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//==- AArch64PromoteConstant.cpp - Promote constant to global for AArch64 --==//
// 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 the AArch64PromoteConstant pass which promotes constants
// to global variables when this is likely to be more efficient. Currently only
// types related to constant vector (i.e., constant vector, array of constant
// vectors, constant structure with a constant vector field, etc.) are promoted
// to global variables. Constant vectors are likely to be lowered in target
// constant pool during instruction selection already; therefore, the access
// will remain the same (memory load), but the structure types are not split
// into different constant pool accesses for each field. A bonus side effect is
// that created globals may be merged by the global merge pass.
// FIXME: This pass may be useful for other targets too.
#include "AArch64.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InlineAsm.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "aarch64-promote-const"
// Stress testing mode - disable heuristics.
static cl::opt<bool> Stress("aarch64-stress-promote-const", cl::Hidden,
cl::desc("Promote all vector constants"));
STATISTIC(NumPromoted, "Number of promoted constants");
STATISTIC(NumPromotedUses, "Number of promoted constants uses");
// AArch64PromoteConstant
namespace {
/// Promotes interesting constant into global variables.
/// The motivating example is:
/// static const uint16_t TableA[32] = {
/// 41944, 40330, 38837, 37450, 36158, 34953, 33826, 32768,
/// 31776, 30841, 29960, 29128, 28340, 27595, 26887, 26215,
/// 25576, 24967, 24386, 23832, 23302, 22796, 22311, 21846,
/// 21400, 20972, 20561, 20165, 19785, 19419, 19066, 18725,
/// };
/// uint8x16x4_t LoadStatic(void) {
/// uint8x16x4_t ret;
/// ret.val[0] = vld1q_u16(TableA + 0);
/// ret.val[1] = vld1q_u16(TableA + 8);
/// ret.val[2] = vld1q_u16(TableA + 16);
/// ret.val[3] = vld1q_u16(TableA + 24);
/// return ret;
/// }
/// The constants in this example are folded into the uses. Thus, 4 different
/// constants are created.
/// As their type is vector the cheapest way to create them is to load them
/// for the memory.
/// Therefore the final assembly final has 4 different loads. With this pass
/// enabled, only one load is issued for the constants.
class AArch64PromoteConstant : public ModulePass {
struct PromotedConstant {
bool ShouldConvert = false;
GlobalVariable *GV = nullptr;
using PromotionCacheTy = SmallDenseMap<Constant *, PromotedConstant, 16>;
struct UpdateRecord {
Constant *C;
Instruction *User;
unsigned Op;
UpdateRecord(Constant *C, Instruction *User, unsigned Op)
: C(C), User(User), Op(Op) {}
static char ID;
AArch64PromoteConstant() : ModulePass(ID) {
StringRef getPassName() const override { return "AArch64 Promote Constant"; }
/// Iterate over the functions and promote the interesting constants into
/// global variables with module scope.
bool runOnModule(Module &M) override {
LLVM_DEBUG(dbgs() << getPassName() << '\n');
if (skipModule(M))
return false;
bool Changed = false;
PromotionCacheTy PromotionCache;
for (auto &MF : M) {
Changed |= runOnFunction(MF, PromotionCache);
return Changed;
/// Look for interesting constants used within the given function.
/// Promote them into global variables, load these global variables within
/// the related function, so that the number of inserted load is minimal.
bool runOnFunction(Function &F, PromotionCacheTy &PromotionCache);
// This transformation requires dominator info
void getAnalysisUsage(AnalysisUsage &AU) const override {
/// Type to store a list of Uses.
using Uses = SmallVector<std::pair<Instruction *, unsigned>, 4>;
/// Map an insertion point to all the uses it dominates.
using InsertionPoints = DenseMap<Instruction *, Uses>;
/// Find the closest point that dominates the given Use.
Instruction *findInsertionPoint(Instruction &User, unsigned OpNo);
/// Check if the given insertion point is dominated by an existing
/// insertion point.
