forked from OSchip/llvm-project
452 lines
18 KiB
C++
452 lines
18 KiB
C++
//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
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//
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// This file implements "aggressive" dead code elimination. ADCE is DCe where
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// values are assumed to be dead until proven otherwise. This is similar to
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// SCCP, except applied to the liveness of values.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Type.h"
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#include "llvm/Analysis/PostDominators.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iPHINode.h"
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#include "llvm/Constant.h"
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#include "llvm/Support/CFG.h"
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#include "Support/Debug.h"
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#include "Support/DepthFirstIterator.h"
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#include "Support/Statistic.h"
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#include "Support/STLExtras.h"
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#include <algorithm>
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namespace {
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Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed");
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Statistic<> NumInstRemoved ("adce", "Number of instructions removed");
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//===----------------------------------------------------------------------===//
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// ADCE Class
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//
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// This class does all of the work of Aggressive Dead Code Elimination.
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// It's public interface consists of a constructor and a doADCE() method.
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//
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class ADCE : public FunctionPass {
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Function *Func; // The function that we are working on
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std::vector<Instruction*> WorkList; // Instructions that just became live
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std::set<Instruction*> LiveSet; // The set of live instructions
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//===--------------------------------------------------------------------===//
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// The public interface for this class
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//
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public:
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// Execute the Aggressive Dead Code Elimination Algorithm
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//
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virtual bool runOnFunction(Function &F) {
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Func = &F;
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bool Changed = doADCE();
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assert(WorkList.empty());
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LiveSet.clear();
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return Changed;
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}
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// getAnalysisUsage - We require post dominance frontiers (aka Control
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// Dependence Graph)
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<PostDominatorTree>();
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AU.addRequired<PostDominanceFrontier>();
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}
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//===--------------------------------------------------------------------===//
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// The implementation of this class
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//
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private:
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// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
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// true if the function was modified.
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//
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bool doADCE();
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void markBlockAlive(BasicBlock *BB);
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// dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
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// instructions in the specified basic block, dropping references on
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// instructions that are dead according to LiveSet.
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bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB);
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TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
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inline void markInstructionLive(Instruction *I) {
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if (LiveSet.count(I)) return;
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DEBUG(std::cerr << "Insn Live: " << I);
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LiveSet.insert(I);
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WorkList.push_back(I);
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}
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inline void markTerminatorLive(const BasicBlock *BB) {
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DEBUG(std::cerr << "Terminat Live: " << BB->getTerminator());
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markInstructionLive((Instruction*)BB->getTerminator());
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}
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};
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RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
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} // End of anonymous namespace
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Pass *createAggressiveDCEPass() { return new ADCE(); }
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void ADCE::markBlockAlive(BasicBlock *BB) {
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// Mark the basic block as being newly ALIVE... and mark all branches that
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// this block is control dependant on as being alive also...
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//
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PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
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PostDominanceFrontier::const_iterator It = CDG.find(BB);
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if (It != CDG.end()) {
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// Get the blocks that this node is control dependant on...
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const PostDominanceFrontier::DomSetType &CDB = It->second;
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for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
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bind_obj(this, &ADCE::markTerminatorLive));
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}
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// If this basic block is live, and it ends in an unconditional branch, then
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// the branch is alive as well...
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if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
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if (BI->isUnconditional())
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markTerminatorLive(BB);
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}
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// dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
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// instructions in the specified basic block, dropping references on
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// instructions that are dead according to LiveSet.
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bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) {
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bool Changed = false;
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for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; )
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if (!LiveSet.count(I)) { // Is this instruction alive?
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I->dropAllReferences(); // Nope, drop references...
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if (PHINode *PN = dyn_cast<PHINode>(I)) {
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// We don't want to leave PHI nodes in the program that have
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// #arguments != #predecessors, so we remove them now.
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//
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PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
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// Delete the instruction...
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I = BB->getInstList().erase(I);
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Changed = true;
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} else {
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++I;
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}
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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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/// convertToUnconditionalBranch - Transform this conditional terminator
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/// instruction into an unconditional branch because we don't care which of the
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/// successors it goes to. This eliminate a use of the condition as well.
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///
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TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
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BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
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BasicBlock *BB = TI->getParent();
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// Remove entries from PHI nodes to avoid confusing ourself later...
