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//===- PromoteMemoryToRegister.cpp - Convert allocas to registers ---------===//
// 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 promotes memory references to be register references. It promotes
// alloca instructions which only have loads and stores as uses. An alloca is
// transformed by using iterated dominator frontiers to place PHI nodes, then
// traversing the function in depth-first order to rewrite loads and stores as
// appropriate.
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/TinyPtrVector.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/IteratedDominanceFrontier.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/Support/Casting.h"
#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "mem2reg"
STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block");
STATISTIC(NumSingleStore, "Number of alloca's promoted with a single store");
STATISTIC(NumDeadAlloca, "Number of dead alloca's removed");
STATISTIC(NumPHIInsert, "Number of PHI nodes inserted");
bool llvm::isAllocaPromotable(const AllocaInst *AI) {
// FIXME: If the memory unit is of pointer or integer type, we can permit
// assignments to subsections of the memory unit.
unsigned AS = AI->getType()->getAddressSpace();
// Only allow direct and non-volatile loads and stores...
for (const User *U : AI->users()) {
if (const LoadInst *LI = dyn_cast<LoadInst>(U)) {
// Note that atomic loads can be transformed; atomic semantics do
// not have any meaning for a local alloca.
if (LI->isVolatile())
return false;
} else if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
if (SI->getOperand(0) == AI)
return false; // Don't allow a store OF the AI, only INTO the AI.
// Note that atomic stores can be transformed; atomic semantics do
// not have any meaning for a local alloca.
if (SI->isVolatile())
return false;
} else if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) {
if (!II->isLifetimeStartOrEnd())
return false;
} else if (const BitCastInst *BCI = dyn_cast<BitCastInst>(U)) {
if (BCI->getType() != Type::getInt8PtrTy(U->getContext(), AS))
return false;
if (!onlyUsedByLifetimeMarkers(BCI))
return false;
} else if (const GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(U)) {
if (GEPI->getType() != Type::getInt8PtrTy(U->getContext(), AS))
return false;
if (!GEPI->hasAllZeroIndices())
return false;
if (!onlyUsedByLifetimeMarkers(GEPI))
return false;
} else {
return false;
return true;
namespace {
struct AllocaInfo {
SmallVector<BasicBlock *, 32> DefiningBlocks;
SmallVector<BasicBlock *, 32> UsingBlocks;
StoreInst *OnlyStore;
BasicBlock *OnlyBlock;
bool OnlyUsedInOneBlock;
TinyPtrVector<DbgVariableIntrinsic *> DbgDeclares;
void clear() {
OnlyStore = nullptr;
OnlyBlock = nullptr;
OnlyUsedInOneBlock = true;
/// Scan the uses of the specified alloca, filling in the AllocaInfo used
/// by the rest of the pass to reason about the uses of this alloca.
void AnalyzeAlloca(AllocaInst *AI) {
// As we scan the uses of the alloca instruction, keep track of stores,
// and decide whether all of the loads and stores to the alloca are within
// the same basic block.
for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
Instruction *User = cast<Instruction>(*UI++);
if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
// Remember the basic blocks which define new values for the alloca
OnlyStore = SI;
} else {
LoadInst *LI = cast<LoadInst>(User);
// Otherwise it must be a load instruction, keep track of variable
// reads.
if (OnlyUsedInOneBlock) {
if (!OnlyBlock)
OnlyBlock = User->getParent();
else if (OnlyBlock != User->getParent())
OnlyUsedInOneBlock = false;
DbgDeclares = FindDbgAddrUses(AI);
/// Data package used by RenamePass().
struct RenamePassData {
using ValVector = std::vector<Value *>;
using LocationVector = std::vector<DebugLoc>;
RenamePassData(BasicBlock *B, BasicBlock *P, ValVector V, LocationVector L)
: BB(B), Pred(P), Values(std::move(V)), Locations(std::move(L)) {}
BasicBlock *BB;
BasicBlock *Pred;
ValVector Values;
LocationVector Locations;
/// This assigns and keeps a per-bb relative ordering of load/store
/// instructions in the block that directly load or store an alloca.
/// This functionality is important because it avoids scanning large basic
/// blocks multiple times when promoting many allocas in the same block.
class LargeBlockInfo {
/// For each instruction that we track, keep the index of the
/// instruction.
/// The index starts out as the number of the instruction from the start of
/// the block.
