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//===- GVNHoist.cpp - Hoist scalar and load expressions -------------------===//
// 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 pass hoists expressions from branches to a common dominator. It uses
// GVN (global value numbering) to discover expressions computing the same
// values. The primary goals of code-hoisting are:
// 1. To reduce the code size.
// 2. In some cases reduce critical path (by exposing more ILP).
// The algorithm factors out the reachability of values such that multiple
// queries to find reachability of values are fast. This is based on finding the
// ANTIC points in the CFG which do not change during hoisting. The ANTIC points
// are basically the dominance-frontiers in the inverse graph. So we introduce a
// data structure (CHI nodes) to keep track of values flowing out of a basic
// block. We only do this for values with multiple occurrences in the function
// as they are the potential hoistable candidates. This approach allows us to
// hoist instructions to a basic block with more than two successors, as well as
// deal with infinite loops in a trivial way.
// Limitations: This pass does not hoist fully redundant expressions because
// they are already handled by GVN-PRE. It is advisable to run gvn-hoist before
// and after gvn-pre because gvn-pre creates opportunities for more instructions
// to be hoisted.
// Hoisting may affect the performance in some cases. To mitigate that, hoisting
// is disabled in the following cases.
// 1. Scalars across calls.
// 2. geps when corresponding load/store cannot be hoisted.
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/IteratedDominanceFrontier.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.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 "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/GVN.h"
#include "llvm/Transforms/Utils/Local.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <memory>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "gvn-hoist"
STATISTIC(NumHoisted, "Number of instructions hoisted");
STATISTIC(NumRemoved, "Number of instructions removed");
STATISTIC(NumLoadsHoisted, "Number of loads hoisted");
STATISTIC(NumLoadsRemoved, "Number of loads removed");
STATISTIC(NumStoresHoisted, "Number of stores hoisted");
STATISTIC(NumStoresRemoved, "Number of stores removed");
STATISTIC(NumCallsHoisted, "Number of calls hoisted");
STATISTIC(NumCallsRemoved, "Number of calls removed");
static cl::opt<int>
MaxHoistedThreshold("gvn-max-hoisted", cl::Hidden, cl::init(-1),
cl::desc("Max number of instructions to hoist "
"(default unlimited = -1)"));
static cl::opt<int> MaxNumberOfBBSInPath(
"gvn-hoist-max-bbs", cl::Hidden, cl::init(4),
cl::desc("Max number of basic blocks on the path between "
"hoisting locations (default = 4, unlimited = -1)"));
static cl::opt<int> MaxDepthInBB(
"gvn-hoist-max-depth", cl::Hidden, cl::init(100),
cl::desc("Hoist instructions from the beginning of the BB up to the "
"maximum specified depth (default = 100, unlimited = -1)"));
static cl::opt<int>
MaxChainLength("gvn-hoist-max-chain-length", cl::Hidden, cl::init(10),
cl::desc("Maximum length of dependent chains to hoist "
"(default = 10, unlimited = -1)"));
namespace llvm {
using BBSideEffectsSet = DenseMap<const BasicBlock *, bool>;
using SmallVecInsn = SmallVector<Instruction *, 4>;
using SmallVecImplInsn = SmallVectorImpl<Instruction *>;
// Each element of a hoisting list contains the basic block where to hoist and
// a list of instructions to be hoisted.
using HoistingPointInfo = std::pair<BasicBlock *, SmallVecInsn>;
using HoistingPointList = SmallVector<HoistingPointInfo, 4>;
// A map from a pair of VNs to all the instructions with those VNs.
using VNType = std::pair<unsigned, uintptr_t>;
using VNtoInsns = DenseMap<VNType, SmallVector<Instruction *, 4>>;
// CHI keeps information about values flowing out of a basic block. It is
// similar to PHI but in the inverse graph, and used for outgoing values on each
// edge. For conciseness, it is computed only for instructions with multiple
// occurrences in the CFG because they are the only hoistable candidates.
// A (CHI[{V, B, I1}, {V, C, I2}]
// / \
// / \
// B(I1) C (I2)
// The Value number for both I1 and I2 is V, the CHI node will save the
// instruction as well as the edge where the value is flowing to.
struct CHIArg {
VNType VN;
// Edge destination (shows the direction of flow), may not be where the I is.
BasicBlock *Dest;
// The instruction (VN) which uses the values flowing out of CHI.
Instruction *I;
bool operator==(const CHIArg &A) const { return VN == A.VN; }
bool operator!=(const CHIArg &A) const { return !(*this == A); }
using CHIIt = SmallVectorImpl<CHIArg>::iterator;
using CHIArgs = iterator_range<CHIIt>;
using OutValuesType = DenseMap<BasicBlock *, SmallVector<CHIArg, 2>>;
using InValuesType =
DenseMap<BasicBlock *, SmallVector<std::pair<VNType, Instruction *>, 2>>;
// An invalid value number Used when inserting a single value number into
// VNtoInsns.
enum : uintptr_t { InvalidVN = ~(uintptr_t)2 };
// Records all scalar instructions candidate for code hoisting.
class InsnInfo {
VNtoInsns VNtoScalars;
// Inserts I and its value number in VNtoScalars.
void insert(Instruction *I, GVNPass::ValueTable &VN) {
// Scalar instruction.
unsigned V = VN.lookupOrAdd(I);
VNtoScalars[{V, InvalidVN}].push_back(I);
const VNtoInsns &getVNTable() const { return VNtoScalars; }
// Records all load instructions candidate for code hoisting.
class LoadInfo {
VNtoInsns VNtoLoads;
// Insert Load and the value number of its memory address in VNtoLoads.
void insert(LoadInst *Load, GVNPass::ValueTable &VN) {
if (Load->isSimple()) {
unsigned V = VN.lookupOrAdd(Load->getPointerOperand());
// With opaque pointers we may have loads from the same pointer with
// different result types, which should be disambiguated.
