LowerProcesses.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
//
//===----------------------------------------------------------------------===//
 
#include "circt/Dialect/Comb/CombOps.h"
#include "circt/Dialect/LLHD/IR/LLHDOps.h"
#include "circt/Dialect/LLHD/Transforms/LLHDPasses.h"
#include "mlir/Analysis/Liveness.h"
#include "mlir/Dialect/ControlFlow/IR/ControlFlowOps.h"
#include "mlir/Dialect/SCF/IR/SCF.h"
#include "mlir/Transforms/RegionUtils.h"
#include "llvm/Support/Debug.h"
 
#define DEBUG_TYPE "llhd-lower-processes"
 
namespace circt {
namespace llhd {
#define GEN_PASS_DEF_LOWERPROCESSESPASS
#include "circt/Dialect/LLHD/Transforms/LLHDPasses.h.inc"
} // namespace llhd
} // namespace circt
 
using namespace mlir;
using namespace circt;
using namespace llhd;
using llvm::SmallDenseSet;
using llvm::SmallSetVector;
 
namespace {
struct Lowering {
  Lowering(ProcessOp processOp) : processOp(processOp) {}
  void lower();
  bool matchControlFlow();
  void markObservedValues();
  bool allOperandsObserved();
  bool isObserved(Value value);
 
  ProcessOp processOp;
  WaitOp waitOp;
  SmallDenseSet<Value> observedValues;
};
} // namespace
 
void Lowering::lower() {
  // Ensure that the process describes combinational logic.
  if (!matchControlFlow())
    return;
  markObservedValues();
  if (!allOperandsObserved())
    return;
  LLVM_DEBUG(llvm::dbgs() << "Lowering process " << processOp.getLoc() << "\n");
 
  // Replace the process.
  OpBuilder builder(processOp);
  auto executeOp = builder.create<scf::ExecuteRegionOp>(
      processOp.getLoc(), processOp.getResultTypes());
  executeOp.getRegion().takeBody(processOp.getBody());
  processOp.replaceAllUsesWith(executeOp);
  processOp.erase();
  processOp = {};
 
  // Replace the `llhd.wait` with an `scf.yield`.
  builder.setInsertionPoint(waitOp);
  builder.create<scf::YieldOp>(waitOp.getLoc(), waitOp.getYieldOperands());
  waitOp.erase();
 
  // Simplify the execute op body region since disconnecting the control flow
  // loop through the wait op has potentially created unreachable blocks.
  IRRewriter rewriter(builder);
  (void)simplifyRegions(rewriter, executeOp->getRegions());
}
 
/// Check that the process' entry block trivially joins a control flow loop
/// immediately after the wait op.
bool Lowering::matchControlFlow() {
  // Ensure that there is only a single wait op in the process and that it has
  // no destination operands.
  for (auto &block : processOp.getBody()) {
    if (auto op = dyn_cast<WaitOp>(block.getTerminator())) {
      if (waitOp) {
        LLVM_DEBUG(llvm::dbgs() << "Skipping process " << processOp.getLoc()
                                << ": multiple wait ops\n");
        return false;
      }
      waitOp = op;
    }
  }
  if (!waitOp) {
    LLVM_DEBUG(llvm::dbgs() << "Skipping process " << processOp.getLoc()
                            << ": no wait op\n");
    return false;
  }
  if (!waitOp.getDestOperands().empty()) {
    LLVM_DEBUG(llvm::dbgs() << "Skipping process " << processOp.getLoc()
                            << ": wait op has destination operands\n");
    return false;
  }
 
  // Helper function to skip across empty blocks with only a single successor.
  auto skipToMergePoint = [&](Block *block) -> std::pair<Block *, ValueRange> {
    ValueRange operands;
    while (auto branchOp = dyn_cast<cf::BranchOp>(block->getTerminator())) {
      if (!block->without_terminator().empty())
        break;
      block = branchOp.getDest();
      operands = branchOp.getDestOperands();
      if (std::distance(block->pred_begin(), block->pred_end()) > 1)
        break;
      if (!operands.empty())
        break;
    }
    return {block, operands};
  };
 
  // Ensure that the entry block and wait op converge on the same block and with
  // the same block arguments.
  auto &entry = processOp.getBody().front();
  auto [entryMergeBlock, entryMergeArgs] = skipToMergePoint(&entry);
  auto [waitMergeBlock, waitMergeArgs] = skipToMergePoint(waitOp.getDest());
  if (entryMergeBlock != waitMergeBlock) {
    LLVM_DEBUG(llvm::dbgs()
               << "Skipping process " << processOp.getLoc()
               << ": control from entry and wait does not converge\n");
    return false;
  }
  if (entryMergeArgs != waitMergeArgs) {
    LLVM_DEBUG(llvm::dbgs() << "Skipping process " << processOp.getLoc()
                            << ": control from entry and wait converges with "
                               "different block arguments\n");
    return false;
  }
 