/// If true, the given use is added to the list of dominated uses for
/// the related existing point.
/// \param NewPt the insertion point to be checked
/// \param User the user of the constant
/// \param OpNo the operand number of the use
/// \param InsertPts existing insertion points
/// \pre NewPt and all instruction in InsertPts belong to the same function
/// \return true if one of the insertion point in InsertPts dominates NewPt,
/// false otherwise
bool isDominated(Instruction *NewPt, Instruction *User, unsigned OpNo,
InsertionPoints &InsertPts);
/// Check if the given insertion point can be merged with an existing
/// insertion point in a common dominator.
/// If true, the given use is added to the list of the created insertion
/// point.
/// \param NewPt the insertion point to be checked
/// \param User the user of the constant
/// \param OpNo the operand number of the use
/// \param InsertPts existing insertion points
/// \pre NewPt and all instruction in InsertPts belong to the same function
/// \pre isDominated returns false for the exact same parameters.
/// \return true if it exists an insertion point in InsertPts that could
/// have been merged with NewPt in a common dominator,
/// false otherwise
bool tryAndMerge(Instruction *NewPt, Instruction *User, unsigned OpNo,
InsertionPoints &InsertPts);
/// Compute the minimal insertion points to dominates all the interesting
/// uses of value.
/// Insertion points are group per function and each insertion point
/// contains a list of all the uses it dominates within the related function
/// \param User the user of the constant
/// \param OpNo the operand number of the constant
/// \param[out] InsertPts output storage of the analysis
void computeInsertionPoint(Instruction *User, unsigned OpNo,
InsertionPoints &InsertPts);
/// Insert a definition of a new global variable at each point contained in
/// InsPtsPerFunc and update the related uses (also contained in
/// InsPtsPerFunc).
void insertDefinitions(Function &F, GlobalVariable &GV,
InsertionPoints &InsertPts);
/// Do the constant promotion indicated by the Updates records, keeping track
/// of globals in PromotionCache.
void promoteConstants(Function &F, SmallVectorImpl<UpdateRecord> &Updates,
PromotionCacheTy &PromotionCache);
/// Transfer the list of dominated uses of IPI to NewPt in InsertPts.
/// Append Use to this list and delete the entry of IPI in InsertPts.
static void appendAndTransferDominatedUses(Instruction *NewPt,
Instruction *User, unsigned OpNo,
InsertionPoints::iterator &IPI,
InsertionPoints &InsertPts) {
// Record the dominated use.
IPI->second.emplace_back(User, OpNo);
// Transfer the dominated uses of IPI to NewPt
// Inserting into the DenseMap may invalidate existing iterator.
// Keep a copy of the key to find the iterator to erase. Keep a copy of the
// value so that we don't have to dereference IPI->second.
Instruction *OldInstr = IPI->first;
Uses OldUses = std::move(IPI->second);
InsertPts[NewPt] = std::move(OldUses);
// Erase IPI.
} // end anonymous namespace
char AArch64PromoteConstant::ID = 0;
INITIALIZE_PASS_BEGIN(AArch64PromoteConstant, "aarch64-promote-const",
"AArch64 Promote Constant Pass", false, false)
INITIALIZE_PASS_END(AArch64PromoteConstant, "aarch64-promote-const",
"AArch64 Promote Constant Pass", false, false)
ModulePass *llvm::createAArch64PromoteConstantPass() {
return new AArch64PromoteConstant();
/// Check if the given type uses a vector type.
static bool isConstantUsingVectorTy(const Type *CstTy) {
if (CstTy->isVectorTy())
return true;
if (CstTy->isStructTy()) {
for (unsigned EltIdx = 0, EndEltIdx = CstTy->getStructNumElements();
EltIdx < EndEltIdx; ++EltIdx)
if (isConstantUsingVectorTy(CstTy->getStructElementType(EltIdx)))
return true;
} else if (CstTy->isArrayTy())
return isConstantUsingVectorTy(CstTy->getArrayElementType());
return false;
// Returns true if \p C contains only ConstantData leafs and no global values,
// block addresses or constant expressions. Traverses ConstantAggregates.
