HWCleanup.cpp

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//===- HWCleanup.cpp - HW Cleanup Pass ------------------------------------===//
//
// 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 transformation pass performs various cleanups and canonicalization
// transformations for hw.module bodies.  This is intended to be used early in
// the HW/SV pipeline to expose optimization opportunities that require global
// analysis.
//
//===----------------------------------------------------------------------===//
 
#include "circt/Dialect/HW/HWOps.h"
#include "circt/Dialect/SV/SVAttributes.h"
#include "circt/Dialect/SV/SVOps.h"
#include "circt/Dialect/SV/SVPasses.h"
#include "mlir/Pass/Pass.h"
 
namespace circt {
namespace sv {
#define GEN_PASS_DEF_HWCLEANUP
#include "circt/Dialect/SV/SVPasses.h.inc"
} // namespace sv
} // namespace circt
 
using namespace circt;
//===----------------------------------------------------------------------===//
// Helper utilities
//===----------------------------------------------------------------------===//
 
namespace {
 
/// Check the equivalence of operations by doing a deep comparison of operands
/// and attributes, but does not compare the content of any regions attached to
/// each op.
struct AlwaysLikeOpInfo : public llvm::DenseMapInfo<Operation *> {
  static unsigned getHashValue(const Operation *opC) {
    return mlir::OperationEquivalence::computeHash(
        const_cast<Operation *>(opC),
        /*hashOperands=*/mlir::OperationEquivalence::directHashValue,
        /*hashResults=*/mlir::OperationEquivalence::ignoreHashValue,
        mlir::OperationEquivalence::IgnoreLocations);
  }
  static bool isEqual(const Operation *lhsC, const Operation *rhsC) {
    auto *lhs = const_cast<Operation *>(lhsC);
    auto *rhs = const_cast<Operation *>(rhsC);
    // Trivially the same.
    if (lhs == rhs)
      return true;
    // Filter out tombstones and empty ops.
    if (lhs == getTombstoneKey() || lhs == getEmptyKey() ||
        rhs == getTombstoneKey() || rhs == getEmptyKey())
      return false;
    // Compare attributes.
    if (lhs->getName() != rhs->getName() ||
        lhs->getAttrDictionary() != rhs->getAttrDictionary() ||
        lhs->getNumOperands() != rhs->getNumOperands())
      return false;
    // Compare operands.
    for (auto operandPair : llvm::zip(lhs->getOperands(), rhs->getOperands())) {
      Value lhsOperand = std::get<0>(operandPair);
      Value rhsOperand = std::get<1>(operandPair);
      if (lhsOperand != rhsOperand)
        return false;
    }
    // The two AlwaysOps are similar enough to be combined.
    return true;
  }
};
 
} // end anonymous namespace
 
// Merge two regions together. These regions must only have a one block.
static void mergeRegions(Region *region1, Region *region2) {
  assert(region1->getBlocks().size() <= 1 && region2->getBlocks().size() <= 1 &&
         "Can only merge regions with a single block");
  if (region1->empty()) {
    // If both regions are empty, move on to the next pair of regions
    if (region2->empty())
      return;
    // If the first region has no block, move the second region's block over.
    region1->getBlocks().splice(region1->end(), region2->getBlocks());
    return;
  }
 
  // If the second region is not empty, splice its block into the start of the
  // first region.
  if (!region2->empty()) {
    auto &block1 = region1->front();
    auto &block2 = region2->front();
    block1.getOperations().splice(block1.begin(), block2.getOperations());
  }
}
 
//===----------------------------------------------------------------------===//
// HWCleanupPass
//===----------------------------------------------------------------------===//
 
namespace {
struct HWCleanupPass : public circt::sv::impl::HWCleanupBase<HWCleanupPass> {
  using sv::impl::HWCleanupBase<HWCleanupPass>::mergeAlwaysBlocks;
 
  void runOnOperation() override;
 
  void runOnRegionsInOp(Operation &op);
  void runOnGraphRegion(Region &region);
  void runOnProceduralRegion(Region &region);
 
private:
  /// Inline all regions from the second operation into the first and delete the
  /// second operation.
  void mergeOperationsIntoFrom(Operation *op1, Operation *op2) {
    // If either op1 or op2 has SV attributues, we cannot merge the ops.
    if (sv::hasSVAttributes(op1) || sv::hasSVAttributes(op2))
      return;
    assert(op1 != op2 && "Cannot merge an op into itself");
    for (size_t i = 0, e = op1->getNumRegions(); i != e; ++i)
      mergeRegions(&op1->getRegion(i), &op2->getRegion(i));
 
    op2->erase();
    anythingChanged = true;
  }
 
  bool anythingChanged;
};
} // end anonymous namespace
 
void HWCleanupPass::runOnOperation() {
  // Keeps track if anything changed during this pass, used to determine if
  // the analyses were preserved.
  anythingChanged = false;
  runOnGraphRegion(getOperation().getBody());
 
