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llvm-6502/lib/Transforms/Scalar/GVN.cpp
Owen Anderson 2cf7537dc6 Don't bother value numbering instructions with void types in GVN. In theory this should allow us to insert 
fewer things into the value numbering maps, but any speedup is beneath the noise threshold on my machine
on 403.gcc.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122844 91177308-0d34-0410-b5e6-96231b3b80d8
2011-01-04 22:15:21 +00:00

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//===- GVN.cpp - Eliminate redundant values and loads ---------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs global value numbering to eliminate fully redundant
// instructions. It also performs simple dead load elimination.
//
// Note that this pass does the value numbering itself; it does not use the
// ValueNumbering analysis passes.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "gvn"
#include "llvm/Transforms/Scalar.h"
#include "llvm/GlobalVariable.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/PHITransAddr.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/IRBuilder.h"
using namespace llvm;
STATISTIC(NumGVNInstr, "Number of instructions deleted");
STATISTIC(NumGVNLoad, "Number of loads deleted");
STATISTIC(NumGVNPRE, "Number of instructions PRE'd");
STATISTIC(NumGVNBlocks, "Number of blocks merged");
STATISTIC(NumPRELoad, "Number of loads PRE'd");
static cl::opt<bool> EnablePRE("enable-pre",
cl::init(true), cl::Hidden);
static cl::opt<bool> EnableLoadPRE("enable-load-pre", cl::init(true));
//===----------------------------------------------------------------------===//
// ValueTable Class
//===----------------------------------------------------------------------===//
/// This class holds the mapping between values and value numbers. It is used
/// as an efficient mechanism to determine the expression-wise equivalence of
/// two values.
namespace {
struct Expression {
uint32_t opcode;
const Type* type;
SmallVector<uint32_t, 4> varargs;
Expression() { }
Expression(uint32_t o) : opcode(o) { }
bool operator==(const Expression &other) const {
if (opcode != other.opcode)
return false;
else if (opcode == ~0U || opcode == ~1U)
return true;
else if (type != other.type)
return false;
else if (varargs != other.varargs)
return false;
return true;
}
};
class ValueTable {
private:
DenseMap<Value*, uint32_t> valueNumbering;
DenseMap<Expression, uint32_t> expressionNumbering;
AliasAnalysis* AA;
MemoryDependenceAnalysis* MD;
DominatorTree* DT;
uint32_t nextValueNumber;
Expression create_expression(Instruction* I);
uint32_t lookup_or_add_call(CallInst* C);
public:
ValueTable() : nextValueNumber(1) { }
uint32_t lookup_or_add(Value *V);
uint32_t lookup(Value *V) const;
void add(Value *V, uint32_t num);
void clear();
void erase(Value *v);
void setAliasAnalysis(AliasAnalysis* A) { AA = A; }
AliasAnalysis *getAliasAnalysis() const { return AA; }
void setMemDep(MemoryDependenceAnalysis* M) { MD = M; }
void setDomTree(DominatorTree* D) { DT = D; }
uint32_t getNextUnusedValueNumber() { return nextValueNumber; }
void verifyRemoved(const Value *) const;
};
}
namespace llvm {
template <> struct DenseMapInfo<Expression> {
static inline Expression getEmptyKey() {
return ~0U;
}
static inline Expression getTombstoneKey() {
return ~1U;
}
static unsigned getHashValue(const Expression e) {
unsigned hash = e.opcode;
hash = ((unsigned)((uintptr_t)e.type >> 4) ^
(unsigned)((uintptr_t)e.type >> 9));
for (SmallVector<uint32_t, 4>::const_iterator I = e.varargs.begin(),
E = e.varargs.end(); I != E; ++I)
hash = *I + hash * 37;
return hash;
}
static bool isEqual(const Expression &LHS, const Expression &RHS) {
return LHS == RHS;
}
};
}
//===----------------------------------------------------------------------===//
// ValueTable Internal Functions
//===----------------------------------------------------------------------===//
Expression ValueTable::create_expression(Instruction *I) {
Expression e;
e.type = I->getType();
e.opcode = I->getOpcode();
for (Instruction::op_iterator OI = I->op_begin(), OE = I->op_end();
OI != OE; ++OI)
e.varargs.push_back(lookup_or_add(*OI));
if (CmpInst *C = dyn_cast<CmpInst>(I))
e.opcode = (C->getOpcode() << 8) | C->getPredicate();
else if (ExtractValueInst *E = dyn_cast<ExtractValueInst>(I)) {
for (ExtractValueInst::idx_iterator II = E->idx_begin(), IE = E->idx_end();
II != IE; ++II)
e.varargs.push_back(*II);
} else if (InsertValueInst *E = dyn_cast<InsertValueInst>(I)) {
for (InsertValueInst::idx_iterator II = E->idx_begin(), IE = E->idx_end();
II != IE; ++II)
e.varargs.push_back(*II);
}
return e;
}
//===----------------------------------------------------------------------===//
// ValueTable External Functions
//===----------------------------------------------------------------------===//
/// add - Insert a value into the table with a specified value number.
void ValueTable::add(Value *V, uint32_t num) {
valueNumbering.insert(std::make_pair(V, num));
}
uint32_t ValueTable::lookup_or_add_call(CallInst* C) {
if (AA->doesNotAccessMemory(C)) {
Expression exp = create_expression(C);
uint32_t& e = expressionNumbering[exp];
if (!e) e = nextValueNumber++;
valueNumbering[C] = e;
return e;
} else if (AA->onlyReadsMemory(C)) {
Expression exp = create_expression(C);
uint32_t& e = expressionNumbering[exp];
if (!e) {
e = nextValueNumber++;
valueNumbering[C] = e;
return e;
}
if (!MD) {
e = nextValueNumber++;
valueNumbering[C] = e;
return e;
}
MemDepResult local_dep = MD->getDependency(C);
if (!local_dep.isDef() && !local_dep.isNonLocal()) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
if (local_dep.isDef()) {
CallInst* local_cdep = cast<CallInst>(local_dep.getInst());
if (local_cdep->getNumArgOperands() != C->getNumArgOperands()) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
for (unsigned i = 0, e = C->getNumArgOperands(); i < e; ++i) {
uint32_t c_vn = lookup_or_add(C->getArgOperand(i));
uint32_t cd_vn = lookup_or_add(local_cdep->getArgOperand(i));
if (c_vn != cd_vn) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
}
uint32_t v = lookup_or_add(local_cdep);
valueNumbering[C] = v;
return v;
}
// Non-local case.
const MemoryDependenceAnalysis::NonLocalDepInfo &deps =
MD->getNonLocalCallDependency(CallSite(C));
// FIXME: call/call dependencies for readonly calls should return def, not
// clobber! Move the checking logic to MemDep!
CallInst* cdep = 0;
// Check to see if we have a single dominating call instruction that is
// identical to C.
for (unsigned i = 0, e = deps.size(); i != e; ++i) {
const NonLocalDepEntry *I = &deps[i];
// Ignore non-local dependencies.
if (I->getResult().isNonLocal())
continue;
// We don't handle non-depedencies. If we already have a call, reject
// instruction dependencies.
if (I->getResult().isClobber() || cdep != 0) {
cdep = 0;
break;
}
CallInst *NonLocalDepCall = dyn_cast<CallInst>(I->getResult().getInst());
// FIXME: All duplicated with non-local case.
if (NonLocalDepCall && DT->properlyDominates(I->getBB(), C->getParent())){
cdep = NonLocalDepCall;
continue;
}
cdep = 0;
break;
}
if (!cdep) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
if (cdep->getNumArgOperands() != C->getNumArgOperands()) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
for (unsigned i = 0, e = C->getNumArgOperands(); i < e; ++i) {
uint32_t c_vn = lookup_or_add(C->getArgOperand(i));
uint32_t cd_vn = lookup_or_add(cdep->getArgOperand(i));
if (c_vn != cd_vn) {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
}
uint32_t v = lookup_or_add(cdep);
valueNumbering[C] = v;
return v;
} else {
valueNumbering[C] = nextValueNumber;
return nextValueNumber++;
}
}
/// lookup_or_add - Returns the value number for the specified value, assigning
/// it a new number if it did not have one before.
