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MemoryAllocation.cpp
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MemoryAllocation.cpp
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//===- MemoryAllocation.cpp -----------------------------------------------===//
//
// 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 "PassDetail.h"
#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/StandardOps/IR/Ops.h"
#include "mlir/IR/Diagnostics.h"
#include "mlir/Pass/Pass.h"
#include "mlir/Transforms/DialectConversion.h"
#include "mlir/Transforms/Passes.h"
#include "llvm/ADT/TypeSwitch.h"
#define DEBUG_TYPE "flang-memory-allocation-opt"
// Number of elements in an array does not determine where it is allocated.
static constexpr std::size_t unlimitedArraySize = ~static_cast<std::size_t>(0);
namespace {
struct MemoryAllocationOptions {
// Always move dynamic array allocations to the heap. This may result in more
// heap fragmentation, so may impact performance negatively.
bool dynamicArrayOnHeap = false;
// Number of elements in array threshold for moving to heap. In environments
// with limited stack size, moving large arrays to the heap can avoid running
// out of stack space.
std::size_t maxStackArraySize = unlimitedArraySize;
};
class ReturnAnalysis {
public:
ReturnAnalysis(mlir::Operation *op) {
if (auto func = mlir::dyn_cast<mlir::FuncOp>(op))
for (mlir::Block &block : func)
for (mlir::Operation &i : block)
if (mlir::isa<mlir::ReturnOp>(i)) {
returnMap[op].push_back(&i);
break;
}
}
llvm::SmallVector<mlir::Operation *> getReturns(mlir::Operation *func) const {
auto iter = returnMap.find(func);
if (iter != returnMap.end())
return iter->second;
return {};
}
private:
llvm::DenseMap<mlir::Operation *, llvm::SmallVector<mlir::Operation *>>
returnMap;
};
} // namespace
/// Return `true` if this allocation is to remain on the stack (`fir.alloca`).
/// Otherwise the allocation should be moved to the heap (`fir.allocmem`).
static inline bool keepStackAllocation(fir::AllocaOp alloca, mlir::Block *entry,
const MemoryAllocationOptions &options) {
// Limitation: only arrays allocated on the stack in the entry block are
// considered for now.
// TODO: Generalize the algorithm and placement of the freemem nodes.
if (alloca->getBlock() != entry)
return true;
if (auto seqTy = alloca.getInType().dyn_cast<fir::SequenceType>()) {
if (fir::hasDynamicSize(seqTy)) {
// Move all arrays with runtime determined size to the heap.
if (options.dynamicArrayOnHeap)
return false;
} else {
std::int64_t numberOfElements = 1;
for (std::int64_t i : seqTy.getShape()) {
numberOfElements *= i;
// If the count is suspicious, then don't change anything here.
if (numberOfElements <= 0)
return true;
}
// If the number of elements exceeds the threshold, move the allocation to
// the heap.