  // If we did not change anything in the graph mark all analysis as
  // preserved.
  if (!anythingChanged)
    markAllAnalysesPreserved();
}
 
/// Recursively process all of the regions in the specified op, dispatching to
/// graph or procedural processing as appropriate.
void HWCleanupPass::runOnRegionsInOp(Operation &op) {
  if (op.hasTrait<sv::ProceduralRegion>()) {
    for (auto &region : op.getRegions())
      runOnProceduralRegion(region);
  } else {
    for (auto &region : op.getRegions())
      runOnGraphRegion(region);
  }
}
 
/// Run simplifications on the specified graph region.
void HWCleanupPass::runOnGraphRegion(Region &region) {
  if (region.getBlocks().size() != 1)
    return;
  Block &body = region.front();
 
  // A set of operations in the current block which are mergable. Any
  // operation in this set is a candidate for another similar operation to
  // merge in to.
  DenseSet<Operation *, AlwaysLikeOpInfo> alwaysFFOpsSeen;
  llvm::SmallDenseMap<Attribute, Operation *, 4> ifdefOps;
  sv::InitialOp initialOpSeen;
 
  for (Operation &op : llvm::make_early_inc_range(body)) {
    // Merge alwaysff and always operations by hashing them to check to see if
    // we've already encountered one.  If so, merge them and reprocess the body.
    if (isa<sv::AlwaysOp, sv::AlwaysFFOp>(op) && mergeAlwaysBlocks) {
      // Merge identical alwaysff's together and delete the old operation.
      auto itAndInserted = alwaysFFOpsSeen.insert(&op);
      if (itAndInserted.second)
        continue;
      auto *existingAlways = *itAndInserted.first;
      mergeOperationsIntoFrom(&op, existingAlways);
 
      *itAndInserted.first = &op;
      continue;
    }
 
    // Merge graph ifdefs anywhere in the module.
    if (auto ifdefOp = dyn_cast<sv::IfDefOp>(op)) {
      auto *&entry = ifdefOps[ifdefOp.getCondAttr()];
      if (entry)
        mergeOperationsIntoFrom(ifdefOp, entry);
 
      entry = ifdefOp;
      continue;
    }
 
    // Merge initial ops anywhere in the module.
    if (auto initialOp = dyn_cast<sv::InitialOp>(op)) {
      if (initialOpSeen)
        mergeOperationsIntoFrom(initialOp, initialOpSeen);
      initialOpSeen = initialOp;
      continue;
    }
  }
 
  for (Operation &op : llvm::make_early_inc_range(body)) {
    // Recursively process any regions in the op.
    if (op.getNumRegions() != 0)
      runOnRegionsInOp(op);
  }
}
 
/// Run simplifications on the specified procedural region.
void HWCleanupPass::runOnProceduralRegion(Region &region) {
  if (region.getBlocks().size() != 1)
    return;
  Block &body = region.front();
 
  Operation *lastSideEffectingOp = nullptr;
  for (Operation &op : llvm::make_early_inc_range(body)) {
    // Merge procedural ifdefs with neighbors in the procedural region.
    if (auto ifdef = dyn_cast<sv::IfDefProceduralOp>(op)) {
      if (auto prevIfDef =
              dyn_cast_or_null<sv::IfDefProceduralOp>(lastSideEffectingOp)) {
        if (ifdef.getCond() == prevIfDef.getCond()) {
          // We know that there are no side effective operations between the
          // two, so merge the first one into this one.
          mergeOperationsIntoFrom(ifdef, prevIfDef);
        }
      }
    }
 
    // Merge 'if' operations with the same condition.
    if (auto ifop = dyn_cast<sv::IfOp>(op)) {
      if (auto prevIf = dyn_cast_or_null<sv::IfOp>(lastSideEffectingOp)) {
        if (ifop.getCond() == prevIf.getCond()) {
          // We know that there are no side effective operations between the
          // two, so merge the first one into this one.
          mergeOperationsIntoFrom(ifop, prevIf);
        }
      }
    }
 
    // Keep track of the last side effecting operation we've seen.
    if (!mlir::isMemoryEffectFree(&op))
      lastSideEffectingOp = &op;
  }
 
  for (Operation &op : llvm::make_early_inc_range(body)) {
    // Recursively process any regions in the op.
    if (op.getNumRegions() != 0)
      runOnRegionsInOp(op);
  }
}
 
std::unique_ptr<Pass> circt::sv::createHWCleanupPass(bool mergeAlwaysBlocks) {
  auto pass = std::make_unique<HWCleanupPass>();
  pass->mergeAlwaysBlocks = mergeAlwaysBlocks;
  return pass;
}