//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
//
-// This pass is a simple loop invariant code motion pass.
+// This pass is a simple loop invariant code motion pass. An interesting aspect
+// of this pass is that it uses alias analysis for two purposes:
+//
+// 1. Moving loop invariant loads out of loops. If we can determine that a
+// load inside of a loop never aliases anything stored to, we can hoist it
+// like any other instruction.
+// 2. Scalar Promotion of Memory - If there is a store instruction inside of
+// the loop, we try to move the store to happen AFTER the loop instead of
+// inside of the loop. This can only happen if a few conditions are true:
+// A. The pointer stored through is loop invariant
+// B. There are no stores or loads in the loop which _may_ alias the
+// 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.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Dominators.h"
-#include "llvm/iOperators.h"
-#include "llvm/iMemory.h"
+#include "llvm/Instructions.h"
+#include "llvm/DerivedTypes.h"
#include "llvm/Support/InstVisitor.h"
-#include "Support/STLExtras.h"
+#include "llvm/Support/CFG.h"
#include "Support/Statistic.h"
+#include "Support/CommandLine.h"
#include "llvm/Assembly/Writer.h"
#include <algorithm>
-using std::string;
namespace {
+ cl::opt<bool> DisablePromotion("disable-licm-promotion", cl::Hidden,
+ cl::desc("Disable memory promotion in LICM pass"));
+
Statistic<> NumHoisted("licm", "Number of instructions hoisted out of loop");
Statistic<> NumHoistedLoads("licm", "Number of load insts hoisted");
+ Statistic<> NumPromoted("licm", "Number of memory locations promoted to registers");
+
+ /// LoopBodyInfo - We recursively traverse loops from most-deeply-nested to
+ /// least-deeply-nested. For all of the loops nested within the current one,
+ /// we keep track of information so that we don't have to repeat queries.
+ ///
+ struct LoopBodyInfo {
+ std::vector<CallInst*> Calls; // Call instructions in loop
+ std::vector<InvokeInst*> Invokes; // Invoke instructions in loop
+
+ // StoredPointers - Targets of store instructions...
+ std::set<Value*> StoredPointers;
+
+ // LoadedPointers - Source pointers for load instructions...
+ std::set<Value*> LoadedPointers;
+
+ enum PointerClass {
+ PointerUnknown = 0, // Nothing is known about this pointer yet
+ PointerMustStore, // Memory is stored to ONLY through this pointer
+ PointerMayStore, // Memory is stored to through this or other pointers
+ PointerNoStore // Memory is not modified in this loop
+ };
+
+ // PointerIsModified - Keep track of information as we find out about it in
+ // the loop body...
+ //
+ std::map<Value*, enum PointerClass> PointerIsModified;
+
+ /// CantModifyAnyPointers - Return true if no memory modifying instructions
+ /// occur in this loop. This is just a conservative approximation, because
+ /// a call may not actually store anything.
+ bool CantModifyAnyPointers() const {
+ return Calls.empty() && Invokes.empty() && StoredPointers.empty();
+ }
+
+ /// incorporate - Incorporate information about a subloop into the current
+ /// loop.
+ void incorporate(const LoopBodyInfo &OtherLBI);
+ void incorporate(BasicBlock &BB); // do the same for a basic block
+
+ PointerClass getPointerInfo(Value *V, AliasAnalysis &AA) {
+ PointerClass &VInfo = PointerIsModified[V];
+ if (VInfo == PointerUnknown)
+ VInfo = calculatePointerInfo(V, AA);
+ return VInfo;
+ }
+ private:
+ /// calculatePointerInfo - Calculate information about the specified
+ /// pointer.
+ PointerClass calculatePointerInfo(Value *V, AliasAnalysis &AA) const;
+ };
+}
+
+/// incorporate - Incorporate information about a subloop into the current loop.
+void LoopBodyInfo::incorporate(const LoopBodyInfo &OtherLBI) {
+ // Do not incorporate NonModifiedPointers (which is just a cache) because it
+ // is too much trouble to make sure it's still valid.