uint32_t ValueTable::lookup_or_add(Value *V) {
DenseMap<Value*, uint32_t>::iterator VI = valueNumbering.find(V);
if (VI != valueNumbering.end())
return VI->second;
if (!isa<Instruction>(V)) {
valueNumbering[V] = nextValueNumber;
return nextValueNumber++;
}
Instruction* I = cast<Instruction>(V);
Expression exp;
switch (I->getOpcode()) {
case Instruction::Call:
return lookup_or_add_call(cast<CallInst>(I));
case Instruction::Add:
case Instruction::FAdd:
case Instruction::Sub:
case Instruction::FSub:
case Instruction::Mul:
case Instruction::FMul:
case Instruction::UDiv:
case Instruction::SDiv:
case Instruction::FDiv:
case Instruction::URem:
case Instruction::SRem:
case Instruction::FRem:
case Instruction::Shl:
case Instruction::LShr:
case Instruction::AShr:
case Instruction::And:
case Instruction::Or :
case Instruction::Xor:
case Instruction::ICmp:
case Instruction::FCmp:
case Instruction::Trunc:
case Instruction::ZExt:
case Instruction::SExt:
case Instruction::FPToUI:
case Instruction::FPToSI:
case Instruction::UIToFP:
case Instruction::SIToFP:
case Instruction::FPTrunc:
case Instruction::FPExt:
case Instruction::PtrToInt:
case Instruction::IntToPtr:
case Instruction::BitCast:
case Instruction::Select:
case Instruction::ExtractElement:
case Instruction::InsertElement:
case Instruction::ShuffleVector:
case Instruction::ExtractValue:
case Instruction::InsertValue:
case Instruction::GetElementPtr:
exp = create_expression(I);
break;
default:
valueNumbering[V] = nextValueNumber;
return nextValueNumber++;
}
uint32_t& e = expressionNumbering[exp];
if (!e) e = nextValueNumber++;
valueNumbering[V] = e;
return e;
}
/// lookup - Returns the value number of the specified value. Fails if
/// the value has not yet been numbered.
uint32_t ValueTable::lookup(Value *V) const {
DenseMap<Value*, uint32_t>::const_iterator VI = valueNumbering.find(V);
assert(VI != valueNumbering.end() && "Value not numbered?");
return VI->second;
}
/// clear - Remove all entries from the ValueTable
void ValueTable::clear() {
valueNumbering.clear();
expressionNumbering.clear();
nextValueNumber = 1;
}
/// erase - Remove a value from the value numbering
void ValueTable::erase(Value *V) {
valueNumbering.erase(V);
}
/// verifyRemoved - Verify that the value is removed from all internal data
/// structures.
void ValueTable::verifyRemoved(const Value *V) const {
for (DenseMap<Value*, uint32_t>::const_iterator
I = valueNumbering.begin(), E = valueNumbering.end(); I != E; ++I) {
assert(I->first != V && "Inst still occurs in value numbering map!");
}
}
//===----------------------------------------------------------------------===//
// GVN Pass
//===----------------------------------------------------------------------===//
namespace {
class GVN : public FunctionPass {
bool runOnFunction(Function &F);
public:
static char ID; // Pass identification, replacement for typeid
explicit GVN(bool noloads = false)
: FunctionPass(ID), NoLoads(noloads), MD(0) {
initializeGVNPass(*PassRegistry::getPassRegistry());
}
private:
bool NoLoads;
MemoryDependenceAnalysis *MD;
DominatorTree *DT;
const TargetData* TD;
ValueTable VN;
/// LeaderTable - A mapping from value numbers to lists of Value*'s that
/// have that value number. Use findLeader to query it.
struct LeaderTableEntry {
Value *Val;
BasicBlock *BB;
LeaderTableEntry *Next;
};
DenseMap<uint32_t, LeaderTableEntry> LeaderTable;
BumpPtrAllocator TableAllocator;
/// addToLeaderTable - Push a new Value to the LeaderTable onto the list for
/// its value number.
void addToLeaderTable(uint32_t N, Value *V, BasicBlock *BB) {
LeaderTableEntry& Curr = LeaderTable[N];
if (!Curr.Val) {
Curr.Val = V;
Curr.BB = BB;
return;
}
LeaderTableEntry* Node = TableAllocator.Allocate<LeaderTableEntry>();
Node->Val = V;
Node->BB = BB;
Node->Next = Curr.Next;
Curr.Next = Node;
}
/// removeFromLeaderTable - Scan the list of values corresponding to a given
/// value number, and remove the given value if encountered.
void removeFromLeaderTable(uint32_t N, Value *V, BasicBlock *BB) {
LeaderTableEntry* Prev = 0;
LeaderTableEntry* Curr = &LeaderTable[N];
while (Curr->Val != V || Curr->BB != BB) {
Prev = Curr;
Curr = Curr->Next;
}
if (Prev) {
Prev->Next = Curr->Next;
} else {
if (!Curr->Next) {
Curr->Val = 0;
Curr->BB = 0;
} else {
LeaderTableEntry* Next = Curr->Next;
Curr->Val = Next->Val;
Curr->BB = Next->BB;
Curr->Next = Next->Next;
}
}
}
// List of critical edges to be split between iterations.
SmallVector<std::pair<TerminatorInst*, unsigned>, 4> toSplit;
// This transformation requires dominator postdominator info
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<DominatorTree>();
if (!NoLoads)
AU.addRequired<MemoryDependenceAnalysis>();
AU.addRequired<AliasAnalysis>();
AU.addPreserved<DominatorTree>();
AU.addPreserved<AliasAnalysis>();
}
// Helper fuctions
// FIXME: eliminate or document these better
bool processLoad(LoadInst* L,
SmallVectorImpl<Instruction*> &toErase);
bool processInstruction(Instruction *I,
SmallVectorImpl<Instruction*> &toErase);
bool processNonLocalLoad(LoadInst* L,
SmallVectorImpl<Instruction*> &toErase);
bool processBlock(BasicBlock *BB);
void dump(DenseMap<uint32_t, Value*>& d);
bool iterateOnFunction(Function &F);
bool performPRE(Function& F);
Value *findLeader(BasicBlock *BB, uint32_t num);
void cleanupGlobalSets();
void verifyRemoved(const Instruction *I) const;
bool splitCriticalEdges();
};
char GVN::ID = 0;
}
// createGVNPass - The public interface to this file...
FunctionPass *llvm::createGVNPass(bool NoLoads) {
return new GVN(NoLoads);
}
INITIALIZE_PASS_BEGIN(GVN, "gvn", "Global Value Numbering", false, false)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(GVN, "gvn", "Global Value Numbering", false, false)
void GVN::dump(DenseMap<uint32_t, Value*>& d) {
errs() << "{\n";
for (DenseMap<uint32_t, Value*>::iterator I = d.begin(),
E = d.end(); I != E; ++I) {
errs() << I->first << "\n";
I->second->dump();
}
errs() << "}\n";
}
/// IsValueFullyAvailableInBlock - Return true if we can prove that the value
/// we're analyzing is fully available in the specified block. As we go, keep
/// track of which blocks we know are fully alive in FullyAvailableBlocks. This
/// map is actually a tri-state map with the following values:
/// 0) we know the block *is not* fully available.
/// 1) we know the block *is* fully available.
/// 2) we do not know whether the block is fully available or not, but we are
/// currently speculating that it will be.
/// 3) we are speculating for this block and have used that to speculate for
/// other blocks.
static bool IsValueFullyAvailableInBlock(BasicBlock *BB,
DenseMap<BasicBlock*, char> &FullyAvailableBlocks) {
// Optimistically assume that the block is fully available and check to see
// if we already know about this block in one lookup.
std::pair<DenseMap<BasicBlock*, char>::iterator, char> IV =
FullyAvailableBlocks.insert(std::make_pair(BB, 2));
// If the entry already existed for this block, return the precomputed value.
if (!IV.second) {
// If this is a speculative "available" value, mark it as being used for
// speculation of other blocks.
if (IV.first->second == 2)
IV.first->second = 3;
return IV.first->second != 0;
}
// Otherwise, see if it is fully available in all predecessors.
pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
// If this block has no predecessors, it isn't live-in here.
if (PI == PE)
goto SpeculationFailure;
for (; PI != PE; ++PI)
// If the value isn't fully available in one of our predecessors, then it
// isn't fully available in this block either. Undo our previous
// optimistic assumption and bail out.
if (!IsValueFullyAvailableInBlock(*PI, FullyAvailableBlocks))
goto SpeculationFailure;
return true;
// SpeculationFailure - If we get here, we found out that this is not, after
// all, a fully-available block. We have a problem if we speculated on this and
// used the speculation to mark other blocks as available.
SpeculationFailure:
char &BBVal = FullyAvailableBlocks[BB];
// If we didn't speculate on this, just return with it set to false.
if (BBVal == 2) {
BBVal = 0;
return false;
}
// If we did speculate on this value, we could have blocks set to 1 that are
// incorrect. Walk the (transitive) successors of this block and mark them as
// 0 if set to one.
SmallVector<BasicBlock*, 32> BBWorklist;
BBWorklist.push_back(BB);
do {
BasicBlock *Entry = BBWorklist.pop_back_val();
// Note that this sets blocks to 0 (unavailable) if they happen to not
// already be in FullyAvailableBlocks. This is safe.
char &EntryVal = FullyAvailableBlocks[Entry];
if (EntryVal == 0) continue; // Already unavailable.
// Mark as unavailable.
