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-rw-r--r--llvm/lib/Transforms/Scalar/LoopUnswitch.cpp126
1 files changed, 79 insertions, 47 deletions
diff --git a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
index a4da2501f9f..5cc7079301b 100644
--- a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
@@ -75,6 +75,7 @@ namespace {
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,
@@ -123,26 +124,50 @@ static bool LoopValuesUsedOutsideLoop(Loop *L) {
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 = Header->begin(), E = Header->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
@@ -165,33 +190,30 @@ static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond,
HeaderTerm->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) {
+ // 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.
+ BasicBlock *LoopExitBlock = 0;
+ if ((LoopExitBlock = isTrivialLoopExitBlock(L, HeaderTerm->getSuccessor(0)))){
if (Val) *Val = ConstantBool::True;
- } else if (HeaderTerm->getSuccessor(0) == Latch)
+ } else if ((LoopExitBlock =
+ isTrivialLoopExitBlock(L, HeaderTerm->getSuccessor(1)))) {
if (Val) *Val = ConstantBool::False;
- else
- return false; // Doesn't branch to latch block.
+ }
+
+ if (!LoopExitBlock)
+ 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;
// 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;
}
@@ -350,6 +372,24 @@ bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
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) {
+ while (isa<PHINode>(SplitPt))
+ ++SplitPt;
+ BasicBlock *New = Old->splitBasicBlock(SplitPt, 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;
@@ -373,24 +413,14 @@ BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) {
// 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;
}
@@ -477,10 +507,12 @@ void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
// 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.
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