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Implement instruction sinking out of loops. This still can do a little bit

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better job, but this is the majority of the work.  This implements
LICM/sink*.ll


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10358 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2003-12-10 06:41:05 +00:00
parent 94046b2cc9
commit a2706518f9
1 changed files with 246 additions and 61 deletions
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307
lib/Transforms/Scalar/LICM.cpp
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@ -54,8 +54,9 @@ namespace {
DisablePromotion("disable-licm-promotion", cl::Hidden,
cl::desc("Disable memory promotion in LICM pass"));
Statistic<> NumSunk("licm", "Number of instructions sunk out of loop");
Statistic<> NumHoisted("licm", "Number of instructions hoisted out of loop");
Statistic<> NumHoistedLoads("licm", "Number of load insts hoisted");
Statistic<> NumMovedLoads("licm", "Number of load insts hoisted or sunk");
Statistic<> NumPromoted("licm",
"Number of memory locations promoted to registers");
@ -110,16 +111,53 @@ namespace {
return false;
}
/// isExitBlockDominatedByBlockInLoop - This method checks to see if the
/// specified exit block of the loop is dominated by the specified block
/// that is in the body of the loop. We use these constraints to
/// dramatically limit the amount of the dominator tree that needs to be
/// searched.
bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock,
BasicBlock *BlockInLoop) const {
// If the block in the loop is the loop header, it must be dominated!
BasicBlock *LoopHeader = CurLoop->getHeader();
if (BlockInLoop == LoopHeader)
return true;
DominatorTree::Node *BlockInLoopNode = DT->getNode(BlockInLoop);
DominatorTree::Node *IDom = DT->getNode(ExitBlock);
// Because the exit block is not in the loop, we know we have to get _at
// least_ it's immediate dominator.
do {
// Get next Immediate Dominator.
IDom = IDom->getIDom();
// If we have got to the header of the loop, then the instructions block
// did not dominate the exit node, so we can't hoist it.
if (IDom->getBlock() == LoopHeader)
return false;
} while (IDom != BlockInLoopNode);
return true;
}
/// sink - When an instruction is found to only be used outside of the loop,
/// this function moves it to the exit blocks and patches up SSA form as
/// needed.
///
void sink(Instruction &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 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.
/// isSafeToExecuteUnconditionally - Only sink or 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);
bool isSafeToExecuteUnconditionally(Instruction &I);
/// pointerInvalidatedByLoop - Return true if the body of this loop may
/// store into the memory location pointed to by V.
@ -136,7 +174,10 @@ namespace {
return !CurLoop->contains(I->getParent());
return true; // All non-instructions are loop invariant
}
bool isLoopInvariantInst(Instruction &Inst);
bool canSinkOrHoistInst(Instruction &I);
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.
@ -205,9 +246,8 @@ void LICM::visitLoop(Loop *L, AliasSetTracker &AST) {
// 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)
for (std::vector<BasicBlock*>::const_iterator I = L->getBlocks().begin(),
E = L->getBlocks().end(); I != E; ++I)
if (LI->getLoopFor(*I) == L) // Ignore blocks in subloops...
AST.add(**I); // Incorporate the specified basic block
@ -244,13 +284,30 @@ void LICM::HoistRegion(DominatorTree::Node *N) {
// If this subregion is not in the top level loop at all, exit.
if (!CurLoop->contains(BB)) return;
// Only need to hoist the contents of this block if it is not part of a
// subloop (which would already have been hoisted)
// Only need to process the contents of this block if it is not part of a
// subloop (which would already have been processed).
if (!inSubLoop(BB))
for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; ) {
Instruction &Inst = *I++;
if (isLoopInvariantInst(Inst) && SafeToHoist(Inst))
hoist(Inst);
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
Instruction &I = *II++;
// We can only handle simple expressions and loads with this code.
if (canSinkOrHoistInst(I)) {
// First check to see if we can sink this instruction to the exit blocks
// of the loop. We can do this if the only users of the instruction are
// outside of the loop. In this case, it doesn't even matter if the
// operands of the instruction are loop invariant.
//
if (isNotUsedInLoop(I))
sink(I);
// If we can't sink the instruction, try hoisting it out to the
// preheader. We can only do this if all of the operands of the
// instruction are loop invariant and if it is safe to hoist the
// instruction.