EntryVal = 0;
for (succ_iterator I = succ_begin(Entry), E = succ_end(Entry); I != E; ++I)
BBWorklist.push_back(*I);
} while (!BBWorklist.empty());
return false;
}
/// CanCoerceMustAliasedValueToLoad - Return true if
/// CoerceAvailableValueToLoadType will succeed.
static bool CanCoerceMustAliasedValueToLoad(Value *StoredVal,
const Type *LoadTy,
const TargetData &TD) {
// If the loaded or stored value is an first class array or struct, don't try
// to transform them. We need to be able to bitcast to integer.
if (LoadTy->isStructTy() || LoadTy->isArrayTy() ||
StoredVal->getType()->isStructTy() ||
StoredVal->getType()->isArrayTy())
return false;
// The store has to be at least as big as the load.
if (TD.getTypeSizeInBits(StoredVal->getType()) <
TD.getTypeSizeInBits(LoadTy))
return false;
return true;
}
/// CoerceAvailableValueToLoadType - If we saw a store of a value to memory, and
/// then a load from a must-aliased pointer of a different type, try to coerce
/// the stored value. LoadedTy is the type of the load we want to replace and
/// InsertPt is the place to insert new instructions.
///
/// If we can't do it, return null.
static Value *CoerceAvailableValueToLoadType(Value *StoredVal,
const Type *LoadedTy,
Instruction *InsertPt,
const TargetData &TD) {
if (!CanCoerceMustAliasedValueToLoad(StoredVal, LoadedTy, TD))
return 0;
const Type *StoredValTy = StoredVal->getType();
uint64_t StoreSize = TD.getTypeStoreSizeInBits(StoredValTy);
uint64_t LoadSize = TD.getTypeSizeInBits(LoadedTy);
// If the store and reload are the same size, we can always reuse it.
if (StoreSize == LoadSize) {
if (StoredValTy->isPointerTy() && LoadedTy->isPointerTy()) {
// Pointer to Pointer -> use bitcast.
return new BitCastInst(StoredVal, LoadedTy, "", InsertPt);
}
// Convert source pointers to integers, which can be bitcast.
if (StoredValTy->isPointerTy()) {
StoredValTy = TD.getIntPtrType(StoredValTy->getContext());
StoredVal = new PtrToIntInst(StoredVal, StoredValTy, "", InsertPt);
}
const Type *TypeToCastTo = LoadedTy;
if (TypeToCastTo->isPointerTy())
TypeToCastTo = TD.getIntPtrType(StoredValTy->getContext());
if (StoredValTy != TypeToCastTo)
StoredVal = new BitCastInst(StoredVal, TypeToCastTo, "", InsertPt);
// Cast to pointer if the load needs a pointer type.
if (LoadedTy->isPointerTy())
StoredVal = new IntToPtrInst(StoredVal, LoadedTy, "", InsertPt);
return StoredVal;
}
// If the loaded value is smaller than the available value, then we can
// extract out a piece from it. If the available value is too small, then we
// can't do anything.
assert(StoreSize >= LoadSize && "CanCoerceMustAliasedValueToLoad fail");
// Convert source pointers to integers, which can be manipulated.
if (StoredValTy->isPointerTy()) {
StoredValTy = TD.getIntPtrType(StoredValTy->getContext());
StoredVal = new PtrToIntInst(StoredVal, StoredValTy, "", InsertPt);
}
// Convert vectors and fp to integer, which can be manipulated.
if (!StoredValTy->isIntegerTy()) {
StoredValTy = IntegerType::get(StoredValTy->getContext(), StoreSize);
StoredVal = new BitCastInst(StoredVal, StoredValTy, "", InsertPt);
}
// If this is a big-endian system, we need to shift the value down to the low
// bits so that a truncate will work.
if (TD.isBigEndian()) {
Constant *Val = ConstantInt::get(StoredVal->getType(), StoreSize-LoadSize);
StoredVal = BinaryOperator::CreateLShr(StoredVal, Val, "tmp", InsertPt);
}
// Truncate the integer to the right size now.
const Type *NewIntTy = IntegerType::get(StoredValTy->getContext(), LoadSize);
StoredVal = new TruncInst(StoredVal, NewIntTy, "trunc", InsertPt);
if (LoadedTy == NewIntTy)
return StoredVal;
// If the result is a pointer, inttoptr.
if (LoadedTy->isPointerTy())
return new IntToPtrInst(StoredVal, LoadedTy, "inttoptr", InsertPt);
// Otherwise, bitcast.
return new BitCastInst(StoredVal, LoadedTy, "bitcast", InsertPt);
}
/// AnalyzeLoadFromClobberingWrite - This function is called when we have a
/// memdep query of a load that ends up being a clobbering memory write (store,
/// memset, memcpy, memmove). This means that the write *may* provide bits used
/// by the load but we can't be sure because the pointers don't mustalias.
///
/// Check this case to see if there is anything more we can do before we give
/// up. This returns -1 if we have to give up, or a byte number in the stored
/// value of the piece that feeds the load.
static int AnalyzeLoadFromClobberingWrite(const Type *LoadTy, Value *LoadPtr,
Value *WritePtr,
uint64_t WriteSizeInBits,
const TargetData &TD) {
// If the loaded or stored value is an first class array or struct, don't try
// to transform them. We need to be able to bitcast to integer.
if (LoadTy->isStructTy() || LoadTy->isArrayTy())
return -1;
int64_t StoreOffset = 0, LoadOffset = 0;
Value *StoreBase = GetPointerBaseWithConstantOffset(WritePtr, StoreOffset,TD);
Value *LoadBase = GetPointerBaseWithConstantOffset(LoadPtr, LoadOffset, TD);
if (StoreBase != LoadBase)
return -1;
// If the load and store are to the exact same address, they should have been
// a must alias. AA must have gotten confused.
// FIXME: Study to see if/when this happens. One case is forwarding a memset
// to a load from the base of the memset.
#if 0
if (LoadOffset == StoreOffset) {
dbgs() << "STORE/LOAD DEP WITH COMMON POINTER MISSED:\n"
<< "Base = " << *StoreBase << "\n"
<< "Store Ptr = " << *WritePtr << "\n"
<< "Store Offs = " << StoreOffset << "\n"
<< "Load Ptr = " << *LoadPtr << "\n";
abort();
}
#endif
// If the load and store don't overlap at all, the store doesn't provide
// anything to the load. In this case, they really don't alias at all, AA
// must have gotten confused.
uint64_t LoadSize = TD.getTypeSizeInBits(LoadTy);
if ((WriteSizeInBits & 7) | (LoadSize & 7))
return -1;
uint64_t StoreSize = WriteSizeInBits >> 3; // Convert to bytes.
LoadSize >>= 3;
bool isAAFailure = false;
if (StoreOffset < LoadOffset)
isAAFailure = StoreOffset+int64_t(StoreSize) <= LoadOffset;
else
isAAFailure = LoadOffset+int64_t(LoadSize) <= StoreOffset;
if (isAAFailure) {
#if 0
dbgs() << "STORE LOAD DEP WITH COMMON BASE:\n"
<< "Base = " << *StoreBase << "\n"
<< "Store Ptr = " << *WritePtr << "\n"
<< "Store Offs = " << StoreOffset << "\n"
<< "Load Ptr = " << *LoadPtr << "\n";
abort();
#endif
return -1;
}
// If the Load isn't completely contained within the stored bits, we don't
// have all the bits to feed it. We could do something crazy in the future
// (issue a smaller load then merge the bits in) but this seems unlikely to be
// valuable.
if (StoreOffset > LoadOffset ||
StoreOffset+StoreSize < LoadOffset+LoadSize)
return -1;
// Okay, we can do this transformation. Return the number of bytes into the
// store that the load is.
return LoadOffset-StoreOffset;
}
/// AnalyzeLoadFromClobberingStore - This function is called when we have a
/// memdep query of a load that ends up being a clobbering store.
static int AnalyzeLoadFromClobberingStore(const Type *LoadTy, Value *LoadPtr,
StoreInst *DepSI,
const TargetData &TD) {
// Cannot handle reading from store of first-class aggregate yet.
if (DepSI->getValueOperand()->getType()->isStructTy() ||
DepSI->getValueOperand()->getType()->isArrayTy())
return -1;
Value *StorePtr = DepSI->getPointerOperand();
uint64_t StoreSize =TD.getTypeSizeInBits(DepSI->getValueOperand()->getType());
return AnalyzeLoadFromClobberingWrite(LoadTy, LoadPtr,
StorePtr, StoreSize, TD);
}
static int AnalyzeLoadFromClobberingMemInst(const Type *LoadTy, Value *LoadPtr,
MemIntrinsic *MI,
const TargetData &TD) {
// If the mem operation is a non-constant size, we can't handle it.