DenseMap<const Instruction *, unsigned> InstNumbers;
/// This code only looks at accesses to allocas.
static bool isInterestingInstruction(const Instruction *I) {
return (isa<LoadInst>(I) && isa<AllocaInst>(I->getOperand(0))) ||
(isa<StoreInst>(I) && isa<AllocaInst>(I->getOperand(1)));
/// Get or calculate the index of the specified instruction.
unsigned getInstructionIndex(const Instruction *I) {
assert(isInterestingInstruction(I) &&
"Not a load/store to/from an alloca?");
// If we already have this instruction number, return it.
DenseMap<const Instruction *, unsigned>::iterator It = InstNumbers.find(I);
if (It != InstNumbers.end())
return It->second;
// Scan the whole block to get the instruction. This accumulates
// information for every interesting instruction in the block, in order to
// avoid gratuitus rescans.
const BasicBlock *BB = I->getParent();
unsigned InstNo = 0;
for (const Instruction &BBI : *BB)
if (isInterestingInstruction(&BBI))
InstNumbers[&BBI] = InstNo++;
It = InstNumbers.find(I);
assert(It != InstNumbers.end() && "Didn't insert instruction?");
return It->second;
void deleteValue(const Instruction *I) { InstNumbers.erase(I); }
void clear() { InstNumbers.clear(); }
struct PromoteMem2Reg {
/// The alloca instructions being promoted.
std::vector<AllocaInst *> Allocas;
DominatorTree &DT;
DIBuilder DIB;
/// A cache of @llvm.assume intrinsics used by SimplifyInstruction.
AssumptionCache *AC;
const SimplifyQuery SQ;
/// Reverse mapping of Allocas.
DenseMap<AllocaInst *, unsigned> AllocaLookup;
/// The PhiNodes we're adding.
/// That map is used to simplify some Phi nodes as we iterate over it, so
/// it should have deterministic iterators. We could use a MapVector, but
/// since we already maintain a map from BasicBlock* to a stable numbering
/// (BBNumbers), the DenseMap is more efficient (also supports removal).
DenseMap<std::pair<unsigned, unsigned>, PHINode *> NewPhiNodes;
/// For each PHI node, keep track of which entry in Allocas it corresponds
/// to.
DenseMap<PHINode *, unsigned> PhiToAllocaMap;
/// For each alloca, we keep track of the dbg.declare intrinsic that
/// describes it, if any, so that we can convert it to a dbg.value
/// intrinsic if the alloca gets promoted.
SmallVector<TinyPtrVector<DbgVariableIntrinsic *>, 8> AllocaDbgDeclares;
/// The set of basic blocks the renamer has already visited.
SmallPtrSet<BasicBlock *, 16> Visited;
/// Contains a stable numbering of basic blocks to avoid non-determinstic
/// behavior.
DenseMap<BasicBlock *, unsigned> BBNumbers;
/// Lazily compute the number of predecessors a block has.
DenseMap<const BasicBlock *, unsigned> BBNumPreds;
PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
AssumptionCache *AC)
: Allocas(Allocas.begin(), Allocas.end()), DT(DT),
DIB(*DT.getRoot()->getParent()->getParent(), /*AllowUnresolved*/ false),
AC(AC), SQ(DT.getRoot()->getParent()->getParent()->getDataLayout(),
nullptr, &DT, AC) {}
void run();
void RemoveFromAllocasList(unsigned &AllocaIdx) {
Allocas[AllocaIdx] = Allocas.back();
unsigned getNumPreds(const BasicBlock *BB) {
unsigned &NP = BBNumPreds[BB];
if (NP == 0)
NP = pred_size(BB) + 1;
return NP - 1;
void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info,
const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
SmallPtrSetImpl<BasicBlock *> &LiveInBlocks);
void RenamePass(BasicBlock *BB, BasicBlock *Pred,
RenamePassData::ValVector &IncVals,
RenamePassData::LocationVector &IncLocs,
std::vector<RenamePassData> &Worklist);
bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version);
} // end anonymous namespace
/// Given a LoadInst LI this adds assume(LI != null) after it.
static void addAssumeNonNull(AssumptionCache *AC, LoadInst *LI) {
Function *AssumeIntrinsic =
Intrinsic::getDeclaration(LI->getModule(), Intrinsic::assume);
ICmpInst *LoadNotNull = new ICmpInst(ICmpInst::ICMP_NE, LI,
CallInst *CI = CallInst::Create(AssumeIntrinsic, {LoadNotNull});
static void removeLifetimeIntrinsicUsers(AllocaInst *AI) {
// Knowing that this alloca is promotable, we know that it's safe to kill all
// instructions except for load and store.
for (auto UI = AI->user_begin(), UE = AI->user_end(); UI != UE;) {
Instruction *I = cast<Instruction>(*UI);
if (isa<LoadInst>(I) || isa<StoreInst>(I))
if (!I->getType()->isVoidTy()) {
// The only users of this bitcast/GEP instruction are lifetime intrinsics.
