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-rw-r--r--contrib/llvm/lib/Transforms/Scalar/LoopDeletion.cpp261
1 files changed, 182 insertions, 79 deletions
diff --git a/contrib/llvm/lib/Transforms/Scalar/LoopDeletion.cpp b/contrib/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
index cca75a3..ac4dd44 100644
--- a/contrib/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
+++ b/contrib/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
@@ -20,6 +20,7 @@
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/IR/Dominators.h"
+#include "llvm/IR/PatternMatch.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/LoopPassManager.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
@@ -29,32 +30,45 @@ using namespace llvm;
STATISTIC(NumDeleted, "Number of loops deleted");
-/// 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 LoopDeletionPass::isLoopDead(Loop *L, ScalarEvolution &SE,
- SmallVectorImpl<BasicBlock *> &exitingBlocks,
- SmallVectorImpl<BasicBlock *> &exitBlocks,
- bool &Changed, BasicBlock *Preheader) {
- BasicBlock *exitBlock = exitBlocks[0];
-
+/// This function deletes dead loops. The caller of this function needs to
+/// guarantee that the loop is infact dead. Here we handle two kinds of dead
+/// loop. The first kind (\p isLoopDead) is where only invariant values from
+/// within the loop are used outside of it. The second kind (\p
+/// isLoopNeverExecuted) is where the loop is provably never executed. We can
+/// always remove never executed loops since they will not cause any difference
+/// to program behaviour.
+///
+/// This also updates the relevant analysis information in \p DT, \p SE, and \p
+/// LI. It also updates the loop PM if an updater struct is provided.
+// TODO: This function will be used by loop-simplifyCFG as well. So, move this
+// to LoopUtils.cpp
+static void deleteDeadLoop(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
+ LoopInfo &LI, LPMUpdater *Updater = nullptr);
+/// Determines 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.
+static bool isLoopDead(Loop *L, ScalarEvolution &SE,
+ SmallVectorImpl<BasicBlock *> &ExitingBlocks,
+ BasicBlock *ExitBlock, bool &Changed,
+ BasicBlock *Preheader) {
// 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();
+ BasicBlock::iterator BI = ExitBlock->begin();
bool AllEntriesInvariant = true;
bool AllOutgoingValuesSame = true;
while (PHINode *P = dyn_cast<PHINode>(BI)) {
- Value *incoming = P->getIncomingValueForBlock(exitingBlocks[0]);
+ Value *incoming = P->getIncomingValueForBlock(ExitingBlocks[0]);
// Make sure all exiting blocks produce the same incoming value for the exit
// block. If there are different incoming values for different exiting
// blocks, then it is impossible to statically determine which value should
// be used.
AllOutgoingValuesSame =
- all_of(makeArrayRef(exitingBlocks).slice(1), [&](BasicBlock *BB) {
+ all_of(makeArrayRef(ExitingBlocks).slice(1), [&](BasicBlock *BB) {
return incoming == P->getIncomingValueForBlock(BB);
});
@@ -78,95 +92,187 @@ bool LoopDeletionPass::isLoopDead(Loop *L, ScalarEvolution &SE,
// 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 (Instruction &I : **LI) {
- if (I.mayHaveSideEffects())
- return false;
- }
- }
-
+ // marked volatile.
+ for (auto &I : L->blocks())
+ if (any_of(*I, [](Instruction &I) { return I.mayHaveSideEffects(); }))
+ return false;
return true;
}
-/// Remove dead loops, by which we mean loops that do not impact the 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 LoopDeletionPass::runImpl(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
- LoopInfo &loopInfo) {
- assert(L->isLCSSAForm(DT) && "Expected LCSSA!");
+/// This function returns true if there is no viable path from the
+/// entry block to the header of \p L. Right now, it only does
+/// a local search to save compile time.
+static bool isLoopNeverExecuted(Loop *L) {
+ using namespace PatternMatch;
- // We can only remove the loop if there is a preheader that we can
- // branch from after removing it.
- BasicBlock *preheader = L->getLoopPreheader();
- if (!preheader)
- return false;
+ auto *Preheader = L->getLoopPreheader();
+ // TODO: We can relax this constraint, since we just need a loop
+ // predecessor.
+ assert(Preheader && "Needs preheader!");
- // If LoopSimplify form is not available, stay out of trouble.
- if (!L->hasDedicatedExits())
+ if (Preheader == &Preheader->getParent()->getEntryBlock())
return false;
+ // All predecessors of the preheader should have a constant conditional
+ // branch, with the loop's preheader as not-taken.
+ for (auto *Pred: predecessors(Preheader)) {
+ BasicBlock *Taken, *NotTaken;
+ ConstantInt *Cond;
+ if (!match(Pred->getTerminator(),
+ m_Br(m_ConstantInt(Cond), Taken, NotTaken)))
+ return false;
+ if (!Cond->getZExtValue())
+ std::swap(Taken, NotTaken);
+ if (Taken == Preheader)
+ return false;
+ }
+ assert(!pred_empty(Preheader) &&
+ "Preheader should have predecessors at this point!");
+ // All the predecessors have the loop preheader as not-taken target.
