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Initial checkin of Tail duplication pass.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6846 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner
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324
lib/Transforms/Scalar/TailDuplication.cpp
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324
lib/Transforms/Scalar/TailDuplication.cpp
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//===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
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//
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// This pass performs a limited form of tail duplication, intended to simplify
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// CFGs by removing some unconditional branches. This pass is necessary to
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// straighten out loops created by the C front-end, but also is capable of
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// making other code nicer. After this pass is run, the CFG simplify pass
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// should be run to clean up the mess.
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//
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// This pass could be enhanced in the future to use profile information to be
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// more aggressive.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Function.h"
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#include "llvm/iPHINode.h"
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#include "llvm/iTerminators.h"
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#include "llvm/Pass.h"
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#include "llvm/Type.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "Support/Statistic.h"
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namespace {
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Statistic<> NumEliminated("tailduplicate",
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"Number of unconditional branches eliminated");
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Statistic<> NumPHINodes("tailduplicate", "Number of phi nodes inserted");
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class TailDup : public FunctionPass {
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bool runOnFunction(Function &F);
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private:
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inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
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inline void eliminateUnconditionalBranch(BranchInst *BI);
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inline void InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
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BasicBlock *NewBlock);
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inline Value *GetValueInBlock(BasicBlock *BB, Value *OrigVal,
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std::map<BasicBlock*, Value*> &ValueMap,
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std::map<BasicBlock*, Value*> &OutValueMap);
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inline Value *GetValueOutBlock(BasicBlock *BB, Value *OrigVal,
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std::map<BasicBlock*, Value*> &ValueMap,
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std::map<BasicBlock*, Value*> &OutValueMap);
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};
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RegisterOpt<TailDup> X("tailduplicate", "Tail Duplication");
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}
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Pass *createTailDuplicationPass() { return new TailDup(); }
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/// runOnFunction - Top level algorithm - Loop over each unconditional branch in
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/// the function, eliminating it if it looks attractive enough.
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///
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bool TailDup::runOnFunction(Function &F) {
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bool Changed = false;
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for (Function::iterator I = F.begin(), E = F.end(); I != E; )
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if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
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eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
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Changed = true;
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} else {
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++I;
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}
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return Changed;
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}
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/// shouldEliminateUnconditionalBranch - Return true if this branch looks
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/// attractive to eliminate. We eliminate the branch if the destination basic
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/// block has <= 5 instructions in it, not counting PHI nodes. In practice,
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/// since one of these is a terminator instruction, this means that we will add
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/// up to 4 instructions to the new block.
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///
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/// We don't count PHI nodes in the count since they will be removed when the
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/// contents of the block are copied over.
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///
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bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
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BranchInst *BI = dyn_cast<BranchInst>(TI);
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if (!BI || !BI->isUnconditional()) return false; // Not an uncond branch!
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BasicBlock *Dest = BI->getSuccessor(0);
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if (Dest == BI->getParent()) return false; // Do not loop infinitely!
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// Do not bother working on dead blocks...
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pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
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if (PI == PE && Dest != Dest->getParent()->begin())
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return false; // It's just a dead block, ignore it...
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// Also, do not bother with blocks with only a single predecessor: simplify
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// CFG will fold these two blocks together!
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++PI;
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if (PI == PE) return false; // Exactly one predecessor!
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BasicBlock::iterator I = Dest->begin();
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while (isa<PHINode>(*I)) ++I;
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for (unsigned Size = 0; I != Dest->end(); ++Size, ++I)
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if (Size == 6) return false; // The block is too large...
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return true;
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}
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/// eliminateUnconditionalBranch - Clone the instructions from the destination
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/// block into the source block, eliminating the specified unconditional branch.
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/// If the destination block defines values used by successors of the dest
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/// block, we may need to insert PHI nodes.
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///
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void TailDup::eliminateUnconditionalBranch(BranchInst *Branch) {
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BasicBlock *SourceBlock = Branch->getParent();
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BasicBlock *DestBlock = Branch->getSuccessor(0);
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assert(SourceBlock != DestBlock && "Our predicate is broken!");
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DEBUG(std::cerr << "TailDuplication[" << SourceBlock->getParent()->getName()
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<< "]: Eliminating branch: " << *Branch);
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// We are going to have to map operands from the original block B to the new
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// copy of the block B'. If there are PHI nodes in the DestBlock, these PHI
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// nodes also define part of this mapping. Loop over these PHI nodes, adding
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// them to our mapping.
