forked from OSchip/llvm-project
[LoopSimplifyCFG] Teach LoopSimplifyCFG to constant-fold branches and switches
This patch introduces infrastructure and the simplest case for constant-folding of branch and switch instructions within loop into unconditional branches. It is useful as a cleanup for such passes as loop unswitching that sometimes produce such branches. Only the simplest case supported in this patch: after the folding, no block should become dead or stop being part of the loop. Support for more sophisticated cases will go separately in follow-up patches. Differential Revision: https://reviews.llvm.org/D54021 Reviewed By: anna llvm-svn: 347183
This commit is contained in:
Max Kazantsev
parent
e7b789b529
commit
8e3e33d138
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@ -41,6 +41,316 @@ using namespace llvm;
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#define DEBUG_TYPE "loop-simplifycfg"
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STATISTIC(NumTerminatorsFolded,
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"Number of terminators folded to unconditional branches");
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/// If \p BB is a switch or a conditional branch, but only one of its successors
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/// can be reached from this block in runtime, return this successor. Otherwise,
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/// return nullptr.
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static BasicBlock *getOnlyLiveSuccessor(BasicBlock *BB) {
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Instruction *TI = BB->getTerminator();
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if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
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if (BI->isUnconditional())
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return nullptr;
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if (BI->getSuccessor(0) == BI->getSuccessor(1))
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return BI->getSuccessor(0);
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ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
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if (!Cond)
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return nullptr;
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return Cond->isZero() ? BI->getSuccessor(1) : BI->getSuccessor(0);
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}
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if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
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auto *CI = dyn_cast<ConstantInt>(SI->getCondition());
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if (!CI)
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return nullptr;
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for (auto Case : SI->cases())
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if (Case.getCaseValue() == CI)
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return Case.getCaseSuccessor();
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return SI->getDefaultDest();
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}
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return nullptr;
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}
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/// Helper class that can turn branches and switches with constant conditions
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/// into unconditional branches.
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class ConstantTerminatorFoldingImpl {
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private:
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Loop &L;
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LoopInfo &LI;
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DominatorTree &DT;
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// Whether or not the current loop will still exist after terminator constant
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// folding will be done. In theory, there are two ways how it can happen:
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// 1. Loop's latch(es) become unreachable from loop header;
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// 2. Loop's header becomes unreachable from method entry.
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// In practice, the second situation is impossible because we only modify the
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// current loop and its preheader and do not affect preheader's reachibility
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// from any other block. So this variable set to true means that loop's latch
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// has become unreachable from loop header.
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bool DeleteCurrentLoop = false;
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// The blocks of the original loop that will still be reachable from entry
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// after the constant folding.
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SmallPtrSet<BasicBlock *, 8> LiveLoopBlocks;
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// The blocks of the original loop that will become unreachable from entry
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// after the constant folding.
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SmallPtrSet<BasicBlock *, 8> DeadLoopBlocks;
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// The exits of the original loop that will still be reachable from entry
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// after the constant folding.
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SmallPtrSet<BasicBlock *, 8> LiveExitBlocks;
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// The exits of the original loop that will become unreachable from entry
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// after the constant folding.
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SmallPtrSet<BasicBlock *, 8> DeadExitBlocks;
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// The blocks that will still be a part of the current loop after folding.
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SmallPtrSet<BasicBlock *, 8> BlocksInLoopAfterFolding;
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// The blocks that have terminators with constant condition that can be
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// folded. Note: fold candidates should be in L but not in any of its
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// subloops to avoid complex LI updates.