//
else if (isLoopInvariantInst(I) && isSafeToExecuteUnconditionally(I))
hoist(I);
}
}
const std::vector<DominatorTree::Node*> &Children = N->getChildren();
@ -258,61 +315,204 @@ void LICM::HoistRegion(DominatorTree::Node *N) {
HoistRegion(Children[i]);
}
bool LICM::isLoopInvariantInst(Instruction &I) {
assert(!isa<TerminatorInst>(I) && "Can't hoist terminator instructions!");
// We can only hoist simple expressions...
if (!isa<BinaryOperator>(I) && !isa<ShiftInst>(I) && !isa<LoadInst>(I) &&
!isa<GetElementPtrInst>(I) && !isa<CastInst>(I) && !isa<VANextInst>(I) &&
!isa<VAArgInst>(I))
return false;
// The instruction is loop invariant if all of its operands are loop-invariant
for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
if (!isLoopInvariant(I.getOperand(i)))
return false;
/// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
/// instruction.
///
bool LICM::canSinkOrHoistInst(Instruction &I) {
// Loads have extra constraints we have to verify before we can hoist them.
if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
if (LI->isVolatile())
return false; // Don't hoist volatile loads!
// Don't hoist loads which have may-aliased stores in loop.
if (pointerInvalidatedByLoop(I.getOperand(0)))
return false;
return !pointerInvalidatedByLoop(LI->getOperand(0));
}
return isa<BinaryOperator>(I) || isa<ShiftInst>(I) || isa<CastInst>(I) ||
isa<GetElementPtrInst>(I) || isa<VANextInst>(I) || isa<VAArgInst>(I);
}
/// isNotUsedInLoop - Return true if the only users of this instruction are
/// outside of the loop. If this is true, we can sink the instruction to the
/// exit blocks of the loop.
///
bool LICM::isNotUsedInLoop(Instruction &I) {
for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI)
if (CurLoop->contains(cast<Instruction>(*UI)->getParent()))
return false;
return true;
}
/// isLoopInvariantInst - Return true if all operands of this instruction are
/// loop invariant. We also filter out non-hoistable instructions here just for
/// efficiency.
///
bool LICM::isLoopInvariantInst(Instruction &I) {
// The instruction is loop invariant if all of its operands are loop-invariant
for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
if (!isLoopInvariant(I.getOperand(i)))
return false;
// If we got this far, the instruction is loop invariant!
return true;
}
/// sink - When an instruction is found to only be used outside of the loop,
/// this function moves it to the exit blocks and patches up SSA form as
/// needed.
///
void LICM::sink(Instruction &I) {
DEBUG(std::cerr << "LICM sinking instruction: " << I);
const std::vector<BasicBlock*> &ExitBlocks = CurLoop->getExitBlocks();
// The case where there is only a single exit node of this loop is common
// enough that we handle it as a special (more efficient) case. It is more
// efficient to handle because there are no PHI nodes that need to be placed.
if (ExitBlocks.size() == 1) {
if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) {
// Instruction is not used, just delete it.
I.getParent()->getInstList().erase(&I);
} else {
// Move the instruction to the start of the exit block, after any PHI
// nodes in it.
I.getParent()->getInstList().remove(&I);
BasicBlock::iterator InsertPt = ExitBlocks[0]->begin();
while (isa<PHINode>(InsertPt)) ++InsertPt;
ExitBlocks[0]->getInstList().insert(InsertPt, &I);
}
} else if (ExitBlocks.size() == 0) {
// The instruction is actually dead if there ARE NO exit blocks.
I.getParent()->getInstList().erase(&I);
return; // Don't count this as a sunk instruction, don't check operands.
} else {
// Otherwise, if we have multiple exits, use the PromoteMem2Reg function to
// do all of the hard work of inserting PHI nodes as necessary. We convert
// the value into a stack object to get it to do this.
// Firstly, we create a stack object to hold the value...
AllocaInst *AI = new AllocaInst(I.getType(), 0, I.getName(),
I.getParent()->getParent()->front().begin());
// Secondly, insert load instructions for each use of the instruction
// outside of the loop.
while (!I.use_empty()) {
Instruction *U = cast<Instruction>(I.use_back());
// If the user is a PHI Node, we actually have to insert load instructions
// in all predecessor blocks, not in the PHI block itself!
if (PHINode *UPN = dyn_cast<PHINode>(U)) {
// Only insert into each predecessor once, so that we don't have
// different incoming values from the same block!
std::map<BasicBlock*, Value*> InsertedBlocks;
for (unsigned i = 0, e = UPN->getNumIncomingValues(); i != e; ++i)
if (UPN->getIncomingValue(i) == &I) {
BasicBlock *Pred = UPN->getIncomingBlock(i);
Value *&PredVal = InsertedBlocks[Pred];
if (!PredVal) {
// Insert a new load instruction right before the terminator in
// the predecessor block.
