| //===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===// |
| // |
| // 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 a sparse conditional constant propagation |
| // in MLIR. It identifies values known to be constant, propagates that |
| // information throughout the IR, and replaces them. This is done with an |
| // optimistic dataflow analysis that assumes that all values are constant until |
| // proven otherwise. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "mlir/Transforms/Passes.h" |
| |
| #include "mlir/Analysis/DataFlow/ConstantPropagationAnalysis.h" |
| #include "mlir/Analysis/DataFlow/DeadCodeAnalysis.h" |
| #include "mlir/IR/Builders.h" |
| #include "mlir/IR/Dialect.h" |
| #include "mlir/Interfaces/SideEffectInterfaces.h" |
| #include "mlir/Pass/Pass.h" |
| #include "mlir/Transforms/FoldUtils.h" |
| |
| namespace mlir { |
| #define GEN_PASS_DEF_SCCP |
| #include "mlir/Transforms/Passes.h.inc" |
| } // namespace mlir |
| |
| using namespace mlir; |
| using namespace mlir::dataflow; |
| |
| //===----------------------------------------------------------------------===// |
| // SCCP Rewrites |
| //===----------------------------------------------------------------------===// |
| |
| /// Replace the given value with a constant if the corresponding lattice |
| /// represents a constant. Returns success if the value was replaced, failure |
| /// otherwise. |
| static LogicalResult replaceWithConstant(DataFlowSolver &solver, |
| OpBuilder &builder, |
| OperationFolder &folder, Value value) { |
| auto *lattice = solver.lookupState<Lattice<ConstantValue>>(value); |
| if (!lattice || lattice->getValue().isUninitialized()) |
| return failure(); |
| const ConstantValue &latticeValue = lattice->getValue(); |
| if (!latticeValue.getConstantValue()) |
| return failure(); |
| |
| // Attempt to materialize a constant for the given value. |
| Dialect *dialect = latticeValue.getConstantDialect(); |
| Value constant = folder.getOrCreateConstant( |
| builder.getInsertionBlock(), dialect, latticeValue.getConstantValue(), |
| value.getType()); |
| if (!constant) |
| return failure(); |
| |
| value.replaceAllUsesWith(constant); |
| return success(); |
| } |
| |
| /// Rewrite the given regions using the computing analysis. This replaces the |
| /// uses of all values that have been computed to be constant, and erases as |
| /// many newly dead operations. |
| static void rewrite(DataFlowSolver &solver, MLIRContext *context, |
| MutableArrayRef<Region> initialRegions) { |
| SmallVector<Block *> worklist; |
| auto addToWorklist = [&](MutableArrayRef<Region> regions) { |
| for (Region ®ion : regions) |
| for (Block &block : llvm::reverse(region)) |
| worklist.push_back(&block); |
| }; |
| |
| // An operation folder used to create and unique constants. |
| OperationFolder folder(context); |
| OpBuilder builder(context); |
| |
| addToWorklist(initialRegions); |
| while (!worklist.empty()) { |
| Block *block = worklist.pop_back_val(); |
| |
| for (Operation &op : llvm::make_early_inc_range(*block)) { |
| builder.setInsertionPoint(&op); |
| |
| // Replace any result with constants. |
| bool replacedAll = op.getNumResults() != 0; |
| for (Value res : op.getResults()) |
| replacedAll &= |
| succeeded(replaceWithConstant(solver, builder, folder, res)); |
| |
| // If all of the results of the operation were replaced, try to erase |
| // the operation completely. |
| if (replacedAll && wouldOpBeTriviallyDead(&op)) { |
| assert(op.use_empty() && "expected all uses to be replaced"); |
| op.erase(); |
| continue; |
| } |
| |
| // Add any the regions of this operation to the worklist. |
| addToWorklist(op.getRegions()); |
| } |
| |
| // Replace any block arguments with constants. |
| builder.setInsertionPointToStart(block); |
| for (BlockArgument arg : block->getArguments()) |
| (void)replaceWithConstant(solver, builder, folder, arg); |
| } |
| } |
| |
| //===----------------------------------------------------------------------===// |
| // SCCP Pass |
| //===----------------------------------------------------------------------===// |
| |
| namespace { |
| struct SCCP : public impl::SCCPBase<SCCP> { |
| void runOnOperation() override; |
| }; |
| } // namespace |
| |
| void SCCP::runOnOperation() { |
| Operation *op = getOperation(); |
| |
| DataFlowSolver solver; |
| solver.load<DeadCodeAnalysis>(); |
| solver.load<SparseConstantPropagation>(); |
| if (failed(solver.initializeAndRun(op))) |
| return signalPassFailure(); |
| rewrite(solver, op->getContext(), op->getRegions()); |
| } |
| |
| std::unique_ptr<Pass> mlir::createSCCPPass() { |
| return std::make_unique<SCCP>(); |
| } |