Statistic<> NumSelects ("loop-unswitch", "Number of selects unswitched");
Statistic<> NumTrivial ("loop-unswitch",
"Number of unswitches that are trivial");
+ Statistic<> NumSimplify("loop-unswitch",
+ "Number of simplifications of unswitched code");
cl::opt<unsigned>
Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
cl::init(10), cl::Hidden);
void VersionLoop(Value *LIC, Constant *OnVal,
Loop *L, Loop *&Out1, Loop *&Out2);
BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To);
+ BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt);
void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val,
bool isEqual);
void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
- BasicBlock *ExitBlock);
+ bool EntersWhenTrue, BasicBlock *ExitBlock);
};
RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
}
return false;
}
-/// FindTrivialLoopExitBlock - We know that we have a branch from the loop
-/// header to the specified latch block. See if one of the successors of the
-/// latch block is an exit, and if so what block it is.
-static BasicBlock *FindTrivialLoopExitBlock(Loop *L, BasicBlock *Latch) {
+/// isTrivialLoopExitBlock - Check to see if all paths from BB either:
+/// 1. Exit the loop with no side effects.
+/// 2. Branch to the latch block with no side-effects.
+///
+/// If these conditions are true, we return true and set ExitBB to the block we
+/// exit through.
+///
+static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
+ BasicBlock *&ExitBB,
+ std::set<BasicBlock*> &Visited) {
BasicBlock *Header = L->getHeader();
- BranchInst *LatchBranch = dyn_cast<BranchInst>(Latch->getTerminator());
- if (!LatchBranch || !LatchBranch->isConditional()) return 0;
-
- // Simple case, the latch block is a conditional branch. The target that
- // doesn't go to the loop header is our block if it is not in the loop.
- if (LatchBranch->getSuccessor(0) == Header) {
- if (L->contains(LatchBranch->getSuccessor(1))) return false;
- return LatchBranch->getSuccessor(1);
- } else {
- assert(LatchBranch->getSuccessor(1) == Header);
- if (L->contains(LatchBranch->getSuccessor(0))) return false;
- return LatchBranch->getSuccessor(0);
+ for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
+ if (!Visited.insert(*SI).second) {
+ // Already visited and Ok, end of recursion.
+ } else if (L->contains(*SI)) {
+ // Check to see if the successor is a trivial loop exit.
+ if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
+ return false;
+ } else {
+ // Otherwise, this is a loop exit, this is fine so long as this is the
+ // first exit.
+ if (ExitBB != 0) return false;
+ ExitBB = *SI;
+ }
}
+
+ // Okay, everything after this looks good, check to make sure that this block
+ // doesn't include any side effects.
+ for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
+ if (I->mayWriteToMemory())
+ return false;
+
+ return true;
}
+static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
+ std::set<BasicBlock*> Visited;
+ Visited.insert(L->getHeader()); // Branches to header are ok.
+ Visited.insert(BB); // Don't revisit BB after we do.
+ BasicBlock *ExitBB = 0;
+ if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
+ return ExitBB;
+ return 0;
+}
/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
/// trivial: that is, that the condition controls whether or not the loop does
/// condition is false. Otherwise, return null to indicate a complex condition.
static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond,
Constant **Val = 0,
+ bool *EntersWhenTrue = 0,
BasicBlock **LoopExit = 0) {
BasicBlock *Header = L->getHeader();
- BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator());
-
- // If the header block doesn't end with a conditional branch on Cond, we can't
- // handle it.
- if (!HeaderTerm || !HeaderTerm->isConditional() ||
- HeaderTerm->getCondition() != Cond)
- return false;
+ TerminatorInst *HeaderTerm = Header->getTerminator();
+
+ BasicBlock *LoopExitBB = 0;
+ if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
+ // If the header block doesn't end with a conditional branch on Cond, we
+ // can't handle it.
+ if (!BI->isConditional() || BI->getCondition() != Cond)
+ return false;
- // Check to see if the conditional branch goes to the latch block. If not,
- // it's not trivial. This also determines the value of Cond that will execute
- // the loop.
- BasicBlock *Latch = L->getLoopLatch();
- if (HeaderTerm->getSuccessor(1) == Latch) {
- if (Val) *Val = ConstantBool::True;
- } else if (HeaderTerm->getSuccessor(0) == Latch)
- if (Val) *Val = ConstantBool::False;
- else
- return false; // Doesn't branch to latch block.
+ // Check to see if a successor of the branch is guaranteed to go to the
+ // latch block or exit through a one exit block without having any
+ // side-effects. If so, determine the value of Cond that causes it to do
+ // this.
