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Initial implementation of Loop invariant memory->scalar promotion

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@5613 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2003-02-24 03:52:32 +00:00
parent 4e31365703
commit 2e6e741b73
1 changed files with 328 additions and 33 deletions
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361
lib/Transforms/Scalar/LICM.cpp
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@ -1,27 +1,147 @@
//===-- 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);
@ -33,18 +153,21 @@ namespace {
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
@ -73,7 +196,10 @@ namespace {
/// 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
///
@ -83,6 +209,22 @@ namespace {
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.
///
@ -116,35 +258,52 @@ Pass *createLICMPass() { return new LICM(); }
/// 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
@ -156,6 +315,11 @@ void LICM::visitLoop(Loop *L) {
//
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;
@ -211,13 +375,144 @@ void LICM::visitLoadInst(LoadInst &LI) {
}
}
/// 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;
}
}
}
}
}