RegisterOptimizer.cpp

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//===- RegisterOptimizer.cpp - Register Optimizer ---------------*- C++ -*-===//
//
// 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 optimized registers as allowed by historic firrtl register
// behaviors.
//
//===----------------------------------------------------------------------===//
 
#include "circt/Dialect/FIRRTL/FIRRTLOps.h"
#include "circt/Dialect/FIRRTL/FIRRTLUtils.h"
#include "circt/Dialect/FIRRTL/Passes.h"
#include "circt/Support/Debug.h"
#include "mlir/IR/Dominance.h"
#include "mlir/Pass/Pass.h"
#include "llvm/Support/Debug.h"
 
#define DEBUG_TYPE "firrtl-register-optimizer"
 
namespace circt {
namespace firrtl {
#define GEN_PASS_DEF_REGISTEROPTIMIZER
#include "circt/Dialect/FIRRTL/Passes.h.inc"
} // namespace firrtl
} // namespace circt
 
using namespace circt;
using namespace firrtl;
 
// Instantiated for RegOp and RegResetOp
template <typename T>
static bool canErase(T op) {
  return !(hasDontTouch(op.getResult()) || op.isForceable() ||
           (op.getAnnotationsAttr() && !op.getAnnotationsAttr().empty()));
}
 
namespace {
 
//===----------------------------------------------------------------------===//
// Pass Infrastructure
//===----------------------------------------------------------------------===//
 
struct RegisterOptimizerPass
    : public circt::firrtl::impl::RegisterOptimizerBase<RegisterOptimizerPass> {
  void runOnOperation() override;
  void checkRegReset(mlir::DominanceInfo &dom,
                     SmallVector<Operation *> &toErase, RegResetOp reg);
  void checkReg(mlir::DominanceInfo &dom, SmallVector<Operation *> &toErase,
                RegOp reg);
};
 
} // namespace
 
void RegisterOptimizerPass::checkReg(mlir::DominanceInfo &dom,
                                     SmallVector<Operation *> &toErase,
                                     RegOp reg) {
  if (!canErase(reg))
    return;
  auto con = getSingleConnectUserOf(reg.getResult());
  if (!con)
    return;
 
  // Register is only written by itself.  Without a time-zero `initial` value
  // its value is unspecified, so replace it with invalid.  With one, the
  // register holds that constant forever, so replace it with the constant.
  if (con.getSrc() == reg.getResult()) {
    auto builder = OpBuilder(reg);
    Value replacement;
    if (auto initial = reg.getInitialAttr())
      replacement =
          ConstantOp::create(builder, reg.getLoc(),
                             type_cast<IntType>(reg.getResult().getType()),
                             initial.getValue())
              .getResult();
    else
      replacement = InvalidValueOp::create(builder, reg.getLoc(),
                                           reg.getResult().getType())
                        .getResult();
    reg.getResult().replaceAllUsesWith(replacement);
    toErase.push_back(reg);
    toErase.push_back(con);
    return;
  }
  // Register is only written by a constant
  if (isConstant(con.getSrc())) {
    // Bail if replacing the register with the constant would change its
    // time-zero simulation value.
    auto cstOp = con.getSrc().getDefiningOp<ConstantOp>();
    if (!preservesInitial(reg.getInitialAttr(),
                          cstOp ? std::optional<APInt>(cstOp.getValue())
                                : std::nullopt))
      return;
    // constant may not dominate the register.  But it might be the next
    // operation, so we can't just move it.  Straight constants can be
    // rematerialized.  Derived constants are piped through wires.
 
    if (cstOp) {
      // Simple constants we can move safely
      auto *fmodb = con->getParentOfType<FModuleOp>().getBodyBlock();
      cstOp->moveBefore(fmodb, fmodb->begin());
      reg.getResult().replaceAllUsesWith(cstOp.getResult());
      toErase.push_back(con);
    } else {
      bool dominatesAll = true;
      for (auto *use : reg->getUsers()) {
        if (use == con)
          continue;
        if (!dom.dominates(con.getSrc(), use)) {
          dominatesAll = false;
          break;
        }
      }
      if (dominatesAll) {
        // Dominance is fine, just replace the op.
        reg.getResult().replaceAllUsesWith(con.getSrc());
        toErase.push_back(con);
      } else {
        auto builder = OpBuilder(reg);
        auto bounce =
            WireOp::create(builder, reg.getLoc(), reg.getResult().getType());
        reg.replaceAllUsesWith(bounce);
      }
    }
    toErase.push_back(reg);
    return;
  }
}
 
void RegisterOptimizerPass::checkRegReset(mlir::DominanceInfo &dom,
                                          SmallVector<Operation *> &toErase,
                                          RegResetOp reg) {
  if (!canErase(reg))
    return;
  auto con = getSingleConnectUserOf(reg.getResult());
  if (!con)
    return;
 
  // Both folds below replace the register with its reset value, so bail if
  // that would change the register's time-zero (`initial`) value.
  auto resetCstOp = reg.getResetValue().getDefiningOp<ConstantOp>();
  if (!preservesInitial(reg.getInitialAttr(),
                        resetCstOp ? std::optional<APInt>(resetCstOp.getValue())
                                   : std::nullopt))
    return;
 
  // Register is only written by itself, and reset with a constant.
  if (reg.getResetValue().getType() == reg.getResult().getType()) {
    if (con.getSrc() == reg.getResult() && isConstant(reg.getResetValue())) {
      // constant obviously dominates the register.
      reg.getResult().replaceAllUsesWith(reg.getResetValue());
      toErase.push_back(reg);
      toErase.push_back(con);
      return;
    }
    // Register is only written by a constant, and reset with the same constant.
    if (con.getSrc() == reg.getResetValue() &&
        isConstant(reg.getResetValue())) {
      // constant obviously dominates the register.
      reg.getResult().replaceAllUsesWith(reg.getResetValue());
      toErase.push_back(reg);
      toErase.push_back(con);
      return;
    }
  }
}
 
void RegisterOptimizerPass::runOnOperation() {
  CIRCT_DEBUG_SCOPED_PASS_LOGGER(this);
 
  auto mod = getOperation();
  SmallVector<Operation *> toErase;
  mlir::DominanceInfo dom(mod);
 
  for (auto &op : *mod.getBodyBlock()) {
    if (auto reg = dyn_cast<RegResetOp>(&op))
      checkRegReset(dom, toErase, reg);
    else if (auto reg = dyn_cast<RegOp>(&op))
      checkReg(dom, toErase, reg);
  }
  for (auto *op : toErase)
    op->erase();
 
  if (!toErase.empty())
    return markAllAnalysesPreserved();
}