static bool containsOnlyConstantData(const Constant *C) {
if (isa<ConstantData>(C))
return true;
if (isa<GlobalValue>(C) || isa<BlockAddress>(C) || isa<ConstantExpr>(C))
return false;
return all_of(C->operands(), [](const Use &U) {
return containsOnlyConstantData(cast<Constant>(&U));
/// Check if the given use (Instruction + OpIdx) of Cst should be converted into
/// a load of a global variable initialized with Cst.
/// A use should be converted if it is legal to do so.
/// For instance, it is not legal to turn the mask operand of a shuffle vector
/// into a load of a global variable.
static bool shouldConvertUse(const Constant *Cst, const Instruction *Instr,
unsigned OpIdx) {
// shufflevector instruction expects a const for the mask argument, i.e., the
// third argument. Do not promote this use in that case.
if (isa<const ShuffleVectorInst>(Instr) && OpIdx == 2)
return false;
// extractvalue instruction expects a const idx.
if (isa<const ExtractValueInst>(Instr) && OpIdx > 0)
return false;
// extractvalue instruction expects a const idx.
if (isa<const InsertValueInst>(Instr) && OpIdx > 1)
return false;
if (isa<const AllocaInst>(Instr) && OpIdx > 0)
return false;
// Alignment argument must be constant.
if (isa<const LoadInst>(Instr) && OpIdx > 0)
return false;
// Alignment argument must be constant.
if (isa<const StoreInst>(Instr) && OpIdx > 1)
return false;
// Index must be constant.
if (isa<const GetElementPtrInst>(Instr) && OpIdx > 0)
return false;
// Personality function and filters must be constant.
// Give up on that instruction.
if (isa<const LandingPadInst>(Instr))
return false;
// Switch instruction expects constants to compare to.
if (isa<const SwitchInst>(Instr))
return false;
// Expected address must be a constant.
if (isa<const IndirectBrInst>(Instr))
return false;
// Do not mess with intrinsics.
if (isa<const IntrinsicInst>(Instr))
return false;
// Do not mess with inline asm.
const CallInst *CI = dyn_cast<const CallInst>(Instr);
return !(CI && CI->isInlineAsm());
/// Check if the given Cst should be converted into
/// a load of a global variable initialized with Cst.
/// A constant should be converted if it is likely that the materialization of
/// the constant will be tricky. Thus, we give up on zero or undef values.
/// \todo Currently, accept only vector related types.
/// Also we give up on all simple vector type to keep the existing
/// behavior. Otherwise, we should push here all the check of the lowering of
/// BUILD_VECTOR. By giving up, we lose the potential benefit of merging
/// constant via global merge and the fact that the same constant is stored
/// only once with this method (versus, as many function that uses the constant
/// for the regular approach, even for float).
/// Again, the simplest solution would be to promote every
/// constant and rematerialize them when they are actually cheap to create.
static bool shouldConvertImpl(const Constant *Cst) {
if (isa<const UndefValue>(Cst))
return false;
// FIXME: In some cases, it may be interesting to promote in memory
// a zero initialized constant.
// E.g., when the type of Cst require more instructions than the
// adrp/add/load sequence or when this sequence can be shared by several
// instances of Cst.
