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-//===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
-//
-// The LLVM Compiler Infrastructure
-//
-// This file 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/ADT/Statistic.h"
-#include "llvm/ADT/SmallPtrSet.h"
-#include "llvm/Analysis/InstructionSimplify.h"
-#include "llvm/Support/CFG.h"
-#include "llvm/Support/CommandLine.h"
-#include "llvm/Support/Debug.h"
-#include "llvm/Support/raw_ostream.h"
-#include "llvm/Transforms/Utils/Local.h"
-#include <map>
-using namespace llvm;
-
-STATISTIC(NumEliminated, "Number of unconditional branches eliminated");
-
-static cl::opt<unsigned>
-TailDupThreshold("taildup-threshold",
- cl::desc("Max block size to tail duplicate"),
- cl::init(1), cl::Hidden);
-
-namespace {
- class TailDup : public FunctionPass {
- bool runOnFunction(Function &F);
- public:
- static char ID; // Pass identification, replacement for typeid
- TailDup() : FunctionPass(ID) {
- initializeTailDupPass(*PassRegistry::getPassRegistry());
- }
-
- private:
- inline bool shouldEliminateUnconditionalBranch(TerminatorInst *, unsigned);
- inline void eliminateUnconditionalBranch(BranchInst *BI);
- SmallPtrSet<BasicBlock*, 4> CycleDetector;
- };
-}
-
-char TailDup::ID = 0;
-INITIALIZE_PASS(TailDup, "tailduplicate", "Tail Duplication", false, false)
-
-// 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. CycleDetector
-/// prevents infinite loops by checking that we aren't redirecting a branch to
-/// a place it already pointed to earlier; see PR 2323.
-bool TailDup::runOnFunction(Function &F) {
- bool Changed = false;
- CycleDetector.clear();
- for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
- if (shouldEliminateUnconditionalBranch(I->getTerminator(),
- TailDupThreshold)) {
- eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
- Changed = true;
- } else {
- ++I;
- CycleDetector.clear();
- }
- }
- 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,
- unsigned Threshold) {
- 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 with blocks with only a single predecessor: simplify
- // CFG will fold these two blocks together!
- pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
- ++PI;
- if (PI == PE) return false; // Exactly one predecessor!
-
- BasicBlock::iterator I = Dest->getFirstNonPHI();
-
- for (unsigned Size = 0; I != Dest->end(); ++I) {
- if (Size == Threshold) return false; // The block is too large.
-
- // Don't tail duplicate call instructions. They are very large compared to
- // other instructions.
- if (isa<CallInst>(I) || isa<InvokeInst>(I)) return false;
-
- // Also alloca and malloc.
- if (isa<AllocaInst>(I)) return false;
-
- // Some vector instructions can expand into a number of instructions.
- if (isa<ShuffleVectorInst>(I) || isa<ExtractElementInst>(I) ||
- isa<InsertElementInst>(I)) return false;
-
- // 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;
- }
-
- // 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;
- }
- }
-
- // Finally, check that we haven't redirected to this target block earlier;
- // there are cases where we loop forever if we don't check this (PR 2323).
- if (!CycleDetector.insert(Dest))
- 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;
- BasicBlock *P = *PI;
- if (P == SrcBlock) {
- if (++PI == PE) return 0;
- DstOtherPred = *PI;
- if (++PI != PE) return 0;
- } else {
- DstOtherPred = P;
- 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!");
-
- DEBUG(dbgs() << "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)) {
- DEBUG(dbgs() << "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->getFirstNonPHI();
- while (!isa<TerminatorInst>(BBI)) {
- Instruction *I = BBI++;
-
- bool CanHoist = I->isSafeToSpeculativelyExecute() &&
- !I->mayReadFromMemory();
- 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);
- DEBUG(dbgs() << "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)
- if (I->isUsedOutsideOfBlock(DestBlock)) {
- // We found a use outside of the tail. Create a new stack slot to
- // break this inter-block usage pattern.
- DemoteRegToStack(*I);
- }
-
- // 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) {
- std::map<Value*, Value*>::const_iterator I =
- ValueMapping.find(BI->getOperand(i));
- if (I != ValueMapping.end())
- BI->setOperand(i, I->second);
- }
-
- // 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...
- std::map<Value*, Value*>::const_iterator I = ValueMapping.find(IV);
- if (I != ValueMapping.end())
- IV = I->second;
- 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()) {
- Instruction *Inst = BI++;
- if (isInstructionTriviallyDead(Inst))
- Inst->eraseFromParent();
- else if (Value *V = SimplifyInstruction(Inst)) {
- Inst->replaceAllUsesWith(V);
- Inst->eraseFromParent();
- }
- }
- }
-
- ++NumEliminated; // We just killed a branch!
-}
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