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SmallVector<BasicBlock *, 8> FoldCandidates;
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void dump() const {
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dbgs() << "Constant terminator folding for loop " << L << "\n";
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dbgs() << "After terminator constant-folding, the loop will";
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if (!DeleteCurrentLoop)
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dbgs() << " not";
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dbgs() << " be destroyed\n";
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dbgs() << "Blocks in which we can constant-fold terminator:\n";
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for (const BasicBlock *BB : FoldCandidates)
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dbgs() << "\t" << BB->getName() << "\n";
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auto PrintOutSet = [&](const char *Message,
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const SmallPtrSetImpl<BasicBlock *> &S) {
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dbgs() << Message << "\n";
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for (const BasicBlock *BB : S)
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dbgs() << "\t" << BB->getName() << "\n";
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};
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PrintOutSet("Live blocks from the original loop:", LiveLoopBlocks);
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PrintOutSet("Dead blocks from the original loop:", DeadLoopBlocks);
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PrintOutSet("Live exit blocks:", LiveExitBlocks);
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PrintOutSet("Dead exit blocks:", DeadExitBlocks);
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if (!DeleteCurrentLoop)
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PrintOutSet("The following blocks will still be part of the loop:",
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BlocksInLoopAfterFolding);
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}
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/// Fill all information about status of blocks and exits of the current loop
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/// if constant folding of all branches will be done.
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void analyze() {
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LoopBlocksDFS DFS(&L);
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DFS.perform(&LI);
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assert(DFS.isComplete() && "DFS is expected to be finished");
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// Collect live and dead loop blocks and exits.
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SmallPtrSet<BasicBlock *, 8> ExitBlocks;
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LiveLoopBlocks.insert(L.getHeader());
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for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I) {
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BasicBlock *BB = *I;
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// If a loop block wasn't marked as live so far, then it's dead.
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if (!LiveLoopBlocks.count(BB)) {
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DeadLoopBlocks.insert(BB);
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continue;
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}
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BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(BB);
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// If a block has only one live successor, it's a candidate on constant
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// folding. Only handle blocks from current loop: branches in child loops
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// are skipped because if they can be folded, they should be folded during
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// the processing of child loops.
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if (TheOnlySucc && LI.getLoopFor(BB) == &L)
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FoldCandidates.push_back(BB);
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// Handle successors.
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auto ProcessSuccessor = [&](BasicBlock *Succ, bool IsLive) {
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if (!L.contains(Succ)) {
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if (IsLive)
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LiveExitBlocks.insert(Succ);
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ExitBlocks.insert(Succ);
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} else if (IsLive)
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LiveLoopBlocks.insert(Succ);
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};
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for (BasicBlock *Succ : successors(BB))
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ProcessSuccessor(Succ, !TheOnlySucc || TheOnlySucc == Succ);
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}
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// Sanity check: amount of dead and live loop blocks should match the total
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// number of blocks in loop.
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assert(L.getNumBlocks() == LiveLoopBlocks.size() + DeadLoopBlocks.size() &&
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"Malformed block sets?");
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// Now, all exit blocks that are not marked as live are dead.
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for (auto *ExitBlock : ExitBlocks)
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if (!LiveExitBlocks.count(ExitBlock))
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DeadExitBlocks.insert(ExitBlock);
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// Whether or not the edge From->To will still be present in graph after the
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// folding.
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auto IsEdgeLive = [&](BasicBlock *From, BasicBlock *To) {
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if (!LiveLoopBlocks.count(From))
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return false;
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BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(From);
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return !TheOnlySucc || TheOnlySucc == To;
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};
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// The loop will not be destroyed if its latch is live.
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DeleteCurrentLoop = !IsEdgeLive(L.getLoopLatch(), L.getHeader());
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// If we are going to delete the current loop completely, no extra analysis
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// is needed.
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if (DeleteCurrentLoop)
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return;
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// Otherwise, we should check which blocks will still be a part of the
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// current loop after the transform.
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BlocksInLoopAfterFolding.insert(L.getLoopLatch());
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// If the loop is live, then we should compute what blocks are still in
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// loop after all branch folding has been done. A block is in loop if
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// it has a live edge to another block that is in the loop; by definition,
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// latch is in the loop.
