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completely rewrite the memory promotion algorithm in LICM. 

Among other things, this uses SSAUpdater instead of 
PromoteMemToReg.

llvm-svn: 112417
This commit is contained in:
Chris Lattner 2010-08-29 06:43:52 +00:00
parent d0c054886c
commit 1dc98b47b5
1 changed files with 218 additions and 203 deletions
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421
llvm/lib/Transforms/Scalar/LICM.cpp
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@ -26,8 +26,7 @@
// pointer. There are no calls in the loop which mod/ref the pointer.
// If these conditions are true, we can promote the loads and stores in the
// loop of the pointer to use a temporary alloca'd variable. We then use
// the mem2reg functionality to construct the appropriate SSA form for the
// variable.
// the SSAUpdater to construct the appropriate SSA form for the value.
//
//===----------------------------------------------------------------------===//
@ -44,7 +43,6 @@
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/CommandLine.h"
@ -205,20 +203,7 @@ namespace {
bool isLoopInvariantInst(Instruction &I);
bool isNotUsedInLoop(Instruction &I);
/// PromoteValuesInLoop - Look at the stores in the loop and promote as many
/// to scalars as we can.
///
void PromoteValuesInLoop();
/// FindPromotableValuesInLoop - Check the current loop for stores to
/// definite pointers, which are not loaded and stored through may aliases.
/// If these are found, create an alloca for the value, add it to the
/// PromotedValues list, and keep track of the mapping from value to
/// alloca...
///
void FindPromotableValuesInLoop(
std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
DenseMap<Value*, AllocaInst*> &Val2AlMap);
void PromoteAliasSet(AliasSet &AS);
};
}
@ -284,10 +269,14 @@ bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
HoistRegion(DT->getNode(L->getHeader()));
// Now that all loop invariants have been removed from the loop, promote any
// memory references to scalars that we can...
if (!DisablePromotion && Preheader && L->hasDedicatedExits())
PromoteValuesInLoop();
// memory references to scalars that we can.
if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
// Loop over all of the alias sets in the tracker object.
for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
I != E; ++I)
PromoteAliasSet(*I);
}
// Clear out loops state information for the next iteration
CurLoop = 0;
Preheader = 0;
@ -622,212 +611,238 @@ bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
return true;
}
/// PromoteValuesInLoop - Try to promote memory values to scalars by sinking
/// PromoteAliasSet - Try to promote memory values to scalars by sinking
/// stores out of the loop and moving loads to before the loop. We do this by
/// looping over the stores in the loop, looking for stores to Must pointers
/// which are loop invariant. We promote these memory locations to use allocas
/// instead. These allocas can easily be raised to register values by the
/// PromoteMem2Reg functionality.
/// which are loop invariant.
///
void LICM::PromoteValuesInLoop() {
// PromotedValues - List of values that are promoted out of the loop. Each
// value has an alloca instruction for it, and a canonical version of the
// pointer.
std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
DenseMap<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca
void LICM::PromoteAliasSet(AliasSet &AS) {
// We can promote this alias set if it has a store, if it is a "Must" alias
// set, if the pointer is loop invariant, and if we are not eliminating any
// volatile loads or stores.
if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
return;
assert(!AS.empty() &&
"Must alias set should have at least one pointer element in it!");
Value *SomePtr = AS.begin()->getValue();
FindPromotableValuesInLoop(PromotedValues, ValueToAllocaMap);
if (ValueToAllocaMap.empty()) return; // If there are values to promote.
Changed = true;
NumPromoted += PromotedValues.size();
std::vector<Value*> PointerValueNumbers;
// Emit a copy from the value into the alloca'd value in the loop preheader
TerminatorInst *LoopPredInst = Preheader->getTerminator();
for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
Value *Ptr = PromotedValues[i].second;
// If we are promoting a pointer value, update alias information for the
// inserted load.
