GenericLUTMapper.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 file implements a generic LUT mapper pass using the cut-based rewriting
// framework. It performs technology mapping by converting combinational logic
// networks into implementations using K-input lookup tables (LUTs).
//
//===----------------------------------------------------------------------===//
 
#include "circt/Dialect/Comb/CombOps.h"
#include "circt/Dialect/HW/HWOps.h"
#include "circt/Dialect/Synth/Transforms/CutRewriter.h"
#include "circt/Dialect/Synth/Transforms/SynthPasses.h"
#include "circt/Support/LLVM.h"
#include "circt/Support/TruthTable.h"
#include "mlir/IR/Builders.h"
#include "mlir/Pass/Pass.h"
#include "llvm/Support/Debug.h"
 
#define DEBUG_TYPE "synth-generic-lut-mapper"
 
using namespace circt;
using namespace circt::synth;
 
namespace circt {
namespace synth {
#define GEN_PASS_DEF_GENERICLUTMAPPER
#include "circt/Dialect/Synth/Transforms/SynthPasses.h.inc"
} // namespace synth
} // namespace circt
 
//===----------------------------------------------------------------------===//
// Generic LUT Pattern
//===----------------------------------------------------------------------===//
 
/// A generic K-input LUT pattern that can implement any boolean function
/// with up to K inputs using a lookup table.
struct GenericLUT : public CutRewritePattern {
  unsigned k; // Maximum number of inputs for this LUT
  SmallVector<DelayType, 8> cachedDelays; // Pre-computed unit delays
 
  GenericLUT(mlir::MLIRContext *context, unsigned k)
      : CutRewritePattern(context), k(k), cachedDelays(k, 1) {}
 
  std::optional<MatchResult> match(CutEnumerator &enumerator,
                                   const Cut &cut) const override {
    // This pattern can implement any cut with at most k inputs
    if (cut.getInputSize() > k || cut.getOutputSize() != 1)
      return std::nullopt;
 
    // Create match result with a reference to cached delays
    return MatchResult(
        1.0, ArrayRef<DelayType>(cachedDelays).take_front(cut.getInputSize()));
  }
 
  llvm::FailureOr<Operation *> rewrite(mlir::OpBuilder &rewriter,
                                       CutEnumerator &enumerator,
                                       const Cut &cut) const override {
    // NOTE: Don't use NPN since it's unnecessary.
    auto truthTable = cut.getTruthTable();
    LLVM_DEBUG({
      llvm::dbgs() << "Rewriting cut with " << cut.getInputSize()
                   << " inputs and " << cut.getInputSize()
                   << " operations to a generic LUT with " << k << " inputs.\n";
      cut.dump(llvm::dbgs());
      llvm::dbgs() << "Truth table details:\n";
      truthTable.dump(llvm::dbgs());
    });
 
    SmallVector<bool> lutTable;
    // Convert the truth table to a LUT table
    for (uint32_t i = 0; i < truthTable.table.getBitWidth(); ++i)
      lutTable.push_back(truthTable.table[i]);
 
    // Reverse the inputs to match the LUT input order
    SmallVector<Value> lutInputs(cut.inputs.rbegin(), cut.inputs.rend());
 
    // Generate comb.truth table operation.
    auto truthTableOp = comb::TruthTableOp::create(
        rewriter, cut.getRoot()->getLoc(), lutInputs, lutTable);
 
    // Replace the root operation with the truth table operation
    return truthTableOp.getOperation();
  }
 
  unsigned getNumOutputs() const override { return 1; }
 
  StringRef getPatternName() const override { return "GenericLUT"; }
};
 
//===----------------------------------------------------------------------===//
// Generic LUT Mapper Pass
//===----------------------------------------------------------------------===//
 
struct GenericLUTMapperPass
    : public impl::GenericLutMapperBase<GenericLUTMapperPass> {
  using GenericLutMapperBase<GenericLUTMapperPass>::GenericLutMapperBase;
 
  void runOnOperation() override {
    auto module = getOperation();
    // Create the cut rewriter options
    CutRewriterOptions options;
    options.strategy = strategy;
    options.maxCutInputSize = maxLutSize;
    options.maxCutSizePerRoot = maxCutsPerRoot;
    options.allowNoMatch = false;
    options.attachDebugTiming = test;
 
    // Create the pattern for generic K-LUT
    SmallVector<std::unique_ptr<CutRewritePattern>, 4> patterns;
    patterns.push_back(
        std::make_unique<GenericLUT>(module->getContext(), maxLutSize));
 
    // Create the pattern set
    CutRewritePatternSet patternSet(std::move(patterns));
 
    // Create the cut rewriter
    CutRewriter rewriter(options, patternSet);
 
    // Apply the rewriting
    if (failed(rewriter.run(module)))
      return signalPassFailure();
  }
};