+ if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(0)))) {
+ if (Val) *Val = ConstantBool::False;
+ } else if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(1)))) {
+ if (Val) *Val = ConstantBool::True;
+ }
+ } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
+ // If this isn't a switch on Cond, we can't handle it.
+ if (SI->getCondition() != Cond) return false;
+
+ // Check to see if a successor of the switch is guaranteed to go to the
+ // latch block or exit through a one exit block without having any
+ // side-effects. If so, determine the value of Cond that causes it to do
+ // this. Note that we can't trivially unswitch on the default case.
+ for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
+ if ((LoopExitBB = isTrivialLoopExitBlock(L, SI->getSuccessor(i)))) {
+ // Okay, we found a trivial case, remember the value that is trivial.
+ if (Val) *Val = SI->getCaseValue(i);
+ if (EntersWhenTrue) *EntersWhenTrue = false;
+ break;
+ }
+ }
+
+ if (!LoopExitBB)
+ return false; // Can't handle this.
- // The latch block must end with a conditional branch where one edge goes to
- // the header (this much we know) and one edge goes OUT of the loop.
- BasicBlock *LoopExitBlock = FindTrivialLoopExitBlock(L, Latch);
- if (!LoopExitBlock) return 0;
- if (LoopExit) *LoopExit = LoopExitBlock;
+ if (LoopExit) *LoopExit = LoopExitBB;
// We already know that nothing uses any scalar values defined inside of this
// loop. As such, we just have to check to see if this loop will execute any
// side-effecting instructions (e.g. stores, calls, volatile loads) in the
- // part of the loop that the code *would* execute.
+ // part of the loop that the code *would* execute. We already checked the
+ // tail, check the header now.
for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
if (I->mayWriteToMemory())
return false;
- for (BasicBlock::iterator I = Latch->begin(), E = Latch->end(); I != E; ++I)
- if (I->mayWriteToMemory())
- return false;
return true;
}
// If this is a trivial condition to unswitch (which results in no code
// duplication), do it now.
Constant *CondVal;
+ bool EntersWhenTrue = true;
BasicBlock *ExitBlock;
- if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal, &ExitBlock)){
- UnswitchTrivialCondition(L, LoopCond, CondVal, ExitBlock);
+ if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal,
+ &EntersWhenTrue, &ExitBlock)) {
+ UnswitchTrivialCondition(L, LoopCond, CondVal, EntersWhenTrue, ExitBlock);
NewLoop1 = L;
} else {
VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2);
return true;
}
+/// SplitBlock - Split the specified block at the specified instruction - every
+/// thing before SplitPt stays in Old and everything starting with SplitPt moves
+/// to a new block. The two blocks are joined by an unconditional branch and
+/// the loop info is updated.
+///
+BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *Old, Instruction *SplitPt) {
+ BasicBlock::iterator SplitIt = SplitPt;
+ while (isa<PHINode>(SplitIt))
+ ++SplitIt;
+ BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
+
+ // The new block lives in whichever loop the old one did.
+ if (Loop *L = LI->getLoopFor(Old))
+ L->addBasicBlockToLoop(New, *LI);
+
+ return New;
+}
+
+
BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) {
TerminatorInst *LatchTerm = BB->getTerminator();
unsigned SuccNum = 0;
// If the edge isn't critical, then BB has a single successor or Succ has a
// single pred. Split the block.
- BasicBlock *BlockToSplit;
BasicBlock::iterator SplitPoint;
if (BasicBlock *SP = Succ->getSinglePredecessor()) {
// If the successor only has a single pred, split the top of the successor
// block.
assert(SP == BB && "CFG broken");
- BlockToSplit = Succ;
- SplitPoint = Succ->begin();
+ return SplitBlock(Succ, Succ->begin());
} else {
// Otherwise, if BB has a single successor, split it at the bottom of the
// block.
assert(BB->getTerminator()->getNumSuccessors() == 1 &&
"Should have a single succ!");
- BlockToSplit = BB;
- SplitPoint = BB->getTerminator();
+ return SplitBlock(BB, BB->getTerminator());
}
-
- BasicBlock *New =
- BlockToSplit->splitBasicBlock(SplitPoint,
- BlockToSplit->getName()+".tail");
- // New now lives in whichever loop that BB used to.