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auto BlockIsInLoop = [&](BasicBlock *BB) {
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return any_of(successors(BB), [&](BasicBlock *Succ) {
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return BlocksInLoopAfterFolding.count(Succ) && IsEdgeLive(BB, Succ);
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});
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};
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for (auto I = DFS.beginPostorder(), E = DFS.endPostorder(); I != E; ++I) {
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BasicBlock *BB = *I;
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if (BlockIsInLoop(BB))
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BlocksInLoopAfterFolding.insert(BB);
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}
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// Sanity check: header must be in loop.
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assert(BlocksInLoopAfterFolding.count(L.getHeader()) &&
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"Header not in loop?");
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}
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/// Constant-fold terminators of blocks acculumated in FoldCandidates into the
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/// unconditional branches.
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void foldTerminators() {
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DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
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for (BasicBlock *BB : FoldCandidates) {
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assert(LI.getLoopFor(BB) == &L && "Should be a loop block!");
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BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(BB);
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assert(TheOnlySucc && "Should have one live successor!");
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LLVM_DEBUG(dbgs() << "Replacing terminator of " << BB->getName()
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<< " with an unconditional branch to the block "
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<< TheOnlySucc->getName() << "\n");
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SmallPtrSet<BasicBlock *, 2> DeadSuccessors;
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// Remove all BB's successors except for the live one.
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for (auto *Succ : successors(BB))
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if (Succ != TheOnlySucc)
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DeadSuccessors.insert(Succ);
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IRBuilder<> Builder(BB->getContext());
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Instruction *Term = BB->getTerminator();
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Builder.SetInsertPoint(Term);
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Builder.CreateBr(TheOnlySucc);
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Term->eraseFromParent();
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for (auto *DeadSucc : DeadSuccessors)
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DTU.deleteEdge(BB, DeadSucc);
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++NumTerminatorsFolded;
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}
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}
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public:
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ConstantTerminatorFoldingImpl(Loop &L, LoopInfo &LI, DominatorTree &DT)
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: L(L), LI(LI), DT(DT) {}
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bool run() {
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assert(L.getLoopLatch() && "Should be single latch!");
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// Collect all available information about status of blocks after constant
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// folding.
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analyze();
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LLVM_DEBUG(dbgs() << "In function " << L.getHeader()->getParent()->getName()
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<< ": ");
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// Nothing to constant-fold.
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if (FoldCandidates.empty()) {
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LLVM_DEBUG(
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dbgs() << "No constant terminator folding candidates found in loop "
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<< L.getHeader()->getName() << "\n");
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return false;
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}
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// TODO: Support deletion of the current loop.
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if (DeleteCurrentLoop) {
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LLVM_DEBUG(
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dbgs()
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<< "Give up constant terminator folding in loop "
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<< L.getHeader()->getName()
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<< ": we don't currently support deletion of the current loop.\n");
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return false;
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}
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// TODO: Support deletion of dead loop blocks.
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if (!DeadLoopBlocks.empty()) {
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LLVM_DEBUG(dbgs() << "Give up constant terminator folding in loop "
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<< L.getHeader()->getName()
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<< ": we don't currently"
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" support deletion of dead in-loop blocks.\n");
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return false;
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}
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// TODO: Support dead loop exits.
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if (!DeadExitBlocks.empty()) {
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LLVM_DEBUG(dbgs() << "Give up constant terminator folding in loop "
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<< L.getHeader()->getName()
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<< ": we don't currently support dead loop exits.\n");
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return false;
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}
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// TODO: Support blocks that are not dead, but also not in loop after the
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// folding.
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if (BlocksInLoopAfterFolding.size() != L.getNumBlocks()) {
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LLVM_DEBUG(
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dbgs() << "Give up constant terminator folding in loop "
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<< L.getHeader()->getName()
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<< ": we don't currently"
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" support blocks that are not dead, but will stop "
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"being a part of the loop after constant-folding.\n");
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return false;
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}
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// Dump analysis results.
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LLVM_DEBUG(dump());
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LLVM_DEBUG(dbgs() << "Constant-folding " << FoldCandidates.size()
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<< " terminators in loop " << L.getHeader()->getName()
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<< "\n");
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// Make the actual transforms.