Value *LoadValue = 0;
if (cast<PointerType>(Ptr->getType())->getElementType()->isPointerTy()) {
// Locate a load or store through the pointer, and assign the same value
// to LI as we are loading or storing. Since we know that the value is
// stored in this loop, this will always succeed.
for (Value::use_iterator UI = Ptr->use_begin(), E = Ptr->use_end();
UI != E; ++UI) {
User *U = *UI;
if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
LoadValue = LI;
break;
} else if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
if (SI->getOperand(1) == Ptr) {
LoadValue = SI->getOperand(0);
break;
}
}
}
assert(LoadValue && "No store through the pointer found!");
PointerValueNumbers.push_back(LoadValue); // Remember this for later.
}
// Load from the memory we are promoting.
LoadInst *LI = new LoadInst(Ptr, Ptr->getName()+".promoted", LoopPredInst);
if (LoadValue) CurAST->copyValue(LoadValue, LI);
// Store into the temporary alloca.
new StoreInst(LI, PromotedValues[i].first, LoopPredInst);
}
// Scan the basic blocks in the loop, replacing uses of our pointers with
// uses of the allocas in question.
// It isn't safe to promote a load/store from the loop if the load/store is
// conditional. For example, turning:
//
for (Loop::block_iterator I = CurLoop->block_begin(),
E = CurLoop->block_end(); I != E; ++I) {
BasicBlock *BB = *I;
// Rewrite all loads and stores in the block of the pointer...
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
if (LoadInst *L = dyn_cast<LoadInst>(II)) {
DenseMap<Value*, AllocaInst*>::iterator
I = ValueToAllocaMap.find(L->getOperand(0));
if (I != ValueToAllocaMap.end())
L->setOperand(0, I->second); // Rewrite load instruction...
} else if (StoreInst *S = dyn_cast<StoreInst>(II)) {
DenseMap<Value*, AllocaInst*>::iterator
I = ValueToAllocaMap.find(S->getOperand(1));
if (I != ValueToAllocaMap.end())
S->setOperand(1, I->second); // Rewrite store instruction...
}
}
}
// Now that the body of the loop uses the allocas instead of the original
// memory locations, insert code to copy the alloca value back into the
// original memory location on all exits from the loop.
SmallVector<BasicBlock*, 8> ExitBlocks;
CurLoop->getUniqueExitBlocks(ExitBlocks);
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
// Copy all of the allocas into their memory locations.
BasicBlock::iterator BI = ExitBlocks[i]->getFirstNonPHI();
Instruction *InsertPos = BI;
unsigned PVN = 0;
for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
// Load from the alloca.
LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
// If this is a pointer type, update alias info appropriately.
if (LI->getType()->isPointerTy())
CurAST->copyValue(PointerValueNumbers[PVN++], LI);
// Store into the memory we promoted.
new StoreInst(LI, PromotedValues[i].second, InsertPos);
}
}
// Now that we have done the deed, use the mem2reg functionality to promote
// all of the new allocas we just created into real SSA registers.
// for () { if (c) *P += 1; }
//
std::vector<AllocaInst*> PromotedAllocas;
PromotedAllocas.reserve(PromotedValues.size());
for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i)
PromotedAllocas.push_back(PromotedValues[i].first);
PromoteMemToReg(PromotedAllocas, *DT, *DF, CurAST);
}
// into:
//
// tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
//
// is not safe, because *P may only be valid to access if 'c' is true.
//
// It is safe to promote P if all uses are direct load/stores and if at
// least one is guaranteed to be executed.
bool GuaranteedToExecute = false;
SmallVector<Instruction*, 64> LoopUses;
SmallPtrSet<Value*, 4> PointerMustAliases;
/// FindPromotableValuesInLoop - Check the current loop for stores to definite
/// pointers, which are not loaded and stored through may aliases and are safe
/// for promotion. If these are found, create an alloca for the value, add it
/// to the PromotedValues list, and keep track of the mapping from value to
/// alloca.
void LICM::FindPromotableValuesInLoop(
std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
DenseMap<Value*, AllocaInst*> &ValueToAllocaMap) {
Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();
// Loop over all of the alias sets in the tracker object.
for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
I != E; ++I) {
AliasSet &AS = *I;
// We can promote this alias set if it has a store, if it is a "Must" alias
// set, if the pointer is loop invariant, and if we are not eliminating any
// volatile loads or stores.
if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
continue;
// Check that all of the pointers in the alias set have the same type. We
// cannot (yet) promote a memory location that is loaded and stored in
// different sizes.
for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
Value *ASIV = ASI->getValue();
PointerMustAliases.insert(ASIV);
assert(!AS.empty() &&
"Must alias set should have at least one pointer element in it!");
Value *V = AS.begin()->getValue();
// Check that all of the pointers in the alias set have the same type. We
// cannot (yet) promote a memory location that is loaded and stored in
// different sizes.