VNtoLoads[{V, (uintptr_t)Load->getType()}].push_back(Load);
const VNtoInsns &getVNTable() const { return VNtoLoads; }
// Records all store instructions candidate for code hoisting.
class StoreInfo {
VNtoInsns VNtoStores;
// Insert the Store and a hash number of the store address and the stored
// value in VNtoStores.
void insert(StoreInst *Store, GVNPass::ValueTable &VN) {
if (!Store->isSimple())
// Hash the store address and the stored value.
Value *Ptr = Store->getPointerOperand();
Value *Val = Store->getValueOperand();
VNtoStores[{VN.lookupOrAdd(Ptr), VN.lookupOrAdd(Val)}].push_back(Store);
const VNtoInsns &getVNTable() const { return VNtoStores; }
// Records all call instructions candidate for code hoisting.
class CallInfo {
VNtoInsns VNtoCallsScalars;
VNtoInsns VNtoCallsLoads;
VNtoInsns VNtoCallsStores;
// Insert Call and its value numbering in one of the VNtoCalls* containers.
void insert(CallInst *Call, GVNPass::ValueTable &VN) {
// A call that doesNotAccessMemory is handled as a Scalar,
// onlyReadsMemory will be handled as a Load instruction,
// all other calls will be handled as stores.
unsigned V = VN.lookupOrAdd(Call);
auto Entry = std::make_pair(V, InvalidVN);
if (Call->doesNotAccessMemory())
else if (Call->onlyReadsMemory())
const VNtoInsns &getScalarVNTable() const { return VNtoCallsScalars; }
const VNtoInsns &getLoadVNTable() const { return VNtoCallsLoads; }
const VNtoInsns &getStoreVNTable() const { return VNtoCallsStores; }
static void combineKnownMetadata(Instruction *ReplInst, Instruction *I) {
static const unsigned KnownIDs[] = {LLVMContext::MD_tbaa,
combineMetadata(ReplInst, I, KnownIDs, true);
// This pass hoists common computations across branches sharing common
// dominator. The primary goal is to reduce the code size, and in some
// cases reduce critical path (by exposing more ILP).
class GVNHoist {
GVNHoist(DominatorTree *DT, PostDominatorTree *PDT, AliasAnalysis *AA,
MemoryDependenceResults *MD, MemorySSA *MSSA)
MSSAUpdater(std::make_unique<MemorySSAUpdater>(MSSA)) {
bool run(Function &F);
// Copied from NewGVN.cpp
// This function provides global ranking of operations so that we can place
// them in a canonical order. Note that rank alone is not necessarily enough
// for a complete ordering, as constants all have the same rank. However,
// generally, we will simplify an operation with all constants so that it
// doesn't matter what order they appear in.
unsigned int rank(const Value *V) const;
GVNPass::ValueTable VN;
DominatorTree *DT;
PostDominatorTree *PDT;
AliasAnalysis *AA;
MemoryDependenceResults *MD;
MemorySSA *MSSA;
std::unique_ptr<MemorySSAUpdater> MSSAUpdater;
DenseMap<const Value *, unsigned> DFSNumber;
BBSideEffectsSet BBSideEffects;
DenseSet<const BasicBlock *> HoistBarrier;
SmallVector<BasicBlock *, 32> IDFBlocks;
unsigned NumFuncArgs;
const bool HoistingGeps = false;
enum InsKind { Unknown, Scalar, Load, Store };
// Return true when there are exception handling in BB.
bool hasEH(const BasicBlock *BB);
// Return true when I1 appears before I2 in the instructions of BB.
bool firstInBB(const Instruction *I1, const Instruction *I2) {
assert(I1->getParent() == I2->getParent());
unsigned I1DFS = DFSNumber.lookup(I1);
unsigned I2DFS = DFSNumber.lookup(I2);
assert(I1DFS && I2DFS);
return I1DFS < I2DFS;
// Return true when there are memory uses of Def in BB.
bool hasMemoryUse(const Instruction *NewPt, MemoryDef *Def,
const BasicBlock *BB);
bool hasEHhelper(const BasicBlock *BB, const BasicBlock *SrcBB,
int &NBBsOnAllPaths);
// Return true when there are exception handling or loads of memory Def
// between Def and NewPt. This function is only called for stores: Def is
// the MemoryDef of the store to be hoisted.
// Decrement by 1 NBBsOnAllPaths for each block between HoistPt and BB, and
// return true when the counter NBBsOnAllPaths reaces 0, except when it is
// initialized to -1 which is unlimited.
bool hasEHOrLoadsOnPath(const Instruction *NewPt, MemoryDef *Def,
int &NBBsOnAllPaths);
// Return true when there are exception handling between HoistPt and BB.