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for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
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TI->getSuccessor(i)->removePredecessor(BB);
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// Delete the old branch itself...
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BB->getInstList().erase(TI);
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return NB;
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}
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// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
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// true if the function was modified.
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//
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bool ADCE::doADCE() {
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bool MadeChanges = false;
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// Iterate over all of the instructions in the function, eliminating trivially
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// dead instructions, and marking instructions live that are known to be
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// needed. Perform the walk in depth first order so that we avoid marking any
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// instructions live in basic blocks that are unreachable. These blocks will
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// be eliminated later, along with the instructions inside.
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//
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for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
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BBI != BBE; ++BBI) {
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BasicBlock *BB = *BBI;
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for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
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if (II->mayWriteToMemory() || II->getOpcode() == Instruction::Ret) {
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markInstructionLive(II);
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++II; // Increment the inst iterator if the inst wasn't deleted
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} else if (isInstructionTriviallyDead(II)) {
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// Remove the instruction from it's basic block...
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II = BB->getInstList().erase(II);
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++NumInstRemoved;
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MadeChanges = true;
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} else {
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++II; // Increment the inst iterator if the inst wasn't deleted
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}
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}
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}
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// Check to ensure we have an exit node for this CFG. If we don't, we won't
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// have any post-dominance information, thus we cannot perform our
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// transformations safely.
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//
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PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
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if (DT[&Func->getEntryNode()] == 0) {
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WorkList.clear();
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return MadeChanges;
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}
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DEBUG(std::cerr << "Processing work list\n");
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// AliveBlocks - Set of basic blocks that we know have instructions that are
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// alive in them...
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//
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std::set<BasicBlock*> AliveBlocks;
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// Process the work list of instructions that just became live... if they
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// became live, then that means that all of their operands are neccesary as
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// well... make them live as well.
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//
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while (!WorkList.empty()) {
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Instruction *I = WorkList.back(); // Get an instruction that became live...
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WorkList.pop_back();
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BasicBlock *BB = I->getParent();
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if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
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AliveBlocks.insert(BB); // Block is now ALIVE!
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markBlockAlive(BB); // Make it so now!
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}
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// PHI nodes are a special case, because the incoming values are actually
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// defined in the predecessor nodes of this block, meaning that the PHI
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// makes the predecessors alive.
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//
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if (PHINode *PN = dyn_cast<PHINode>(I))
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for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
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if (!AliveBlocks.count(*PI)) {
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AliveBlocks.insert(BB); // Block is now ALIVE!
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markBlockAlive(*PI);
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}
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// Loop over all of the operands of the live instruction, making sure that
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// they are known to be alive as well...
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//
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for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
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if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
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markInstructionLive(Operand);
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}
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DEBUG(
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std::cerr << "Current Function: X = Live\n";
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for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
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std::cerr << I->getName() << ":\t"
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<< (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
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for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
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if (LiveSet.count(BI)) std::cerr << "X ";
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std::cerr << *BI;
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}
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});
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// Find the first postdominator of the entry node that is alive. Make it the
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// new entry node...
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//
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if (AliveBlocks.size() == Func->size()) { // No dead blocks?
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for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
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// Loop over all of the instructions in the function, telling dead
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// instructions to drop their references. This is so that the next sweep
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// over the program can safely delete dead instructions without other dead
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// instructions still refering to them.
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//
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dropReferencesOfDeadInstructionsInLiveBlock(I);
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// Check to make sure the terminator instruction is live. If it isn't,
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// this means that the condition that it branches on (we know it is not an
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// unconditional branch), is not needed to make the decision of where to
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// go to, because all outgoing edges go to the same place. We must remove
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// the use of the condition (because it's probably dead), so we convert
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// the terminator to a conditional branch.
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//
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TerminatorInst *TI = I->getTerminator();
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if (!LiveSet.count(TI))
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convertToUnconditionalBranch(TI);
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}
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} else { // If there are some blocks dead...
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// If the entry node is dead, insert a new entry node to eliminate the entry
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// node as a special case.
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//
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if (!AliveBlocks.count(&Func->front())) {
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BasicBlock *NewEntry = new BasicBlock();
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NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
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Func->getBasicBlockList().push_front(NewEntry);
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AliveBlocks.insert(NewEntry); // This block is always alive!