{
bool PointerOk = true;
for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
if (V->getType() != I->getValue()->getType()) {
PointerOk = false;
break;
}
if (!PointerOk)
continue;
}
// It isn't safe to promote a load/store from the loop if the load/store is
// conditional. For example, turning:
//
// for () { if (c) *P += 1; }
//
// into:
//
// tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
//
// is not safe, because *P may only be valid to access if 'c' is true.
//
// It is safe to promote P if all uses are direct load/stores and if at
// least one is guaranteed to be executed.
bool GuaranteedToExecute = false;
bool InvalidInst = false;
for (Value::use_iterator UI = V->use_begin(), UE = V->use_end();
if (SomePtr->getType() != ASIV->getType())
return;
for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
UI != UE; ++UI) {
// Ignore instructions not in this loop.
// Ignore instructions that are outside the loop.
Instruction *Use = dyn_cast<Instruction>(*UI);
if (!Use || !CurLoop->contains(Use))
continue;
if (!isa<LoadInst>(Use) && !isa<StoreInst>(Use)) {
InvalidInst = true;
break;
}
// If there is an non-load/store instruction in the loop, we can't promote
// it.
if (isa<LoadInst>(Use))
assert(!cast<LoadInst>(Use)->isVolatile() && "AST broken");
else if (isa<StoreInst>(Use))
assert(!cast<StoreInst>(Use)->isVolatile() &&
Use->getOperand(0) != ASIV && "AST broken");
else
return; // Not a load or store.
if (!GuaranteedToExecute)
GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use);
LoopUses.push_back(Use);
}
}
// If there isn't a guaranteed-to-execute instruction, we can't promote.
if (!GuaranteedToExecute)
return;
// Otherwise, this is safe to promote, lets do it!
DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
Changed = true;
++NumPromoted;
// We use the SSAUpdater interface to insert phi nodes as required.
SmallVector<PHINode*, 16> NewPHIs;
SSAUpdater SSA(&NewPHIs);
// It wants to know some value of the same type as what we'll be inserting.
Value *SomeValue;
if (isa<LoadInst>(LoopUses[0]))
SomeValue = LoopUses[0];
else
SomeValue = cast<StoreInst>(LoopUses[0])->getOperand(0);
SSA.Initialize(SomeValue);
// First step: bucket up uses of the pointers by the block they occur in.
// This is important because we have to handle multiple defs/uses in a block
// ourselves: SSAUpdater is purely for cross-block references.
// FIXME: Want a TinyVector<Instruction*> since there is usually 0/1 element.
DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock;
for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
Instruction *User = LoopUses[i];
UsesByBlock[User->getParent()].push_back(User);
}
// Okay, now we can iterate over all the blocks in the loop with uses,
// processing them. Keep track of which loads are loading a live-in value.
SmallVector<LoadInst*, 32> LiveInLoads;
for (unsigned LoopUse = 0, e = LoopUses.size(); LoopUse != e; ++LoopUse) {
Instruction *User = LoopUses[LoopUse];
std::vector<Instruction*> &BlockUses = UsesByBlock[User->getParent()];
// If this block has already been processed, ignore this repeat use.
if (BlockUses.empty()) continue;
// Okay, this is the first use in the block. If this block just has a
// single user in it, we can rewrite it trivially.
if (BlockUses.size() == 1) {
// If it is a store, it is a trivial def of the value in the block.
if (isa<StoreInst>(User)) {
SSA.AddAvailableValue(User->getParent(),
cast<StoreInst>(User)->getOperand(0));
} else {
// Otherwise it is a load, queue it to rewrite as a live-in load.
