| //===- TailDuplication.cpp - Simplify CFG through tail duplication --------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file was developed by the LLVM research group and is distributed under |
| // the University of Illinois Open Source License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This pass performs a limited form of tail duplication, intended to simplify |
| // CFGs by removing some unconditional branches. This pass is necessary to |
| // straighten out loops created by the C front-end, but also is capable of |
| // making other code nicer. After this pass is run, the CFG simplify pass |
| // should be run to clean up the mess. |
| // |
| // This pass could be enhanced in the future to use profile information to be |
| // more aggressive. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "tailduplicate" |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/Constant.h" |
| #include "llvm/Function.h" |
| #include "llvm/Instructions.h" |
| #include "llvm/IntrinsicInst.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Type.h" |
| #include "llvm/Support/CFG.h" |
| #include "llvm/Transforms/Utils/Local.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Compiler.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/ADT/Statistic.h" |
| using namespace llvm; |
| |
| STATISTIC(NumEliminated, "Number of unconditional branches eliminated"); |
| |
| namespace { |
| cl::opt<unsigned> |
| Threshold("taildup-threshold", cl::desc("Max block size to tail duplicate"), |
| cl::init(6), cl::Hidden); |
| class VISIBILITY_HIDDEN TailDup : public FunctionPass { |
| bool runOnFunction(Function &F); |
| private: |
| inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI); |
| inline void eliminateUnconditionalBranch(BranchInst *BI); |
| }; |
| RegisterPass<TailDup> X("tailduplicate", "Tail Duplication"); |
| } |
| |
| // Public interface to the Tail Duplication pass |
| FunctionPass *llvm::createTailDuplicationPass() { return new TailDup(); } |
| |
| /// runOnFunction - Top level algorithm - Loop over each unconditional branch in |
| /// the function, eliminating it if it looks attractive enough. |
| /// |
| bool TailDup::runOnFunction(Function &F) { |
| bool Changed = false; |
| for (Function::iterator I = F.begin(), E = F.end(); I != E; ) |
| if (shouldEliminateUnconditionalBranch(I->getTerminator())) { |
| eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator())); |
| Changed = true; |
| } else { |
| ++I; |
| } |
| return Changed; |
| } |
| |
| /// shouldEliminateUnconditionalBranch - Return true if this branch looks |
| /// attractive to eliminate. We eliminate the branch if the destination basic |
| /// block has <= 5 instructions in it, not counting PHI nodes. In practice, |
| /// since one of these is a terminator instruction, this means that we will add |
| /// up to 4 instructions to the new block. |
| /// |
| /// We don't count PHI nodes in the count since they will be removed when the |
| /// contents of the block are copied over. |
| /// |
| bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) { |
| BranchInst *BI = dyn_cast<BranchInst>(TI); |
| if (!BI || !BI->isUnconditional()) return false; // Not an uncond branch! |
| |
| BasicBlock *Dest = BI->getSuccessor(0); |
| if (Dest == BI->getParent()) return false; // Do not loop infinitely! |
| |
| // Do not inline a block if we will just get another branch to the same block! |
| TerminatorInst *DTI = Dest->getTerminator(); |
| if (BranchInst *DBI = dyn_cast<BranchInst>(DTI)) |
| if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest) |
| return false; // Do not loop infinitely! |
| |
| // FIXME: DemoteRegToStack cannot yet demote invoke instructions to the stack, |
| // because doing so would require breaking critical edges. This should be |
| // fixed eventually. |
| if (!DTI->use_empty()) |
| return false; |
| |
| // Do not bother working on dead blocks... |
| pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest); |
| if (PI == PE && Dest != Dest->getParent()->begin()) |
| return false; // It's just a dead block, ignore it... |
| |
| // Also, do not bother with blocks with only a single predecessor: simplify |
| // CFG will fold these two blocks together! |
| ++PI; |
| if (PI == PE) return false; // Exactly one predecessor! |
| |
| BasicBlock::iterator I = Dest->begin(); |
| while (isa<PHINode>(*I)) ++I; |
| |
| for (unsigned Size = 0; I != Dest->end(); ++I) { |
| if (Size == Threshold) return false; // The block is too large. |
| // Only count instructions that are not debugger intrinsics. |
| if (!isa<DbgInfoIntrinsic>(I)) ++Size; |
| } |
| |
| // Do not tail duplicate a block that has thousands of successors into a block |
| // with a single successor if the block has many other predecessors. This can |
| // cause an N^2 explosion in CFG edges (and PHI node entries), as seen in |