+ Calls.insert (Calls.end(), OtherLBI.Calls.begin(), OtherLBI.Calls.end());
+ Invokes.insert(Invokes.end(),OtherLBI.Invokes.begin(),OtherLBI.Invokes.end());
+ StoredPointers.insert(OtherLBI.StoredPointers.begin(),
+ OtherLBI.StoredPointers.end());
+ LoadedPointers.insert(OtherLBI.LoadedPointers.begin(),
+ OtherLBI.LoadedPointers.end());
+}
+
+void LoopBodyInfo::incorporate(BasicBlock &BB) {
+ for (BasicBlock::iterator I = BB.begin(), E = --BB.end(); I != E; ++I)
+ if (CallInst *CI = dyn_cast<CallInst>(&*I))
+ Calls.push_back(CI);
+ else if (StoreInst *SI = dyn_cast<StoreInst>(&*I))
+ StoredPointers.insert(SI->getOperand(1));
+ else if (LoadInst *LI = dyn_cast<LoadInst>(&*I))
+ LoadedPointers.insert(LI->getOperand(0));
+
+ if (InvokeInst *II = dyn_cast<InvokeInst>(BB.getTerminator()))
+ Invokes.push_back(II);
+}
+
+
+// calculatePointerInfo - Calculate information about the specified pointer.
+LoopBodyInfo::PointerClass LoopBodyInfo::calculatePointerInfo(Value *V,
+ AliasAnalysis &AA) const {
+ for (unsigned i = 0, e = Calls.size(); i != e; ++i)
+ if (AA.canCallModify(*Calls[i], V))
+ return PointerMayStore;
+
+ for (unsigned i = 0, e = Invokes.size(); i != e; ++i)
+ if (AA.canInvokeModify(*Invokes[i], V))
+ return PointerMayStore;
+
+ PointerClass Result = PointerNoStore;
+ for (std::set<Value*>::const_iterator I = StoredPointers.begin(),
+ E = StoredPointers.end(); I != E; ++I)
+ if (AA.alias(V, *I))
+ if (V == *I)
+ Result = PointerMustStore; // If this is the only alias, return must
+ else
+ return PointerMayStore; // We have to return may
+ return Result;
+}
+namespace {
struct LICM : public FunctionPass, public InstVisitor<LICM> {
virtual bool runOnFunction(Function &F);
AU.addRequiredID(LoopPreheadersID);
AU.addRequired<LoopInfo>();
AU.addRequired<DominatorTree>();
+ AU.addRequired<DominanceFrontier>();
AU.addRequired<AliasAnalysis>();
}
private:
- Loop *CurLoop; // The current loop we are working on...
- BasicBlock *Preheader; // The preheader block of the current loop...
- bool Changed; // Set to true when we change anything.
- AliasAnalysis *AA; // Currently AliasAnalysis information
+ LoopInfo *LI; // Current LoopInfo
+ AliasAnalysis *AA; // Current AliasAnalysis information
+ bool Changed; // Set to true when we change anything.
+ BasicBlock *Preheader; // The preheader block of the current loop...
+ Loop *CurLoop; // The current loop we are working on...
+ LoopBodyInfo *CurLBI; // Information about the current loop...
/// visitLoop - Hoist expressions out of the specified loop...
///
- void visitLoop(Loop *L);
+ void visitLoop(Loop *L, LoopBodyInfo &LBI);
/// HoistRegion - Walk the specified region of the CFG (defined by all
/// blocks dominated by the specified block, and that are in the current
/// pointerInvalidatedByLoop - Return true if the body of this loop may
/// store into the memory location pointed to by V.
///
- bool pointerInvalidatedByLoop(Value *V);
+ bool pointerInvalidatedByLoop(Value *V) {
+ // Check to see if any of the basic blocks in CurLoop invalidate V.
+ return CurLBI->getPointerInfo(V, *AA) != LoopBodyInfo::PointerNoStore;
+ }
/// isLoopInvariant - Return true if the specified value is loop invariant
///
return true; // All non-instructions are loop invariant
}
+ /// 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
+ /// definate 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,
+ std::map<Value*, AllocaInst*> &Val2AlMap);
+
+
/// Instruction visitation handlers... these basically control whether or
/// not the specified instruction types are hoisted.