- if (Loop *L = LI->getLoopFor(BlockToSplit))
- L->addBasicBlockToLoop(New, *LI);
- return New;
}
/// side-effects), unswitch it. This doesn't involve any code duplication, just
/// moving the conditional branch outside of the loop and updating loop info.
void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
- Constant *Val,
+ Constant *Val, bool EntersWhenTrue,
BasicBlock *ExitBlock) {
DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %"
<< L->getHeader()->getName() << " [" << L->getBlocks().size()
<< " blocks] in Function " << L->getHeader()->getParent()->getName()
- << " on cond:" << *Cond << "\n");
+ << " on cond: " << *Val << (EntersWhenTrue ? " == " : " != ") <<
+ *Cond << "\n");
// First step, split the preheader, so that we know that there is a safe place
// to insert the conditional branch. We will change 'OrigPH' to have a
// to branch to: this is the exit block out of the loop that we should
// short-circuit to.
- // Split this edge now, so that the loop maintains its exit block.
+ // Split this block now, so that the loop maintains its exit block, and so
+ // that the jump from the preheader can execute the contents of the exit block
+ // without actually branching to it (the exit block should be dominated by the
+ // loop header, not the preheader).
assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
- BasicBlock *NewExit = SplitEdge(L->getLoopLatch(), ExitBlock);
- assert(NewExit != ExitBlock && "Edge not split!");
+ BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin());
// Okay, now we have a position to branch from and a position to branch to,
// insert the new conditional branch.
- EmitPreheaderBranchOnCondition(Cond, Val, NewPH, NewExit,
- OrigPH->getTerminator());
+ {
+ BasicBlock *TrueDest = NewPH, *FalseDest = NewExit;
+ if (!EntersWhenTrue) std::swap(TrueDest, FalseDest);
+ EmitPreheaderBranchOnCondition(Cond, Val, TrueDest, FalseDest,
+ OrigPH->getTerminator());
+ }
OrigPH->getTerminator()->eraseFromParent();
// Now that we know that the loop is never entered when this condition is a
// particular value, rewrite the loop with this info. We know that this will
// at least eliminate the old branch.
- RewriteLoopBodyWithConditionConstant(L, Cond, Val, true);
+ RewriteLoopBodyWithConditionConstant(L, Cond, Val, EntersWhenTrue);
++NumTrivial;
}
Out2 = NewLoop;
}
+/// RemoveFromWorklist - Remove all instances of I from the worklist vector
+/// specified.
+static void RemoveFromWorklist(Instruction *I,
+ std::vector<Instruction*> &Worklist) {
+ std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
+ Worklist.end(), I);
+ while (WI != Worklist.end()) {
+ unsigned Offset = WI-Worklist.begin();
+ Worklist.erase(WI);
+ WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
+ }
+}
+
+/// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
+/// program, replacing all uses with V and update the worklist.
+static void ReplaceUsesOfWith(Instruction *I, Value *V,
+ std::vector<Instruction*> &Worklist) {
+ DEBUG(std::cerr << "Replace with '" << *V << "': " << *I);
+
+ // Add uses to the worklist, which may be dead now.
+ for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
+ if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
+ Worklist.push_back(Use);
+
+ // Add users to the worklist which may be simplified now.
+ for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
+ UI != E; ++UI)
+ Worklist.push_back(cast<Instruction>(*UI));
+ I->replaceAllUsesWith(V);
+ I->eraseFromParent();
+ RemoveFromWorklist(I, Worklist);
+ ++NumSimplify;
+}
+
+
+
// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
// the value specified by Val in the specified loop, or we know it does NOT have
// that value. Rewrite any uses of LIC or of properties correlated to it.
// FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
// selects, switches.
std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
+
+ std::vector<Instruction*> Worklist;
- // Haha, this loop could be unswitched. Get it? The unswitch pass could
- // unswitch itself. Amazing.
- for (unsigned i = 0, e = Users.size(); i != e; ++i)
- if (Instruction *U = cast<Instruction>(Users[i]))
- if (L->contains(U->getParent()))
- if (IsEqual) {
- U->replaceUsesOfWith(LIC, Val);
- } else if (NotVal) {
- U->replaceUsesOfWith(LIC, NotVal);
- } else {
- // If we know that LIC is not Val, use this info to simplify code.
- if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
- for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
- if (SI->getCaseValue(i) == Val) {
- // Found a dead case value. Don't remove PHI nodes in the
- // successor if they become single-entry, those PHI nodes may
- // be in the Users list.
- SI->getSuccessor(i)->removePredecessor(SI->getParent(), true);
- SI->removeCase(i);
- break;
- }
+ // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
+ // in the loop with the appropriate one directly.
