//===- SROA.h - Scalar Replacement Of Aggregates ----------------*- C++ -*-===//
//
// 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
//
//===----------------------------------------------------------------------===//
/// \file
/// This file provides the interface for LLVM's Scalar Replacement of
/// Aggregates pass. This pass provides both aggregate splitting and the
/// primary SSA formation used in the compiler.
///
//===----------------------------------------------------------------------===//
#ifndef LLVM_TRANSFORMS_SCALAR_SROA_H
#define LLVM_TRANSFORMS_SCALAR_SROA_H
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/ValueHandle.h"
#include <vector>
namespace llvm {
class AllocaInst;
class LoadInst;
class StoreInst;
class AssumptionCache;
class DominatorTree;
class DomTreeUpdater;
class Function;
class LLVMContext;
class PHINode;
class SelectInst;
class Use;
/// A private "module" namespace for types and utilities used by SROA. These
/// are implementation details and should not be used by clients.
namespace sroa LLVM_LIBRARY_VISIBILITY {
class AllocaSliceRewriter;
class AllocaSlices;
class Partition;
class SROALegacyPass;
class SelectHandSpeculativity {
unsigned char Storage = 0; // None are speculatable by default.
using TrueVal = Bitfield::Element<bool, 0, 1>; // Low 0'th bit.
using FalseVal = Bitfield::Element<bool, 1, 1>; // Low 1'th bit.
public:
SelectHandSpeculativity() = default;
SelectHandSpeculativity &setAsSpeculatable(bool isTrueVal);
bool isSpeculatable(bool isTrueVal) const;
bool areAllSpeculatable() const;
bool areAnySpeculatable() const;
bool areNoneSpeculatable() const;
// For interop as int half of PointerIntPair.
explicit operator intptr_t() const { return static_cast<intptr_t>(Storage); }
explicit SelectHandSpeculativity(intptr_t Storage_) : Storage(Storage_) {}
};
static_assert(sizeof(SelectHandSpeculativity) == sizeof(unsigned char));
using PossiblySpeculatableLoad =
PointerIntPair<LoadInst *, 2, sroa::SelectHandSpeculativity>;
using UnspeculatableStore = StoreInst *;
using RewriteableMemOp =
std::variant<PossiblySpeculatableLoad, UnspeculatableStore>;
using RewriteableMemOps = SmallVector<RewriteableMemOp, 2>;
} // end namespace sroa
enum class SROAOptions : bool { ModifyCFG, PreserveCFG };
/// An optimization pass providing Scalar Replacement of Aggregates.
///
/// This pass takes allocations which can be completely analyzed (that is, they
/// don't escape) and tries to turn them into scalar SSA values. There are
/// a few steps to this process.
///
/// 1) It takes allocations of aggregates and analyzes the ways in which they
/// are used to try to split them into smaller allocations, ideally of
/// a single scalar data type. It will split up memcpy and memset accesses
/// as necessary and try to isolate individual scalar accesses.
/// 2) It will transform accesses into forms which are suitable for SSA value
/// promotion. This can be replacing a memset with a scalar store of an
/// integer value, or it can involve speculating operations on a PHI or
/// select to be a PHI or select of the results.
/// 3) Finally, this will try to detect a pattern of accesses which map cleanly
/// onto insert and extract operations on a vector value, and convert them to
/// this form. By doing so, it will enable promotion of vector aggregates to
/// SSA vector values.
class SROAPass : public PassInfoMixin<SROAPass> {
LLVMContext *C = nullptr;
DomTreeUpdater *DTU = nullptr;
AssumptionCache *AC = nullptr;
const bool PreserveCFG;
/// Worklist of alloca instructions to simplify.
///
/// Each alloca in the function is added to this. Each new alloca formed gets
/// added to it as well to recursively simplify unless that alloca can be
/// directly promoted. Finally, each time we rewrite a use of an alloca other
/// the one being actively rewritten, we add it back onto the list if not
/// already present to ensure it is re-visited.
SetVector<AllocaInst *, SmallVector<AllocaInst *, 16>> Worklist;
/// A collection of instructions to delete.
/// We try to batch deletions to simplify code and make things a bit more
/// efficient. We also make sure there is no dangling pointers.
SmallVector<WeakVH, 8> DeadInsts;
/// Post-promotion worklist.
///
/// Sometimes we discover an alloca which has a high probability of becoming
/// viable for SROA after a round of promotion takes place. In those cases,
/// the alloca is enqueued here for re-processing.
///
/// Note that we have to be very careful to clear allocas out of this list in
/// the event they are deleted.
SetVector<AllocaInst *, SmallVector<AllocaInst *, 16>> PostPromotionWorklist;
/// A collection of alloca instructions we can directly promote.
std::vector<AllocaInst *> PromotableAllocas;
/// A worklist of PHIs to speculate prior to promoting allocas.
///
/// All of these PHIs have been checked for the safety of speculation and by
/// being speculated will allow promoting allocas currently in the promotable
/// queue.
SetVector<PHINode *, SmallVector<PHINode *, 8>> SpeculatablePHIs;
/// A worklist of select instructions to rewrite prior to promoting
/// allocas.
SmallMapVector<SelectInst *, sroa::RewriteableMemOps, 8> SelectsToRewrite;
/// Select instructions that use an alloca and are subsequently loaded can be
/// rewritten to load both input pointers and then select between the result,
/// allowing the load of the alloca to be promoted.
/// From this:
/// %P2 = select i1 %cond, ptr %Alloca, ptr %Other
/// %V = load <type>, ptr %P2
/// to:
/// %V1 = load <type>, ptr %Alloca -> will be mem2reg'd
/// %V2 = load <type>, ptr %Other
/// %V = select i1 %cond, <type> %V1, <type> %V2
///
/// We can do this to a select if its only uses are loads
/// and if either the operand to the select can be loaded unconditionally,
/// or if we are allowed to perform CFG modifications.
/// If found an intervening bitcast with a single use of the load,
/// allow the promotion.
static std::optional<sroa::RewriteableMemOps>
isSafeSelectToSpeculate(SelectInst &SI, bool PreserveCFG);
public:
/// If \p PreserveCFG is set, then the pass is not allowed to modify CFG
/// in any way, even if it would update CFG analyses.
SROAPass(SROAOptions PreserveCFG);
/// Run the pass over the function.
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
void printPipeline(raw_ostream &OS,
function_ref<StringRef(StringRef)> MapClassName2PassName);
private:
friend class sroa::AllocaSliceRewriter;
friend class sroa::SROALegacyPass;
/// Helper used by both the public run method and by the legacy pass.
PreservedAnalyses runImpl(Function &F, DomTreeUpdater &RunDTU,
AssumptionCache &RunAC);
PreservedAnalyses runImpl(Function &F, DominatorTree &RunDT,
AssumptionCache &RunAC);
bool presplitLoadsAndStores(AllocaInst &AI, sroa::AllocaSlices &AS);
AllocaInst *rewritePartition(AllocaInst &AI, sroa::AllocaSlices &AS,
sroa::Partition &P);
bool splitAlloca(AllocaInst &AI, sroa::AllocaSlices &AS);
std::pair<bool /*Changed*/, bool /*CFGChanged*/> runOnAlloca(AllocaInst &AI);
void clobberUse(Use &U);
bool deleteDeadInstructions(SmallPtrSetImpl<AllocaInst *> &DeletedAllocas);
bool promoteAllocas(Function &F);
};
} // end namespace llvm
#endif // LLVM_TRANSFORMS_SCALAR_SROA_H