// Decrement by 1 NBBsOnAllPaths for each block between HoistPt and BB, and
// return true when the counter NBBsOnAllPaths reaches 0, except when it is
// initialized to -1 which is unlimited.
bool hasEHOnPath(const BasicBlock *HoistPt, const BasicBlock *SrcBB,
int &NBBsOnAllPaths);
// Return true when it is safe to hoist a memory load or store U from OldPt
// to NewPt.
bool safeToHoistLdSt(const Instruction *NewPt, const Instruction *OldPt,
MemoryUseOrDef *U, InsKind K, int &NBBsOnAllPaths);
// Return true when it is safe to hoist scalar instructions from all blocks in
// WL to HoistBB.
bool safeToHoistScalar(const BasicBlock *HoistBB, const BasicBlock *BB,
int &NBBsOnAllPaths) {
return !hasEHOnPath(HoistBB, BB, NBBsOnAllPaths);
// In the inverse CFG, the dominance frontier of basic block (BB) is the
// point where ANTIC needs to be computed for instructions which are going
// to be hoisted. Since this point does not change during gvn-hoist,
// we compute it only once (on demand).
// The ides is inspired from:
// "Partial Redundancy Elimination in SSA Form"
// They use similar idea in the forward graph to find fully redundant and
// partially redundant expressions, here it is used in the inverse graph to
// find fully anticipable instructions at merge point (post-dominator in
// the inverse CFG).
// Returns the edge via which an instruction in BB will get the values from.
// Returns true when the values are flowing out to each edge.
bool valueAnticipable(CHIArgs C, Instruction *TI) const;
// Check if it is safe to hoist values tracked by CHI in the range
// [Begin, End) and accumulate them in Safe.
void checkSafety(CHIArgs C, BasicBlock *BB, InsKind K,
SmallVectorImpl<CHIArg> &Safe);
using RenameStackType = DenseMap<VNType, SmallVector<Instruction *, 2>>;
// Push all the VNs corresponding to BB into RenameStack.
void fillRenameStack(BasicBlock *BB, InValuesType &ValueBBs,
RenameStackType &RenameStack);
void fillChiArgs(BasicBlock *BB, OutValuesType &CHIBBs,
RenameStackType &RenameStack);
// Walk the post-dominator tree top-down and use a stack for each value to
// store the last value you see. When you hit a CHI from a given edge, the
// value to use as the argument is at the top of the stack, add the value to
// CHI and pop.
void insertCHI(InValuesType &ValueBBs, OutValuesType &CHIBBs) {
auto Root = PDT->getNode(nullptr);
if (!Root)
// Depth first walk on PDom tree to fill the CHIargs at each PDF.
for (auto *Node : depth_first(Root)) {
BasicBlock *BB = Node->getBlock();
if (!BB)
RenameStackType RenameStack;
// Collect all values in BB and push to stack.
fillRenameStack(BB, ValueBBs, RenameStack);
// Fill outgoing values in each CHI corresponding to BB.
fillChiArgs(BB, CHIBBs, RenameStack);
// Walk all the CHI-nodes to find ones which have a empty-entry and remove
// them Then collect all the instructions which are safe to hoist and see if
// they form a list of anticipable values. OutValues contains CHIs
// corresponding to each basic block.
void findHoistableCandidates(OutValuesType &CHIBBs, InsKind K,
HoistingPointList &HPL);
// Compute insertion points for each values which can be fully anticipated at
// a dominator. HPL contains all such values.
void computeInsertionPoints(const VNtoInsns &Map, HoistingPointList &HPL,
InsKind K) {
// Sort VNs based on their rankings
std::vector<VNType> Ranks;
for (const auto &Entry : Map) {
// TODO: Remove fully-redundant expressions.
// Get instruction from the Map, assume that all the Instructions
// with same VNs have same rank (this is an approximation).
llvm::sort(Ranks, [this, &Map](const VNType &r1, const VNType &r2) {
return (rank(*Map.lookup(r1).begin()) < rank(*Map.lookup(r2).begin()));
// - Sort VNs according to their rank, and start with lowest ranked VN
// - Take a VN and for each instruction with same VN
// - Find the dominance frontier in the inverse graph (PDF)
// - Insert the chi-node at PDF
// - Remove the chi-nodes with missing entries
// - Remove values from CHI-nodes which do not truly flow out, e.g.,
// modified along the path.
// - Collect the remaining values that are still anticipable
SmallVector<BasicBlock *, 2> IDFBlocks;
ReverseIDFCalculator IDFs(*PDT);
OutValuesType OutValue;
InValuesType InValue;
for (const auto &R : Ranks) {
const SmallVecInsn &V = Map.lookup(R);
if (V.size() < 2)
const VNType &VN = R;
SmallPtrSet<BasicBlock *, 2> VNBlocks;
for (const auto &I : V) {
BasicBlock *BBI = I->getParent();
if (!hasEH(BBI))
// Compute the Post Dominance Frontiers of each basic block
// The dominance frontier of a live block X in the reverse
// control graph is the set of blocks upon which X is control
// dependent. The following sequence computes the set of blocks
// which currently have dead terminators that are control
// dependence sources of a block which is in NewLiveBlocks.
// Make a map of BB vs instructions to be hoisted.
for (unsigned i = 0; i < V.size(); ++i) {
InValue[V[i]->getParent()].push_back(std::make_pair(VN, V[i]));
// Insert empty CHI node for this VN. This is used to factor out
// basic blocks where the ANTIC can potentially change.