LiveInLoads.push_back(cast<LoadInst>(User));
}
BlockUses.clear();
continue;
}
// Otherwise, check to see if this block is all loads. If so, we can queue
// them all as live in loads.
bool HasStore = false;
for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) {
if (isa<StoreInst>(BlockUses[i])) {
HasStore = true;
break;
}
}
if (!HasStore) {
for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
BlockUses.clear();
continue;
}
// If there is an non-load/store instruction in the loop, we can't promote
// it. If there isn't a guaranteed-to-execute instruction, we can't
// promote.
if (InvalidInst || !GuaranteedToExecute)
continue;
// Otherwise, we have mixed loads and stores (or just a bunch of stores).
// Since SSAUpdater is purely for cross-block values, we need to determine
// the order of these instructions in the block. If the first use in the
// block is a load, then it uses the live in value. The last store defines
// the live out value. We handle this by doing a linear scan of the block.
BasicBlock *BB = User->getParent();
Value *StoredValue = 0;
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
if (LoadInst *L = dyn_cast<LoadInst>(II)) {
// If this is a load to an unrelated pointer, ignore it.
if (!PointerMustAliases.count(L->getOperand(0))) continue;
// If we haven't seen a store yet, this is a live in use, otherwise
// use the stored value.
if (StoredValue)
L->replaceAllUsesWith(StoredValue);
else
LiveInLoads.push_back(L);
continue;
}
if (StoreInst *S = dyn_cast<StoreInst>(II)) {
// If this is a load to an unrelated pointer, ignore it.
if (!PointerMustAliases.count(S->getOperand(1))) continue;
// Remember that this is the active value in the block.
StoredValue = S->getOperand(0);
}
}
const Type *Ty = cast<PointerType>(V->getType())->getElementType();
AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart);
PromotedValues.push_back(std::make_pair(AI, V));
// Update the AST and alias analysis.
CurAST->copyValue(V, AI);
for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
ValueToAllocaMap[I->getValue()] = AI;
DEBUG(dbgs() << "LICM: Promoting value: " << *V << "\n");
// The last stored value that happened is the live-out for the block.
assert(StoredValue && "Already checked that there is a store in block");
SSA.AddAvailableValue(BB, StoredValue);
BlockUses.clear();
}
// Now that all the intra-loop values are classified, set up the preheader.
// It gets a load of the pointer we're promoting, and it is the live-out value
// from the preheader.
LoadInst *PreheaderLoad = new LoadInst(SomePtr,SomePtr->getName()+".promoted",
Preheader->getTerminator());
SSA.AddAvailableValue(Preheader, PreheaderLoad);
// Now that the preheader is good to go, set up the exit blocks. Each exit
// block gets a store of the live-out values that feed them. Since we've
// already told the SSA updater about the defs in the loop and the preheader
// definition, it is all set and we can start using it.
SmallVector<BasicBlock*, 8> ExitBlocks;
CurLoop->getUniqueExitBlocks(ExitBlocks);
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
BasicBlock *ExitBlock = ExitBlocks[i];
Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
Instruction *InsertPos = ExitBlock->getFirstNonPHI();
new StoreInst(LiveInValue, SomePtr, InsertPos);
}
// Okay, now we rewrite all loads that use live-in values in the loop,
// inserting PHI nodes as necessary.
for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) {
LoadInst *ALoad = LiveInLoads[i];
ALoad->replaceAllUsesWith(SSA.GetValueInMiddleOfBlock(ALoad->getParent()));
}
// Now that everything is rewritten, delete the old instructions from the body
// of the loop. They should all be dead now.
for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
Instruction *User = LoopUses[i];
CurAST->deleteValue(User);
User->eraseFromParent();
}
// If the preheader load is itself a pointer, we need to tell alias analysis
// about the new pointer we created in the preheader block and about any PHI
// nodes that just got inserted.
if (PreheaderLoad->getType()->isPointerTy()) {
// Copy any value stored to or loaded from a must-alias of the pointer.
CurAST->copyValue(SomeValue, PreheaderLoad);
for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
CurAST->copyValue(SomeValue, NewPHIs[i]);
}
// fwew, we're done!
}
/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
AliasSetTracker *AST = LoopToAliasMap[L];