rewrite for forwarding index constants through convert (#407)

Author: rjnw

flang/include/flang/Optimizer/Transforms/RewritePatterns.td

@@ -15,13 +15,16 @@
 #define FIR_REWRITE_PATTERNS
 
 include "mlir/IR/OpBase.td"
+include "mlir/Dialect/StandardOps/IR/Ops.td"
 include "flang/Optimizer/Dialect/FIROps.td"
 
 def IdenticalTypePred : Constraint<CPred<"$0.getType() == $1.getType()">>;
 def IntegerTypePred : Constraint<CPred<"fir::isa_integer($0.getType())">>;
+def IndexTypePred : Constraint<CPred<"$0.getType().isa<mlir::IndexType>()">>;
 
 def SmallerWidthPred
-    : Constraint<CPred<"$0.getType().getIntOrFloatBitWidth() < $1.getType().getIntOrFloatBitWidth()">>;
+    : Constraint<CPred<"$0.getType().getIntOrFloatBitWidth() "
+                       "<= $1.getType().getIntOrFloatBitWidth()">>;
 
 def ConvertConvertOptPattern
     : Pat<(fir_ConvertOp (fir_ConvertOp $arg)),
@@ -42,4 +45,14 @@ def CombineConvertOptPattern
           ,(IntegerTypePred $irm)
           ,(SmallerWidthPred $arg, $irm)]>;
 
+def createConstantOp
+    : NativeCodeCall<"$_builder.create<mlir::ConstantOp>"
+                     "($_loc, $_builder.getIndexType(), "
+                     "rewriter.getIndexAttr($1.dyn_cast<IntegerAttr>().getInt()))">;
+
+def ForwardConstantConvertPattern
+    : Pat<(fir_ConvertOp:$res (ConstantOp $attr)),
+          (createConstantOp $res, $attr),
+          [(IndexTypePred $res)]>;
+
 #endif // FIR_REWRITE_PATTERNS

flang/lib/Optimizer/Dialect/FIROps.cpp

@@ -323,7 +323,7 @@ mlir::ParseResult fir::parseCmpcOp(mlir::OpAsmParser &parser,
 void fir::ConvertOp::getCanonicalizationPatterns(
     OwningRewritePatternList &results, MLIRContext *context) {
   results.insert<ConvertConvertOptPattern, RedundantConvertOptPattern,
-                 CombineConvertOptPattern>(context);
+                 CombineConvertOptPattern, ForwardConstantConvertPattern>(context);
 }
 
 mlir::OpFoldResult fir::ConvertOp::fold(llvm::ArrayRef<mlir::Attribute> opnds) {