CHIArg EmptyChi = {VN, nullptr, nullptr};
for (auto *IDFBB : IDFBlocks) {
for (unsigned i = 0; i < V.size(); ++i) {
// Ignore spurious PDFs.
if (DT->properlyDominates(IDFBB, V[i]->getParent())) {
LLVM_DEBUG(dbgs() << "\nInserting a CHI for BB: "
<< IDFBB->getName() << ", for Insn: " << *V[i]);
// Insert CHI args at each PDF to iterate on factored graph of
// control dependence.
insertCHI(InValue, OutValue);
// Using the CHI args inserted at each PDF, find fully anticipable values.
findHoistableCandidates(OutValue, K, HPL);
// Return true when all operands of Instr are available at insertion point
// HoistPt. When limiting the number of hoisted expressions, one could hoist
// a load without hoisting its access function. So before hoisting any
// expression, make sure that all its operands are available at insert point.
bool allOperandsAvailable(const Instruction *I,
const BasicBlock *HoistPt) const;
// Same as allOperandsAvailable with recursive check for GEP operands.
bool allGepOperandsAvailable(const Instruction *I,
const BasicBlock *HoistPt) const;
// Make all operands of the GEP available.
void makeGepsAvailable(Instruction *Repl, BasicBlock *HoistPt,
const SmallVecInsn &InstructionsToHoist,
Instruction *Gep) const;
void updateAlignment(Instruction *I, Instruction *Repl);
// Remove all the instructions in Candidates and replace their usage with
// Repl. Returns the number of instructions removed.
unsigned rauw(const SmallVecInsn &Candidates, Instruction *Repl,
MemoryUseOrDef *NewMemAcc);
// Replace all Memory PHI usage with NewMemAcc.
void raMPHIuw(MemoryUseOrDef *NewMemAcc);
// Remove all other instructions and replace them with Repl.
unsigned removeAndReplace(const SmallVecInsn &Candidates, Instruction *Repl,
BasicBlock *DestBB, bool MoveAccess);
// In the case Repl is a load or a store, we make all their GEPs
// available: GEPs are not hoisted by default to avoid the address
// computations to be hoisted without the associated load or store.
bool makeGepOperandsAvailable(Instruction *Repl, BasicBlock *HoistPt,
const SmallVecInsn &InstructionsToHoist) const;
std::pair<unsigned, unsigned> hoist(HoistingPointList &HPL);
// Hoist all expressions. Returns Number of scalars hoisted
// and number of non-scalars hoisted.
std::pair<unsigned, unsigned> hoistExpressions(Function &F);
class GVNHoistLegacyPass : public FunctionPass {
static char ID;
GVNHoistLegacyPass() : FunctionPass(ID) {
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto &PDT = getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
auto &MD = getAnalysis<MemoryDependenceWrapperPass>().getMemDep();
auto &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA();
GVNHoist G(&DT, &PDT, &AA, &MD, &MSSA);
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool GVNHoist::run(Function &F) {
NumFuncArgs = F.arg_size();
bool Res = false;
// Perform DFS Numbering of instructions.
unsigned BBI = 0;
for (const BasicBlock *BB : depth_first(&F.getEntryBlock())) {
DFSNumber[BB] = ++BBI;
unsigned I = 0;
for (const auto &Inst : *BB)
DFSNumber[&Inst] = ++I;
int ChainLength = 0;
// FIXME: use lazy evaluation of VN to avoid the fix-point computation.
while (true) {
if (MaxChainLength != -1 && ++ChainLength >= MaxChainLength)
return Res;
auto HoistStat = hoistExpressions(F);
if (HoistStat.first + HoistStat.second == 0)
return Res;
if (HoistStat.second > 0)
// To address a limitation of the current GVN, we need to rerun the
// hoisting after we hoisted loads or stores in order to be able to
// hoist all scalars dependent on the hoisted ld/st.
Res = true;
return Res;
unsigned int GVNHoist::rank(const Value *V) const {
// Prefer constants to undef to anything else
// Undef is a constant, have to check it first.
// Prefer smaller constants to constantexprs
if (isa<ConstantExpr>(V))
return 2;
if (isa<UndefValue>(V))
return 1;
if (isa<Constant>(V))
return 0;
else if (auto *A = dyn_cast<Argument>(V))
return 3 + A->getArgNo();
// Need to shift the instruction DFS by number of arguments + 3 to account
// for the constant and argument ranking above.
auto Result = DFSNumber.lookup(V);
if (Result > 0)
return 4 + NumFuncArgs + Result;
// Unreachable or something else, just return a really large number.
return ~0;
bool GVNHoist::hasEH(const BasicBlock *BB) {
auto It = BBSideEffects.find(BB);
if (It != BBSideEffects.end())
return It->second;
if (BB->isEHPad() || BB->hasAddressTaken()) {
BBSideEffects[BB] = true;
return true;
if (BB->getTerminator()->mayThrow()) {
BBSideEffects[BB] = true;
return true;
BBSideEffects[BB] = false;
return false;
bool GVNHoist::hasMemoryUse(const Instruction *NewPt, MemoryDef *Def,
const BasicBlock *BB) {
const MemorySSA::AccessList *Acc = MSSA->getBlockAccesses(BB);
if (!Acc)
return false;
Instruction *OldPt = Def->getMemoryInst();
const BasicBlock *OldBB = OldPt->getParent();
const BasicBlock *NewBB = NewPt->getParent();
bool ReachedNewPt = false;
for (const MemoryAccess &MA : *Acc)
if (const MemoryUse *MU = dyn_cast<MemoryUse>(&MA)) {
Instruction *Insn = MU->getMemoryInst();
// Do not check whether MU aliases Def when MU occurs after OldPt.