ConstantInt *SizeCst = dyn_cast<ConstantInt>(MI->getLength());
if (SizeCst == 0) return -1;
uint64_t MemSizeInBits = SizeCst->getZExtValue()*8;
// If this is memset, we just need to see if the offset is valid in the size
// of the memset..
if (MI->getIntrinsicID() == Intrinsic::memset)
return AnalyzeLoadFromClobberingWrite(LoadTy, LoadPtr, MI->getDest(),
MemSizeInBits, TD);
// If we have a memcpy/memmove, the only case we can handle is if this is a
// copy from constant memory. In that case, we can read directly from the
// constant memory.
MemTransferInst *MTI = cast<MemTransferInst>(MI);
Constant *Src = dyn_cast<Constant>(MTI->getSource());
if (Src == 0) return -1;
GlobalVariable *GV = dyn_cast<GlobalVariable>(GetUnderlyingObject(Src));
if (GV == 0 || !GV->isConstant()) return -1;
// See if the access is within the bounds of the transfer.
int Offset = AnalyzeLoadFromClobberingWrite(LoadTy, LoadPtr,
MI->getDest(), MemSizeInBits, TD);
if (Offset == -1)
return Offset;
// Otherwise, see if we can constant fold a load from the constant with the
// offset applied as appropriate.
Src = ConstantExpr::getBitCast(Src,
llvm::Type::getInt8PtrTy(Src->getContext()));
Constant *OffsetCst =
ConstantInt::get(Type::getInt64Ty(Src->getContext()), (unsigned)Offset);
Src = ConstantExpr::getGetElementPtr(Src, &OffsetCst, 1);
Src = ConstantExpr::getBitCast(Src, PointerType::getUnqual(LoadTy));
if (ConstantFoldLoadFromConstPtr(Src, &TD))
return Offset;
return -1;
}
/// GetStoreValueForLoad - This function is called when we have a
/// memdep query of a load that ends up being a clobbering store. This means
/// that the store *may* provide bits used by the load but we can't be sure
/// because the pointers don't mustalias. Check this case to see if there is
/// anything more we can do before we give up.
static Value *GetStoreValueForLoad(Value *SrcVal, unsigned Offset,
const Type *LoadTy,
Instruction *InsertPt, const TargetData &TD){
LLVMContext &Ctx = SrcVal->getType()->getContext();
uint64_t StoreSize = (TD.getTypeSizeInBits(SrcVal->getType()) + 7) / 8;
uint64_t LoadSize = (TD.getTypeSizeInBits(LoadTy) + 7) / 8;
IRBuilder<> Builder(InsertPt->getParent(), InsertPt);
// Compute which bits of the stored value are being used by the load. Convert
// to an integer type to start with.
if (SrcVal->getType()->isPointerTy())
SrcVal = Builder.CreatePtrToInt(SrcVal, TD.getIntPtrType(Ctx), "tmp");
if (!SrcVal->getType()->isIntegerTy())
SrcVal = Builder.CreateBitCast(SrcVal, IntegerType::get(Ctx, StoreSize*8),
"tmp");
// Shift the bits to the least significant depending on endianness.
unsigned ShiftAmt;
if (TD.isLittleEndian())
ShiftAmt = Offset*8;
else
ShiftAmt = (StoreSize-LoadSize-Offset)*8;
if (ShiftAmt)
SrcVal = Builder.CreateLShr(SrcVal, ShiftAmt, "tmp");
if (LoadSize != StoreSize)
SrcVal = Builder.CreateTrunc(SrcVal, IntegerType::get(Ctx, LoadSize*8),
"tmp");
return CoerceAvailableValueToLoadType(SrcVal, LoadTy, InsertPt, TD);
}
/// GetMemInstValueForLoad - This function is called when we have a
/// memdep query of a load that ends up being a clobbering mem intrinsic.
static Value *GetMemInstValueForLoad(MemIntrinsic *SrcInst, unsigned Offset,
const Type *LoadTy, Instruction *InsertPt,
const TargetData &TD){
LLVMContext &Ctx = LoadTy->getContext();
uint64_t LoadSize = TD.getTypeSizeInBits(LoadTy)/8;
IRBuilder<> Builder(InsertPt->getParent(), InsertPt);
// We know that this method is only called when the mem transfer fully
// provides the bits for the load.
if (MemSetInst *MSI = dyn_cast<MemSetInst>(SrcInst)) {
// memset(P, 'x', 1234) -> splat('x'), even if x is a variable, and
// independently of what the offset is.
Value *Val = MSI->getValue();
if (LoadSize != 1)
Val = Builder.CreateZExt(Val, IntegerType::get(Ctx, LoadSize*8));
Value *OneElt = Val;
// Splat the value out to the right number of bits.
for (unsigned NumBytesSet = 1; NumBytesSet != LoadSize; ) {
// If we can double the number of bytes set, do it.
if (NumBytesSet*2 <= LoadSize) {
Value *ShVal = Builder.CreateShl(Val, NumBytesSet*8);
Val = Builder.CreateOr(Val, ShVal);
NumBytesSet <<= 1;
continue;
}
// Otherwise insert one byte at a time.
Value *ShVal = Builder.CreateShl(Val, 1*8);
Val = Builder.CreateOr(OneElt, ShVal);
++NumBytesSet;
}
return CoerceAvailableValueToLoadType(Val, LoadTy, InsertPt, TD);
}
// Otherwise, this is a memcpy/memmove from a constant global.
MemTransferInst *MTI = cast<MemTransferInst>(SrcInst);
Constant *Src = cast<Constant>(MTI->getSource());
// Otherwise, see if we can constant fold a load from the constant with the
// offset applied as appropriate.
Src = ConstantExpr::getBitCast(Src,
llvm::Type::getInt8PtrTy(Src->getContext()));
Constant *OffsetCst =
ConstantInt::get(Type::getInt64Ty(Src->getContext()), (unsigned)Offset);
Src = ConstantExpr::getGetElementPtr(Src, &OffsetCst, 1);
Src = ConstantExpr::getBitCast(Src, PointerType::getUnqual(LoadTy));
return ConstantFoldLoadFromConstPtr(Src, &TD);
}
namespace {
struct AvailableValueInBlock {
/// BB - The basic block in question.
BasicBlock *BB;
enum ValType {
SimpleVal, // A simple offsetted value that is accessed.
MemIntrin // A memory intrinsic which is loaded from.
};
/// V - The value that is live out of the block.
PointerIntPair<Value *, 1, ValType> Val;
/// Offset - The byte offset in Val that is interesting for the load query.
unsigned Offset;
static AvailableValueInBlock get(BasicBlock *BB, Value *V,
unsigned Offset = 0) {
AvailableValueInBlock Res;
Res.BB = BB;
Res.Val.setPointer(V);
Res.Val.setInt(SimpleVal);
Res.Offset = Offset;
return Res;
}
static AvailableValueInBlock getMI(BasicBlock *BB, MemIntrinsic *MI,
unsigned Offset = 0) {
AvailableValueInBlock Res;
Res.BB = BB;
Res.Val.setPointer(MI);
Res.Val.setInt(MemIntrin);
Res.Offset = Offset;
return Res;
}
bool isSimpleValue() const { return Val.getInt() == SimpleVal; }
Value *getSimpleValue() const {
assert(isSimpleValue() && "Wrong accessor");
return Val.getPointer();
}
MemIntrinsic *getMemIntrinValue() const {
assert(!isSimpleValue() && "Wrong accessor");
return cast<MemIntrinsic>(Val.getPointer());
}
/// MaterializeAdjustedValue - Emit code into this block to adjust the value
/// defined here to the specified type. This handles various coercion cases.
Value *MaterializeAdjustedValue(const Type *LoadTy,
const TargetData *TD) const {
Value *Res;
if (isSimpleValue()) {
Res = getSimpleValue();
if (Res->getType() != LoadTy) {
assert(TD && "Need target data to handle type mismatch case");
Res = GetStoreValueForLoad(Res, Offset, LoadTy, BB->getTerminator(),
*TD);
DEBUG(errs() << "GVN COERCED NONLOCAL VAL:\nOffset: " << Offset << " "
<< *getSimpleValue() << '\n'
<< *Res << '\n' << "\n\n\n");
}
} else {
Res = GetMemInstValueForLoad(getMemIntrinValue(), Offset,
LoadTy, BB->getTerminator(), *TD);
DEBUG(errs() << "GVN COERCED NONLOCAL MEM INTRIN:\nOffset: " << Offset
<< " " << *getMemIntrinValue() << '\n'
<< *Res << '\n' << "\n\n\n");
}
return Res;
}
};
}
/// ConstructSSAForLoadSet - Given a set of loads specified by ValuesPerBlock,
/// construct SSA form, allowing us to eliminate LI. This returns the value
/// that should be used at LI's definition site.
static Value *ConstructSSAForLoadSet(LoadInst *LI,
SmallVectorImpl<AvailableValueInBlock> &ValuesPerBlock,
const TargetData *TD,
const DominatorTree &DT,
AliasAnalysis *AA) {
// Check for the fully redundant, dominating load case. In this case, we can
// just use the dominating value directly.
if (ValuesPerBlock.size() == 1 &&
DT.properlyDominates(ValuesPerBlock[0].BB, LI->getParent()))
return ValuesPerBlock[0].MaterializeAdjustedValue(LI->getType(), TD);
// Otherwise, we have to construct SSA form.