// Follow the use/def chain to erase them now instead of leaving it for
// dead code elimination later.
for (auto UUI = I->user_begin(), UUE = I->user_end(); UUI != UUE;) {
Instruction *Inst = cast<Instruction>(*UUI);
/// Rewrite as many loads as possible given a single store.
/// When there is only a single store, we can use the domtree to trivially
/// replace all of the dominated loads with the stored value. Do so, and return
/// true if this has successfully promoted the alloca entirely. If this returns
/// false there were some loads which were not dominated by the single store
/// and thus must be phi-ed with undef. We fall back to the standard alloca
/// promotion algorithm in that case.
static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info,
LargeBlockInfo &LBI, const DataLayout &DL,
DominatorTree &DT, AssumptionCache *AC) {
StoreInst *OnlyStore = Info.OnlyStore;
bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0));
BasicBlock *StoreBB = OnlyStore->getParent();
int StoreIndex = -1;
// Clear out UsingBlocks. We will reconstruct it here if needed.
for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
Instruction *UserInst = cast<Instruction>(*UI++);
if (UserInst == OnlyStore)
LoadInst *LI = cast<LoadInst>(UserInst);
// Okay, if we have a load from the alloca, we want to replace it with the
// only value stored to the alloca. We can do this if the value is
// dominated by the store. If not, we use the rest of the mem2reg machinery
// to insert the phi nodes as needed.
if (!StoringGlobalVal) { // Non-instructions are always dominated.
if (LI->getParent() == StoreBB) {
// If we have a use that is in the same block as the store, compare the
// indices of the two instructions to see which one came first. If the
// load came before the store, we can't handle it.
if (StoreIndex == -1)
StoreIndex = LBI.getInstructionIndex(OnlyStore);
if (unsigned(StoreIndex) > LBI.getInstructionIndex(LI)) {
// Can't handle this load, bail out.
} else if (!DT.dominates(StoreBB, LI->getParent())) {
// If the load and store are in different blocks, use BB dominance to
// check their relationships. If the store doesn't dom the use, bail
// out.
// Otherwise, we *can* safely rewrite this load.
Value *ReplVal = OnlyStore->getOperand(0);
// If the replacement value is the load, this must occur in unreachable
// code.
if (ReplVal == LI)
ReplVal = UndefValue::get(LI->getType());
// If the load was marked as nonnull we don't want to lose
// that information when we erase this Load. So we preserve
// it with an assume.
if (AC && LI->getMetadata(LLVMContext::MD_nonnull) &&
!isKnownNonZero(ReplVal, DL, 0, AC, LI, &DT))
addAssumeNonNull(AC, LI);
// Finally, after the scan, check to see if the store is all that is left.
if (!Info.UsingBlocks.empty())
return false; // If not, we'll have to fall back for the remainder.
// Record debuginfo for the store and remove the declaration's
// debuginfo.
for (DbgVariableIntrinsic *DII : Info.DbgDeclares) {
DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false);
ConvertDebugDeclareToDebugValue(DII, Info.OnlyStore, DIB);
// Remove the (now dead) store and alloca.
return true;
/// Many allocas are only used within a single basic block. If this is the
/// case, avoid traversing the CFG and inserting a lot of potentially useless
/// PHI nodes by just performing a single linear pass over the basic block
/// using the Alloca.
/// If we cannot promote this alloca (because it is read before it is written),
/// return false. This is necessary in cases where, due to control flow, the
/// alloca is undefined only on some control flow paths. e.g. code like
/// this is correct in LLVM IR:
/// // A is an alloca with no stores so far
/// for (...) {
/// int t = *A;
/// if (!first_iteration)
/// use(t);
/// *A = 42;
/// }
static bool promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info,
LargeBlockInfo &LBI,
const DataLayout &DL,
DominatorTree &DT,
AssumptionCache *AC) {
// The trickiest case to handle is when we have large blocks. Because of this,
// this code is optimized assuming that large blocks happen. This does not
// significantly pessimize the small block case. This uses LargeBlockInfo to
// make it efficient to get the index of various operations in the block.