| // cases that have a large number of indirect gotos. |
| unsigned NumSuccs = DTI->getNumSuccessors(); |
| if (NumSuccs > 8) { |
| unsigned TooMany = 128; |
| if (NumSuccs >= TooMany) return false; |
| TooMany = TooMany/NumSuccs; |
| for (; PI != PE; ++PI) |
| if (TooMany-- == 0) return false; |
| } |
| |
| // Finally, if this unconditional branch is a fall-through, be careful about |
| // tail duplicating it. In particular, we don't want to taildup it if the |
| // original block will still be there after taildup is completed: doing so |
| // would eliminate the fall-through, requiring unconditional branches. |
| Function::iterator DestI = Dest; |
| if (&*--DestI == BI->getParent()) { |
| // The uncond branch is a fall-through. Tail duplication of the block is |
| // will eliminate the fall-through-ness and end up cloning the terminator |
| // at the end of the Dest block. Since the original Dest block will |
| // continue to exist, this means that one or the other will not be able to |
| // fall through. One typical example that this helps with is code like: |
| // if (a) |
| // foo(); |
| // if (b) |
| // foo(); |
| // Cloning the 'if b' block into the end of the first foo block is messy. |
| |
| // The messy case is when the fall-through block falls through to other |
| // blocks. This is what we would be preventing if we cloned the block. |
| DestI = Dest; |
| if (++DestI != Dest->getParent()->end()) { |
| BasicBlock *DestSucc = DestI; |
| // If any of Dest's successors are fall-throughs, don't do this xform. |
| for (succ_iterator SI = succ_begin(Dest), SE = succ_end(Dest); |
| SI != SE; ++SI) |
| if (*SI == DestSucc) |
| return false; |
| } |
| } |
| |
| return true; |
| } |
| |
| /// FindObviousSharedDomOf - We know there is a branch from SrcBlock to |
| /// DestBlock, and that SrcBlock is not the only predecessor of DstBlock. If we |
| /// can find a predecessor of SrcBlock that is a dominator of both SrcBlock and |
| /// DstBlock, return it. |
| static BasicBlock *FindObviousSharedDomOf(BasicBlock *SrcBlock, |
| BasicBlock *DstBlock) { |
| // SrcBlock must have a single predecessor. |
| pred_iterator PI = pred_begin(SrcBlock), PE = pred_end(SrcBlock); |
| if (PI == PE || ++PI != PE) return 0; |
| |
| BasicBlock *SrcPred = *pred_begin(SrcBlock); |
| |
| // Look at the predecessors of DstBlock. One of them will be SrcBlock. If |
| // there is only one other pred, get it, otherwise we can't handle it. |
| PI = pred_begin(DstBlock); PE = pred_end(DstBlock); |
| BasicBlock *DstOtherPred = 0; |
| if (*PI == SrcBlock) { |
| if (++PI == PE) return 0; |
| DstOtherPred = *PI; |
| if (++PI != PE) return 0; |
| } else { |
| DstOtherPred = *PI; |
| if (++PI == PE || *PI != SrcBlock || ++PI != PE) return 0; |
| } |
| |
| // We can handle two situations here: "if then" and "if then else" blocks. An |
| // 'if then' situation is just where DstOtherPred == SrcPred. |
| if (DstOtherPred == SrcPred) |
| return SrcPred; |
| |
| // Check to see if we have an "if then else" situation, which means that |
| // DstOtherPred will have a single predecessor and it will be SrcPred. |
| PI = pred_begin(DstOtherPred); PE = pred_end(DstOtherPred); |
| if (PI != PE && *PI == SrcPred) { |
| if (++PI != PE) return 0; // Not a single pred. |
| return SrcPred; // Otherwise, it's an "if then" situation. Return the if. |
| } |
| |
| // Otherwise, this is something we can't handle. |
| return 0; |
| } |
| |
| |
| /// eliminateUnconditionalBranch - Clone the instructions from the destination |
| /// block into the source block, eliminating the specified unconditional branch. |
| /// If the destination block defines values used by successors of the dest |
| /// block, we may need to insert PHI nodes. |
| /// |
| void TailDup::eliminateUnconditionalBranch(BranchInst *Branch) { |
| BasicBlock *SourceBlock = Branch->getParent(); |
| BasicBlock *DestBlock = Branch->getSuccessor(0); |
| assert(SourceBlock != DestBlock && "Our predicate is broken!"); |
| |
| DOUT << "TailDuplication[" << SourceBlock->getParent()->getName() |
| << "]: Eliminating branch: " << *Branch; |
| |
| // See if we can avoid duplicating code by moving it up to a dominator of both |
| // blocks. |
| if (BasicBlock *DomBlock = FindObviousSharedDomOf(SourceBlock, DestBlock)) { |
| DOUT << "Found shared dominator: " << DomBlock->getName() << "\n"; |
| |
| // If there are non-phi instructions in DestBlock that have no operands |
| // defined in DestBlock, and if the instruction has no side effects, we can |
| // move the instruction to DomBlock instead of duplicating it. |
| BasicBlock::iterator BBI = DestBlock->begin(); |
| while (isa<PHINode>(BBI)) ++BBI; |
| while (!isa<TerminatorInst>(BBI)) { |
| Instruction *I = BBI++; |
| |
| bool CanHoist = !I->isTrapping() && !I->mayWriteToMemory(); |
| if (CanHoist) { |