///
/// function, hoisting expressions out of loops if possible.
///
bool LICM::runOnFunction(Function &) {
- // Get information about the top level loops in the function...
- const std::vector<Loop*> &TopLevelLoops =
- getAnalysis<LoopInfo>().getTopLevelLoops();
+ Changed = false;
- // Get our alias analysis information...
+ // Get our Loop and Alias Analysis information...
+ LI = &getAnalysis<LoopInfo>();
AA = &getAnalysis<AliasAnalysis>();
- // Traverse loops in postorder, hoisting expressions out of the deepest loops
- // first.
- //
- Changed = false;
- std::for_each(TopLevelLoops.begin(), TopLevelLoops.end(),
- bind_obj(this, &LICM::visitLoop));
+ // Hoist expressions out of all of the top-level loops.
+ const std::vector<Loop*> &TopLevelLoops = LI->getTopLevelLoops();
+ for (std::vector<Loop*>::const_iterator I = TopLevelLoops.begin(),
+ E = TopLevelLoops.end(); I != E; ++I) {
+ LoopBodyInfo LBI;
+ LICM::visitLoop(*I, LBI);
+ }
return Changed;
}
/// visitLoop - Hoist expressions out of the specified loop...
///
-void LICM::visitLoop(Loop *L) {
+void LICM::visitLoop(Loop *L, LoopBodyInfo &LBI) {
// Recurse through all subloops before we process this loop...
- std::for_each(L->getSubLoops().begin(), L->getSubLoops().end(),
- bind_obj(this, &LICM::visitLoop));
+ for (std::vector<Loop*>::const_iterator I = L->getSubLoops().begin(),
+ E = L->getSubLoops().end(); I != E; ++I) {
+ LoopBodyInfo SubLBI;
+ LICM::visitLoop(*I, SubLBI);
+
+ // Incorporate information about the subloops into this loop...
+ LBI.incorporate(SubLBI);
+ }
CurLoop = L;
+ CurLBI = &LBI;
// Get the preheader block to move instructions into...
Preheader = L->getLoopPreheader();
assert(Preheader&&"Preheader insertion pass guarantees we have a preheader!");
+ // Loop over the body of this loop, looking for calls, invokes, and stores.
+ // Because subloops have already been incorporated into LBI, we skip blocks in
+ // subloops.
+ //
+ const std::vector<BasicBlock*> &LoopBBs = L->getBlocks();
+ for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
+ E = LoopBBs.end(); I != E; ++I)
+ if (LI->getLoopFor(*I) == L) // Ignore blocks in subloops...
+ LBI.incorporate(**I); // Incorporate the specified basic block
+
// We want to visit all of the instructions in this loop... that are not parts
// of our subloops (they have already had their invariants hoisted out of
// their loop, into this loop, so there is no need to process the BODIES of
//
HoistRegion(getAnalysis<DominatorTree>()[L->getHeader()]);
+ // Now that all loop invariants have been removed from the loop, promote any
+ // memory references to scalars that we can...
+ if (!DisablePromotion)
+ PromoteValuesInLoop();
+
// Clear out loops state information for the next iteration
CurLoop = 0;
Preheader = 0;
}
}
-/// pointerInvalidatedByLoop - Return true if the body of this loop may store
-/// into the memory location pointed to by V.
-///
-bool LICM::pointerInvalidatedByLoop(Value *V) {
- // Check to see if any of the basic blocks in CurLoop invalidate V.
- for (unsigned i = 0, e = CurLoop->getBlocks().size(); i != e; ++i)
- if (AA->canBasicBlockModify(*CurLoop->getBlocks()[i], V))
- return true;
- return false;
+/// PromoteValuesInLoop - 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.
+///
+void LICM::PromoteValuesInLoop() {
+ // PromotedValues - List of values that are promoted out of the loop. Each
+ // value has an alloca instruction for it, and a cannonical version of the
+ // pointer.
