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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16432 91177308-0d34-0410-b5e6-96231b3b80d8
247 lines
9.8 KiB
C++
247 lines
9.8 KiB
C++
//===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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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/Constant.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.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 "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/Statistic.h"
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using namespace llvm;
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namespace {
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cl::opt<unsigned>
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Threshold("taildup-threshold", cl::desc("Max block size to tail duplicate"),
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cl::init(6), cl::Hidden);
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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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};
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RegisterOpt<TailDup> X("tailduplicate", "Tail Duplication");
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}
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// Public interface to the Tail Duplication pass
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FunctionPass *llvm::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 inline a block if we will just get another branch to the same block!
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TerminatorInst *DTI = Dest->getTerminator();
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if (BranchInst *DBI = dyn_cast<BranchInst>(DTI))
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if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest)
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return false; // Do not loop infinitely!
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// FIXME: DemoteRegToStack cannot yet demote invoke instructions to the stack,
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// because doing so would require breaking critical edges. This should be
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// fixed eventually.
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if (!DTI->use_empty())
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return false;
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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 == Threshold) return false; // The block is too large...
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// Do not tail duplicate a block that has thousands of successors into a block
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// with a single successor if the block has many other predecessors. This can
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// cause an N^2 explosion in CFG edges (and PHI node entries), as seen in
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// cases that have a large number of indirect gotos.
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if (DTI->getNumSuccessors() > 8)
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if (std::distance(PI, PE) * DTI->getNumSuccessors() > 128)
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return false;
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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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// Tail duplication can not update SSA properties correctly if the values
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// defined in the duplicated tail are used outside of the tail itself. For
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// this reason, we spill all values that are used outside of the tail to the
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// stack.
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for (BasicBlock::iterator I = DestBlock->begin(); I != DestBlock->end(); ++I)
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for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
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++UI) {
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bool ShouldDemote = false;
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if (cast<Instruction>(*UI)->getParent() != DestBlock) {
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// We must allow our successors to use tail values in their PHI nodes
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// (if the incoming value corresponds to the tail block).
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if (PHINode *PN = dyn_cast<PHINode>(*UI)) {
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingValue(i) == I &&
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PN->getIncomingBlock(i) != DestBlock) {
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ShouldDemote = true;
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break;
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}
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} else {
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ShouldDemote = true;
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}
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} else if (PHINode *PN = dyn_cast<PHINode>(cast<Instruction>(*UI))) {
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// If the user of this instruction is a PHI node in the current block,
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// which has an entry from another block using the value, spill it.
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingValue(i) == I &&
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PN->getIncomingBlock(i) != DestBlock) {
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ShouldDemote = true;
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break;
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}
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}
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if (ShouldDemote) {
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// We found a use outside of the tail. Create a new stack slot to
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// break this inter-block usage pattern.
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DemoteRegToStack(*I);
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break;
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}
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}
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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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//
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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(); isa<PHINode>(PNI); ++PNI) {
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PHINode *PN = cast<PHINode>(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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// Next, remove the old branch instruction, and any PHI node entries that we
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// had.
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BI = Branch; ++BI; // Get an iterator to the first new instruction
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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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// 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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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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++NumEliminated; // We just killed a branch!
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}
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