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//===- SSAUpdater.cpp - Unstructured SSA Update Tool ----------------------===//
// 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 SSAUpdater class.
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/TinyPtrVector.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
#include <cassert>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "ssaupdater"
using AvailableValsTy = DenseMap<BasicBlock *, Value *>;
static AvailableValsTy &getAvailableVals(void *AV) {
return *static_cast<AvailableValsTy*>(AV);
SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode *> *NewPHI)
: InsertedPHIs(NewPHI) {}
SSAUpdater::~SSAUpdater() {
delete static_cast<AvailableValsTy*>(AV);
void SSAUpdater::Initialize(Type *Ty, StringRef Name) {
if (!AV)
AV = new AvailableValsTy();
ProtoType = Ty;
ProtoName = std::string(Name);
bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
return getAvailableVals(AV).count(BB);
Value *SSAUpdater::FindValueForBlock(BasicBlock *BB) const {
return getAvailableVals(AV).lookup(BB);
void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
assert(ProtoType && "Need to initialize SSAUpdater");
assert(ProtoType == V->getType() &&
"All rewritten values must have the same type");
getAvailableVals(AV)[BB] = V;
static bool IsEquivalentPHI(PHINode *PHI,
SmallDenseMap<BasicBlock *, Value *, 8> &ValueMapping) {
unsigned PHINumValues = PHI->getNumIncomingValues();
if (PHINumValues != ValueMapping.size())
return false;
// Scan the phi to see if it matches.
for (unsigned i = 0, e = PHINumValues; i != e; ++i)
if (ValueMapping[PHI->getIncomingBlock(i)] !=
PHI->getIncomingValue(i)) {
return false;
return true;
Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
Value *Res = GetValueAtEndOfBlockInternal(BB);
return Res;
Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
// If there is no definition of the renamed variable in this block, just use
// GetValueAtEndOfBlock to do our work.
if (!HasValueForBlock(BB))
return GetValueAtEndOfBlock(BB);
// Otherwise, we have the hard case. Get the live-in values for each
// predecessor.
SmallVector<std::pair<BasicBlock *, Value *>, 8> PredValues;
Value *SingularValue = nullptr;
// We can get our predecessor info by walking the pred_iterator list, but it
// is relatively slow. If we already have PHI nodes in this block, walk one
// of them to get the predecessor list instead.
if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
Value *PredVal = GetValueAtEndOfBlock(PredBB);
PredValues.push_back(std::make_pair(PredBB, PredVal));
// Compute SingularValue.
if (i == 0)
SingularValue = PredVal;
else if (PredVal != SingularValue)
SingularValue = nullptr;
} else {
bool isFirstPred = true;
for (BasicBlock *PredBB : predecessors(BB)) {
Value *PredVal = GetValueAtEndOfBlock(PredBB);
PredValues.push_back(std::make_pair(PredBB, PredVal));
// Compute SingularValue.
if (isFirstPred) {
SingularValue = PredVal;
isFirstPred = false;
} else if (PredVal != SingularValue)
SingularValue = nullptr;
// If there are no predecessors, just return undef.
if (PredValues.empty())
return UndefValue::get(ProtoType);
// Otherwise, if all the merged values are the same, just use it.
if (SingularValue)
return SingularValue;
// Otherwise, we do need a PHI: check to see if we already have one available
// in this block that produces the right value.
if (isa<PHINode>(BB->begin())) {
SmallDenseMap<BasicBlock *, Value *, 8> ValueMapping(PredValues.begin(),
for (PHINode &SomePHI : BB->phis()) {
if (IsEquivalentPHI(&SomePHI, ValueMapping))
return &SomePHI;
// Ok, we have no way out, insert a new one now.
PHINode *InsertedPHI = PHINode::Create(ProtoType, PredValues.size(),
ProtoName, &BB->front());
// Fill in all the predecessors of the PHI.
for (const auto &PredValue : PredValues)
InsertedPHI->addIncoming(PredValue.second, PredValue.first);
// See if the PHI node can be merged to a single value. This can happen in
// loop cases when we get a PHI of itself and one other value.
if (Value *V =
SimplifyInstruction(InsertedPHI, BB->getModule()->getDataLayout())) {
return V;
// Set the DebugLoc of the inserted PHI, if available.