// Ideally, we could promote this into a global and rematerialize the constant
// when it was a bad idea.
if (Cst->isZeroValue())
return false;
if (Stress)
return true;
// FIXME: see function \todo
if (Cst->getType()->isVectorTy())
return false;
return isConstantUsingVectorTy(Cst->getType());
static bool
shouldConvert(Constant &C,
AArch64PromoteConstant::PromotionCacheTy &PromotionCache) {
auto Converted = PromotionCache.insert(
std::make_pair(&C, AArch64PromoteConstant::PromotedConstant()));
if (Converted.second)
Converted.first->second.ShouldConvert = shouldConvertImpl(&C);
return Converted.first->second.ShouldConvert;
Instruction *AArch64PromoteConstant::findInsertionPoint(Instruction &User,
unsigned OpNo) {
// If this user is a phi, the insertion point is in the related
// incoming basic block.
if (PHINode *PhiInst = dyn_cast<PHINode>(&User))
return PhiInst->getIncomingBlock(OpNo)->getTerminator();
return &User;
bool AArch64PromoteConstant::isDominated(Instruction *NewPt, Instruction *User,
unsigned OpNo,
InsertionPoints &InsertPts) {
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>(
// Traverse all the existing insertion points and check if one is dominating
// NewPt. If it is, remember that.
for (auto &IPI : InsertPts) {
if (NewPt == IPI.first || DT.dominates(IPI.first, NewPt) ||
// When IPI.first is a terminator instruction, DT may think that
// the result is defined on the edge.
// Here we are testing the insertion point, not the definition.
(IPI.first->getParent() != NewPt->getParent() &&
DT.dominates(IPI.first->getParent(), NewPt->getParent()))) {
// No need to insert this point. Just record the dominated use.
LLVM_DEBUG(dbgs() << "Insertion point dominated by:\n");
LLVM_DEBUG(dbgs() << '\n');
IPI.second.emplace_back(User, OpNo);
return true;
return false;
bool AArch64PromoteConstant::tryAndMerge(Instruction *NewPt, Instruction *User,
unsigned OpNo,
InsertionPoints &InsertPts) {
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>(
BasicBlock *NewBB = NewPt->getParent();
// Traverse all the existing insertion point and check if one is dominated by
// NewPt and thus useless or can be combined with NewPt into a common
// dominator.
for (InsertionPoints::iterator IPI = InsertPts.begin(),
EndIPI = InsertPts.end();
IPI != EndIPI; ++IPI) {
BasicBlock *CurBB = IPI->first->getParent();
if (NewBB == CurBB) {
// Instructions are in the same block.
// By construction, NewPt is dominating the other.
// Indeed, isDominated returned false with the exact same arguments.
LLVM_DEBUG(dbgs() << "Merge insertion point with:\n");
LLVM_DEBUG(dbgs() << "\nat considered insertion point.\n");
appendAndTransferDominatedUses(NewPt, User, OpNo, IPI, InsertPts);
return true;
// Look for a common dominator
BasicBlock *CommonDominator = DT.findNearestCommonDominator(NewBB, CurBB);
// If none exists, we cannot merge these two points.
if (!CommonDominator)
if (CommonDominator != NewBB) {
// By construction, the CommonDominator cannot be CurBB.
assert(CommonDominator != CurBB &&
"Instruction has not been rejected during isDominated check!");
// Take the last instruction of the CommonDominator as insertion point
NewPt = CommonDominator->getTerminator();
// else, CommonDominator is the block of NewBB, hence NewBB is the last
// possible insertion point in that block.
LLVM_DEBUG(dbgs() << "Merge insertion point with:\n");
LLVM_DEBUG(dbgs() << '\n');
LLVM_DEBUG(dbgs() << '\n');
appendAndTransferDominatedUses(NewPt, User, OpNo, IPI, InsertPts);
return true;
return false;
void AArch64PromoteConstant::computeInsertionPoint(
Instruction *User, unsigned OpNo, InsertionPoints &InsertPts) {
LLVM_DEBUG(dbgs() << "Considered use, opidx " << OpNo << ":\n");
LLVM_DEBUG(dbgs() << '\n');
Instruction *InsertionPoint = findInsertionPoint(*User, OpNo);
LLVM_DEBUG(dbgs() << "Considered insertion point:\n");
LLVM_DEBUG(dbgs() << '\n');
if (isDominated(InsertionPoint, User, OpNo, InsertPts))
// This insertion point is useful, check if we can merge some insertion
// point in a common dominator or if NewPt dominates an existing one.
