[mlir] Add ConstantLike trait to LLVM::ConstantOp

This make LLVM dialect constants to work with `m_constant` matches. Implement
the folding hook for this operation as required by the trait. This in turn
allows LLVM::ConstantOp to properly participate in constant-folding.

Depends On D116757

Reviewed By: wsmoses

Differential Revision: https://reviews.llvm.org/D116758

mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td

@@ -1364,7 +1364,7 @@ def LLVM_UndefOp : LLVM_Op<"mlir.undef", [NoSideEffect]>,
 }
 
 def LLVM_ConstantOp
-    : LLVM_Op<"mlir.constant", [NoSideEffect]>,
+    : LLVM_Op<"mlir.constant", [NoSideEffect, ConstantLike]>,
       LLVM_Builder<[{$res = getLLVMConstant($_resultType, $value, $_location,
                                             moduleTranslation);}]>
 {
@@ -1403,6 +1403,7 @@ def LLVM_ConstantOp
   let builders = [LLVM_OneResultOpBuilder];
   let assemblyFormat = "`(` $value `)` attr-dict `:` type($res)";
   let verifier = [{ return ::verify(*this); }];
+  let hasFolder = 1;
 }
 
 // Operations that correspond to LLVM intrinsics. With MLIR operation set being

mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp

@@ -2086,6 +2086,9 @@ static LogicalResult verify(LLVM::ConstantOp op) {
   return success();
 }
 
+// Constant op constant-folds to its value.
+OpFoldResult LLVM::ConstantOp::fold(ArrayRef<Attribute>) { return getValue(); }
+
 //===----------------------------------------------------------------------===//
 // Utility functions for parsing atomic ops
 //===----------------------------------------------------------------------===//

mlir/test/Dialect/LLVMIR/canonicalize.mlir

@@ -40,6 +40,7 @@ llvm.func @no_fold_extractvalue(%arr: !llvm.array<4xf32>) -> f32 {
 }
 
 // -----
+
 // CHECK-LABEL: fold_bitcast
 // CHECK-SAME: %[[a0:arg[0-9]+]]
 // CHECK-NEXT: llvm.return %[[a0]]
@@ -87,3 +88,19 @@ llvm.func @fold_gep(%x : !llvm.ptr<i8>) -> !llvm.ptr<i8> {
   llvm.return %c : !llvm.ptr<i8>
 }
 
+// -----
+
+// Check that LLVM constants participate in cross-dialect constant folding. The
+// resulting constant is created in the arith dialect because the last folded
+// operation belongs to it.
+// CHECK-LABEL: llvm_constant
+func @llvm_constant() -> i32 {
+  // CHECK-NOT: llvm.mlir.constant
+  %0 = llvm.mlir.constant(40 : i32) : i32
+  %1 = llvm.mlir.constant(42 : i32) : i32
+  // CHECK: %[[RES:.*]] = arith.constant 82 : i32
+  // CHECK-NOT: arith.addi
+  %2 = arith.addi %0, %1 : i32
+  // CHECK: return %[[RES]]
+  return %2 : i32
+}