DebugLoc DL;
if (const Instruction *I = BB->getFirstNonPHI())
DL = I->getDebugLoc();
// If the client wants to know about all new instructions, tell it.
if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
LLVM_DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n");
return InsertedPHI;
void SSAUpdater::RewriteUse(Use &U) {
Instruction *User = cast<Instruction>(U.getUser());
Value *V;
if (PHINode *UserPN = dyn_cast<PHINode>(User))
V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
V = GetValueInMiddleOfBlock(User->getParent());
void SSAUpdater::RewriteUseAfterInsertions(Use &U) {
Instruction *User = cast<Instruction>(U.getUser());
Value *V;
if (PHINode *UserPN = dyn_cast<PHINode>(User))
V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
V = GetValueAtEndOfBlock(User->getParent());
namespace llvm {
class SSAUpdaterTraits<SSAUpdater> {
using BlkT = BasicBlock;
using ValT = Value *;
using PhiT = PHINode;
using BlkSucc_iterator = succ_iterator;
static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); }
static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); }
class PHI_iterator {
unsigned idx;
explicit PHI_iterator(PHINode *P) // begin iterator
: PHI(P), idx(0) {}
PHI_iterator(PHINode *P, bool) // end iterator
: PHI(P), idx(PHI->getNumIncomingValues()) {}
PHI_iterator &operator++() { ++idx; return *this; }
bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
Value *getIncomingValue() { return PHI->getIncomingValue(idx); }
BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); }
static PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
static PHI_iterator PHI_end(PhiT *PHI) {
return PHI_iterator(PHI, true);
/// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds
/// vector, set Info->NumPreds, and allocate space in Info->Preds.
static void FindPredecessorBlocks(BasicBlock *BB,
SmallVectorImpl<BasicBlock *> *Preds) {
// We can get our predecessor info by walking the pred_iterator list,
// but it is relatively slow. If we already have PHI nodes in this
// block, walk one of them to get the predecessor list instead.
if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin()))
append_range(*Preds, SomePhi->blocks());
append_range(*Preds, predecessors(BB));
/// GetUndefVal - Get an undefined value of the same type as the value
/// being handled.
static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) {
return UndefValue::get(Updater->ProtoType);
/// CreateEmptyPHI - Create a new PHI instruction in the specified block.
/// Reserve space for the operands but do not fill them in yet.
static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds,
SSAUpdater *Updater) {
PHINode *PHI = PHINode::Create(Updater->ProtoType, NumPreds,
Updater->ProtoName, &BB->front());
return PHI;
/// AddPHIOperand - Add the specified value as an operand of the PHI for
/// the specified predecessor block.
static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) {
PHI->addIncoming(Val, Pred);
/// ValueIsPHI - Check if a value is a PHI.
static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) {
return dyn_cast<PHINode>(Val);
/// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
/// operands, i.e., it was just added.
static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) {
PHINode *PHI = ValueIsPHI(Val, Updater);
if (PHI && PHI->getNumIncomingValues() == 0)
return PHI;
return nullptr;
/// GetPHIValue - For the specified PHI instruction, return the value
/// that it defines.
static Value *GetPHIValue(PHINode *PHI) {
return PHI;
} // end namespace llvm
/// Check to see if AvailableVals has an entry for the specified BB and if so,
/// return it. If not, construct SSA form by first calculating the required
/// placement of PHIs and then inserting new PHIs where needed.
Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
AvailableValsTy &AvailableVals = getAvailableVals(AV);
if (Value *V = AvailableVals[BB])
return V;
SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
return Impl.GetValue(BB);
// LoadAndStorePromoter Implementation
LoadAndStorePromoter(ArrayRef<const Instruction *> Insts,
SSAUpdater &S, StringRef BaseName) : SSA(S) {
if (Insts.empty()) return;
const Value *SomeVal;
if (const LoadInst *LI = dyn_cast<LoadInst>(Insts[0]))
SomeVal = LI;
SomeVal = cast<StoreInst>(Insts[0])->getOperand(0);
if (BaseName.empty())
BaseName = SomeVal->getName();
SSA.Initialize(SomeVal->getType(), BaseName);
void LoadAndStorePromoter::run(const SmallVectorImpl<Instruction *> &Insts) {
// First step: bucket up uses of the alloca by the block they occur in.