if (BB == OldBB && firstInBB(OldPt, Insn))
// Do not check whether MU aliases Def when MU occurs before NewPt.
if (BB == NewBB) {
if (!ReachedNewPt) {
if (firstInBB(Insn, NewPt))
ReachedNewPt = true;
if (MemorySSAUtil::defClobbersUseOrDef(Def, MU, *AA))
return true;
return false;
bool GVNHoist::hasEHhelper(const BasicBlock *BB, const BasicBlock *SrcBB,
int &NBBsOnAllPaths) {
// Stop walk once the limit is reached.
if (NBBsOnAllPaths == 0)
return true;
// Impossible to hoist with exceptions on the path.
if (hasEH(BB))
return true;
// No such instruction after HoistBarrier in a basic block was
// selected for hoisting so instructions selected within basic block with
// a hoist barrier can be hoisted.
if ((BB != SrcBB) && HoistBarrier.count(BB))
return true;
return false;
bool GVNHoist::hasEHOrLoadsOnPath(const Instruction *NewPt, MemoryDef *Def,
int &NBBsOnAllPaths) {
const BasicBlock *NewBB = NewPt->getParent();
const BasicBlock *OldBB = Def->getBlock();
assert(DT->dominates(NewBB, OldBB) && "invalid path");
assert(DT->dominates(Def->getDefiningAccess()->getBlock(), NewBB) &&
"def does not dominate new hoisting point");
// Walk all basic blocks reachable in depth-first iteration on the inverse
// CFG from OldBB to NewBB. These blocks are all the blocks that may be
// executed between the execution of NewBB and OldBB. Hoisting an expression
// from OldBB into NewBB has to be safe on all execution paths.
for (auto I = idf_begin(OldBB), E = idf_end(OldBB); I != E;) {
const BasicBlock *BB = *I;
if (BB == NewBB) {
// Stop traversal when reaching HoistPt.
if (hasEHhelper(BB, OldBB, NBBsOnAllPaths))
return true;
// Check that we do not move a store past loads.
if (hasMemoryUse(NewPt, Def, BB))
return true;
// -1 is unlimited number of blocks on all paths.
if (NBBsOnAllPaths != -1)
return false;
bool GVNHoist::hasEHOnPath(const BasicBlock *HoistPt, const BasicBlock *SrcBB,
int &NBBsOnAllPaths) {
assert(DT->dominates(HoistPt, SrcBB) && "Invalid path");
// Walk all basic blocks reachable in depth-first iteration on
// the inverse CFG from BBInsn to NewHoistPt. These blocks are all the
// blocks that may be executed between the execution of NewHoistPt and
// BBInsn. Hoisting an expression from BBInsn into NewHoistPt has to be safe
// on all execution paths.
for (auto I = idf_begin(SrcBB), E = idf_end(SrcBB); I != E;) {
const BasicBlock *BB = *I;
if (BB == HoistPt) {
// Stop traversal when reaching NewHoistPt.
if (hasEHhelper(BB, SrcBB, NBBsOnAllPaths))
return true;
// -1 is unlimited number of blocks on all paths.
if (NBBsOnAllPaths != -1)
return false;
bool GVNHoist::safeToHoistLdSt(const Instruction *NewPt,
const Instruction *OldPt, MemoryUseOrDef *U,
GVNHoist::InsKind K, int &NBBsOnAllPaths) {
// In place hoisting is safe.
if (NewPt == OldPt)
return true;
const BasicBlock *NewBB = NewPt->getParent();
const BasicBlock *OldBB = OldPt->getParent();
const BasicBlock *UBB = U->getBlock();
// Check for dependences on the Memory SSA.
MemoryAccess *D = U->getDefiningAccess();
BasicBlock *DBB = D->getBlock();
if (DT->properlyDominates(NewBB, DBB))
// Cannot move the load or store to NewBB above its definition in DBB.
return false;
if (NewBB == DBB && !MSSA->isLiveOnEntryDef(D))
if (auto *UD = dyn_cast<MemoryUseOrDef>(D))
if (!firstInBB(UD->getMemoryInst(), NewPt))
// Cannot move the load or store to NewPt above its definition in D.
return false;
// Check for unsafe hoistings due to side effects.
if (K == InsKind::Store) {
if (hasEHOrLoadsOnPath(NewPt, cast<MemoryDef>(U), NBBsOnAllPaths))
return false;
} else if (hasEHOnPath(NewBB, OldBB, NBBsOnAllPaths))
return false;
if (UBB == NewBB) {
if (DT->properlyDominates(DBB, NewBB))
return true;
assert(UBB == DBB);
assert(MSSA->locallyDominates(D, U));
// No side effects: it is safe to hoist.
return true;
bool GVNHoist::valueAnticipable(CHIArgs C, Instruction *TI) const {
if (TI->getNumSuccessors() > (unsigned)size(C))
return false; // Not enough args in this CHI.
for (auto CHI : C) {
// Find if all the edges have values flowing out of BB.
if (!llvm::is_contained(successors(TI), CHI.Dest))
return false;
return true;
void GVNHoist::checkSafety(CHIArgs C, BasicBlock *BB, GVNHoist::InsKind K,
SmallVectorImpl<CHIArg> &Safe) {
int NumBBsOnAllPaths = MaxNumberOfBBSInPath;
for (auto CHI : C) {
Instruction *Insn = CHI.I;
if (!Insn) // No instruction was inserted in this CHI.
if (K == InsKind::Scalar) {
if (safeToHoistScalar(BB, Insn->getParent(), NumBBsOnAllPaths))
} else {
auto *T = BB->getTerminator();
if (MemoryUseOrDef *UD = MSSA->getMemoryAccess(Insn))
if (safeToHoistLdSt(T, Insn, UD, K, NumBBsOnAllPaths))
void GVNHoist::fillRenameStack(BasicBlock *BB, InValuesType &ValueBBs,
GVNHoist::RenameStackType &RenameStack) {
auto it1 = ValueBBs.find(BB);
if (it1 != ValueBBs.end()) {
// Iterate in reverse order to keep lower ranked values on the top.
