llvm-6502/lib/Transforms/Scalar/LICM.cpp
2003-08-05 20:39:02 +00:00

457 lines
18 KiB
C++

//===-- LICM.cpp - 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/AliasSetTracker.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Instructions.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/InstVisitor.h"
#include "llvm/Support/CFG.h"
#include "Support/CommandLine.h"
#include "Support/Debug.h"
#include "Support/Statistic.h"
#include "llvm/Assembly/Writer.h"
#include <algorithm>
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");
struct LICM : public FunctionPass, public InstVisitor<LICM> {
virtual bool runOnFunction(Function &F);
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG...
///
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequiredID(LoopPreheadersID);
AU.addRequired<LoopInfo>();
AU.addRequired<DominatorTree>();
AU.addRequired<DominanceFrontier>(); // For scalar promotion (mem2reg)
AU.addRequired<AliasAnalysis>();
}
private:
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...
AliasSetTracker *CurAST; // AliasSet information for the current loop...
DominatorTree *DT; // Dominator Tree for the current Loop...
/// visitLoop - Hoist expressions out of the specified loop...
///
void visitLoop(Loop *L, AliasSetTracker &AST);
/// HoistRegion - Walk the specified region of the CFG (defined by all
/// blocks dominated by the specified block, and that are in the current
/// loop) in depth first order w.r.t the DominatorTree. This allows us to
/// visit defintions before uses, allowing us to hoist a loop body in one
/// pass without iteration.
///
void HoistRegion(DominatorTree::Node *N);
/// inSubLoop - Little predicate that returns true if the specified basic
/// block is in a subloop of the current one, not the current one itself.
///
bool inSubLoop(BasicBlock *BB) {
assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
for (unsigned i = 0, e = CurLoop->getSubLoops().size(); i != e; ++i)
if (CurLoop->getSubLoops()[i]->contains(BB))
return true; // A subloop actually contains this block!
return false;
}
/// hoist - When an instruction is found to only use loop invariant operands
/// that is safe to hoist, this instruction is called to do the dirty work.
///
void hoist(Instruction &I);
/// SafeToHoist - Only hoist an instruction if it is not a trapping instruction
/// or if it is a trapping instruction and is guaranteed to execute
///
bool SafeToHoist(Instruction &I);
/// pointerInvalidatedByLoop - Return true if the body of this loop may
/// store into the memory location pointed to by V.
///
bool pointerInvalidatedByLoop(Value *V) {
// Check to see if any of the basic blocks in CurLoop invalidate *V.
return CurAST->getAliasSetForPointer(V, 0).isMod();
}
/// isLoopInvariant - Return true if the specified value is loop invariant
///
inline bool isLoopInvariant(Value *V) {
if (Instruction *I = dyn_cast<Instruction>(V))
return !CurLoop->contains(I->getParent());
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.
///
friend class InstVisitor<LICM>;
void visitBinaryOperator(Instruction &I) {
if (isLoopInvariant(I.getOperand(0)) && isLoopInvariant(I.getOperand(1)) && SafeToHoist(I))
hoist(I);
}
void visitCastInst(CastInst &CI) {
Instruction &I = (Instruction&)CI;
if (isLoopInvariant(I.getOperand(0)) && SafeToHoist(CI)) hoist(I);
}
void visitShiftInst(ShiftInst &I) { visitBinaryOperator((Instruction&)I); }
void visitLoadInst(LoadInst &LI);
void visitGetElementPtrInst(GetElementPtrInst &GEPI) {
Instruction &I = (Instruction&)GEPI;
for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
if (!isLoopInvariant(I.getOperand(i))) return;
if(SafeToHoist(GEPI))
hoist(I);
}
};
RegisterOpt<LICM> X("licm", "Loop Invariant Code Motion");
}
Pass *createLICMPass() { return new LICM(); }
/// runOnFunction - For LICM, this simply traverses the loop structure of the
/// function, hoisting expressions out of loops if possible.
///
bool LICM::runOnFunction(Function &) {
Changed = false;
// Get our Loop and Alias Analysis information...
LI = &getAnalysis<LoopInfo>();
AA = &getAnalysis<AliasAnalysis>();
DT = &getAnalysis<DominatorTree>();
// 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) {
AliasSetTracker AST(*AA);
LICM::visitLoop(*I, AST);
}
return Changed;
}
/// visitLoop - Hoist expressions out of the specified loop...