// Walk the use-def list of the alloca, getting the locations of all stores.
using StoresByIndexTy = SmallVector<std::pair<unsigned, StoreInst *>, 64>;
StoresByIndexTy StoresByIndex;
for (User *U : AI->users())
if (StoreInst *SI = dyn_cast<StoreInst>(U))
StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI));
// Sort the stores by their index, making it efficient to do a lookup with a
// binary search.
llvm::sort(StoresByIndex, less_first());
// Walk all of the loads from this alloca, replacing them with the nearest
// store above them, if any.
for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
LoadInst *LI = dyn_cast<LoadInst>(*UI++);
if (!LI)
unsigned LoadIdx = LBI.getInstructionIndex(LI);
// Find the nearest store that has a lower index than this load.
StoresByIndexTy::iterator I = llvm::lower_bound(
std::make_pair(LoadIdx, static_cast<StoreInst *>(nullptr)),
if (I == StoresByIndex.begin()) {
if (StoresByIndex.empty())
// If there are no stores, the load takes the undef value.
// There is no store before this load, bail out (load may be affected
// by the following stores - see main comment).
return false;
} else {
// Otherwise, there was a store before this load, the load takes its value.
// Note, if the load was marked as nonnull we don't want to lose that
// information when we erase it. So we preserve it with an assume.
Value *ReplVal = std::prev(I)->second->getOperand(0);
if (AC && LI->getMetadata(LLVMContext::MD_nonnull) &&
!isKnownNonZero(ReplVal, DL, 0, AC, LI, &DT))
addAssumeNonNull(AC, LI);
// If the replacement value is the load, this must occur in unreachable
// code.
if (ReplVal == LI)
ReplVal = UndefValue::get(LI->getType());
// Remove the (now dead) stores and alloca.
while (!AI->use_empty()) {
StoreInst *SI = cast<StoreInst>(AI->user_back());
// Record debuginfo for the store before removing it.
for (DbgVariableIntrinsic *DII : Info.DbgDeclares) {
DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false);
ConvertDebugDeclareToDebugValue(DII, SI, DIB);
// The alloca's debuginfo can be removed as well.
for (DbgVariableIntrinsic *DII : Info.DbgDeclares)
return true;
void PromoteMem2Reg::run() {
Function &F = *DT.getRoot()->getParent();
AllocaInfo Info;
LargeBlockInfo LBI;
ForwardIDFCalculator IDF(DT);
for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
AllocaInst *AI = Allocas[AllocaNum];
assert(isAllocaPromotable(AI) && "Cannot promote non-promotable alloca!");
assert(AI->getParent()->getParent() == &F &&
"All allocas should be in the same function, which is same as DF!");
if (AI->use_empty()) {
// If there are no uses of the alloca, just delete it now.
// Remove the alloca from the Allocas list, since it has been processed
// Calculate the set of read and write-locations for each alloca. This is
// analogous to finding the 'uses' and 'definitions' of each variable.
// If there is only a single store to this value, replace any loads of
// it that are directly dominated by the definition with the value stored.
if (Info.DefiningBlocks.size() == 1) {
if (rewriteSingleStoreAlloca(AI, Info, LBI, SQ.DL, DT, AC)) {
// The alloca has been processed, move on.
// If the alloca is only read and written in one basic block, just perform a
// linear sweep over the block to eliminate it.
if (Info.OnlyUsedInOneBlock &&
promoteSingleBlockAlloca(AI, Info, LBI, SQ.DL, DT, AC)) {
// The alloca has been processed, move on.
// If we haven't computed a numbering for the BB's in the function, do so
// now.
if (BBNumbers.empty()) {
unsigned ID = 0;
for (auto &BB : F)
BBNumbers[&BB] = ID++;
// Remember the dbg.declare intrinsic describing this alloca, if any.
if (!Info.DbgDeclares.empty())
AllocaDbgDeclares[AllocaNum] = Info.DbgDeclares;
// Keep the reverse mapping of the 'Allocas' array for the rename pass.
AllocaLookup[Allocas[AllocaNum]] = AllocaNum;
// At this point, we're committed to promoting the alloca using IDF's, and
// the standard SSA construction algorithm. Determine which blocks need PHI
// nodes and see if we can optimize out some work by avoiding insertion of
// dead phi nodes.