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foldTerminators();
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#ifndef NDEBUG
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// Make sure that we have preserved all data structures after the transform.
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DT.verify();
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assert(DT.isReachableFromEntry(L.getHeader()));
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LI.verify(DT);
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#endif
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return true;
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}
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};
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/// Turn branches and switches with known constant conditions into unconditional
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/// branches.
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static bool constantFoldTerminators(Loop &L, DominatorTree &DT, LoopInfo &LI) {
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// To keep things simple, only process loops with single latch. We
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// canonicalize most loops to this form. We can support multi-latch if needed.
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if (!L.getLoopLatch())
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return false;
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ConstantTerminatorFoldingImpl BranchFolder(L, LI, DT);
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return BranchFolder.run();
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}
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static bool mergeBlocksIntoPredecessors(Loop &L, DominatorTree &DT,
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LoopInfo &LI, MemorySSAUpdater *MSSAU) {
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bool Changed = false;
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@ -73,6 +383,9 @@ static bool simplifyLoopCFG(Loop &L, DominatorTree &DT, LoopInfo &LI,
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ScalarEvolution &SE, MemorySSAUpdater *MSSAU) {
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bool Changed = false;
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// Constant-fold terminators with known constant conditions.
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Changed |= constantFoldTerminators(L, DT, LI);
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// Eliminate unconditional branches by merging blocks into their predecessors.
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Changed |= mergeBlocksIntoPredecessors(L, DT, LI, MSSAU);
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@ -1,10 +1,53 @@
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; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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; RUN: opt -S -loop-simplifycfg < %s | FileCheck %s
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; RUN: opt -S -passes='require<domtree>,loop(simplify-cfg)' < %s | FileCheck %s
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; RUN: opt -S -loop-simplifycfg -enable-mssa-loop-dependency=true -verify-memoryssa < %s | FileCheck %s
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; RUN: opt -S -loop-simplifycfg -debug-only=loop-simplifycfg 2>&1 < %s | FileCheck %s
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; RUN: opt -S -passes='require<domtree>,loop(simplify-cfg)' -debug-only=loop-simplifycfg 2>&1 < %s | FileCheck %s
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; RUN: opt -S -loop-simplifycfg -enable-mssa-loop-dependency=true -verify-memoryssa -debug-only=loop-simplifycfg 2>&1 < %s | FileCheck %s
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target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
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; CHECK-LABEL: In function dead_backedge_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support blocks that are not dead, but will stop being a part of the loop after constant-folding.
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; CHECK-LABEL: In function dead_backedge_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support blocks that are not dead, but will stop being a part of the loop after constant-folding.
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; CHECK-LABEL: In function dead_block_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
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; CHECK-LABEL: In function dead_block_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
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; CHECK-LABEL: In function dead_block_propogate_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
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; CHECK-LABEL: In function dead_block_propogate_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
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; CHECK-LABEL: In function dead_exit_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support dead loop exits.
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; CHECK-LABEL: In function dead_exit_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support dead loop exits.