SmallVector<PHINode*, 8> NewPHIs;
SSAUpdater SSAUpdate(&NewPHIs);
SSAUpdate.Initialize(LI->getType(), LI->getName());
const Type *LoadTy = LI->getType();
for (unsigned i = 0, e = ValuesPerBlock.size(); i != e; ++i) {
const AvailableValueInBlock &AV = ValuesPerBlock[i];
BasicBlock *BB = AV.BB;
if (SSAUpdate.HasValueForBlock(BB))
continue;
SSAUpdate.AddAvailableValue(BB, AV.MaterializeAdjustedValue(LoadTy, TD));
}
// Perform PHI construction.
Value *V = SSAUpdate.GetValueInMiddleOfBlock(LI->getParent());
// If new PHI nodes were created, notify alias analysis.
if (V->getType()->isPointerTy())
for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
AA->copyValue(LI, NewPHIs[i]);
// Now that we've copied information to the new PHIs, scan through
// them again and inform alias analysis that we've added potentially
// escaping uses to any values that are operands to these PHIs.
for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i) {
PHINode *P = NewPHIs[i];
for (unsigned ii = 0, ee = P->getNumIncomingValues(); ii != ee; ++ii)
AA->addEscapingUse(P->getOperandUse(2*ii));
}
return V;
}
static bool isLifetimeStart(const Instruction *Inst) {
if (const IntrinsicInst* II = dyn_cast<IntrinsicInst>(Inst))
return II->getIntrinsicID() == Intrinsic::lifetime_start;
return false;
}
/// processNonLocalLoad - Attempt to eliminate a load whose dependencies are
/// non-local by performing PHI construction.
bool GVN::processNonLocalLoad(LoadInst *LI,
SmallVectorImpl<Instruction*> &toErase) {
// Find the non-local dependencies of the load.
SmallVector<NonLocalDepResult, 64> Deps;
AliasAnalysis::Location Loc = VN.getAliasAnalysis()->getLocation(LI);
MD->getNonLocalPointerDependency(Loc, true, LI->getParent(), Deps);
//DEBUG(dbgs() << "INVESTIGATING NONLOCAL LOAD: "
// << Deps.size() << *LI << '\n');
// If we had to process more than one hundred blocks to find the
// dependencies, this load isn't worth worrying about. Optimizing
// it will be too expensive.
if (Deps.size() > 100)
return false;
// If we had a phi translation failure, we'll have a single entry which is a
// clobber in the current block. Reject this early.
if (Deps.size() == 1 && Deps[0].getResult().isClobber()) {
DEBUG(
dbgs() << "GVN: non-local load ";
WriteAsOperand(dbgs(), LI);
dbgs() << " is clobbered by " << *Deps[0].getResult().getInst() << '\n';
);
return false;
}
// Filter out useless results (non-locals, etc). Keep track of the blocks
// where we have a value available in repl, also keep track of whether we see
// dependencies that produce an unknown value for the load (such as a call
// that could potentially clobber the load).
SmallVector<AvailableValueInBlock, 16> ValuesPerBlock;
SmallVector<BasicBlock*, 16> UnavailableBlocks;
for (unsigned i = 0, e = Deps.size(); i != e; ++i) {
BasicBlock *DepBB = Deps[i].getBB();
MemDepResult DepInfo = Deps[i].getResult();
if (DepInfo.isClobber()) {
// The address being loaded in this non-local block may not be the same as
// the pointer operand of the load if PHI translation occurs. Make sure
// to consider the right address.
Value *Address = Deps[i].getAddress();
// If the dependence is to a store that writes to a superset of the bits
// read by the load, we can extract the bits we need for the load from the
// stored value.
if (StoreInst *DepSI = dyn_cast<StoreInst>(DepInfo.getInst())) {
if (TD && Address) {
int Offset = AnalyzeLoadFromClobberingStore(LI->getType(), Address,
DepSI, *TD);
if (Offset != -1) {
ValuesPerBlock.push_back(AvailableValueInBlock::get(DepBB,
DepSI->getValueOperand(),
Offset));
continue;
}
}
}
// If the clobbering value is a memset/memcpy/memmove, see if we can
// forward a value on from it.
if (MemIntrinsic *DepMI = dyn_cast<MemIntrinsic>(DepInfo.getInst())) {
if (TD && Address) {
int Offset = AnalyzeLoadFromClobberingMemInst(LI->getType(), Address,
DepMI, *TD);
if (Offset != -1) {
ValuesPerBlock.push_back(AvailableValueInBlock::getMI(DepBB, DepMI,
Offset));
continue;
}
}
}
UnavailableBlocks.push_back(DepBB);
continue;
}
Instruction *DepInst = DepInfo.getInst();
// Loading the allocation -> undef.
if (isa<AllocaInst>(DepInst) || isMalloc(DepInst) ||
// Loading immediately after lifetime begin -> undef.
isLifetimeStart(DepInst)) {
ValuesPerBlock.push_back(AvailableValueInBlock::get(DepBB,
UndefValue::get(LI->getType())));
continue;
}
if (StoreInst *S = dyn_cast<StoreInst>(DepInst)) {
// Reject loads and stores that are to the same address but are of
// different types if we have to.
if (S->getValueOperand()->getType() != LI->getType()) {
// If the stored value is larger or equal to the loaded value, we can
// reuse it.
if (TD == 0 || !CanCoerceMustAliasedValueToLoad(S->getValueOperand(),
LI->getType(), *TD)) {
UnavailableBlocks.push_back(DepBB);
continue;
}
}
ValuesPerBlock.push_back(AvailableValueInBlock::get(DepBB,
S->getValueOperand()));
continue;
}
if (LoadInst *LD = dyn_cast<LoadInst>(DepInst)) {
// If the types mismatch and we can't handle it, reject reuse of the load.
if (LD->getType() != LI->getType()) {
// If the stored value is larger or equal to the loaded value, we can
// reuse it.
if (TD == 0 || !CanCoerceMustAliasedValueToLoad(LD, LI->getType(),*TD)){
UnavailableBlocks.push_back(DepBB);
continue;
}
}
ValuesPerBlock.push_back(AvailableValueInBlock::get(DepBB, LD));
continue;
}
UnavailableBlocks.push_back(DepBB);
continue;
}
// If we have no predecessors that produce a known value for this load, exit
// early.
if (ValuesPerBlock.empty()) return false;
// If all of the instructions we depend on produce a known value for this
// load, then it is fully redundant and we can use PHI insertion to compute
// its value. Insert PHIs and remove the fully redundant value now.
if (UnavailableBlocks.empty()) {
DEBUG(dbgs() << "GVN REMOVING NONLOCAL LOAD: " << *LI << '\n');
// Perform PHI construction.
Value *V = ConstructSSAForLoadSet(LI, ValuesPerBlock, TD, *DT,
VN.getAliasAnalysis());
LI->replaceAllUsesWith(V);
if (isa<PHINode>(V))
V->takeName(LI);
if (V->getType()->isPointerTy())
MD->invalidateCachedPointerInfo(V);
VN.erase(LI);
toErase.push_back(LI);
++NumGVNLoad;
return true;
}
if (!EnablePRE || !EnableLoadPRE)
return false;
// Okay, we have *some* definitions of the value. This means that the value
// is available in some of our (transitive) predecessors. Lets think about
// doing PRE of this load. This will involve inserting a new load into the
// predecessor when it's not available. We could do this in general, but
// prefer to not increase code size. As such, we only do this when we know
// that we only have to insert *one* load (which means we're basically moving
// the load, not inserting a new one).
SmallPtrSet<BasicBlock *, 4> Blockers;
for (unsigned i = 0, e = UnavailableBlocks.size(); i != e; ++i)
Blockers.insert(UnavailableBlocks[i]);
// Lets find first basic block with more than one predecessor. Walk backwards
// through predecessors if needed.
BasicBlock *LoadBB = LI->getParent();
BasicBlock *TmpBB = LoadBB;
bool isSinglePred = false;
bool allSingleSucc = true;
while (TmpBB->getSinglePredecessor()) {
isSinglePred = true;
TmpBB = TmpBB->getSinglePredecessor();
if (TmpBB == LoadBB) // Infinite (unreachable) loop.
return false;
if (Blockers.count(TmpBB))
return false;
// If any of these blocks has more than one successor (i.e. if the edge we
// just traversed was critical), then there are other paths through this
// block along which the load may not be anticipated. Hoisting the load
// above this block would be adding the load to execution paths along
// which it was not previously executed.
if (TmpBB->getTerminator()->getNumSuccessors() != 1)
return false;
}
assert(TmpBB);
LoadBB = TmpBB;
// FIXME: It is extremely unclear what this loop is doing, other than
// artificially restricting loadpre.
if (isSinglePred) {
bool isHot = false;
for (unsigned i = 0, e = ValuesPerBlock.size(); i != e; ++i) {
const AvailableValueInBlock &AV = ValuesPerBlock[i];
if (AV.isSimpleValue())
// "Hot" Instruction is in some loop (because it dominates its dep.