+ return true;
+}
+/// Remove a loop if it is dead.
+///
+/// A loop is considered dead if it does not impact the observable behavior of
+/// the program other than finite running time. This never removes a loop that
+/// might be infinite (unless it is never executed), as doing so could change
+/// the halting/non-halting nature of a program.
+///
+/// This entire process relies pretty heavily on LoopSimplify form and LCSSA in
+/// order to make various safety checks work.
+///
+/// \returns true if any changes were made. This may mutate the loop even if it
+/// is unable to delete it due to hoisting trivially loop invariant
+/// instructions out of the loop.
+static bool deleteLoopIfDead(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
+ LoopInfo &LI, LPMUpdater *Updater = nullptr) {
+ assert(L->isLCSSAForm(DT) && "Expected LCSSA!");
+
+ // We can only remove the loop if there is a preheader that we can branch from
+ // after removing it. Also, if LoopSimplify form is not available, stay out
+ // of trouble.
+ BasicBlock *Preheader = L->getLoopPreheader();
+ if (!Preheader || !L->hasDedicatedExits()) {
+ DEBUG(dbgs()
+ << "Deletion requires Loop with preheader and dedicated exits.\n");
+ return false;
+ }
// We can't remove loops that contain subloops. If the subloops were dead,
// they would already have been removed in earlier executions of this pass.
- if (L->begin() != L->end())
+ if (L->begin() != L->end()) {
+ DEBUG(dbgs() << "Loop contains subloops.\n");
return false;
+ }
- SmallVector<BasicBlock *, 4> exitingBlocks;
- L->getExitingBlocks(exitingBlocks);
- SmallVector<BasicBlock *, 4> exitBlocks;
- L->getUniqueExitBlocks(exitBlocks);
+ BasicBlock *ExitBlock = L->getUniqueExitBlock();
+
+ if (ExitBlock && isLoopNeverExecuted(L)) {
+ DEBUG(dbgs() << "Loop is proven to never execute, delete it!");
+ // Set incoming value to undef for phi nodes in the exit block.
+ BasicBlock::iterator BI = ExitBlock->begin();
+ while (PHINode *P = dyn_cast<PHINode>(BI)) {
+ for (unsigned i = 0; i < P->getNumIncomingValues(); i++)
+ P->setIncomingValue(i, UndefValue::get(P->getType()));
+ BI++;
+ }
+ deleteDeadLoop(L, DT, SE, LI, Updater);
+ ++NumDeleted;
+ return true;
+ }
+
+ // The remaining checks below are for a loop being dead because all statements
+ // in the loop are invariant.
+ SmallVector<BasicBlock *, 4> ExitingBlocks;
+ L->getExitingBlocks(ExitingBlocks);
// 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 branched to, or trying to preserve the branching logic in
// a loop invariant manner.
- if (exitBlocks.size() != 1)
+ if (!ExitBlock) {
+ DEBUG(dbgs() << "Deletion requires single exit block\n");
return false;
-
+ }
// Finally, we have to check that the loop really is dead.
bool Changed = false;
- if (!isLoopDead(L, SE, exitingBlocks, exitBlocks, Changed, preheader))
+ if (!isLoopDead(L, SE, ExitingBlocks, ExitBlock, Changed, Preheader)) {
+ DEBUG(dbgs() << "Loop is not invariant, cannot delete.\n");
return Changed;
+ }
// 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.
const SCEV *S = SE.getMaxBackedgeTakenCount(L);
- if (isa<SCEVCouldNotCompute>(S))
+ if (isa<SCEVCouldNotCompute>(S)) {
+ DEBUG(dbgs() << "Could not compute SCEV MaxBackedgeTakenCount.\n");
return Changed;
+ }
+
+ DEBUG(dbgs() << "Loop is invariant, delete it!");
+ deleteDeadLoop(L, DT, SE, LI, Updater);
+ ++NumDeleted;
+
+ return true;
+}
+
+static void deleteDeadLoop(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
+ LoopInfo &LI, LPMUpdater *Updater) {
+ assert(L->isLCSSAForm(DT) && "Expected LCSSA!");
+ auto *Preheader = L->getLoopPreheader();
+ assert(Preheader && "Preheader should exist!");
// Now that we know the removal is safe, remove the loop by changing the
// branch from the preheader to go to the single exit block.
- BasicBlock *exitBlock = exitBlocks[0];
-
+ //
// Because we're deleting a large chunk of code at once, the sequence in which
- // we remove things is very important to avoid invalidation issues. Don't
- // mess with this unless you have good reason and know what you're doing.
+ // we remove things is very important to avoid invalidation issues.
+
+ // If we have an LPM updater, tell it about the loop being removed.