// This is important because we have to handle multiple defs/uses in a block
// ourselves: SSAUpdater is purely for cross-block references.
DenseMap<BasicBlock *, TinyPtrVector<Instruction *>> UsesByBlock;
for (Instruction *User : Insts)
// Okay, now we can iterate over all the blocks in the function with uses,
// processing them. Keep track of which loads are loading a live-in value.
// Walk the uses in the use-list order to be determinstic.
SmallVector<LoadInst *, 32> LiveInLoads;
DenseMap<Value *, Value *> ReplacedLoads;
for (Instruction *User : Insts) {
BasicBlock *BB = User->getParent();
TinyPtrVector<Instruction *> &BlockUses = UsesByBlock[BB];
// If this block has already been processed, ignore this repeat use.
if (BlockUses.empty()) continue;
// Okay, this is the first use in the block. If this block just has a
// single user in it, we can rewrite it trivially.
if (BlockUses.size() == 1) {
// If it is a store, it is a trivial def of the value in the block.
if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
SSA.AddAvailableValue(BB, SI->getOperand(0));
} else
// Otherwise it is a load, queue it to rewrite as a live-in load.
// Otherwise, check to see if this block is all loads.
bool HasStore = false;
for (Instruction *I : BlockUses) {
if (isa<StoreInst>(I)) {
HasStore = true;
// If so, we can queue them all as live in loads. We don't have an
// efficient way to tell which on is first in the block and don't want to
// scan large blocks, so just add all loads as live ins.
if (!HasStore) {
for (Instruction *I : BlockUses)
// Otherwise, we have mixed loads and stores (or just a bunch of stores).
// Since SSAUpdater is purely for cross-block values, we need to determine
// the order of these instructions in the block. If the first use in the
// block is a load, then it uses the live in value. The last store defines
// the live out value. We handle this by doing a linear scan of the block.
Value *StoredValue = nullptr;
for (Instruction &I : *BB) {
if (LoadInst *L = dyn_cast<LoadInst>(&I)) {
// If this is a load from an unrelated pointer, ignore it.
if (!isInstInList(L, Insts)) continue;
// If we haven't seen a store yet, this is a live in use, otherwise
// use the stored value.
if (StoredValue) {
replaceLoadWithValue(L, StoredValue);
ReplacedLoads[L] = StoredValue;
} else {
if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
// If this is a store to an unrelated pointer, ignore it.
if (!isInstInList(SI, Insts)) continue;
// Remember that this is the active value in the block.
StoredValue = SI->getOperand(0);
// The last stored value that happened is the live-out for the block.
assert(StoredValue && "Already checked that there is a store in block");
SSA.AddAvailableValue(BB, StoredValue);
// Okay, now we rewrite all loads that use live-in values in the loop,
// inserting PHI nodes as necessary.
for (LoadInst *ALoad : LiveInLoads) {
Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
replaceLoadWithValue(ALoad, NewVal);
// Avoid assertions in unreachable code.
if (NewVal == ALoad) NewVal = UndefValue::get(NewVal->getType());
ReplacedLoads[ALoad] = NewVal;
// Allow the client to do stuff before we start nuking things.
// Now that everything is rewritten, delete the old instructions from the
// function. They should all be dead now.
for (Instruction *User : Insts) {
// If this is a load that still has uses, then the load must have been added
// as a live value in the SSAUpdate data structure for a block (e.g. because
// the loaded value was stored later). In this case, we need to recursively
// propagate the updates until we get to the real value.
if (!User->use_empty()) {
Value *NewVal = ReplacedLoads[User];
assert(NewVal && "not a replaced load?");
// Propagate down to the ultimate replacee. The intermediately loads
// could theoretically already have been deleted, so we don't want to
// dereference the Value*'s.
DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
while (RLI != ReplacedLoads.end()) {
NewVal = RLI->second;
RLI = ReplacedLoads.find(NewVal);
replaceLoadWithValue(cast<LoadInst>(User), NewVal);
LoadAndStorePromoter::isInstInList(Instruction *I,
const SmallVectorImpl<Instruction *> &Insts)
const {
return is_contained(Insts, I);