// instruction).
if (Instruction *I = dyn_cast<Instruction>(AV.getSimpleValue()))
if (DT->dominates(LI, I)) {
isHot = true;
break;
}
}
// We are interested only in "hot" instructions. We don't want to do any
// mis-optimizations here.
if (!isHot)
return false;
}
// Check to see how many predecessors have the loaded value fully
// available.
DenseMap<BasicBlock*, Value*> PredLoads;
DenseMap<BasicBlock*, char> FullyAvailableBlocks;
for (unsigned i = 0, e = ValuesPerBlock.size(); i != e; ++i)
FullyAvailableBlocks[ValuesPerBlock[i].BB] = true;
for (unsigned i = 0, e = UnavailableBlocks.size(); i != e; ++i)
FullyAvailableBlocks[UnavailableBlocks[i]] = false;
SmallVector<std::pair<TerminatorInst*, unsigned>, 4> NeedToSplit;
for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB);
PI != E; ++PI) {
BasicBlock *Pred = *PI;
if (IsValueFullyAvailableInBlock(Pred, FullyAvailableBlocks)) {
continue;
}
PredLoads[Pred] = 0;
if (Pred->getTerminator()->getNumSuccessors() != 1) {
if (isa<IndirectBrInst>(Pred->getTerminator())) {
DEBUG(dbgs() << "COULD NOT PRE LOAD BECAUSE OF INDBR CRITICAL EDGE '"
<< Pred->getName() << "': " << *LI << '\n');
return false;
}
unsigned SuccNum = GetSuccessorNumber(Pred, LoadBB);
NeedToSplit.push_back(std::make_pair(Pred->getTerminator(), SuccNum));
}
}
if (!NeedToSplit.empty()) {
toSplit.append(NeedToSplit.begin(), NeedToSplit.end());
return false;
}
// Decide whether PRE is profitable for this load.
unsigned NumUnavailablePreds = PredLoads.size();
assert(NumUnavailablePreds != 0 &&
"Fully available value should be eliminated above!");
// If this load is unavailable in multiple predecessors, reject it.
// FIXME: If we could restructure the CFG, we could make a common pred with
// all the preds that don't have an available LI and insert a new load into
// that one block.
if (NumUnavailablePreds != 1)
return false;
// Check if the load can safely be moved to all the unavailable predecessors.
bool CanDoPRE = true;
SmallVector<Instruction*, 8> NewInsts;
for (DenseMap<BasicBlock*, Value*>::iterator I = PredLoads.begin(),
E = PredLoads.end(); I != E; ++I) {
BasicBlock *UnavailablePred = I->first;
// Do PHI translation to get its value in the predecessor if necessary. The
// returned pointer (if non-null) is guaranteed to dominate UnavailablePred.
// If all preds have a single successor, then we know it is safe to insert
// the load on the pred (?!?), so we can insert code to materialize the
// pointer if it is not available.
PHITransAddr Address(LI->getPointerOperand(), TD);
Value *LoadPtr = 0;
if (allSingleSucc) {
LoadPtr = Address.PHITranslateWithInsertion(LoadBB, UnavailablePred,
*DT, NewInsts);
} else {
Address.PHITranslateValue(LoadBB, UnavailablePred, DT);
LoadPtr = Address.getAddr();
}
// If we couldn't find or insert a computation of this phi translated value,
// we fail PRE.
if (LoadPtr == 0) {
DEBUG(dbgs() << "COULDN'T INSERT PHI TRANSLATED VALUE OF: "
<< *LI->getPointerOperand() << "\n");
CanDoPRE = false;
break;
}
// Make sure it is valid to move this load here. We have to watch out for:
// @1 = getelementptr (i8* p, ...
// test p and branch if == 0
// load @1
// It is valid to have the getelementptr before the test, even if p can
// be 0, as getelementptr only does address arithmetic.
// If we are not pushing the value through any multiple-successor blocks
// we do not have this case. Otherwise, check that the load is safe to
// put anywhere; this can be improved, but should be conservatively safe.
if (!allSingleSucc &&
// FIXME: REEVALUTE THIS.
!isSafeToLoadUnconditionally(LoadPtr,
UnavailablePred->getTerminator(),
LI->getAlignment(), TD)) {
CanDoPRE = false;
break;
}
I->second = LoadPtr;
}
if (!CanDoPRE) {
while (!NewInsts.empty())
NewInsts.pop_back_val()->eraseFromParent();
return false;
}
// Okay, we can eliminate this load by inserting a reload in the predecessor
// and using PHI construction to get the value in the other predecessors, do
// it.
DEBUG(dbgs() << "GVN REMOVING PRE LOAD: " << *LI << '\n');
DEBUG(if (!NewInsts.empty())
dbgs() << "INSERTED " << NewInsts.size() << " INSTS: "
<< *NewInsts.back() << '\n');
// Assign value numbers to the new instructions.
for (unsigned i = 0, e = NewInsts.size(); i != e; ++i) {
// FIXME: We really _ought_ to insert these value numbers into their
// parent's availability map. However, in doing so, we risk getting into
// ordering issues. If a block hasn't been processed yet, we would be
// marking a value as AVAIL-IN, which isn't what we intend.
VN.lookup_or_add(NewInsts[i]);
}
for (DenseMap<BasicBlock*, Value*>::iterator I = PredLoads.begin(),
E = PredLoads.end(); I != E; ++I) {
BasicBlock *UnavailablePred = I->first;
Value *LoadPtr = I->second;
Instruction *NewLoad = new LoadInst(LoadPtr, LI->getName()+".pre", false,
LI->getAlignment(),
UnavailablePred->getTerminator());
// Transfer the old load's TBAA tag to the new load.
if (MDNode *Tag = LI->getMetadata(LLVMContext::MD_tbaa))
NewLoad->setMetadata(LLVMContext::MD_tbaa, Tag);
// Add the newly created load.
ValuesPerBlock.push_back(AvailableValueInBlock::get(UnavailablePred,
NewLoad));
MD->invalidateCachedPointerInfo(LoadPtr);
DEBUG(dbgs() << "GVN INSERTED " << *NewLoad << '\n');
}
// Perform PHI construction.
Value *V = ConstructSSAForLoadSet(LI, ValuesPerBlock, TD, *DT,
VN.getAliasAnalysis());
LI->replaceAllUsesWith(V);
if (isa<PHINode>(V))
V->takeName(LI);
if (V->getType()->isPointerTy())
MD->invalidateCachedPointerInfo(V);
VN.erase(LI);
toErase.push_back(LI);
++NumPRELoad;
return true;
}
/// processLoad - Attempt to eliminate a load, first by eliminating it
/// locally, and then attempting non-local elimination if that fails.
bool GVN::processLoad(LoadInst *L, SmallVectorImpl<Instruction*> &toErase) {
if (!MD)
return false;
if (L->isVolatile())
return false;
// ... to a pointer that has been loaded from before...
MemDepResult Dep = MD->getDependency(L);
// If the value isn't available, don't do anything!
if (Dep.isClobber()) {
// Check to see if we have something like this:
// store i32 123, i32* %P
// %A = bitcast i32* %P to i8*
// %B = gep i8* %A, i32 1
// %C = load i8* %B
//
// We could do that by recognizing if the clobber instructions are obviously
// a common base + constant offset, and if the previous store (or memset)
// completely covers this load. This sort of thing can happen in bitfield
// access code.
Value *AvailVal = 0;
if (StoreInst *DepSI = dyn_cast<StoreInst>(Dep.getInst()))
if (TD) {
int Offset = AnalyzeLoadFromClobberingStore(L->getType(),
L->getPointerOperand(),
DepSI, *TD);
if (Offset != -1)
AvailVal = GetStoreValueForLoad(DepSI->getValueOperand(), Offset,
L->getType(), L, *TD);
}
// If the clobbering value is a memset/memcpy/memmove, see if we can forward
// a value on from it.
if (MemIntrinsic *DepMI = dyn_cast<MemIntrinsic>(Dep.getInst())) {
if (TD) {
int Offset = AnalyzeLoadFromClobberingMemInst(L->getType(),
L->getPointerOperand(),
DepMI, *TD);
if (Offset != -1)
AvailVal = GetMemInstValueForLoad(DepMI, Offset, L->getType(), L,*TD);
}
}
if (AvailVal) {
DEBUG(dbgs() << "GVN COERCED INST:\n" << *Dep.getInst() << '\n'
<< *AvailVal << '\n' << *L << "\n\n\n");
// Replace the load!