LLVM_DEBUG(dbgs() << "\nVisiting: " << BB->getName()
<< " for pushing instructions on stack";);
for (std::pair<VNType, Instruction *> &VI : reverse(it1->second)) {
// Get the value of instruction I
LLVM_DEBUG(dbgs() << "\nPushing on stack: " << *VI.second);
void GVNHoist::fillChiArgs(BasicBlock *BB, OutValuesType &CHIBBs,
GVNHoist::RenameStackType &RenameStack) {
// For each *predecessor* (because Post-DOM) of BB check if it has a CHI
for (auto *Pred : predecessors(BB)) {
auto P = CHIBBs.find(Pred);
if (P == CHIBBs.end()) {
LLVM_DEBUG(dbgs() << "\nLooking at CHIs in: " << Pred->getName(););
// A CHI is found (BB -> Pred is an edge in the CFG)
// Pop the stack until Top(V) = Ve.
auto &VCHI = P->second;
for (auto It = VCHI.begin(), E = VCHI.end(); It != E;) {
CHIArg &C = *It;
if (!C.Dest) {
auto si = RenameStack.find(C.VN);
// The Basic Block where CHI is must dominate the value we want to
// track in a CHI. In the PDom walk, there can be values in the
// stack which are not control dependent e.g., nested loop.
if (si != RenameStack.end() && si->second.size() &&
DT->properlyDominates(Pred, si->second.back()->getParent())) {
C.Dest = BB; // Assign the edge
C.I = si->second.pop_back_val(); // Assign the argument
<< "\nCHI Inserted in BB: " << C.Dest->getName() << *C.I
<< ", VN: " << C.VN.first << ", " << C.VN.second);
// Move to next CHI of a different value
It = std::find_if(It, VCHI.end(), [It](CHIArg &A) { return A != *It; });
} else
void GVNHoist::findHoistableCandidates(OutValuesType &CHIBBs,
GVNHoist::InsKind K,
HoistingPointList &HPL) {
auto cmpVN = [](const CHIArg &A, const CHIArg &B) { return A.VN < B.VN; };
// CHIArgs now have the outgoing values, so check for anticipability and
// accumulate hoistable candidates in HPL.
for (std::pair<BasicBlock *, SmallVector<CHIArg, 2>> &A : CHIBBs) {
BasicBlock *BB = A.first;
SmallVectorImpl<CHIArg> &CHIs = A.second;
// Vector of PHIs contains PHIs for different instructions.
// Sort the args according to their VNs, such that identical
// instructions are together.
llvm::stable_sort(CHIs, cmpVN);
auto TI = BB->getTerminator();
auto B = CHIs.begin();
// [PreIt, PHIIt) form a range of CHIs which have identical VNs.
auto PHIIt = llvm::find_if(CHIs, [B](CHIArg &A) { return A != *B; });
auto PrevIt = CHIs.begin();
while (PrevIt != PHIIt) {
// Collect values which satisfy safety checks.
SmallVector<CHIArg, 2> Safe;
// We check for safety first because there might be multiple values in
// the same path, some of which are not safe to be hoisted, but overall
// each edge has at least one value which can be hoisted, making the
// value anticipable along that path.
checkSafety(make_range(PrevIt, PHIIt), BB, K, Safe);
// List of safe values should be anticipable at TI.
if (valueAnticipable(make_range(Safe.begin(), Safe.end()), TI)) {
HPL.push_back({BB, SmallVecInsn()});
SmallVecInsn &V = HPL.back().second;
for (auto B : Safe)
// Check other VNs
PrevIt = PHIIt;
PHIIt = std::find_if(PrevIt, CHIs.end(),
[PrevIt](CHIArg &A) { return A != *PrevIt; });
bool GVNHoist::allOperandsAvailable(const Instruction *I,
const BasicBlock *HoistPt) const {
for (const Use &Op : I->operands())
if (const auto *Inst = dyn_cast<Instruction>(&Op))
if (!DT->dominates(Inst->getParent(), HoistPt))
return false;
return true;
bool GVNHoist::allGepOperandsAvailable(const Instruction *I,
const BasicBlock *HoistPt) const {
for (const Use &Op : I->operands())
if (const auto *Inst = dyn_cast<Instruction>(&Op))
if (!DT->dominates(Inst->getParent(), HoistPt)) {
if (const GetElementPtrInst *GepOp =
dyn_cast<GetElementPtrInst>(Inst)) {
if (!allGepOperandsAvailable(GepOp, HoistPt))
return false;
// Gep is available if all operands of GepOp are available.
