[Transforms][MapArithToComb] Added best-effort lowering for Arith to Comb (#9380)

Resolve failed legalization of `arith.fptosi` and `arith.fptoui`.

Author: KavyaChopra04

include/circt/Transforms/Passes.h

@@ -39,7 +39,8 @@ enum class OpCountEmissionFormat {
 void populateArithToCombPatterns(mlir::RewritePatternSet &patterns,
                                  TypeConverter &typeConverter);
 
-std::unique_ptr<mlir::Pass> createMapArithToCombPass();
+std::unique_ptr<mlir::Pass>
+createMapArithToCombPass(bool enableBestEffortLowering = false);
 std::unique_ptr<mlir::Pass> createConvertIndexToUIntPass();
 std::unique_ptr<mlir::Pass> createFlattenMemRefPass();
 std::unique_ptr<mlir::Pass> createFlattenMemRefCallsPass();

include/circt/Transforms/Passes.td

@@ -66,7 +66,14 @@ def MapArithToCombPass : Pass<"map-arith-to-comb"> {
     Rather, it provides a simple way (mostly for testing purposes) to map
     `arith` operations.
   }];
-  let constructor = "circt::createMapArithToCombPass()";
+  let options = [
+    Option<"enableBestEffortLowering", "enable-best-effort-lowering",
+           "bool", /*default=*/"false",
+           "Enable best effort lowering of operations"
+           "(eg, existing arith operations that have no lowering stay as is)">
+  ];
+  
+  let constructor = "circt::createMapArithToCombPass(/*enableBestEffortLowering=*/false)";
   let dependentDialects = ["circt::hw::HWDialect, mlir::arith::ArithDialect, circt::comb::CombDialect"];
 }
 

lib/Conversion/ImportVerilog/ImportVerilog.cpp

@@ -484,7 +484,7 @@ void circt::populateLlhdToCorePipeline(
 
   // Convert `arith.select` generated by some of the control flow canonicalizers
   // to `comb.mux`.
-  modulePM.addPass(createMapArithToCombPass());
+  modulePM.addPass(createMapArithToCombPass(true));
 
   // Simplify module-level signals.
   modulePM.addPass(llhd::createCombineDrivesPass());

lib/Transforms/MapArithToComb.cpp

@@ -155,12 +155,40 @@ class CompConversionPattern : public OpConversionPattern<arith::CmpIOp> {
 struct MapArithToCombPass
     : public circt::impl::MapArithToCombPassBase<MapArithToCombPass> {
 public:
+  MapArithToCombPass(bool enableBestEffortLowering) {
+    this->enableBestEffortLowering = enableBestEffortLowering;
+  }
+
   void runOnOperation() override {
     auto *ctx = &getContext();
 
     ConversionTarget target(*ctx);
     target.addLegalDialect<comb::CombDialect, hw::HWDialect>();
-    target.addIllegalDialect<arith::ArithDialect>();
+    if (!enableBestEffortLowering) {
+      target.addIllegalDialect<arith::ArithDialect>();
+    } else {
+      // We make all arith operations with a potential lowering here (as
+      // specified in circt::populateArithToCombPatterns) illegal
+      target.addIllegalOp<arith::AddIOp>();
+      target.addIllegalOp<arith::SubIOp>();
+      target.addIllegalOp<arith::MulIOp>();
+      target.addIllegalOp<arith::DivSIOp>();
+      target.addIllegalOp<arith::DivUIOp>();
+      target.addIllegalOp<arith::RemSIOp>();
+      target.addIllegalOp<arith::RemUIOp>();
+      target.addIllegalOp<arith::AndIOp>();
+      target.addIllegalOp<arith::OrIOp>();
+      target.addIllegalOp<arith::XOrIOp>();
+      target.addIllegalOp<arith::ShLIOp>();
+      target.addIllegalOp<arith::ShRSIOp>();
+      target.addIllegalOp<arith::ShRUIOp>();
+      target.addIllegalOp<arith::ConstantOp>();
+      target.addIllegalOp<arith::SelectOp>();
+      target.addIllegalOp<arith::ExtSIOp>();
+      target.addIllegalOp<arith::ExtUIOp>();
+      target.addIllegalOp<arith::TruncIOp>();
+      target.addIllegalOp<arith::CmpIOp>();
+    }
     MapArithTypeConverter typeConverter;
     RewritePatternSet patterns(ctx);
     populateArithToCombPatterns(patterns, typeConverter);
@@ -195,6 +223,7 @@ void circt::populateArithToCombPatterns(mlir::RewritePatternSet &patterns,
       typeConverter, patterns.getContext());
 }
 
-std::unique_ptr<mlir::Pass> circt::createMapArithToCombPass() {
-  return std::make_unique<MapArithToCombPass>();
+std::unique_ptr<mlir::Pass>
+circt::createMapArithToCombPass(bool enableBestEffortLowering) {
+  return std::make_unique<MapArithToCombPass>(enableBestEffortLowering);
 }