[llvm-commits] [llvm] r112417 - /llvm/trunk/lib/Transforms/Scalar/LICM.cpp
Chris Lattner
sabre at nondot.org
Sat Aug 28 23:43:52 PDT 2010
Author: lattner
Date: Sun Aug 29 01:43:52 2010
New Revision: 112417
URL: http://llvm.org/viewvc/llvm-project?rev=112417&view=rev
Log:
completely rewrite the memory promotion algorithm in LICM.
Among other things, this uses SSAUpdater instead of
PromoteMemToReg.
Modified:
llvm/trunk/lib/Transforms/Scalar/LICM.cpp
Modified: llvm/trunk/lib/Transforms/Scalar/LICM.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LICM.cpp?rev=112417&r1=112416&r2=112417&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LICM.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LICM.cpp Sun Aug 29 01:43:52 2010
@@ -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 @@
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 @@
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 @@
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
-
- 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);
- }
+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();
- // 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; }
+ //
+ // into:
+ //
+ // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
//
- 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);
-}
-
-/// 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();
+ // 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;
- // 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;
- // 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)
+ // 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;
+ }
+ }
- 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));
+ if (!HasStore) {
+ for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
+ LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
+ BlockUses.clear();
+ continue;
+ }
- // Update the AST and alias analysis.
- CurAST->copyValue(V, AI);
+ // 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;
- for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
- ValueToAllocaMap[I->getValue()] = AI;
+ // 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;
- DEBUG(dbgs() << "LICM: Promoting value: " << *V << "\n");
+ // Remember that this is the active value in the block.
+ StoredValue = S->getOperand(0);
+ }
+ }
+
+ // The last stored value that happened is the live-out for the block.
+ assert(StoredValue && "Already checked that there is a store in block");
+ SSA.AddAvailableValue(BB, StoredValue);
+ 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];
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