+ if (IsEqual || NotVal) {
+ Value *Replacement = NotVal ? NotVal : Val;
+
+ for (unsigned i = 0, e = Users.size(); i != e; ++i)
+ if (Instruction *U = cast<Instruction>(Users[i])) {
+ if (!L->contains(U->getParent()))
+ continue;
+ U->replaceUsesOfWith(LIC, Replacement);
+ Worklist.push_back(U);
+ }
+ } else {
+ // Otherwise, we don't know the precise value of LIC, but we do know that it
+ // is certainly NOT "Val". As such, simplify any uses in the loop that we
+ // can. This case occurs when we unswitch switch statements.
+ for (unsigned i = 0, e = Users.size(); i != e; ++i)
+ if (Instruction *U = cast<Instruction>(Users[i])) {
+ if (!L->contains(U->getParent()))
+ continue;
+
+ Worklist.push_back(U);
+
+ // If we know that LIC is not Val, use this info to simplify code.
+ if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
+ for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
+ if (SI->getCaseValue(i) == Val) {
+ // Found a dead case value. Don't remove PHI nodes in the
+ // successor if they become single-entry, those PHI nodes may
+ // be in the Users list.
+ SI->getSuccessor(i)->removePredecessor(SI->getParent(), true);
+ SI->removeCase(i);
+ break;
}
}
-
- // TODO: We could simplify stuff like X == C.
}
+
+ // TODO: We could do other simplifications, for example, turning
+ // LIC == Val -> false.
+ }
+ }
+
+ // Okay, now that we have simplified some instructions in the loop, walk over
+ // it and constant prop, dce, and fold control flow where possible. Note that
+ // this is effectively a very simple loop-structure-aware optimizer.
+ while (!Worklist.empty()) {
+ Instruction *I = Worklist.back();
+ Worklist.pop_back();
+
+ // Simple constant folding.
+ if (Constant *C = ConstantFoldInstruction(I)) {
+ ReplaceUsesOfWith(I, C, Worklist);
+ continue;
+ }
+
+ // Simple DCE.
+ if (isInstructionTriviallyDead(I)) {
+ DEBUG(std::cerr << "Remove dead instruction '" << *I);
+
+ // Add uses to the worklist, which may be dead now.
+ for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
+ if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
+ Worklist.push_back(Use);
+ I->eraseFromParent();
+ RemoveFromWorklist(I, Worklist);
+ ++NumSimplify;
+ continue;
+ }
+
+ // Special case hacks that appear commonly in unswitched code.
+ switch (I->getOpcode()) {
+ case Instruction::Select:
+ if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(0))) {
+ ReplaceUsesOfWith(I, I->getOperand(!CB->getValue()+1), Worklist);
+ continue;
+ }
+ break;
+ case Instruction::And:
+ if (isa<ConstantBool>(I->getOperand(0))) // constant -> RHS
+ cast<BinaryOperator>(I)->swapOperands();
+ if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) {
+ if (CB->getValue()) // X & 1 -> X
+ ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
+ else // X & 0 -> 0
+ ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
+ continue;
+ }
+ break;
+ case Instruction::Or:
+ if (isa<ConstantBool>(I->getOperand(0))) // constant -> RHS
+ cast<BinaryOperator>(I)->swapOperands();
+ if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) {
+ if (CB->getValue()) // X | 1 -> 1
+ ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
+ else // X | 0 -> X
+ ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
+ continue;
+ }
+ break;
+ case Instruction::Br: {
+ BranchInst *BI = cast<BranchInst>(I);
+ if (BI->isUnconditional()) {
+ // If BI's parent is the only pred of the successor, fold the two blocks
+ // together.
+ BasicBlock *Pred = BI->getParent();
+ BasicBlock *Succ = BI->getSuccessor(0);
+ BasicBlock *SinglePred = Succ->getSinglePredecessor();
+ if (!SinglePred) continue; // Nothing to do.
+ assert(SinglePred == Pred && "CFG broken");
+
+ DEBUG(std::cerr << "Merging blocks: " << Pred->getName() << " <- "
+ << Succ->getName() << "\n");
+
+ // Resolve any single entry PHI nodes in Succ.
+ while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
+ ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist);
+
+ // Move all of the successor contents from Succ to Pred.
+ Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
+ Succ->end());
+ BI->eraseFromParent();
+ RemoveFromWorklist(BI, Worklist);
+
+ // If Succ has any successors with PHI nodes, update them to have
+ // entries coming from Pred instead of Succ.
+ Succ->replaceAllUsesWith(Pred);
+
+ // Remove Succ from the loop tree.
+ LI->removeBlock(Succ);
+ Succ->eraseFromParent();
+ break;
+ }
+ break;
+ }
+ }
+ }
}
projects / oota-llvm.git / blobdiff