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A lot of cleanups and documentation improvements, as well as a few corner case fixes. Most

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of this was suggested by Chris.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@50441 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Owen Anderson 2008-04-29 20:59:33 +00:00
parent 0396cd33ca
commit 9862f31533
1 changed files with 76 additions and 59 deletions
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135
lib/Transforms/Scalar/LoopDeletion.cpp
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@ -7,7 +7,10 @@
//
//===----------------------------------------------------------------------===//
//
// This file implements the Dead Loop Elimination Pass.
// This file implements the Dead Loop Elimination Pass. This pass is
// responsible for eliminating loops with non-infinite computable trip counts
// that have no side effects or volatile instructions, and do not contribute
// to the computation of the function's return value.
//
//===----------------------------------------------------------------------===//
@ -31,8 +34,10 @@ namespace {
// Possibly eliminate loop L if it is dead.
bool runOnLoop(Loop* L, LPPassManager& LPM);
bool SingleDominatingExit(Loop* L);
bool IsLoopDead(Loop* L);
bool SingleDominatingExit(Loop* L,
SmallVector<BasicBlock*, 4>& exitingBlocks);
bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks,
SmallVector<BasicBlock*, 4>& exitBlocks);
bool IsLoopInvariantInst(Instruction *I, Loop* L);
virtual void getAnalysisUsage(AnalysisUsage& AU) const {
@ -56,24 +61,34 @@ LoopPass* llvm::createLoopDeletionPass() {
return new LoopDeletion();
}
bool LoopDeletion::SingleDominatingExit(Loop* L) {
SmallVector<BasicBlock*, 4> exitingBlocks;
L->getExitingBlocks(exitingBlocks);
/// SingleDominatingExit - Checks that there is only a single blocks that
/// branches out of the loop, and that it also dominates the latch block. Loops
/// with multiple or non-latch-dominating exiting blocks could be dead, but we'd
/// have to do more extensive analysis to make sure, for instance, that the
/// control flow logic involves was or could be made loop-invariant.
bool LoopDeletion::SingleDominatingExit(Loop* L,
SmallVector<BasicBlock*, 4>& exitingBlocks) {
if (exitingBlocks.size() != 1)
return 0;
return false;
BasicBlock* latch = L->getLoopLatch();
if (!latch)
return 0;
return false;
DominatorTree& DT = getAnalysis<DominatorTree>();
if (DT.dominates(exitingBlocks[0], latch))
return exitingBlocks[0];
return true;
else
return 0;
return false;
}
/// IsLoopInvariantInst - Checks if an instruction is invariant with respect to
/// a loop, which is defined as being true if all of its operands are defined
/// outside of the loop. These instructions can be hoisted out of the loop
/// if their results are needed. This could be made more aggressive by
/// recursively checking the operands for invariance, but it's not clear that
/// it's worth it.
bool LoopDeletion::IsLoopInvariantInst(Instruction *I, Loop* L) {
// PHI nodes are not loop invariant if defined in the loop.
if (isa<PHINode>(I) && L->contains(I->getParent()))
@ -88,19 +103,20 @@ bool LoopDeletion::IsLoopInvariantInst(Instruction *I, Loop* L) {
return true;
}
bool LoopDeletion::IsLoopDead(Loop* L) {
SmallVector<BasicBlock*, 1> exitingBlocks;
L->getExitingBlocks(exitingBlocks);
/// IsLoopDead - Determined if a loop is dead. This assumes that we've already
/// checked for unique exit and exiting blocks, and that the code is in LCSSA
/// form.
bool LoopDeletion::IsLoopDead(Loop* L,
SmallVector<BasicBlock*, 4>& exitingBlocks,
SmallVector<BasicBlock*, 4>& exitBlocks) {
BasicBlock* exitingBlock = exitingBlocks[0];
// Get the set of out-of-loop blocks that the exiting block branches to.
SmallVector<BasicBlock*, 8> exitBlocks;
L->getUniqueExitBlocks(exitBlocks);
if (exitBlocks.size() > 1)
return false;
BasicBlock* exitBlock = exitBlocks[0];
// Make sure that all PHI entries coming from the loop are loop invariant.
// Because the code is in LCSSA form, any values used outside of the loop
// must pass through a PHI in the exit block, meaning that this check is
// sufficient to guarantee that no loop-variant values are used outside
// of the loop.
BasicBlock::iterator BI = exitBlock->begin();
while (PHINode* P = dyn_cast<PHINode>(BI)) {
Value* incoming = P->getIncomingValueForBlock(exitingBlock);
@ -112,12 +128,18 @@ bool LoopDeletion::IsLoopDead(Loop* L) {
}
// Make sure that no instructions in the block have potential side-effects.
