end of block 'dest'. The
   /// source block will be deleted and must have no uses. 'argValues' is used to
   /// replace the block arguments of 'source'
   ///
   /// The dest block must have no successors. Otherwise, the resulting IR would
   /// have unreachable operation.
-  void mergeBlocks(Block *source, Block *dest,
-                   ValueRange argValues = std::nullopt);
+  void mergeBlocks(Block *source, Block *dest, ValueRange argValues = {});
 
   /// Split the operations starting at "before" (inclusive) out of the given
   /// block into a new block, and return it.

mlir/include/mlir/IR/Region.h

@@ -353,7 +353,7 @@ class RegionRange
 public:
   using RangeBaseT::RangeBaseT;
 
-  RegionRange(MutableArrayRef<Region> regions = std::nullopt);
+  RegionRange(MutableArrayRef<Region> regions = {});
 
   template <typename Arg, typename = std::enable_if_t<std::is_constructible<
                               ArrayRef<std::unique_ptr<Region>>, Arg>::value>>

mlir/include/mlir/IR/SymbolTable.h

@@ -411,7 +411,8 @@ class SymbolUserMap {
   /// Return the users of the provided symbol operation.
   ArrayRef<Operation *> getUsers(Operation *symbol) const {
     auto it = symbolToUsers.find(symbol);
-    return it != symbolToUsers.end() ? it->second.getArrayRef() : std::nullopt;
+    return it != symbolToUsers.end() ? it->second.getArrayRef()
+                                     : ArrayRef<Operation *>();
   }
 
   /// Return true if the given symbol has no uses.