} else {
// Gep is not available if it has operands other than GEPs that are
// defined in blocks not dominating HoistPt.
return false;
return true;
void GVNHoist::makeGepsAvailable(Instruction *Repl, BasicBlock *HoistPt,
const SmallVecInsn &InstructionsToHoist,
Instruction *Gep) const {
assert(allGepOperandsAvailable(Gep, HoistPt) && "GEP operands not available");
Instruction *ClonedGep = Gep->clone();
for (unsigned i = 0, e = Gep->getNumOperands(); i != e; ++i)
if (Instruction *Op = dyn_cast<Instruction>(Gep->getOperand(i))) {
// Check whether the operand is already available.
if (DT->dominates(Op->getParent(), HoistPt))
// As a GEP can refer to other GEPs, recursively make all the operands
// of this GEP available at HoistPt.
if (GetElementPtrInst *GepOp = dyn_cast<GetElementPtrInst>(Op))
makeGepsAvailable(ClonedGep, HoistPt, InstructionsToHoist, GepOp);
// Copy Gep and replace its uses in Repl with ClonedGep.
// Conservatively discard any optimization hints, they may differ on the
// other paths.
// If we have optimization hints which agree with each other along different
// paths, preserve them.
for (const Instruction *OtherInst : InstructionsToHoist) {
const GetElementPtrInst *OtherGep;
if (auto *OtherLd = dyn_cast<LoadInst>(OtherInst))
OtherGep = cast<GetElementPtrInst>(OtherLd->getPointerOperand());
OtherGep = cast<GetElementPtrInst>(
// Replace uses of Gep with ClonedGep in Repl.
Repl->replaceUsesOfWith(Gep, ClonedGep);
void GVNHoist::updateAlignment(Instruction *I, Instruction *Repl) {
if (auto *ReplacementLoad = dyn_cast<LoadInst>(Repl)) {
std::min(ReplacementLoad->getAlign(), cast<LoadInst>(I)->getAlign()));
} else if (auto *ReplacementStore = dyn_cast<StoreInst>(Repl)) {
std::min(ReplacementStore->getAlign(), cast<StoreInst>(I)->getAlign()));
} else if (auto *ReplacementAlloca = dyn_cast<AllocaInst>(Repl)) {
} else if (isa<CallInst>(Repl)) {
unsigned GVNHoist::rauw(const SmallVecInsn &Candidates, Instruction *Repl,
MemoryUseOrDef *NewMemAcc) {
unsigned NR = 0;
for (Instruction *I : Candidates) {
if (I != Repl) {
updateAlignment(I, Repl);
if (NewMemAcc) {
// Update the uses of the old MSSA access with NewMemAcc.
MemoryAccess *OldMA = MSSA->getMemoryAccess(I);
combineKnownMetadata(Repl, I);
// Also invalidate the Alias Analysis cache.
return NR;
void GVNHoist::raMPHIuw(MemoryUseOrDef *NewMemAcc) {
SmallPtrSet<MemoryPhi *, 4> UsePhis;
for (User *U : NewMemAcc->users())
if (MemoryPhi *Phi = dyn_cast<MemoryPhi>(U))
for (MemoryPhi *Phi : UsePhis) {
auto In = Phi->incoming_values();
if (llvm::all_of(In, [&](Use &U) { return U == NewMemAcc; })) {
unsigned GVNHoist::removeAndReplace(const SmallVecInsn &Candidates,
Instruction *Repl, BasicBlock *DestBB,
bool MoveAccess) {
MemoryUseOrDef *NewMemAcc = MSSA->getMemoryAccess(Repl);
if (MoveAccess && NewMemAcc) {
// The definition of this ld/st will not change: ld/st hoisting is
// legal when the ld/st is not moved past its current definition.
MSSAUpdater->moveToPlace(NewMemAcc, DestBB, MemorySSA::BeforeTerminator);
// Replace all other instructions with Repl with memory access NewMemAcc.
unsigned NR = rauw(Candidates, Repl, NewMemAcc);
// Remove MemorySSA phi nodes with the same arguments.
if (NewMemAcc)
return NR;
bool GVNHoist::makeGepOperandsAvailable(
Instruction *Repl, BasicBlock *HoistPt,
const SmallVecInsn &InstructionsToHoist) const {
// Check whether the GEP of a ld/st can be synthesized at HoistPt.
GetElementPtrInst *Gep = nullptr;
Instruction *Val = nullptr;
if (auto *Ld = dyn_cast<LoadInst>(Repl)) {
Gep = dyn_cast<GetElementPtrInst>(Ld->getPointerOperand());
} else if (auto *St = dyn_cast<StoreInst>(Repl)) {
Gep = dyn_cast<GetElementPtrInst>(St->getPointerOperand());
Val = dyn_cast<Instruction>(St->getValueOperand());
// Check that the stored value is available.
if (Val) {
if (isa<GetElementPtrInst>(Val)) {
// Check whether we can compute the GEP at HoistPt.
if (!allGepOperandsAvailable(Val, HoistPt))
return false;
} else if (!DT->dominates(Val->getParent(), HoistPt))
return false;
// Check whether we can compute the Gep at HoistPt.
if (!Gep || !allGepOperandsAvailable(Gep, HoistPt))
return false;
makeGepsAvailable(Repl, HoistPt, InstructionsToHoist, Gep);
if (Val && isa<GetElementPtrInst>(Val))
makeGepsAvailable(Repl, HoistPt, InstructionsToHoist, Val);
return true;
std::pair<unsigned, unsigned> GVNHoist::hoist(HoistingPointList &HPL) {
unsigned NI = 0, NL = 0, NS = 0, NC = 0, NR = 0;
for (const HoistingPointInfo &HP : HPL) {
// Find out whether we already have one of the instructions in HoistPt,
// in which case we do not have to move it.