// Unique the set of defining blocks for efficient lookup.
SmallPtrSet<BasicBlock *, 32> DefBlocks(Info.DefiningBlocks.begin(),
// Determine which blocks the value is live in. These are blocks which lead
// to uses.
SmallPtrSet<BasicBlock *, 32> LiveInBlocks;
ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks);
// At this point, we're committed to promoting the alloca using IDF's, and
// the standard SSA construction algorithm. Determine which blocks need phi
// nodes and see if we can optimize out some work by avoiding insertion of
// dead phi nodes.
SmallVector<BasicBlock *, 32> PHIBlocks;
llvm::sort(PHIBlocks, [this](BasicBlock *A, BasicBlock *B) {
return BBNumbers.find(A)->second < BBNumbers.find(B)->second;
unsigned CurrentVersion = 0;
for (BasicBlock *BB : PHIBlocks)
QueuePhiNode(BB, AllocaNum, CurrentVersion);
if (Allocas.empty())
return; // All of the allocas must have been trivial!
// Set the incoming values for the basic block to be null values for all of
// the alloca's. We do this in case there is a load of a value that has not
// been stored yet. In this case, it will get this null value.
RenamePassData::ValVector Values(Allocas.size());
for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
Values[i] = UndefValue::get(Allocas[i]->getAllocatedType());
// When handling debug info, treat all incoming values as if they have unknown
// locations until proven otherwise.
RenamePassData::LocationVector Locations(Allocas.size());
// Walks all basic blocks in the function performing the SSA rename algorithm
// and inserting the phi nodes we marked as necessary
std::vector<RenamePassData> RenamePassWorkList;
RenamePassWorkList.emplace_back(&F.front(), nullptr, std::move(Values),
do {
RenamePassData RPD = std::move(RenamePassWorkList.back());
// RenamePass may add new worklist entries.
RenamePass(RPD.BB, RPD.Pred, RPD.Values, RPD.Locations, RenamePassWorkList);
} while (!RenamePassWorkList.empty());
// The renamer uses the Visited set to avoid infinite loops. Clear it now.
// Remove the allocas themselves from the function.
for (Instruction *A : Allocas) {
// If there are any uses of the alloca instructions left, they must be in
// unreachable basic blocks that were not processed by walking the dominator
// tree. Just delete the users now.
if (!A->use_empty())
// Remove alloca's dbg.declare instrinsics from the function.
for (auto &Declares : AllocaDbgDeclares)
for (auto *DII : Declares)
// Loop over all of the PHI nodes and see if there are any that we can get
// rid of because they merge all of the same incoming values. This can
// happen due to undef values coming into the PHI nodes. This process is
// iterative, because eliminating one PHI node can cause others to be removed.
bool EliminatedAPHI = true;
while (EliminatedAPHI) {
EliminatedAPHI = false;
// Iterating over NewPhiNodes is deterministic, so it is safe to try to
// simplify and RAUW them as we go. If it was not, we could add uses to
// the values we replace with in a non-deterministic order, thus creating
// non-deterministic def->use chains.
for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
I = NewPhiNodes.begin(),
E = NewPhiNodes.end();
I != E;) {
PHINode *PN = I->second;
// If this PHI node merges one value and/or undefs, get the value.
if (Value *V = SimplifyInstruction(PN, SQ)) {
EliminatedAPHI = true;
// At this point, the renamer has added entries to PHI nodes for all reachable
// code. Unfortunately, there may be unreachable blocks which the renamer
// hasn't traversed. If this is the case, the PHI nodes may not
// have incoming values for all predecessors. Loop over all PHI nodes we have
// created, inserting undef values if they are missing any incoming values.
for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
I = NewPhiNodes.begin(),
E = NewPhiNodes.end();
I != E; ++I) {
// We want to do this once per basic block. As such, only process a block
// when we find the PHI that is the first entry in the block.
PHINode *SomePHI = I->second;
BasicBlock *BB = SomePHI->getParent();
if (&BB->front() != SomePHI)
// Only do work here if there the PHI nodes are missing incoming values. We
// know that all PHI nodes that were inserted in a block will have the same
// number of incoming values, so we can just check any of them.
if (SomePHI->getNumIncomingValues() == getNumPreds(BB))
// Get the preds for BB.