L->replaceAllUsesWith(AvailVal);
if (AvailVal->getType()->isPointerTy())
MD->invalidateCachedPointerInfo(AvailVal);
VN.erase(L);
toErase.push_back(L);
++NumGVNLoad;
return true;
}
DEBUG(
// fast print dep, using operator<< on instruction would be too slow
dbgs() << "GVN: load ";
WriteAsOperand(dbgs(), L);
Instruction *I = Dep.getInst();
dbgs() << " is clobbered by " << *I << '\n';
);
return false;
}
// If it is defined in another block, try harder.
if (Dep.isNonLocal())
return processNonLocalLoad(L, toErase);
Instruction *DepInst = Dep.getInst();
if (StoreInst *DepSI = dyn_cast<StoreInst>(DepInst)) {
Value *StoredVal = DepSI->getValueOperand();
// The store and load are to a must-aliased pointer, but they may not
// actually have the same type. See if we know how to reuse the stored
// value (depending on its type).
if (StoredVal->getType() != L->getType()) {
if (TD) {
StoredVal = CoerceAvailableValueToLoadType(StoredVal, L->getType(),
L, *TD);
if (StoredVal == 0)
return false;
DEBUG(dbgs() << "GVN COERCED STORE:\n" << *DepSI << '\n' << *StoredVal
<< '\n' << *L << "\n\n\n");
}
else
return false;
}
// Remove it!
L->replaceAllUsesWith(StoredVal);
if (StoredVal->getType()->isPointerTy())
MD->invalidateCachedPointerInfo(StoredVal);
VN.erase(L);
toErase.push_back(L);
++NumGVNLoad;
return true;
}
if (LoadInst *DepLI = dyn_cast<LoadInst>(DepInst)) {
Value *AvailableVal = DepLI;
// The loads are of a must-aliased pointer, but they may not actually have
// the same type. See if we know how to reuse the previously loaded value
// (depending on its type).
if (DepLI->getType() != L->getType()) {
if (TD) {
AvailableVal = CoerceAvailableValueToLoadType(DepLI, L->getType(), L,*TD);
if (AvailableVal == 0)
return false;
DEBUG(dbgs() << "GVN COERCED LOAD:\n" << *DepLI << "\n" << *AvailableVal
<< "\n" << *L << "\n\n\n");
}
else
return false;
}
// Remove it!
L->replaceAllUsesWith(AvailableVal);
if (DepLI->getType()->isPointerTy())
MD->invalidateCachedPointerInfo(DepLI);
VN.erase(L);
toErase.push_back(L);
++NumGVNLoad;
return true;
}
// If this load really doesn't depend on anything, then we must be loading an
// undef value. This can happen when loading for a fresh allocation with no
// intervening stores, for example.
if (isa<AllocaInst>(DepInst) || isMalloc(DepInst)) {
L->replaceAllUsesWith(UndefValue::get(L->getType()));
VN.erase(L);
toErase.push_back(L);
++NumGVNLoad;
return true;
}
// If this load occurs either right after a lifetime begin,
// then the loaded value is undefined.
if (IntrinsicInst* II = dyn_cast<IntrinsicInst>(DepInst)) {
if (II->getIntrinsicID() == Intrinsic::lifetime_start) {
L->replaceAllUsesWith(UndefValue::get(L->getType()));
VN.erase(L);
toErase.push_back(L);
++NumGVNLoad;
return true;
}
}
return false;
}
// findLeader - In order to find a leader for a given value number at a
// specific basic block, we first obtain the list of all Values for that number,
// and then scan the list to find one whose block dominates the block in
// question. This is fast because dominator tree queries consist of only
// a few comparisons of DFS numbers.
Value *GVN::findLeader(BasicBlock *BB, uint32_t num) {
LeaderTableEntry Vals = LeaderTable[num];
if (!Vals.Val) return 0;
Value *Val = 0;
if (DT->dominates(Vals.BB, BB)) {
Val = Vals.Val;
if (isa<Constant>(Val)) return Val;
}
LeaderTableEntry* Next = Vals.Next;
while (Next) {
if (DT->dominates(Next->BB, BB)) {
if (isa<Constant>(Next->Val)) return Next->Val;
if (!Val) Val = Next->Val;
}
Next = Next->Next;
}
return Val;
}
/// processInstruction - When calculating availability, handle an instruction
/// by inserting it into the appropriate sets
bool GVN::processInstruction(Instruction *I,
SmallVectorImpl<Instruction*> &toErase) {
// Ignore dbg info intrinsics.
if (isa<DbgInfoIntrinsic>(I))
return false;
// If the instruction can be easily simplified then do so now in preference
// to value numbering it. Value numbering often exposes redundancies, for
// example if it determines that %y is equal to %x then the instruction
// "%z = and i32 %x, %y" becomes "%z = and i32 %x, %x" which we now simplify.
if (Value *V = SimplifyInstruction(I, TD, DT)) {
I->replaceAllUsesWith(V);
if (MD && V->getType()->isPointerTy())
MD->invalidateCachedPointerInfo(V);
VN.erase(I);
toErase.push_back(I);
return true;
}
if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
bool Changed = processLoad(LI, toErase);
if (!Changed) {
unsigned Num = VN.lookup_or_add(LI);
addToLeaderTable(Num, LI, LI->getParent());
}
return Changed;
}
// For conditions branches, we can perform simple conditional propagation on
// the condition value itself.
if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
if (!BI->isConditional() || isa<Constant>(BI->getCondition()))
return false;
Value *BranchCond = BI->getCondition();
uint32_t CondVN = VN.lookup_or_add(BranchCond);
BasicBlock *TrueSucc = BI->getSuccessor(0);
BasicBlock *FalseSucc = BI->getSuccessor(1);
if (TrueSucc->getSinglePredecessor())
addToLeaderTable(CondVN,
ConstantInt::getTrue(TrueSucc->getContext()),
TrueSucc);
if (FalseSucc->getSinglePredecessor())
addToLeaderTable(CondVN,
ConstantInt::getFalse(TrueSucc->getContext()),
FalseSucc);
return false;
}
// Instructions with void type don't return a value, so there's
// no point in trying to find redudancies in them.
if (I->getType()->isVoidTy()) return false;
uint32_t NextNum = VN.getNextUnusedValueNumber();
unsigned Num = VN.lookup_or_add(I);
// Allocations are always uniquely numbered, so we can save time and memory
// by fast failing them.
if (isa<AllocaInst>(I) || isa<TerminatorInst>(I) || isa<PHINode>(I)) {
addToLeaderTable(Num, I, I->getParent());
return false;
}
// If the number we were assigned was a brand new VN, then we don't
// need to do a lookup to see if the number already exists
// somewhere in the domtree: it can't!
if (Num == NextNum) {
addToLeaderTable(Num, I, I->getParent());
return false;
}
// Perform fast-path value-number based elimination of values inherited from
// dominators.
Value *repl = findLeader(I->getParent(), Num);
if (repl == 0) {
// Failure, just remember this instance for future use.
addToLeaderTable(Num, I, I->getParent());
return false;
}
// Remove it!
VN.erase(I);
I->replaceAllUsesWith(repl);
if (MD && repl->getType()->isPointerTy())
MD->invalidateCachedPointerInfo(repl);
toErase.push_back(I);
return true;
}
/// runOnFunction - This is the main transformation entry point for a function.
bool GVN::runOnFunction(Function& F) {
if (!NoLoads)
MD = &getAnalysis<MemoryDependenceAnalysis>();
DT = &getAnalysis<DominatorTree>();
TD = getAnalysisIfAvailable<TargetData>();
VN.setAliasAnalysis(&getAnalysis<AliasAnalysis>());
VN.setMemDep(MD);
VN.setDomTree(DT);
bool Changed = false;
bool ShouldContinue = true;
// Merge unconditional branches, allowing PRE to catch more
// optimization opportunities.
for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ) {
BasicBlock *BB = FI;
++FI;
bool removedBlock = MergeBlockIntoPredecessor(BB, this);
if (removedBlock) ++NumGVNBlocks;
Changed |= removedBlock;
}
unsigned Iteration = 0;
while (ShouldContinue) {
DEBUG(dbgs() << "GVN iteration: " << Iteration << "\n");
ShouldContinue = iterateOnFunction(F);
if (splitCriticalEdges())
ShouldContinue = true;
Changed |= ShouldContinue;
++Iteration;
}
if (EnablePRE) {
bool PREChanged = true;
while (PREChanged) {
PREChanged = performPRE(F);
Changed |= PREChanged;
}
}
// FIXME: Should perform GVN again after PRE does something. PRE can move
// computations into blocks where they become fully redundant. Note that
// we can't do this until PRE's critical edge splitting updates memdep.