BasicBlock *DestBB = HP.first;
const SmallVecInsn &InstructionsToHoist = HP.second;
Instruction *Repl = nullptr;
for (Instruction *I : InstructionsToHoist)
if (I->getParent() == DestBB)
// If there are two instructions in HoistPt to be hoisted in place:
// update Repl to be the first one, such that we can rename the uses
// of the second based on the first.
if (!Repl || firstInBB(I, Repl))
Repl = I;
// Keep track of whether we moved the instruction so we know whether we
// should move the MemoryAccess.
bool MoveAccess = true;
if (Repl) {
// Repl is already in HoistPt: it remains in place.
assert(allOperandsAvailable(Repl, DestBB) &&
"instruction depends on operands that are not available");
MoveAccess = false;
} else {
// When we do not find Repl in HoistPt, select the first in the list
// and move it to HoistPt.
Repl = InstructionsToHoist.front();
// We can move Repl in HoistPt only when all operands are available.
// The order in which hoistings are done may influence the availability
// of operands.
if (!allOperandsAvailable(Repl, DestBB)) {
// When HoistingGeps there is nothing more we can do to make the
// operands available: just continue.
if (HoistingGeps)
// When not HoistingGeps we need to copy the GEPs.
if (!makeGepOperandsAvailable(Repl, DestBB, InstructionsToHoist))
// Move the instruction at the end of HoistPt.
Instruction *Last = DestBB->getTerminator();
DFSNumber[Repl] = DFSNumber[Last]++;
// Drop debug location as per debug info update guide.
NR += removeAndReplace(InstructionsToHoist, Repl, DestBB, MoveAccess);
if (isa<LoadInst>(Repl))
else if (isa<StoreInst>(Repl))
else if (isa<CallInst>(Repl))
else // Scalar
if (MSSA && VerifyMemorySSA)
NumHoisted += NL + NS + NC + NI;
NumRemoved += NR;
NumLoadsHoisted += NL;
NumStoresHoisted += NS;
NumCallsHoisted += NC;
return {NI, NL + NC + NS};
std::pair<unsigned, unsigned> GVNHoist::hoistExpressions(Function &F) {
InsnInfo II;
LoadInfo LI;
StoreInfo SI;
CallInfo CI;
for (BasicBlock *BB : depth_first(&F.getEntryBlock())) {
int InstructionNb = 0;
for (Instruction &I1 : *BB) {
// If I1 cannot guarantee progress, subsequent instructions
// in BB cannot be hoisted anyways.
if (!isGuaranteedToTransferExecutionToSuccessor(&I1)) {
// Only hoist the first instructions in BB up to MaxDepthInBB. Hoisting
// deeper may increase the register pressure and compilation time.
if (MaxDepthInBB != -1 && InstructionNb++ >= MaxDepthInBB)
// Do not value number terminator instructions.
if (I1.isTerminator())
if (auto *Load = dyn_cast<LoadInst>(&I1))
LI.insert(Load, VN);
else if (auto *Store = dyn_cast<StoreInst>(&I1))
SI.insert(Store, VN);
else if (auto *Call = dyn_cast<CallInst>(&I1)) {
if (auto *Intr = dyn_cast<IntrinsicInst>(Call)) {
if (isa<DbgInfoIntrinsic>(Intr) ||
Intr->getIntrinsicID() == Intrinsic::assume ||
Intr->getIntrinsicID() == Intrinsic::sideeffect)
if (Call->mayHaveSideEffects())
if (Call->isConvergent())
CI.insert(Call, VN);
} else if (HoistingGeps || !isa<GetElementPtrInst>(&I1))
// Do not hoist scalars past calls that may write to memory because
// that could result in spills later. geps are handled separately.
// TODO: We can relax this for targets like AArch64 as they have more
// registers than X86.
II.insert(&I1, VN);
HoistingPointList HPL;
computeInsertionPoints(II.getVNTable(), HPL, InsKind::Scalar);
computeInsertionPoints(LI.getVNTable(), HPL, InsKind::Load);
computeInsertionPoints(SI.getVNTable(), HPL, InsKind::Store);
computeInsertionPoints(CI.getScalarVNTable(), HPL, InsKind::Scalar);
computeInsertionPoints(CI.getLoadVNTable(), HPL, InsKind::Load);
computeInsertionPoints(CI.getStoreVNTable(), HPL, InsKind::Store);
return hoist(HPL);
} // end namespace llvm
PreservedAnalyses GVNHoistPass::run(Function &F, FunctionAnalysisManager &AM) {
DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
PostDominatorTree &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);
AliasAnalysis &AA = AM.getResult<AAManager>(F);
MemoryDependenceResults &MD = AM.getResult<MemoryDependenceAnalysis>(F);
MemorySSA &MSSA = AM.getResult<MemorySSAAnalysis>(F).getMSSA();
GVNHoist G(&DT, &PDT, &AA, &MD, &MSSA);
if (!
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
char GVNHoistLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(GVNHoistLegacyPass, "gvn-hoist",
"Early GVN Hoisting of Expressions", false, false)
INITIALIZE_PASS_END(GVNHoistLegacyPass, "gvn-hoist",
"Early GVN Hoisting of Expressions", false, false)
FunctionPass *llvm::createGVNHoistPass() { return new GVNHoistLegacyPass(); }