PredVal = new LoadInst(AI, "", Pred->getTerminator());
}
UPN->setIncomingValue(i, PredVal);
}
} else {
LoadInst *L = new LoadInst(AI, "", U);
U->replaceUsesOfWith(&I, L);
}
}
// Thirdly, insert a copy of the instruction in each exit block of the loop
// that is dominated by the instruction, storing the result into the memory
// location. Be careful not to insert the instruction into any particular
// basic block more than once.
std::set<BasicBlock*> InsertedBlocks;
BasicBlock *InstOrigBB = I.getParent();
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
BasicBlock *ExitBlock = ExitBlocks[i];
if (isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB)) {
std::set<BasicBlock*>::iterator SI =
InsertedBlocks.lower_bound(ExitBlock);
// If we haven't already processed this exit block, do so now.
if (SI == InsertedBlocks.end() || *SI != ExitBlock) {
// Insert the code after the last PHI node...
BasicBlock::iterator InsertPt = ExitBlock->begin();
while (isa<PHINode>(InsertPt)) ++InsertPt;
// If this is the first exit block processed, just move the original
// instruction, otherwise clone the original instruction and insert
// the copy.
Instruction *New;
if (InsertedBlocks.empty()) {
I.getParent()->getInstList().remove(&I);
ExitBlock->getInstList().insert(InsertPt, &I);
New = &I;
} else {
New = I.clone();
New->setName(I.getName()+".le");
ExitBlock->getInstList().insert(InsertPt, New);
}
// Now that we have inserted the instruction, store it into the alloca
new StoreInst(New, AI, InsertPt);
// Remember we processed this block
InsertedBlocks.insert(SI, ExitBlock);
}
}
}
// Finally, promote the fine value to SSA form.
std::vector<AllocaInst*> Allocas;
Allocas.push_back(AI);
PromoteMemToReg(Allocas, *DT, *DF, AA->getTargetData());
}
if (isa<LoadInst>(I)) ++NumMovedLoads;
++NumSunk;
Changed = true;
// Since we just sunk an instruction, check to see if any other instructions
// used by this instruction are now sinkable. If so, sink them too.
for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
if (Instruction *OpI = dyn_cast<Instruction>(I.getOperand(i)))
if (CurLoop->contains(OpI->getParent()) && canSinkOrHoistInst(*OpI) &&
isNotUsedInLoop(*OpI) &&
isSafeToExecuteUnconditionally(*OpI))
sink(*OpI);
}
/// 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) {
void LICM::hoist(Instruction &I) {
DEBUG(std::cerr << "LICM hoisting to";
WriteAsOperand(std::cerr, Preheader, false);
std::cerr << ": " << Inst);
if (isa<LoadInst>(Inst))
++NumHoistedLoads;
std::cerr << ": " << I);
// Remove the instruction from its current basic block... but don't delete the
// instruction.
Inst.getParent()->getInstList().remove(&Inst);
I.getParent()->getInstList().remove(&I);
// Insert the new node in Preheader, before the terminator.
Preheader->getInstList().insert(Preheader->getTerminator(), &Inst);
Preheader->getInstList().insert(Preheader->getTerminator(), &I);
if (isa<LoadInst>(I)) ++NumMovedLoads;
++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
/// isSafeToExecuteUnconditionally - Only sink or 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) {
bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
// If it is not a trapping instruction, it is always safe to hoist.
if (!Inst.isTrapping()) return true;
@ -323,32 +523,17 @@ bool LICM::SafeToHoist(Instruction &Inst) {
// If the instruction is in the header block for the loop (which is very
// common), it is always guaranteed to dominate the exit blocks. Since this
// is a common case, and can save some work, check it now.
BasicBlock *LoopHeader = CurLoop->getHeader();
if (Inst.getParent() == LoopHeader)
if (Inst.getParent() == CurLoop->getHeader())
return true;
// Get the Dominator Tree Node for the instruction's basic block.
DominatorTree::Node *InstDTNode = DT->getNode(Inst.getParent());
// Get the exit blocks for the current loop.
const std::vector<BasicBlock* > &ExitBlocks = CurLoop->getExitBlocks();
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, e = ExitBlocks.size(); i != e; ++i) {
DominatorTree::Node *IDom = DT->getNode(ExitBlocks[i]);
do {
// Get next Immediate Dominator.
IDom = IDom->getIDom();
// If we have got to the header of the loop, then the instructions block
// did not dominate the exit node, so we can't hoist it.
if (IDom->getBlock() == LoopHeader)
return false;
} while(IDom != InstDTNode);
}
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent()))
return false;
return true;
}