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; CHECK-LABEL: In function dead_loop_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
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; CHECK-LABEL: In function dead_loop_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
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; CHECK-LABEL: In function dead_sub_loop_test_branch_loop: No constant terminator folding candidates found in loop dead_loop
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; CHECK-LABEL: In function dead_sub_loop_test_branch_loop: No constant terminator folding candidates found in loop live_loop
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; CHECK-LABEL: In function dead_sub_loop_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
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; CHECK-LABEL: In function dead_sub_loop_test_switch_loop: No constant terminator folding candidates found in loop live_loop
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; CHECK-LABEL: In function dead_sub_loop_test_switch_loop: No constant terminator folding candidates found in loop dead_loop
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; CHECK-LABEL: In function dead_sub_loop_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
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; CHECK-LABEL: In function inf_loop_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
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; CHECK-LABEL: In function inf_loop_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
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; CHECK-LABEL: In function live_block_test_branch_loop: Constant terminator folding for loop Loop at depth 1 containing: %header<header>,%check,%live,%backedge<latch><exiting>
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; CHECK: Replacing terminator of check with an unconditional branch to the block backedge
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; CHECK-LABEL: In function live_block_test_switch_loop: Constant terminator folding for loop Loop at depth 1 containing: %header<header>,%check,%live,%backedge<latch><exiting>
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; CHECK: Replacing terminator of check with an unconditional branch to the block backedge
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; CHECK-LABEL: In function partial_sub_loop_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
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; CHECK-LABEL: In function partial_sub_loop_test_branch_loop: No constant terminator folding candidates found in loop outer_header
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; CHECK-LABEL: In function partial_sub_loop_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
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; CHECK-LABEL: In function partial_sub_loop_test_switch_loop: No constant terminator folding candidates found in loop outer_header
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; CHECK-LABEL: In function full_sub_loop_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
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; CHECK-LABEL: In function full_sub_loop_test_branch_loop: No constant terminator folding candidates found in loop outer_header
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; CHECK-LABEL: In function full_sub_loop_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
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; CHECK-LABEL: In function full_sub_loop_test_switch_loop: No constant terminator folding candidates found in loop outer_header
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; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_1: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
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; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_1: No constant terminator folding candidates found in loop outer_header
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; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_1: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
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; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_1: No constant terminator folding candidates found in loop outer_header
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; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_2: Give up constant terminator folding in loop header: we don't currently support dead loop exits.
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; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_2: No constant terminator folding candidates found in loop outer_header
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; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_2: Give up constant terminator folding in loop header: we don't currently support dead loop exits.
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; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_2: No constant terminator folding candidates found in loop outer_header
|
||||
; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_3: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
|
||||
; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_3: No constant terminator folding candidates found in loop outer_header
|
||||
; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_3: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
|
||||
; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_3: No constant terminator folding candidates found in loop outer_header
|
||||
|
||||
; Make sure that we can eliminate a provably dead backedge.
|
||||
define i32 @dead_backedge_test_branch_loop(i32 %end) {
|
||||
; CHECK-LABEL: @dead_backedge_test_branch_loop(
|
||||
|
|
@ -707,7 +750,7 @@ define i32 @live_block_test_branch_loop(i1 %c, i32 %end) {
|
|||
; CHECK-NEXT: [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
|
||||
; CHECK-NEXT: br i1 [[C:%.*]], label [[CHECK:%.*]], label [[LIVE:%.*]]
|
||||
; CHECK: check:
|
||||
; CHECK-NEXT: br i1 true, label [[BACKEDGE]], label [[LIVE]]
|
||||
; CHECK-NEXT: br label [[BACKEDGE]]
|
||||
; CHECK: live:
|
||||
; CHECK-NEXT: [[I_2:%.*]] = add i32 [[I]], 1
|
||||
; CHECK-NEXT: br label [[BACKEDGE]]
|
||||
|
|
@ -752,11 +795,7 @@ define i32 @live_block_test_switch_loop(i1 %c, i32 %end) {
|
|||
; CHECK-NEXT: [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
|
||||
; CHECK-NEXT: br i1 [[C:%.*]], label [[CHECK:%.*]], label [[LIVE:%.*]]
|
||||
; CHECK: check:
|
||||
; CHECK-NEXT: switch i32 1, label [[LIVE]] [
|
||||
; CHECK-NEXT: i32 0, label [[LIVE]]
|
||||
; CHECK-NEXT: i32 1, label [[BACKEDGE]]
|
||||
; CHECK-NEXT: i32 2, label [[LIVE]]
|
||||
; CHECK-NEXT: ]
|
||||
; CHECK-NEXT: br label [[BACKEDGE]]
|
||||
; CHECK: live:
|
||||
; CHECK-NEXT: [[I_2:%.*]] = add i32 [[I]], 1
|
||||
; CHECK-NEXT: br label [[BACKEDGE]]
|
||||
|
|
|
|||
Loading…
Reference in New Issue