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//===- SymbolDCE.cpp - Pass to delete dead symbols ------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements an algorithm for eliminating symbol operations that are
// known to be dead.
#include "PassDetail.h"
#include "mlir/Transforms/Passes.h"
using namespace mlir;
namespace {
struct SymbolDCE : public SymbolDCEBase<SymbolDCE> {
void runOnOperation() override;
/// Compute the liveness of the symbols within the given symbol table.
/// `symbolTableIsHidden` is true if this symbol table is known to be
/// unaccessible from operations in its parent regions.
LogicalResult computeLiveness(Operation *symbolTableOp,
SymbolTableCollection &symbolTable,
bool symbolTableIsHidden,
DenseSet<Operation *> &liveSymbols);
} // end anonymous namespace
void SymbolDCE::runOnOperation() {
Operation *symbolTableOp = getOperation();
// SymbolDCE should only be run on operations that define a symbol table.
if (!symbolTableOp->hasTrait<OpTrait::SymbolTable>()) {
<< " was scheduled to run under SymbolDCE, but does not define a "
"symbol table";
return signalPassFailure();
// A flag that signals if the top level symbol table is hidden, i.e. not
// accessible from parent scopes.
bool symbolTableIsHidden = true;
SymbolOpInterface symbol = dyn_cast<SymbolOpInterface>(symbolTableOp);
if (symbolTableOp->getParentOp() && symbol)
symbolTableIsHidden = symbol.isPrivate();
// Compute the set of live symbols within the symbol table.
DenseSet<Operation *> liveSymbols;
SymbolTableCollection symbolTable;
if (failed(computeLiveness(symbolTableOp, symbolTable, symbolTableIsHidden,
return signalPassFailure();
// After computing the liveness, delete all of the symbols that were found to
// be dead.
symbolTableOp->walk([&](Operation *nestedSymbolTable) {
if (!nestedSymbolTable->hasTrait<OpTrait::SymbolTable>())
for (auto &block : nestedSymbolTable->getRegion(0)) {
for (Operation &op : llvm::make_early_inc_range(block)) {
if (isa<SymbolOpInterface>(&op) && !liveSymbols.count(&op))
/// Compute the liveness of the symbols within the given symbol table.
/// `symbolTableIsHidden` is true if this symbol table is known to be
/// unaccessible from operations in its parent regions.
LogicalResult SymbolDCE::computeLiveness(Operation *symbolTableOp,
SymbolTableCollection &symbolTable,
bool symbolTableIsHidden,
DenseSet<Operation *> &liveSymbols) {
// A worklist of live operations to propagate uses from.
SmallVector<Operation *, 16> worklist;
// Walk the symbols within the current symbol table, marking the symbols that
// are known to be live.
for (auto &block : symbolTableOp->getRegion(0)) {
// Add all non-symbols or symbols that can't be discarded.
for (Operation &op : block) {
SymbolOpInterface symbol = dyn_cast<SymbolOpInterface>(&op);
if (!symbol) {
bool isDiscardable = (symbolTableIsHidden || symbol.isPrivate()) &&
if (!isDiscardable && liveSymbols.insert(&op).second)
// Process the set of symbols that were known to be live, adding new symbols
// that are referenced within.
while (!worklist.empty()) {
Operation *op = worklist.pop_back_val();
// If this is a symbol table, recursively compute its liveness.
if (op->hasTrait<OpTrait::SymbolTable>()) {
// The internal symbol table is hidden if the parent is, if its not a
// symbol, or if it is a private symbol.
SymbolOpInterface symbol = dyn_cast<SymbolOpInterface>(op);
bool symIsHidden = symbolTableIsHidden || !symbol || symbol.isPrivate();
if (failed(computeLiveness(op, symbolTable, symIsHidden, liveSymbols)))
return failure();
// Collect the uses held by this operation.
Optional<SymbolTable::UseRange> uses = SymbolTable::getSymbolUses(op);
if (!uses) {
return op->emitError()
<< "operation contains potentially unknown symbol table, "
"meaning that we can't reliable compute symbol uses";
SmallVector<Operation *, 4> resolvedSymbols;
for (const SymbolTable::SymbolUse &use : *uses) {
// Lookup the symbols referenced by this use.
if (failed(symbolTable.lookupSymbolIn(
op->getParentOp(), use.getSymbolRef(), resolvedSymbols))) {
return use.getUser()->emitError()
<< "unable to resolve reference to symbol "
<< use.getSymbolRef();
// Mark each of the resolved symbols as live.
for (Operation *resolvedSymbol : resolvedSymbols)
if (liveSymbols.insert(resolvedSymbol).second)
return success();
std::unique_ptr<Pass> mlir::createSymbolDCEPass() {
return std::make_unique<SymbolDCE>();