+ if (Updater)
+ Updater->markLoopAsDeleted(*L);
// Tell ScalarEvolution that the loop is deleted. Do this before
// deleting the loop so that ScalarEvolution can look at the loop
// to determine what it needs to clean up.
SE.forgetLoop(L);
- // Connect the preheader directly to the exit block.
- TerminatorInst *TI = preheader->getTerminator();
- TI->replaceUsesOfWith(L->getHeader(), exitBlock);
+ auto *ExitBlock = L->getUniqueExitBlock();
+ assert(ExitBlock && "Should have a unique exit block!");
- // Rewrite phis in the exit block to get their inputs from
- // the preheader instead of the exiting block.
- BasicBlock *exitingBlock = exitingBlocks[0];
- BasicBlock::iterator BI = exitBlock->begin();
+ assert(L->hasDedicatedExits() && "Loop should have dedicated exits!");
+
+ // Connect the preheader directly to the exit block.
+ // Even when the loop is never executed, we cannot remove the edge from the
+ // source block to the exit block. Consider the case where the unexecuted loop
+ // branches back to an outer loop. If we deleted the loop and removed the edge
+ // coming to this inner loop, this will break the outer loop structure (by
+ // deleting the backedge of the outer loop). If the outer loop is indeed a
+ // non-loop, it will be deleted in a future iteration of loop deletion pass.
+ Preheader->getTerminator()->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)) {
- int j = P->getBasicBlockIndex(exitingBlock);
- assert(j >= 0 && "Can't find exiting block in exit block's phi node!");
- P->setIncomingBlock(j, preheader);
- for (unsigned i = 1; i < exitingBlocks.size(); ++i)
- P->removeIncomingValue(exitingBlocks[i]);
+ // Set the zero'th element of Phi to be from the preheader and remove all
+ // other incoming values. Given the loop has dedicated exits, all other
+ // incoming values must be from the exiting blocks.
+ int PredIndex = 0;
+ P->setIncomingBlock(PredIndex, Preheader);
+ // Removes all incoming values from all other exiting blocks (including
+ // duplicate values from an exiting block).
+ // Nuke all entries except the zero'th entry which is the preheader entry.
+ // NOTE! We need to remove Incoming Values in the reverse order as done
+ // below, to keep the indices valid for deletion (removeIncomingValues
+ // updates getNumIncomingValues and shifts all values down into the operand
+ // being deleted).
+ for (unsigned i = 0, e = P->getNumIncomingValues() - 1; i != e; ++i)
+ P->removeIncomingValue(e-i, false);
+
+ assert((P->getNumIncomingValues() == 1 &&
+ P->getIncomingBlock(PredIndex) == Preheader) &&
+ "Should have exactly one value and that's from the preheader!");
++BI;
}
@@ -175,11 +281,11 @@ bool LoopDeletionPass::runImpl(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
SmallVector<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
+ // Move all of the block's children to be children of the Preheader, which
// allows us to remove the domtree entry for the block.
ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
for (DomTreeNode *ChildNode : ChildNodes) {
- DT.changeImmediateDominator(ChildNode, DT[preheader]);
+ DT.changeImmediateDominator(ChildNode, DT[Preheader]);
}
ChildNodes.clear();
@@ -204,22 +310,19 @@ bool LoopDeletionPass::runImpl(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
SmallPtrSet<BasicBlock *, 8> blocks;
blocks.insert(L->block_begin(), L->block_end());
for (BasicBlock *BB : blocks)
- loopInfo.removeBlock(BB);
+ LI.removeBlock(BB);
// The last step is to update LoopInfo now that we've eliminated this loop.
- loopInfo.markAsRemoved(L);
- Changed = true;
-
- ++NumDeleted;
-
- return Changed;
+ LI.markAsRemoved(L);
}
PreservedAnalyses LoopDeletionPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
- LPMUpdater &) {
- bool Changed = runImpl(&L, AR.DT, AR.SE, AR.LI);
- if (!Changed)
+ LPMUpdater &Updater) {
+
+ DEBUG(dbgs() << "Analyzing Loop for deletion: ");
+ DEBUG(L.dump());
+ if (!deleteLoopIfDead(&L, AR.DT, AR.SE, AR.LI, &Updater))
return PreservedAnalyses::all();
return getLoopPassPreservedAnalyses();
@@ -254,11 +357,11 @@ Pass *llvm::createLoopDeletionPass() { return new LoopDeletionLegacyPass(); }
bool LoopDeletionLegacyPass::runOnLoop(Loop *L, LPPassManager &) {
if (skipLoop(L))
return false;
-
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
- LoopInfo &loopInfo = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+ LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
- LoopDeletionPass Impl;
- return Impl.runImpl(L, DT, SE, loopInfo);
+ DEBUG(dbgs() << "Analyzing Loop for deletion: ");
+ DEBUG(L->dump());
+ return deleteLoopIfDead(L, DT, SE, LI);
}
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