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std::map<Value*, Value*> ValueMapping;
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BasicBlock::iterator BI = DestBlock->begin();
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bool HadPHINodes = isa<PHINode>(BI);
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for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
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ValueMapping[PN] = PN->getIncomingValueForBlock(SourceBlock);
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// Clone the non-phi instructions of the dest block into the source block,
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// keeping track of the mapping...
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//
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for (; BI != DestBlock->end(); ++BI) {
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Instruction *New = BI->clone();
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New->setName(BI->getName());
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SourceBlock->getInstList().push_back(New);
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ValueMapping[BI] = New;
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}
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// Now that we have built the mapping information and cloned all of the
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// instructions (giving us a new terminator, among other things), walk the new
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// instructions, rewriting references of old instructions to use new
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// instructions.
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//
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BI = Branch; ++BI; // Get an iterator to the first new instruction
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for (; BI != SourceBlock->end(); ++BI)
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for (unsigned i = 0, e = BI->getNumOperands(); i != e; ++i)
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if (Value *Remapped = ValueMapping[BI->getOperand(i)])
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BI->setOperand(i, Remapped);
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// Next we check to see if any of the successors of DestBlock had PHI nodes.
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// If so, we need to add entries to the PHI nodes for SourceBlock now.
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for (succ_iterator SI = succ_begin(DestBlock), SE = succ_end(DestBlock);
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SI != SE; ++SI) {
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BasicBlock *Succ = *SI;
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for (BasicBlock::iterator PNI = Succ->begin();
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PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
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// Ok, we have a PHI node. Figure out what the incoming value was for the
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// DestBlock.
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Value *IV = PN->getIncomingValueForBlock(DestBlock);
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// Remap the value if necessary...
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if (Value *MappedIV = ValueMapping[IV])
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IV = MappedIV;
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PN->addIncoming(IV, SourceBlock);
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}
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}
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// Now that all of the instructions are correctly copied into the SourceBlock,
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// we have one more minor problem: the successors of the original DestBB may
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// use the values computed in DestBB either directly (if DestBB dominated the
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// block), or through a PHI node. In either case, we need to insert PHI nodes
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// into any successors of DestBB (which are now our successors) for each value
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// that is computed in DestBB, but is used outside of it. All of these uses
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// we have to rewrite with the new PHI node.
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//
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if (succ_begin(SourceBlock) != succ_end(SourceBlock)) // Avoid wasting time...
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for (BI = DestBlock->begin(); BI != DestBlock->end(); ++BI)
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if (BI->getType() != Type::VoidTy)
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InsertPHINodesIfNecessary(BI, ValueMapping[BI], SourceBlock);
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// Final step: now that we have finished everything up, walk the cloned
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// instructions one last time, constant propagating and DCE'ing them, because
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// they may not be needed anymore.
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//
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BI = Branch; ++BI; // Get an iterator to the first new instruction
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if (HadPHINodes)
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while (BI != SourceBlock->end())
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if (!dceInstruction(BI) && !doConstantPropagation(BI))
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++BI;
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DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes...
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SourceBlock->getInstList().erase(Branch); // Destroy the uncond branch...
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++NumEliminated; // We just killed a branch!
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}
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/// InsertPHINodesIfNecessary - So at this point, we cloned the OrigInst
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/// instruction into the NewBlock with the value of NewInst. If OrigInst was
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/// used outside of its defining basic block, we need to insert a PHI nodes into
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/// the successors.
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///
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void TailDup::InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
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BasicBlock *NewBlock) {
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// Loop over all of the uses of OrigInst, rewriting them to be newly inserted
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// PHI nodes, unless they are in the same basic block as OrigInst.
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BasicBlock *OrigBlock = OrigInst->getParent();
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std::vector<Instruction*> Users;
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Users.reserve(OrigInst->use_size());
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for (Value::use_iterator I = OrigInst->use_begin(), E = OrigInst->use_end();
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I != E; ++I) {
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Instruction *In = cast<Instruction>(*I);
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if (In->getParent() != OrigBlock) // Don't modify uses in the orig block!
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Users.push_back(In);
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}
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// The common case is that the instruction is only used within the block that
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// defines it. If we have this case, quick exit.
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//
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if (Users.empty()) return;
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// Otherwise, we have a more complex case, handle it now. This requires the
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// construction of a mapping between a basic block and the value to use when
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// in the scope of that basic block. This map will map to the original and
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// new values when in the original or new block, but will map to inserted PHI
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// nodes when in other blocks.