// Actually, when this happens, we should just fully integrate PRE into GVN.
cleanupGlobalSets();
return Changed;
}
bool GVN::processBlock(BasicBlock *BB) {
// FIXME: Kill off toErase by doing erasing eagerly in a helper function (and
// incrementing BI before processing an instruction).
SmallVector<Instruction*, 8> toErase;
bool ChangedFunction = false;
for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
BI != BE;) {
ChangedFunction |= processInstruction(BI, toErase);
if (toErase.empty()) {
++BI;
continue;
}
// If we need some instructions deleted, do it now.
NumGVNInstr += toErase.size();
// Avoid iterator invalidation.
bool AtStart = BI == BB->begin();
if (!AtStart)
--BI;
for (SmallVector<Instruction*, 4>::iterator I = toErase.begin(),
E = toErase.end(); I != E; ++I) {
DEBUG(dbgs() << "GVN removed: " << **I << '\n');
if (MD) MD->removeInstruction(*I);
(*I)->eraseFromParent();
DEBUG(verifyRemoved(*I));
}
toErase.clear();
if (AtStart)
BI = BB->begin();
else
++BI;
}
return ChangedFunction;
}
/// performPRE - Perform a purely local form of PRE that looks for diamond
/// control flow patterns and attempts to perform simple PRE at the join point.
bool GVN::performPRE(Function &F) {
bool Changed = false;
DenseMap<BasicBlock*, Value*> predMap;
for (df_iterator<BasicBlock*> DI = df_begin(&F.getEntryBlock()),
DE = df_end(&F.getEntryBlock()); DI != DE; ++DI) {
BasicBlock *CurrentBlock = *DI;
// Nothing to PRE in the entry block.
if (CurrentBlock == &F.getEntryBlock()) continue;
for (BasicBlock::iterator BI = CurrentBlock->begin(),
BE = CurrentBlock->end(); BI != BE; ) {
Instruction *CurInst = BI++;
if (isa<AllocaInst>(CurInst) ||
isa<TerminatorInst>(CurInst) || isa<PHINode>(CurInst) ||
CurInst->getType()->isVoidTy() ||
CurInst->mayReadFromMemory() || CurInst->mayHaveSideEffects() ||
isa<DbgInfoIntrinsic>(CurInst))
continue;
// We don't currently value number ANY inline asm calls.
if (CallInst *CallI = dyn_cast<CallInst>(CurInst))
if (CallI->isInlineAsm())
continue;
uint32_t ValNo = VN.lookup(CurInst);
// Look for the predecessors for PRE opportunities. We're
// only trying to solve the basic diamond case, where
// a value is computed in the successor and one predecessor,
// but not the other. We also explicitly disallow cases
// where the successor is its own predecessor, because they're
// more complicated to get right.
unsigned NumWith = 0;
unsigned NumWithout = 0;
BasicBlock *PREPred = 0;
predMap.clear();
for (pred_iterator PI = pred_begin(CurrentBlock),
PE = pred_end(CurrentBlock); PI != PE; ++PI) {
BasicBlock *P = *PI;
// We're not interested in PRE where the block is its
// own predecessor, or in blocks with predecessors
// that are not reachable.
if (P == CurrentBlock) {
NumWithout = 2;
break;
} else if (!DT->dominates(&F.getEntryBlock(), P)) {
NumWithout = 2;
break;
}
Value* predV = findLeader(P, ValNo);
if (predV == 0) {
PREPred = P;
++NumWithout;
} else if (predV == CurInst) {
NumWithout = 2;
} else {
predMap[P] = predV;
++NumWith;
}
}
// Don't do PRE when it might increase code size, i.e. when
// we would need to insert instructions in more than one pred.
if (NumWithout != 1 || NumWith == 0)
continue;
// Don't do PRE across indirect branch.
if (isa<IndirectBrInst>(PREPred->getTerminator()))
continue;
// We can't do PRE safely on a critical edge, so instead we schedule
// the edge to be split and perform the PRE the next time we iterate
// on the function.
unsigned SuccNum = GetSuccessorNumber(PREPred, CurrentBlock);
if (isCriticalEdge(PREPred->getTerminator(), SuccNum)) {
toSplit.push_back(std::make_pair(PREPred->getTerminator(), SuccNum));
continue;
}
// Instantiate the expression in the predecessor that lacked it.
// Because we are going top-down through the block, all value numbers
// will be available in the predecessor by the time we need them. Any
// that weren't originally present will have been instantiated earlier
// in this loop.
Instruction *PREInstr = CurInst->clone();
bool success = true;
for (unsigned i = 0, e = CurInst->getNumOperands(); i != e; ++i) {
Value *Op = PREInstr->getOperand(i);
if (isa<Argument>(Op) || isa<Constant>(Op) || isa<GlobalValue>(Op))
continue;
if (Value *V = findLeader(PREPred, VN.lookup(Op))) {
PREInstr->setOperand(i, V);
} else {
success = false;
break;
}
}
// Fail out if we encounter an operand that is not available in
// the PRE predecessor. This is typically because of loads which
// are not value numbered precisely.
if (!success) {
delete PREInstr;
DEBUG(verifyRemoved(PREInstr));
continue;
}
PREInstr->insertBefore(PREPred->getTerminator());
PREInstr->setName(CurInst->getName() + ".pre");
predMap[PREPred] = PREInstr;
VN.add(PREInstr, ValNo);
++NumGVNPRE;
// Update the availability map to include the new instruction.
addToLeaderTable(ValNo, PREInstr, PREPred);
// Create a PHI to make the value available in this block.
PHINode* Phi = PHINode::Create(CurInst->getType(),
CurInst->getName() + ".pre-phi",
CurrentBlock->begin());
for (pred_iterator PI = pred_begin(CurrentBlock),
PE = pred_end(CurrentBlock); PI != PE; ++PI) {
BasicBlock *P = *PI;
Phi->addIncoming(predMap[P], P);
}
VN.add(Phi, ValNo);
addToLeaderTable(ValNo, Phi, CurrentBlock);
CurInst->replaceAllUsesWith(Phi);
if (Phi->getType()->isPointerTy()) {
// Because we have added a PHI-use of the pointer value, it has now
// "escaped" from alias analysis' perspective. We need to inform
// AA of this.
for (unsigned ii = 0, ee = Phi->getNumIncomingValues(); ii != ee; ++ii)
VN.getAliasAnalysis()->addEscapingUse(Phi->getOperandUse(2*ii));
if (MD)
MD->invalidateCachedPointerInfo(Phi);
}
VN.erase(CurInst);
removeFromLeaderTable(ValNo, CurInst, CurrentBlock);
DEBUG(dbgs() << "GVN PRE removed: " << *CurInst << '\n');
if (MD) MD->removeInstruction(CurInst);
CurInst->eraseFromParent();
DEBUG(verifyRemoved(CurInst));
Changed = true;
}
}
if (splitCriticalEdges())
Changed = true;
return Changed;
}
/// splitCriticalEdges - Split critical edges found during the previous
/// iteration that may enable further optimization.
bool GVN::splitCriticalEdges() {
if (toSplit.empty())
return false;
do {
std::pair<TerminatorInst*, unsigned> Edge = toSplit.pop_back_val();
SplitCriticalEdge(Edge.first, Edge.second, this);
} while (!toSplit.empty());
if (MD) MD->invalidateCachedPredecessors();
return true;
}
/// iterateOnFunction - Executes one iteration of GVN
bool GVN::iterateOnFunction(Function &F) {
cleanupGlobalSets();
// Top-down walk of the dominator tree
bool Changed = false;
#if 0
// Needed for value numbering with phi construction to work.
ReversePostOrderTraversal<Function*> RPOT(&F);
for (ReversePostOrderTraversal<Function*>::rpo_iterator RI = RPOT.begin(),
RE = RPOT.end(); RI != RE; ++RI)
Changed |= processBlock(*RI);
#else
for (df_iterator<DomTreeNode*> DI = df_begin(DT->getRootNode()),
DE = df_end(DT->getRootNode()); DI != DE; ++DI)
Changed |= processBlock(DI->getBlock());
#endif
return Changed;
}
void GVN::cleanupGlobalSets() {
VN.clear();
LeaderTable.clear();
TableAllocator.Reset();
}
/// verifyRemoved - Verify that the specified instruction does not occur in our
/// internal data structures.
void GVN::verifyRemoved(const Instruction *Inst) const {
VN.verifyRemoved(Inst);
// Walk through the value number scope to make sure the instruction isn't
// ferreted away in it.
for (DenseMap<uint32_t, LeaderTableEntry>::const_iterator
I = LeaderTable.begin(), E = LeaderTable.end(); I != E; ++I) {
const LeaderTableEntry *Node = &I->second;
assert(Node->Val != Inst && "Inst still in value numbering scope!");
while (Node->Next) {
Node = Node->Next;
assert(Node->Val != Inst && "Inst still in value numbering scope!");
}
}
}
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