[llvm-commits] [llvm] r50441 - /llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp
Owen Anderson
resistor at mac.com
Tue Apr 29 13:59:34 PDT 2008
Author: resistor
Date: Tue Apr 29 15:59:33 2008
New Revision: 50441
URL: http://llvm.org/viewvc/llvm-project?rev=50441&view=rev
Log:
A lot of cleanups and documentation improvements, as well as a few corner case fixes. Most
of this was suggested by Chris.
Modified:
llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp
Modified: llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp?rev=50441&r1=50440&r2=50441&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopDeletion.cpp Tue Apr 29 15:59:33 2008
@@ -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 @@
// 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 @@
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 @@
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 @@
}
// 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 @@
/// 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 @@
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 @@
// 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 @@
// 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.
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