| for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) |
| if (Instruction *OpI = dyn_cast<Instruction>(I->getOperand(op))) |
| if (OpI->getParent() == DestBlock || |
| (isa<InvokeInst>(OpI) && OpI->getParent() == DomBlock)) { |
| CanHoist = false; |
| break; |
| } |
| if (CanHoist) { |
| // Remove from DestBlock, move right before the term in DomBlock. |
| DestBlock->getInstList().remove(I); |
| DomBlock->getInstList().insert(DomBlock->getTerminator(), I); |
| DOUT << "Hoisted: " << *I; |
| } |
| } |
| } |
| } |
| |
| // Tail duplication can not update SSA properties correctly if the values |
| // defined in the duplicated tail are used outside of the tail itself. For |
| // this reason, we spill all values that are used outside of the tail to the |
| // stack. |
| for (BasicBlock::iterator I = DestBlock->begin(); I != DestBlock->end(); ++I) |
| for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; |
| ++UI) { |
| bool ShouldDemote = false; |
| if (cast<Instruction>(*UI)->getParent() != DestBlock) { |
| // We must allow our successors to use tail values in their PHI nodes |
| // (if the incoming value corresponds to the tail block). |
| if (PHINode *PN = dyn_cast<PHINode>(*UI)) { |
| for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) |
| if (PN->getIncomingValue(i) == I && |
| PN->getIncomingBlock(i) != DestBlock) { |
| ShouldDemote = true; |
| break; |
| } |
| |
| } else { |
| ShouldDemote = true; |
| } |
| } else if (PHINode *PN = dyn_cast<PHINode>(cast<Instruction>(*UI))) { |
| // If the user of this instruction is a PHI node in the current block, |
| // which has an entry from another block using the value, spill it. |
| for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) |
| if (PN->getIncomingValue(i) == I && |
| PN->getIncomingBlock(i) != DestBlock) { |
| ShouldDemote = true; |
| break; |
| } |
| } |
| |
| if (ShouldDemote) { |
| // We found a use outside of the tail. Create a new stack slot to |
| // break this inter-block usage pattern. |
| DemoteRegToStack(*I); |
| break; |
| } |
| } |
| |
| // We are going to have to map operands from the original block B to the new |
| // copy of the block B'. If there are PHI nodes in the DestBlock, these PHI |
| // nodes also define part of this mapping. Loop over these PHI nodes, adding |
| // them to our mapping. |
| // |
| std::map<Value*, Value*> ValueMapping; |
| |
| BasicBlock::iterator BI = DestBlock->begin(); |
| bool HadPHINodes = isa<PHINode>(BI); |
| for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) |
| ValueMapping[PN] = PN->getIncomingValueForBlock(SourceBlock); |
| |
| // Clone the non-phi instructions of the dest block into the source block, |
| // keeping track of the mapping... |
| // |
| for (; BI != DestBlock->end(); ++BI) { |
| Instruction *New = BI->clone(); |
| New->setName(BI->getName()); |
| SourceBlock->getInstList().push_back(New); |
| ValueMapping[BI] = New; |
| } |
| |
| // Now that we have built the mapping information and cloned all of the |
| // instructions (giving us a new terminator, among other things), walk the new |
| // instructions, rewriting references of old instructions to use new |
| // instructions. |
| // |
| BI = Branch; ++BI; // Get an iterator to the first new instruction |
| for (; BI != SourceBlock->end(); ++BI) |
| for (unsigned i = 0, e = BI->getNumOperands(); i != e; ++i) |
| if (Value *Remapped = ValueMapping[BI->getOperand(i)]) |
| BI->setOperand(i, Remapped); |
| |
| // Next we check to see if any of the successors of DestBlock had PHI nodes. |
| // If so, we need to add entries to the PHI nodes for SourceBlock now. |
| for (succ_iterator SI = succ_begin(DestBlock), SE = succ_end(DestBlock); |
| SI != SE; ++SI) { |
| BasicBlock *Succ = *SI; |
| for (BasicBlock::iterator PNI = Succ->begin(); isa<PHINode>(PNI); ++PNI) { |
| PHINode *PN = cast<PHINode>(PNI); |
| // Ok, we have a PHI node. Figure out what the incoming value was for the |
| // DestBlock. |
| Value *IV = PN->getIncomingValueForBlock(DestBlock); |
| |
| // Remap the value if necessary... |
| if (Value *MappedIV = ValueMapping[IV]) |
| IV = MappedIV; |
| PN->addIncoming(IV, SourceBlock); |
| } |
| } |
| |
| // Next, remove the old branch instruction, and any PHI node entries that we |
| // had. |
| BI = Branch; ++BI; // Get an iterator to the first new instruction |
| DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes... |
| SourceBlock->getInstList().erase(Branch); // Destroy the uncond branch... |
| |
| // Final step: now that we have finished everything up, walk the cloned |
| // instructions one last time, constant propagating and DCE'ing them, because |
| // they may not be needed anymore. |
| // |
| if (HadPHINodes) |
| while (BI != SourceBlock->end()) |
| if (!dceInstruction(BI) && !doConstantPropagation(BI)) |
| ++BI; |
| |
| ++NumEliminated; // We just killed a branch! |
| } |