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//===- MemRefDataFlowOpt.cpp - Memory DataFlow Optimization 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
//
//===----------------------------------------------------------------------===//
#include "flang/Optimizer/Dialect/FIRDialect.h"
#include "flang/Optimizer/Dialect/FIROps.h"
#include "flang/Optimizer/Dialect/FIRType.h"
#include "flang/Optimizer/Transforms/Passes.h"
#include "mlir/Dialect/Func/IR/FuncOps.h"
#include "mlir/IR/Dominance.h"
#include "mlir/IR/Operation.h"
#include "mlir/Transforms/Passes.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include <optional>
namespace fir {
#define GEN_PASS_DEF_MEMREFDATAFLOWOPT
#include "flang/Optimizer/Transforms/Passes.h.inc"
} // namespace fir
#define DEBUG_TYPE "fir-memref-dataflow-opt"
using namespace mlir;
namespace {
template <typename OpT>
static std::vector<OpT> getSpecificUsers(mlir::Value v) {
std::vector<OpT> ops;
for (mlir::Operation *user : v.getUsers())
if (auto op = dyn_cast<OpT>(user))
ops.push_back(op);
return ops;
}
/// This is based on MLIR's MemRefDataFlowOpt which is specialized on AffineRead
/// and AffineWrite interface
template <typename ReadOp, typename WriteOp>
class LoadStoreForwarding {
public:
LoadStoreForwarding(mlir::DominanceInfo *di) : domInfo(di) {}
// FIXME: This algorithm has a bug. It ignores escaping references between a
// store and a load.
std::optional<WriteOp> findStoreToForward(ReadOp loadOp,
std::vector<WriteOp> &&storeOps) {
llvm::SmallVector<WriteOp> candidateSet;
for (auto storeOp : storeOps)
if (domInfo->dominates(storeOp, loadOp))
candidateSet.push_back(storeOp);
if (candidateSet.empty())
return {};
std::optional<WriteOp> nearestStore;
for (auto candidate : candidateSet) {
auto nearerThan = [&](WriteOp otherStore) {
if (candidate == otherStore)
return false;
bool rv = domInfo->properlyDominates(candidate, otherStore);
if (rv) {
LLVM_DEBUG(llvm::dbgs()
<< "candidate " << candidate << " is not the nearest to "
<< loadOp << " because " << otherStore << " is closer\n");
}
return rv;
};
if (!llvm::any_of(candidateSet, nearerThan)) {
nearestStore = mlir::cast<WriteOp>(candidate);
break;
}
}
if (!nearestStore) {
LLVM_DEBUG(
llvm::dbgs()
<< "load " << loadOp << " has " << candidateSet.size()
<< " store candidates, but this algorithm can't find a best.\n");
}
return nearestStore;
}
std::optional<ReadOp> findReadForWrite(WriteOp storeOp,
std::vector<ReadOp> &&loadOps) {
for (auto &loadOp : loadOps) {
if (domInfo->dominates(storeOp, loadOp))
return loadOp;
}
return {};
}
private:
mlir::DominanceInfo *domInfo;
};
class MemDataFlowOpt : public fir::impl::MemRefDataFlowOptBase<MemDataFlowOpt> {
public:
void runOnOperation() override {
mlir::func::FuncOp f = getOperation();
auto *domInfo = &getAnalysis<mlir::DominanceInfo>();
LoadStoreForwarding<fir::LoadOp, fir::StoreOp> lsf(domInfo);
f.walk([&](fir::LoadOp loadOp) {
auto maybeStore = lsf.findStoreToForward(
loadOp, getSpecificUsers<fir::StoreOp>(loadOp.getMemref()));
if (maybeStore) {
auto storeOp = *maybeStore;
LLVM_DEBUG(llvm::dbgs() << "FlangMemDataFlowOpt: In " << f.getName()
<< " erasing load " << loadOp
<< " with value from " << storeOp << '\n');
loadOp.getResult().replaceAllUsesWith(storeOp.getValue());
loadOp.erase();
}
});
f.walk([&](fir::AllocaOp alloca) {
for (auto &storeOp : getSpecificUsers<fir::StoreOp>(alloca.getResult())) {
if (!lsf.findReadForWrite(
storeOp, getSpecificUsers<fir::LoadOp>(storeOp.getMemref()))) {
LLVM_DEBUG(llvm::dbgs() << "FlangMemDataFlowOpt: In " << f.getName()
<< " erasing store " << storeOp << '\n');
storeOp.erase();
}
}
});
}
};
} // namespace
std::unique_ptr<mlir::Pass> fir::createMemDataFlowOptPass() {
return std::make_unique<MemDataFlowOpt>();
}
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