[llvm-commits] CVS: llvm/lib/Transforms/Scalar/LICM.cpp
Chris Lattner
lattner at cs.uiuc.edu
Wed Dec 10 00:42:05 PST 2003
Changes in directory llvm/lib/Transforms/Scalar:
LICM.cpp updated: 1.43 -> 1.44
---
Log message:
Implement instruction sinking out of loops. This still can do a little bit
better job, but this is the majority of the work. This implements
LICM/sink*.ll
---
Diffs of the changes: (+246 -61)
Index: llvm/lib/Transforms/Scalar/LICM.cpp
diff -u llvm/lib/Transforms/Scalar/LICM.cpp:1.43 llvm/lib/Transforms/Scalar/LICM.cpp:1.44
--- llvm/lib/Transforms/Scalar/LICM.cpp:1.43 Tue Dec 9 13:32:44 2003
+++ llvm/lib/Transforms/Scalar/LICM.cpp Wed Dec 10 00:41:05 2003
@@ -54,8 +54,9 @@
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 @@
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 @@
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 @@
// 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 @@
// 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 @@
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 @@
// 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;
}
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