if (static_cast<std::size_t>(numberOfElements) >
options.maxStackArraySize) {
LLVM_DEBUG(llvm::dbgs()
<< "memory allocation opt: found " << alloca << '\n');
return false;
}
}
}
return true;
}
namespace {
class AllocaOpConversion : public mlir::OpRewritePattern<fir::AllocaOp> {
public:
using OpRewritePattern::OpRewritePattern;
AllocaOpConversion(mlir::MLIRContext *ctx,
llvm::ArrayRef<mlir::Operation *> rets)
: OpRewritePattern(ctx), returnOps(rets) {}
mlir::LogicalResult
matchAndRewrite(fir::AllocaOp alloca,
mlir::PatternRewriter &rewriter) const override {
auto loc = alloca.getLoc();
mlir::Type varTy = alloca.getInType();
auto unpackName =
[](llvm::Optional<llvm::StringRef> opt) -> llvm::StringRef {
if (opt)
return *opt;
return {};
};
auto uniqName = unpackName(alloca.uniq_name());
auto bindcName = unpackName(alloca.bindc_name());
auto heap = rewriter.create<fir::AllocMemOp>(
loc, varTy, uniqName, bindcName, alloca.typeparams(), alloca.shape());
auto insPt = rewriter.saveInsertionPoint();
for (mlir::Operation *retOp : returnOps) {
rewriter.setInsertionPoint(retOp);
[[maybe_unused]] auto free = rewriter.create<fir::FreeMemOp>(loc, heap);
LLVM_DEBUG(llvm::dbgs() << "memory allocation opt: add free " << free
<< " for " << heap << '\n');
}
rewriter.restoreInsertionPoint(insPt);
rewriter.replaceOpWithNewOp<fir::ConvertOp>(
alloca, fir::ReferenceType::get(varTy), heap);
LLVM_DEBUG(llvm::dbgs() << "memory allocation opt: replaced " << alloca
<< " with " << heap << '\n');
return mlir::success();
}
private:
llvm::ArrayRef<mlir::Operation *> returnOps;
};
/// This pass can reclassify memory allocations (fir.alloca, fir.allocmem) based
/// on heuristics and settings. The intention is to allow better performance and
/// workarounds for conditions such as environments with limited stack space.
///
/// Currently, implements two conversions from stack to heap allocation.
/// 1. If a stack allocation is an array larger than some threshold value
/// make it a heap allocation.
/// 2. If a stack allocation is an array with a runtime evaluated size make
/// it a heap allocation.
class MemoryAllocationOpt
: public fir::MemoryAllocationOptBase<MemoryAllocationOpt> {
public:
MemoryAllocationOpt() {
// Set options with default values. (See Passes.td.) Note that the
// command-line options, e.g. dynamicArrayOnHeap, are not set yet.
options = {dynamicArrayOnHeap, maxStackArraySize};
}
MemoryAllocationOpt(bool dynOnHeap, std::size_t maxStackSize) {
// Set options with default values. (See Passes.td.)
options = {dynOnHeap, maxStackSize};
}
/// Override `options` if command-line options have been set.
inline void useCommandLineOptions() {
if (dynamicArrayOnHeap)
options.dynamicArrayOnHeap = dynamicArrayOnHeap;
if (maxStackArraySize != unlimitedArraySize)
options.maxStackArraySize = maxStackArraySize;
}
void runOnOperation() override {
auto *context = &getContext();
auto func = getOperation();
mlir::RewritePatternSet patterns(context);
mlir::ConversionTarget target(*context);
useCommandLineOptions();
LLVM_DEBUG(llvm::dbgs()
<< "dynamic arrays on heap: " << options.dynamicArrayOnHeap
<< "\nmaximum number of elements of array on stack: "
<< options.maxStackArraySize << '\n');
// If func is a declaration, skip it.
if (func.empty())
return;
const auto &analysis = getAnalysis<ReturnAnalysis>();
target.addLegalDialect<fir::FIROpsDialect, mlir::arith::ArithmeticDialect,
mlir::StandardOpsDialect>();
target.addDynamicallyLegalOp<fir::AllocaOp>([&](fir::AllocaOp alloca) {
return keepStackAllocation(alloca, &func.front(), options);
});
patterns.insert<AllocaOpConversion>(context, analysis.getReturns(func));
if (mlir::failed(
mlir::applyPartialConversion(func, target, std::move(patterns)))) {
mlir::emitError(func.getLoc(),
"error in memory allocation optimization\n");
signalPassFailure();
}
}
private:
MemoryAllocationOptions options;
};
} // namespace
std::unique_ptr<mlir::Pass> fir::createMemoryAllocationPass() {
return std::make_unique<MemoryAllocationOpt>();
}
std::unique_ptr<mlir::Pass>
fir::createMemoryAllocationPass(bool dynOnHeap, std::size_t maxStackSize) {
return std::make_unique<MemoryAllocationOpt>(dynOnHeap, maxStackSize);
}