///
void LICM::visitLoop(Loop *L, AliasSetTracker &AST) {
// Recurse through all subloops before we process this loop...
for (std::vector<Loop*>::const_iterator I = L->getSubLoops().begin(),
E = L->getSubLoops().end(); I != E; ++I) {
AliasSetTracker SubAST(*AA);
LICM::visitLoop(*I, SubAST);
// Incorporate information about the subloops into this loop...
AST.add(SubAST);
}
CurLoop = L;
CurAST = &AST;
// 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 AST, 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...
AST.add(**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
// the subloops).
//
// Traverse the body of the loop in depth first order on the dominator tree so
// that we are guaranteed to see definitions before we see uses. This allows
// us to perform the LICM transformation in one pass, without iteration.
//
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)
PromoteValuesInLoop();
// Clear out loops state information for the next iteration
CurLoop = 0;
Preheader = 0;
}
/// HoistRegion - Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in depth
/// first order w.r.t the DominatorTree. This allows us to visit defintions
/// before uses, allowing us to hoist a loop body in one pass without iteration.
///
void LICM::HoistRegion(DominatorTree::Node *N) {
assert(N != 0 && "Null dominator tree node?");
// If this subregion is not in the top level loop at all, exit.
if (!CurLoop->contains(N->getNode())) return;
// Only need to hoist the contents of this block if it is not part of a
// subloop (which would already have been hoisted)
if (!inSubLoop(N->getNode()))
visit(*N->getNode());
const std::vector<DominatorTree::Node*> &Children = N->getChildren();
for (unsigned i = 0, e = Children.size(); i != e; ++i)
HoistRegion(Children[i]);
}
/// hoist - When an instruction is found to only use loop invariant operands
/// that is safe to hoist, this instruction is called to do the dirty work.
///
void LICM::hoist(Instruction &Inst) {
DEBUG(std::cerr << "LICM hoisting to";
WriteAsOperand(std::cerr, Preheader, false);
std::cerr << ": " << Inst);
// Remove the instruction from its current basic block... but don't delete the
// instruction.
Inst.getParent()->getInstList().remove(&Inst);
// Insert the new node in Preheader, before the terminator.
Preheader->getInstList().insert(Preheader->getTerminator(), &Inst);
++NumHoisted;
Changed = true;
}
/// SafeToHoist - Only hoist an instruction if it is not a trapping instruction
/// or if it is a trapping instruction and is guaranteed to execute
///
bool LICM::SafeToHoist(Instruction &Inst) {
//If it is a trapping instruction, then check if its guaranteed to execute.
if(Inst.isTrapping()) {
//Get the instruction's basic block.
BasicBlock *InstBB = Inst.getParent();
//Get the Dominator Tree Node for the instruction's basic block/
DominatorTree::Node *InstDTNode = DT->getNode(InstBB);
//Get the exit blocks for the current loop.
const std::vector<BasicBlock* > &ExitBlocks = CurLoop->getExitBlocks();
//For each exit block, get the DT node and walk up the DT until
//the instruction's basic block is found or we exit the loop.
for(unsigned i=0; i < ExitBlocks.size(); ++i) {
DominatorTree::Node *IDom = DT->getNode(ExitBlocks[i]);
while(IDom != InstDTNode) {
//Get next Immediate Dominator.
IDom = IDom->getIDom();
//See if we exited the loop.
if(!CurLoop->contains(IDom->getNode()))
return false;
}
}
}
return true;
}
void LICM::visitLoadInst(LoadInst &LI) {
if (isLoopInvariant(LI.getOperand(0)) &&
!pointerInvalidatedByLoop(LI.getOperand(0)) && SafeToHoist(LI)) {
hoist(LI);
++NumHoistedLoads;
}
}
/// 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...
BasicBlock::iterator BI = (*SI)->begin();
while (isa<PHINode>(*BI))
++BI; // Skip over all of the phi nodes in the block...
Instruction *InsertPos = BI;
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>(),
AA->getTargetData());
}
/// 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();
// 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, and if the pointer is loop invariant.
if (!AS.isForwardingAliasSet() && AS.isMod() && AS.isMustAlias() &&
isLoopInvariant(AS.begin()->first)) {
assert(AS.begin() != AS.end() &&
"Must alias set should have at least one pointer element in it!");
Value *V = AS.begin()->first;
// 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->first->getType()) {
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));
for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
ValueToAllocaMap.insert(std::make_pair(I->first, AI));
DEBUG(std::cerr << "LICM: Promoting value: " << *V << "\n");
}
}
}
}