  // Ensure that no values are live across the wait op.
  Liveness liveness(processOp);
  for (auto value : liveness.getLiveOut(waitOp->getBlock())) {
    if (value.getParentRegion()->isProperAncestor(&processOp.getBody()))
      continue;
    LLVM_DEBUG({
      llvm::dbgs() << "Skipping process " << processOp.getLoc() << ": value ";
      value.print(llvm::dbgs(), OpPrintingFlags().skipRegions());
      llvm::dbgs() << " live across wait\n";
    });
    return false;
  }
 
  return true;
}
 
/// Mark values the process observes that are defined outside the process.
void Lowering::markObservedValues() {
  SmallVector<Value> worklist;
  auto markObserved = [&](Value value) {
    if (observedValues.insert(value).second)
      worklist.push_back(value);
  };
 
  for (auto value : waitOp.getObserved())
    if (value.getParentRegion()->isProperAncestor(&processOp.getBody()))
      markObserved(value);
 
  while (!worklist.empty()) {
    auto value = worklist.pop_back_val();
    auto *op = value.getDefiningOp();
    if (!op)
      continue;
 
    // Look through probe ops to mark the probe signal as well, just in case
    // there may be multiple probes of the same signal.
    if (auto probeOp = dyn_cast<PrbOp>(op))
      markObserved(probeOp.getSignal());
 
    // Look through operations that simply reshape incoming values into an
    // aggregate form from which any changes remain apparent.
    if (isa<hw::ArrayCreateOp, hw::StructCreateOp, comb::ConcatOp,
            hw::BitcastOp>(op))
      for (auto operand : op->getOperands())
        markObserved(operand);
  }
}
 
/// Ensure that any value defined outside the process that is used inside the
/// process is derived entirely from an observed value.
bool Lowering::allOperandsObserved() {
  // Collect all ancestor regions such that we can easily check if a value is
  // defined outside the process.
  SmallPtrSet<Region *, 4> properAncestors;
  for (auto *region = processOp->getParentRegion(); region;
       region = region->getParentRegion())
    properAncestors.insert(region);
 
  // Walk all operations under the process and check each operand.
  auto walkResult = processOp.walk([&](Operation *op) {
    for (auto operand : op->getOperands()) {
      // We only care about values defined outside the process.
      if (!properAncestors.count(operand.getParentRegion()))
        continue;
 
      // If the value is observed, all is well.
      if (isObserved(operand))
        continue;
 
      // Otherwise complain and abort.
      LLVM_DEBUG({
        llvm::dbgs() << "Skipping process " << processOp.getLoc()
                     << ": unobserved value ";
        operand.print(llvm::dbgs(), OpPrintingFlags().skipRegions());
        llvm::dbgs() << "\n";
      });
      return WalkResult::interrupt();
    }
    return WalkResult::advance();
  });
  return !walkResult.wasInterrupted();
}
 
/// Check if a value is observed by the wait op, or all its operands are only
/// derived from observed values.
bool Lowering::isObserved(Value value) {
  // Check if the value is trivially observed.
  if (observedValues.contains(value))
    return true;
 
  // Otherwise get the operation that defines it such that we can check if the
  // value is derived from purely observed values. If it isn't define by an op,
  // the value is unobserved.
  auto *defOp = value.getDefiningOp();
  if (!defOp)
    return false;
 
  // Otherwise visit all ops in the fan-in cone and ensure that they are
  // observed. If any value is unobserved, immediately return false.
  SmallDenseSet<Operation *> seenOps;
  SmallVector<Operation *> worklist;
  seenOps.insert(defOp);
  worklist.push_back(defOp);
  while (!worklist.empty()) {
    auto *op = worklist.pop_back_val();
 
    // Give up on ops with nested regions.
    if (op->getNumRegions() != 0)
      return false;
 
    // Otherwise check that all operands are observed. If we haven't seen an
    // operand before, and it is not a signal, add it to the worklist to be
    // checked.
    for (auto operand : op->getOperands()) {
      if (observedValues.contains(operand))
        continue;
      if (isa<hw::InOutType>(operand.getType()))
        return false;
      auto *defOp = operand.getDefiningOp();
      if (!defOp || !isMemoryEffectFree(defOp))
        return false;
      if (seenOps.insert(defOp).second)
        worklist.push_back(defOp);
    }
  }
 
  // If we arrive at this point, we weren't able to reach an unobserved value.
  // Therefore we consider this value derived from only observed values.
  observedValues.insert(value);
  return true;
}
 
namespace {
struct LowerProcessesPass
    : public llhd::impl::LowerProcessesPassBase<LowerProcessesPass> {
  void runOnOperation() override;
};
} // namespace
 
void LowerProcessesPass::runOnOperation() {
  SmallVector<ProcessOp> processOps(getOperation().getOps<ProcessOp>());
  for (auto processOp : processOps)
    Lowering(processOp).lower();
}