SmallVector<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
// Ok, now we know that all of the PHI nodes are missing entries for some
// basic blocks. Start by sorting the incoming predecessors for efficient
// access.
auto CompareBBNumbers = [this](BasicBlock *A, BasicBlock *B) {
return BBNumbers.find(A)->second < BBNumbers.find(B)->second;
llvm::sort(Preds, CompareBBNumbers);
// Now we loop through all BB's which have entries in SomePHI and remove
// them from the Preds list.
for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
// Do a log(n) search of the Preds list for the entry we want.
SmallVectorImpl<BasicBlock *>::iterator EntIt = llvm::lower_bound(
Preds, SomePHI->getIncomingBlock(i), CompareBBNumbers);
assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i) &&
"PHI node has entry for a block which is not a predecessor!");
// Remove the entry
// At this point, the blocks left in the preds list must have dummy
// entries inserted into every PHI nodes for the block. Update all the phi
// nodes in this block that we are inserting (there could be phis before
// mem2reg runs).
unsigned NumBadPreds = SomePHI->getNumIncomingValues();
BasicBlock::iterator BBI = BB->begin();
while ((SomePHI = dyn_cast<PHINode>(BBI++)) &&
SomePHI->getNumIncomingValues() == NumBadPreds) {
Value *UndefVal = UndefValue::get(SomePHI->getType());
for (BasicBlock *Pred : Preds)
SomePHI->addIncoming(UndefVal, Pred);
/// Determine which blocks the value is live in.
/// These are blocks which lead to uses. Knowing this allows us to avoid
/// inserting PHI nodes into blocks which don't lead to uses (thus, the
/// inserted phi nodes would be dead).
void PromoteMem2Reg::ComputeLiveInBlocks(
AllocaInst *AI, AllocaInfo &Info,
const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
SmallPtrSetImpl<BasicBlock *> &LiveInBlocks) {
// To determine liveness, we must iterate through the predecessors of blocks
// where the def is live. Blocks are added to the worklist if we need to
// check their predecessors. Start with all the using blocks.
SmallVector<BasicBlock *, 64> LiveInBlockWorklist(Info.UsingBlocks.begin(),
// If any of the using blocks is also a definition block, check to see if the
// definition occurs before or after the use. If it happens before the use,
// the value isn't really live-in.
for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) {
BasicBlock *BB = LiveInBlockWorklist[i];
if (!DefBlocks.count(BB))
// Okay, this is a block that both uses and defines the value. If the first
// reference to the alloca is a def (store), then we know it isn't live-in.
for (BasicBlock::iterator I = BB->begin();; ++I) {
if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
if (SI->getOperand(1) != AI)
// We found a store to the alloca before a load. The alloca is not
// actually live-in here.
LiveInBlockWorklist[i] = LiveInBlockWorklist.back();
if (LoadInst *LI = dyn_cast<LoadInst>(I))
// Okay, we found a load before a store to the alloca. It is actually
// live into this block.
if (LI->getOperand(0) == AI)
// Now that we have a set of blocks where the phi is live-in, recursively add
// their predecessors until we find the full region the value is live.
while (!LiveInBlockWorklist.empty()) {
BasicBlock *BB = LiveInBlockWorklist.pop_back_val();
// The block really is live in here, insert it into the set. If already in
// the set, then it has already been processed.
if (!LiveInBlocks.insert(BB).second)
// Since the value is live into BB, it is either defined in a predecessor or
// live into it to. Add the preds to the worklist unless they are a
// defining block.
for (BasicBlock *P : predecessors(BB)) {
// The value is not live into a predecessor if it defines the value.
if (DefBlocks.count(P))
// Otherwise it is, add to the worklist.
/// Queue a phi-node to be added to a basic-block for a specific Alloca.
/// Returns true if there wasn't already a phi-node for that variable
bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo,
unsigned &Version) {
// Look up the basic-block in question.
PHINode *&PN = NewPhiNodes[std::make_pair(BBNumbers[BB], AllocaNo)];
// If the BB already has a phi node added for the i'th alloca then we're done!
if (PN)
return false;
// Create a PhiNode using the dereferenced type... and add the phi-node to the
// BasicBlock.
PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(), getNumPreds(BB),
Allocas[AllocaNo]->getName() + "." + Twine(Version++),
PhiToAllocaMap[PN] = AllocaNo;
return true;
/// Update the debug location of a phi. \p ApplyMergedLoc indicates whether to
/// create a merged location incorporating \p DL, or to set \p DL directly.
static void updateForIncomingValueLocation(PHINode *PN, DebugLoc DL,
bool ApplyMergedLoc) {
if (ApplyMergedLoc)
PN->applyMergedLocation(PN->getDebugLoc(), DL);
/// Recursively traverse the CFG of the function, renaming loads and
/// stores to the allocas which we are promoting.