+ std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
+ std::map<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();
+
+ // 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) {
+ // Load from the memory we are promoting...
+ LoadInst *LI = new LoadInst(PromotedValues[i].second,
+ PromotedValues[i].second->getName()+".promoted",
+ LoopPredInst);
+ // 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. If we find a branch that exits the
+ // loop, make sure to put reload code into all of the successors of the
+ // loop.
+ //
+ const std::vector<BasicBlock*> &LoopBBs = CurLoop->getBlocks();
+ for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
+ E = LoopBBs.end(); I != E; ++I) {
+ // Rewrite all loads and stores in the block of the pointer...
+ for (BasicBlock::iterator II = (*I)->begin(), E = (*I)->end();
+ II != E; ++II) {
+ if (LoadInst *L = dyn_cast<LoadInst>(&*II)) {
+ std::map<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)) {
+ std::map<Value*, AllocaInst*>::iterator
+ I = ValueToAllocaMap.find(S->getOperand(1));
+ if (I != ValueToAllocaMap.end())
+ S->setOperand(1, I->second); // Rewrite store instruction...
+ }
+ }
+
+ // Check to see if any successors of this block are outside of the loop.
+ // If so, we need to copy the value from the alloca back into the memory
+ // location...
+ //
+ for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
+ if (!CurLoop->contains(*SI)) {
+ // Copy all of the allocas into their memory locations...
+ Instruction *InsertPos = (*SI)->begin();
+ for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
+ // Load from the alloca...
+ LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
+ // 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...
+ //
+ 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, getAnalysis<DominanceFrontier>());
+}
+
+/// findPromotableValuesInLoop - Check the current loop for stores to definate
+/// 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 LICM::findPromotableValuesInLoop(
+ std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
+ std::map<Value*, AllocaInst*> &ValueToAllocaMap) {
+ Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();
+
+ for (std::set<Value*>::iterator I = CurLBI->StoredPointers.begin(),
+ E = CurLBI->StoredPointers.end(); I != E; ++I) {
+ Value *V = *I;
+ if (isLoopInvariant(V) &&
+ CurLBI->getPointerInfo(V, *AA) == LoopBodyInfo::PointerMustStore) {
+
+ // Don't add a new entry for this stored pointer if it aliases something
+ // we have already processed.
+ std::map<Value*, AllocaInst*>::iterator V2AMI =
+ ValueToAllocaMap.lower_bound(V);
+ if (V2AMI == ValueToAllocaMap.end() || V2AMI->first != V) {
+ // Check to make sure that any loads in the loop are either NO or MUST
+ // aliases. We cannot rewrite loads that _might_ come from this memory
+ // location.
+
+ bool PointerOk = true;
+ for (std::set<Value*>::const_iterator I =CurLBI->LoadedPointers.begin(),
+ E = CurLBI->LoadedPointers.end(); I != E; ++I)
+ if (AA->alias(V, *I) == AliasAnalysis::MayAlias) {
+ PointerOk = false;
+ break;
+ }
+
+ if (PointerOk) {
+ 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));
+ ValueToAllocaMap.insert(V2AMI, std::make_pair(V, AI));
+
+ DEBUG(std::cerr << "LICM: Promoting value: " << *V << "\n");
+
+ // Loop over all of the loads and stores that alias this pointer,
+ // adding them to the Value2AllocaMap as well...
+ for (std::set<Value*>::const_iterator
+ I = CurLBI->LoadedPointers.begin(),
+ E = CurLBI->LoadedPointers.end(); I != E; ++I)
+ if (AA->alias(V, *I) == AliasAnalysis::MustAlias)
+ ValueToAllocaMap[*I] = AI;
+
+ for (std::set<Value*>::const_iterator
+ I = CurLBI->StoredPointers.begin(),
+ E = CurLBI->StoredPointers.end(); I != E; ++I)
+ if (AA->alias(V, *I) == AliasAnalysis::MustAlias)
+ ValueToAllocaMap[*I] = AI;
+ }
+ }
+ }
+ }
}
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