// This includes instructions that could write to memory, and loads that are
// marked volatile. This could be made more aggressive by using aliasing
// information to identify readonly and readnone calls.
for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
LI != LE; ++LI) {
for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
BI != BE; ++BI) {
if (BI->mayWriteToMemory())
return false;
else if (LoadInst* L = dyn_cast<LoadInst>(BI))
if (L->isVolatile())
return false;
}
}
@ -128,10 +150,20 @@ bool LoopDeletion::IsLoopDead(Loop* L) {
/// observable behavior of the program other than finite running time. Note
/// we do ensure that this never remove a loop that might be infinite, as doing
/// so could change the halting/non-halting nature of a program.
/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA
/// in order to make various safety checks work.
bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
// Don't remove loops for which we can't solve the trip count.
// They could be infinite, in which case we'd be changing program behavior.
if (L->getTripCount())
SmallVector<BasicBlock*, 4> exitingBlocks;
L->getExitingBlocks(exitingBlocks);
SmallVector<BasicBlock*, 4> exitBlocks;
L->getUniqueExitBlocks(exitBlocks);
// We require that the loop only have a single exit block. Otherwise, we'd
// be in the situation of needing to be able to solve statically which exit
// block will be branced to, or trying to preserve the branching logic in
// a loop invariant manner.
if (exitBlocks.size() != 1)
return false;
// We can only remove the loop if there is a preheader that we can
@ -145,23 +177,22 @@ bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
if (L->begin() != L->end())
return false;
// Don't remove loops for which we can't solve the trip count.
// They could be infinite, in which case we'd be changing program behavior.
if (L->getTripCount())
return false;
// Loops with multiple exits or exits that don't dominate the latch
// are too complicated to handle correctly.
if (!SingleDominatingExit(L))
if (!SingleDominatingExit(L, exitingBlocks))
return false;
// Finally, we have to check that the loop really is dead.
if (!IsLoopDead(L))
if (!IsLoopDead(L, exitingBlocks, exitBlocks))
return false;
// Now that we know the removal is safe, change the branch from the preheader
// to go to the single exiting block.
SmallVector<BasicBlock*, 1> exitingBlocks;
L->getExitingBlocks(exitingBlocks);
BasicBlock* exitingBlock = exitingBlocks[0];
SmallVector<BasicBlock*, 1> exitBlocks;
L->getUniqueExitBlocks(exitBlocks);
// Now that we know the removal is safe, remove the loop by changing the
// branch from the preheader to go to the single exiting block.
BasicBlock* exitBlock = exitBlocks[0];
// Because we're deleting a large chunk of code at once, the sequence in which
@ -181,39 +212,30 @@ bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
// Connect the preheader directly to the exit block.
TerminatorInst* TI = preheader->getTerminator();
if (BranchInst* BI = dyn_cast<BranchInst>(TI)) {
if (BI->isUnconditional())
BI->setSuccessor(0, exitBlock);
else if (L->contains(BI->getSuccessor(0)))
BI->setSuccessor(0, exitBlock);
else
BI->setSuccessor(1, exitBlock);
} else {
// FIXME: Support switches
return false;
}
TI->replaceUsesOfWith(L->getHeader(), exitBlock);
// Rewrite phis in the exit block to get their inputs from
// the preheader instead of the exiting block.
BasicBlock::iterator BI = exitBlock->begin();
while (PHINode* P = dyn_cast<PHINode>(BI)) {
unsigned i = P->getBasicBlockIndex(exitingBlock);
P->setIncomingBlock(i, preheader);
P->replaceUsesOfWith(exitBlock, preheader);
BI++;
}
// Update lots of internal structures...
// Update the dominator tree and remove the instructions and blocks that will
// be deleted from the reference counting scheme.
DominatorTree& DT = getAnalysis<DominatorTree>();
SmallPtrSet<DomTreeNode*, 8> ChildNodes;
for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
LI != LE; ++LI) {
// Move all of the block's children to be children of the preheader, which
// allows us to remove the domtree entry for the block.
SmallPtrSet<DomTreeNode*, 8> childNodes;
childNodes.insert(DT[*LI]->begin(), DT[*LI]->end());
for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = childNodes.begin(),
DE = childNodes.end(); DI != DE; ++DI)
ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end());
for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(),
DE = ChildNodes.end(); DI != DE; ++DI)
DT.changeImmediateDominator(*DI, DT[preheader]);
ChildNodes.clear();
DT.eraseNode(*LI);
// Drop all references between the instructions and the block so
@ -228,16 +250,11 @@ bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) {
// Erase the instructions and the blocks without having to worry
// about ordering because we already dropped the references.
// NOTE: This iteration is safe because erasing the block does not remove its
// entry from the loop's block list. We do that in the next section.
for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
LI != LE; ++LI) {
for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end();
BI != BE; ) {
Instruction* I = BI++;
I->eraseFromParent();
}
LI != LE; ++LI)
(*LI)->eraseFromParent();
}
// Finally, the blocks from loopinfo. This has to happen late because
// otherwise our loop iterators won't work.