if (tryAndMerge(InsertionPoint, User, OpNo, InsertPts))
LLVM_DEBUG(dbgs() << "Keep considered insertion point\n");
// It is definitely useful by its own
InsertPts[InsertionPoint].emplace_back(User, OpNo);
static void ensurePromotedGV(Function &F, Constant &C,
AArch64PromoteConstant::PromotedConstant &PC) {
assert(PC.ShouldConvert &&
"Expected that we should convert this to a global");
if (PC.GV)
PC.GV = new GlobalVariable(
*F.getParent(), C.getType(), true, GlobalValue::InternalLinkage, nullptr,
"_PromotedConst", nullptr, GlobalVariable::NotThreadLocal);
LLVM_DEBUG(dbgs() << "Global replacement: ");
LLVM_DEBUG(dbgs() << '\n');
void AArch64PromoteConstant::insertDefinitions(Function &F,
GlobalVariable &PromotedGV,
InsertionPoints &InsertPts) {
#ifndef NDEBUG
// Do more checking for debug purposes.
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>(F).getDomTree();
assert(!InsertPts.empty() && "Empty uses does not need a definition");
for (const auto &IPI : InsertPts) {
// Create the load of the global variable.
IRBuilder<> Builder(IPI.first);
LoadInst *LoadedCst =
Builder.CreateLoad(PromotedGV.getValueType(), &PromotedGV);
LLVM_DEBUG(dbgs() << "**********\n");
LLVM_DEBUG(dbgs() << "New def: ");
LLVM_DEBUG(dbgs() << '\n');
// Update the dominated uses.
for (auto Use : IPI.second) {
#ifndef NDEBUG
findInsertionPoint(*Use.first, Use.second)) &&
"Inserted definition does not dominate all its uses!");
dbgs() << "Use to update " << Use.second << ":";
dbgs() << '\n';
Use.first->setOperand(Use.second, LoadedCst);
void AArch64PromoteConstant::promoteConstants(
Function &F, SmallVectorImpl<UpdateRecord> &Updates,
PromotionCacheTy &PromotionCache) {
// Promote the constants.
for (auto U = Updates.begin(), E = Updates.end(); U != E;) {
LLVM_DEBUG(dbgs() << "** Compute insertion points **\n");
auto First = U;
Constant *C = First->C;
InsertionPoints InsertPts;
do {
computeInsertionPoint(U->User, U->Op, InsertPts);
} while (++U != E && U->C == C);
auto &Promotion = PromotionCache[C];
ensurePromotedGV(F, *C, Promotion);
insertDefinitions(F, *Promotion.GV, InsertPts);
bool AArch64PromoteConstant::runOnFunction(Function &F,
PromotionCacheTy &PromotionCache) {
// Look for instructions using constant vector. Promote that constant to a
// global variable. Create as few loads of this variable as possible and
// update the uses accordingly.
SmallVector<UpdateRecord, 64> Updates;
for (Instruction &I : instructions(&F)) {
// Traverse the operand, looking for constant vectors. Replace them by a
// load of a global variable of constant vector type.
for (Use &U : I.operands()) {
Constant *Cst = dyn_cast<Constant>(U);
// There is no point in promoting global values as they are already
// global. Do not promote constants containing constant expression, global
// values or blockaddresses either, as they may require some code
// expansion.
if (!Cst || isa<GlobalValue>(Cst) || !containsOnlyConstantData(Cst))
// Check if this constant is worth promoting.
if (!shouldConvert(*Cst, PromotionCache))
// Check if this use should be promoted.
unsigned OpNo = &U - I.op_begin();
if (!shouldConvertUse(Cst, &I, OpNo))
Updates.emplace_back(Cst, &I, OpNo);
if (Updates.empty())
return false;
promoteConstants(F, Updates, PromotionCache);
return true;