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//
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std::map<BasicBlock*, Value*> ValueMap;
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std::map<BasicBlock*, Value*> OutValueMap; // The outgoing value map
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OutValueMap[OrigBlock] = OrigInst;
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OutValueMap[NewBlock ] = NewInst; // Seed the initial values...
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DEBUG(std::cerr << " ** Inserting PHI nodes for " << OrigInst);
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while (!Users.empty()) {
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Instruction *User = Users.back(); Users.pop_back();
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if (PHINode *PN = dyn_cast<PHINode>(User)) {
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// PHI nodes must be handled specially here, because their operands are
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// actually defined in predecessor basic blocks, NOT in the block that the
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// PHI node lives in. Note that we have already added entries to PHI nods
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// which are in blocks that are immediate successors of OrigBlock, so
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// don't modify them again.
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingValue(i) == OrigInst &&
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PN->getIncomingBlock(i) != OrigBlock) {
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Value *V = GetValueOutBlock(PN->getIncomingBlock(i), OrigInst,
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ValueMap, OutValueMap);
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PN->setIncomingValue(i, V);
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}
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} else {
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// Any other user of the instruction can just replace any uses with the
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// new value defined in the block it resides in.
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Value *V = GetValueInBlock(User->getParent(), OrigInst, ValueMap,
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OutValueMap);
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User->replaceUsesOfWith(OrigInst, V);
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}
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}
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}
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/// GetValueInBlock - This is a recursive method which inserts PHI nodes into
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/// the function until there is a value available in basic block BB.
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///
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Value *TailDup::GetValueInBlock(BasicBlock *BB, Value *OrigVal,
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std::map<BasicBlock*, Value*> &ValueMap,
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std::map<BasicBlock*, Value*> &OutValueMap) {
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Value*& BBVal = ValueMap[BB];
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if (BBVal) return BBVal; // Value already computed for this block?
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assert(pred_begin(BB) != pred_end(BB) &&
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"Propagating PHI nodes to unreachable blocks?");
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// If there is no value already available in this basic block, we need to
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// either reuse a value from an incoming, dominating, basic block, or we need
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// to create a new PHI node to merge in different incoming values. Because we
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// don't know if we're part of a loop at this point or not, we create a PHI
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// node, even if we will ultimately eliminate it.
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PHINode *PN = new PHINode(OrigVal->getType(), OrigVal->getName()+".pn",
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BB->begin());
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BBVal = PN; // Insert this into the BBVal slot in case of cycles...
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Value*& BBOutVal = OutValueMap[BB];
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if (BBOutVal == 0) BBOutVal = PN;
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// Now that we have created the PHI node, loop over all of the predecessors of
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// this block, computing an incoming value for the predecessor.
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std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
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for (unsigned i = 0, e = Preds.size(); i != e; ++i)
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PN->addIncoming(GetValueOutBlock(Preds[i], OrigVal, ValueMap, OutValueMap),
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Preds[i]);
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// The PHI node is complete. In many cases, however the PHI node was
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// ultimately unnecessary: we could have just reused a dominating incoming
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// value. If this is the case, nuke the PHI node and replace the map entry
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// with the dominating value.
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//
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assert(PN->getNumIncomingValues() > 0 && "No predecessors?");
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// Check to see if all of the elements in the PHI node are either the PHI node
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// itself or ONE particular value.
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unsigned i = 0;
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Value *ReplVal = PN->getIncomingValue(i);
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for (; ReplVal == PN && i != PN->getNumIncomingValues(); ++i)
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ReplVal = PN->getIncomingValue(i); // Skip values equal to the PN
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for (; i != PN->getNumIncomingValues(); ++i)
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if (PN->getIncomingValue(i) != PN && PN->getIncomingValue(i) != ReplVal) {
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ReplVal = 0;
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break;
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}
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// Found a value to replace the PHI node with?
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if (ReplVal) {
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PN->replaceAllUsesWith(ReplVal);
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BBVal = ReplVal;
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if (BBOutVal == PN) BBOutVal = ReplVal;
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BB->getInstList().erase(PN); // Erase the PHI node...
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} else {
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++NumPHINodes;
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}
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return BBVal;
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}
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Value *TailDup::GetValueOutBlock(BasicBlock *BB, Value *OrigVal,
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std::map<BasicBlock*, Value*> &ValueMap,
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std::map<BasicBlock*, Value*> &OutValueMap) {
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Value*& BBVal = OutValueMap[BB];
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if (BBVal) return BBVal; // Value already computed for this block?
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return BBVal = GetValueInBlock(BB, OrigVal, ValueMap, OutValueMap);
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}
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