ProcessLoweringPass.cpp

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//===- ProcessLoweringPass.cpp - Implement Process Lowering 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
//
//===----------------------------------------------------------------------===//
//
// Implement Pass to transform combinational processes to entities.
//
//===----------------------------------------------------------------------===//
 
#include "circt/Dialect/LLHD/IR/LLHDOps.h"
#include "circt/Dialect/LLHD/Transforms/LLHDPasses.h"
#include "mlir/Dialect/ControlFlow/IR/ControlFlowOps.h"
#include "mlir/IR/PatternMatch.h"
#include "mlir/IR/Visitors.h"
#include "mlir/Pass/Pass.h"
#include "llvm/Support/Debug.h"
 
#define DEBUG_TYPE "llhd-process-lowering"
 
namespace circt {
namespace llhd {
#define GEN_PASS_DEF_PROCESSLOWERING
#include "circt/Dialect/LLHD/Transforms/LLHDPasses.h.inc"
} // namespace llhd
} // namespace circt
 
using namespace mlir;
using namespace circt;
 
namespace {
 
struct ProcessLoweringPass
    : public circt::llhd::impl::ProcessLoweringBase<ProcessLoweringPass> {
  void runOnOperation() override;
};
 
static LogicalResult isProcValidToLower(llhd::ProcessOp op) {
  if (op.getBody().getBlocks().size() != 2) {
    LLVM_DEBUG({
      llvm::dbgs() << "process-lowering only supports processes with "
                      "two basic blocks where the first "
                      "contains a 'cf.br' terminator and the second one is "
                      "terminated by a 'llhd.wait' operation\n";
    });
    return failure();
  }
 
  Block &first = op.getBody().front();
  Block &last = op.getBody().back();
 
  if (!last.getArguments().empty()) {
    LLVM_DEBUG({
      llvm::dbgs() << "the second block (containing the "
                      "llhd.wait) is not allowed to have arguments\n";
    });
    return failure();
  }
 
  if (!isa<cf::BranchOp>(first.getTerminator())) {
    LLVM_DEBUG({
      llvm::dbgs() << "the first block has to "
                      "be terminated by a cf.br operation\n";
    });
    return failure();
  }
 
  if (auto wait = dyn_cast<llhd::WaitOp>(last.getTerminator())) {
    // No optional time argument is allowed
    if (wait.getTime()) {
      LLVM_DEBUG({
        llvm::dbgs() << "llhd.wait terminators with optional time "
                        "argument cannot be lowered to structural LLHD\n";
      });
      return failure();
    }
 
    SmallVector<Value> observedSignals;
    for (Value obs : wait.getObserved())
      if (auto prb = obs.getDefiningOp<llhd::PrbOp>())
        if (!op.getBody().isAncestor(prb->getParentRegion()))
          observedSignals.push_back(prb.getSignal());
 
    // Every probed signal has to occur in the observed signals list in
    // the wait instruction
    WalkResult result = op.walk([&](Operation *operation) -> WalkResult {
      // TODO: value does not need to be observed if all values this value is
      // a combinatorial result of are observed.
      for (Value operand : operation->getOperands()) {
        if (op.getBody().isAncestor(operand.getParentRegion()))
          continue;
        if (llvm::is_contained(wait.getObserved(), operand))
          continue;
        if (llvm::is_contained(observedSignals, operand))
          continue;
        if (auto *defOp = operand.getDefiningOp();
            defOp && defOp->hasTrait<OpTrait::ConstantLike>())
          continue;
        if (auto bitcastOp = operand.getDefiningOp<hw::BitcastOp>())
          if (auto *defOp = bitcastOp.getInput().getDefiningOp();
              defOp && defOp->hasTrait<OpTrait::ConstantLike>())
            continue;
 
        LLVM_DEBUG({
          llvm::dbgs() << "the wait terminator is required to "
                          "have values used in the process as arguments\n";
        });
        return failure();
      }
 
      return WalkResult::advance();
    });
 
    return failure(result.wasInterrupted());
  }
 
  LLVM_DEBUG({
    llvm::dbgs() << "the second block must be "
                    "terminated by llhd.wait\n";
  });
  return failure();
}
 
void ProcessLoweringPass::runOnOperation() {
  ModuleOp module = getOperation();
 
  module.walk([](llhd::ProcessOp op) {
    LLVM_DEBUG({ llvm::dbgs() << "\n=== Process\n"; });
    // Check invariants
    if (failed(isProcValidToLower(op)))
      return;
 
    // In the case that wait is used to suspend the process, we need to merge
    // the two blocks as we needed the second block to have a target for wait
    // (the entry block cannot be targeted).
    if (op.getBody().getBlocks().size() == 2) {
      Block &first = op.getBody().front();
      Block &second = op.getBody().back();
      // Delete the BranchOp operation in the entry block
      first.getTerminator()->erase();
      // Move operations of second block in entry block.
      first.getOperations().splice(first.end(), second.getOperations());
      // Drop all references to the second block and delete it.
      second.dropAllReferences();
      second.dropAllDefinedValueUses();
      second.erase();
    }
 
    // Remove the remaining llhd.halt or llhd.wait terminator
    op.getBody().front().getTerminator()->erase();
 
    IRRewriter rewriter(op);
    rewriter.inlineBlockBefore(&op.getBody().front(), op);
    op.erase();
 
    LLVM_DEBUG({ llvm::dbgs() << "Process lowered successfully!\n"; });
  });
}
} // namespace