/// IncomingVals indicates what value each Alloca contains on exit from the
/// predecessor block Pred.
void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
RenamePassData::ValVector &IncomingVals,
RenamePassData::LocationVector &IncomingLocs,
std::vector<RenamePassData> &Worklist) {
// If we are inserting any phi nodes into this BB, they will already be in the
// block.
if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) {
// If we have PHI nodes to update, compute the number of edges from Pred to
// BB.
if (PhiToAllocaMap.count(APN)) {
// We want to be able to distinguish between PHI nodes being inserted by
// this invocation of mem2reg from those phi nodes that already existed in
// the IR before mem2reg was run. We determine that APN is being inserted
// because it is missing incoming edges. All other PHI nodes being
// inserted by this pass of mem2reg will have the same number of incoming
// operands so far. Remember this count.
unsigned NewPHINumOperands = APN->getNumOperands();
unsigned NumEdges = std::count(succ_begin(Pred), succ_end(Pred), BB);
assert(NumEdges && "Must be at least one edge from Pred to BB!");
// Add entries for all the phis.
BasicBlock::iterator PNI = BB->begin();
do {
unsigned AllocaNo = PhiToAllocaMap[APN];
// Update the location of the phi node.
updateForIncomingValueLocation(APN, IncomingLocs[AllocaNo],
APN->getNumIncomingValues() > 0);
// Add N incoming values to the PHI node.
for (unsigned i = 0; i != NumEdges; ++i)
APN->addIncoming(IncomingVals[AllocaNo], Pred);
// The currently active variable for this block is now the PHI.
IncomingVals[AllocaNo] = APN;
for (DbgVariableIntrinsic *DII : AllocaDbgDeclares[AllocaNo])
ConvertDebugDeclareToDebugValue(DII, APN, DIB);
// Get the next phi node.
APN = dyn_cast<PHINode>(PNI);
if (!APN)
// Verify that it is missing entries. If not, it is not being inserted
// by this mem2reg invocation so we want to ignore it.
} while (APN->getNumOperands() == NewPHINumOperands);
// Don't revisit blocks.
if (!Visited.insert(BB).second)
for (BasicBlock::iterator II = BB->begin(); !II->isTerminator();) {
Instruction *I = &*II++; // get the instruction, increment iterator
if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand());
if (!Src)
DenseMap<AllocaInst *, unsigned>::iterator AI = AllocaLookup.find(Src);
if (AI == AllocaLookup.end())
Value *V = IncomingVals[AI->second];
// If the load was marked as nonnull we don't want to lose
// that information when we erase this Load. So we preserve
// it with an assume.
if (AC && LI->getMetadata(LLVMContext::MD_nonnull) &&
!isKnownNonZero(V, SQ.DL, 0, AC, LI, &DT))
addAssumeNonNull(AC, LI);
// Anything using the load now uses the current value.
} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
// Delete this instruction and mark the name as the current holder of the
// value
AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand());
if (!Dest)
DenseMap<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
if (ai == AllocaLookup.end())
// what value were we writing?
unsigned AllocaNo = ai->second;
IncomingVals[AllocaNo] = SI->getOperand(0);
// Record debuginfo for the store before removing it.
IncomingLocs[AllocaNo] = SI->getDebugLoc();
for (DbgVariableIntrinsic *DII : AllocaDbgDeclares[ai->second])
ConvertDebugDeclareToDebugValue(DII, SI, DIB);
// 'Recurse' to our successors.
succ_iterator I = succ_begin(BB), E = succ_end(BB);
if (I == E)
// Keep track of the successors so we don't visit the same successor twice
SmallPtrSet<BasicBlock *, 8> VisitedSuccs;
// Handle the first successor without using the worklist.
Pred = BB;
BB = *I;
for (; I != E; ++I)
if (VisitedSuccs.insert(*I).second)
Worklist.emplace_back(*I, Pred, IncomingVals, IncomingLocs);
goto NextIteration;
void llvm::PromoteMemToReg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
AssumptionCache *AC) {
// If there is nothing to do, bail out...
if (Allocas.empty())
PromoteMem2Reg(Allocas, DT, AC).run();