HWBypassInnerSymbols.cpp

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//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
//  This pass moves inner symbols from ports to wires, then bypasses wire
//  operations with inner symbols by replacing uses with their inputs while
//  keeping the wire to preserve the symbol. This enables optimizations to
//  cross symbol boundaries while maintaining symbol references.
//
//  Note: This transformation assumes that values associated with inner
//  symbols are not mutated through inner symbols (e.g. force). This assumption
//  may not hold in design verification contexts, but is safe in synthesis.
//
//===----------------------------------------------------------------------===//
 
#include "circt/Dialect/HW/HWOps.h"
#include "circt/Dialect/HW/HWPasses.h"
#include "circt/Dialect/HW/PortImplementation.h"
#include "mlir/IR/PatternMatch.h"
#include "mlir/Transforms/WalkPatternRewriteDriver.h"
 
namespace circt {
namespace hw {
#define GEN_PASS_DEF_HWBYPASSINNERSYMBOLS
#include "circt/Dialect/HW/Passes.h.inc"
} // namespace hw
} // namespace circt
 
using namespace circt;
using namespace circt::hw;
 
namespace {
struct HWBypassInnerSymbolsPass
    : public impl::HWBypassInnerSymbolsBase<HWBypassInnerSymbolsPass> {
  void runOnOperation() override;
  using HWBypassInnerSymbolsBase<
      HWBypassInnerSymbolsPass>::HWBypassInnerSymbolsBase;
};
 
/// Pattern to bypass wire operations with inner symbols.
struct BypassWireWithInnerSym : public OpRewritePattern<WireOp> {
  using OpRewritePattern<WireOp>::OpRewritePattern;
 
  LogicalResult matchAndRewrite(WireOp wire,
                                PatternRewriter &rewriter) const override {
    // Only bypass wires that have an inner symbol or use
    if (!wire.getInnerSymAttr() || wire.use_empty())
      return failure();
 
    // Replace all uses of the wire with its input
    rewriter.modifyOpInPlace(wire, [&] {
      wire->replaceAllUsesWith(ArrayRef<Value>{wire.getInput()});
    });
    return success();
  }
};
} // namespace
 
void HWBypassInnerSymbolsPass::runOnOperation() {
  auto module = getOperation();
 
  // First move the inner symbol from the port to allow constprop and other
  // optimizations to cross the boundary.
  hw::ModulePortInfo portList(module.getPortList());
  auto *outputOp = module.getBodyBlock()->getTerminator();
  OpBuilder builder(&getContext());
  bool hasInnerSym = false;
  auto moduleType = module.getModuleType();
  for (auto [index, port] : llvm::enumerate(portList)) {
    auto sym = port.getSym();
    if (!sym)
      continue;
    hasInnerSym = true;
    ++numPortsMoved;
    if (port.isOutput()) {
      auto value = outputOp->getOperand(moduleType.getOutputIdForPortId(index));
      builder.setInsertionPointAfterValue(value);
      auto wire = WireOp::create(builder, value.getLoc(), value);
      wire.setInnerSymAttr(sym);
    } else {
      auto arg = module.getBodyBlock()->getArgument(
          moduleType.getInputIdForPortId(index));
      builder.setInsertionPointToStart(module.getBodyBlock());
      auto wire = WireOp::create(builder, arg.getLoc(), arg);
      wire.setInnerSymAttr(sym);
    }
  }
 
  if (hasInnerSym) {
    // Bulk clear all port symbols.
    auto innerSymAttr = StringAttr::get(
        &getContext(), hw::HWModuleLike::getPortSymbolAttrName());
    SmallVector<Attribute, 0> newAttrs(portList.size(), {});
    cast<HWModuleLike>(*module).setPortAttrs(innerSymAttr, newAttrs);
  }
 
  RewritePatternSet patterns(&getContext());
  patterns.add<BypassWireWithInnerSym>(&getContext());
  mlir::walkAndApplyPatterns(module, std::move(patterns));
}