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LiveSet.insert(NewEntry->getTerminator()); // The branch is live
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}
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// Loop over all of the alive blocks in the function. If any successor
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// blocks are not alive, we adjust the outgoing branches to branch to the
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// first live postdominator of the live block, adjusting any PHI nodes in
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// the block to reflect this.
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//
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for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
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if (AliveBlocks.count(I)) {
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BasicBlock *BB = I;
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TerminatorInst *TI = BB->getTerminator();
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// If the terminator instruction is alive, but the block it is contained
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// in IS alive, this means that this terminator is a conditional branch
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// on a condition that doesn't matter. Make it an unconditional branch
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// to ONE of the successors. This has the side effect of dropping a use
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// of the conditional value, which may also be dead.
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if (!LiveSet.count(TI))
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TI = convertToUnconditionalBranch(TI);
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// Loop over all of the successors, looking for ones that are not alive.
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// We cannot save the number of successors in the terminator instruction
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// here because we may remove them if we don't have a postdominator...
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//
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for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
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if (!AliveBlocks.count(TI->getSuccessor(i))) {
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// Scan up the postdominator tree, looking for the first
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// postdominator that is alive, and the last postdominator that is
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// dead...
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//
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PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
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// There is a special case here... if there IS no post-dominator for
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// the block we have no owhere to point our branch to. Instead,
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// convert it to a return. This can only happen if the code
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// branched into an infinite loop. Note that this may not be
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// desirable, because we _are_ altering the behavior of the code.
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// This is a well known drawback of ADCE, so in the future if we
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// choose to revisit the decision, this is where it should be.
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//
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if (LastNode == 0) { // No postdominator!
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// Call RemoveSuccessor to transmogrify the terminator instruction
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// to not contain the outgoing branch, or to create a new
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// terminator if the form fundementally changes (ie unconditional
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// branch to return). Note that this will change a branch into an
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// infinite loop into a return instruction!
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//
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RemoveSuccessor(TI, i);
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// RemoveSuccessor may replace TI... make sure we have a fresh
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// pointer... and e variable.
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//
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TI = BB->getTerminator();
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// Rescan this successor...
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--i;
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} else {
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PostDominatorTree::Node *NextNode = LastNode->getIDom();
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while (!AliveBlocks.count(NextNode->getNode())) {
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LastNode = NextNode;
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NextNode = NextNode->getIDom();
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}
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// Get the basic blocks that we need...
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BasicBlock *LastDead = LastNode->getNode();
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BasicBlock *NextAlive = NextNode->getNode();
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// Make the conditional branch now go to the next alive block...
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TI->getSuccessor(i)->removePredecessor(BB);
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TI->setSuccessor(i, NextAlive);
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// If there are PHI nodes in NextAlive, we need to add entries to
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// the PHI nodes for the new incoming edge. The incoming values
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// should be identical to the incoming values for LastDead.
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//
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for (BasicBlock::iterator II = NextAlive->begin();
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PHINode *PN = dyn_cast<PHINode>(II); ++II)
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if (LiveSet.count(PN)) { // Only modify live phi nodes
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// Get the incoming value for LastDead...
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int OldIdx = PN->getBasicBlockIndex(LastDead);
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assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
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Value *InVal = PN->getIncomingValue(OldIdx);
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// Add an incoming value for BB now...
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PN->addIncoming(InVal, BB);
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}
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}
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}
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// Now loop over all of the instructions in the basic block, telling
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// dead instructions to drop their references. This is so that the next
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// sweep over the program can safely delete dead instructions without
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// other dead instructions still refering to them.
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//
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dropReferencesOfDeadInstructionsInLiveBlock(BB);
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}
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}
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// We make changes if there are any dead blocks in the function...
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if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) {
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MadeChanges = true;
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NumBlockRemoved += NumDeadBlocks;
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}
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// Loop over all of the basic blocks in the function, removing control flow
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// edges to live blocks (also eliminating any entries in PHI functions in
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// referenced blocks).
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//
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for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
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if (!AliveBlocks.count(BB)) {
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// Remove all outgoing edges from this basic block and convert the
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// terminator into a return instruction.
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std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
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if (!Succs.empty()) {
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// Loop over all of the successors, removing this block from PHI node
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// entries that might be in the block...
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while (!Succs.empty()) {
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Succs.back()->removePredecessor(BB);
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Succs.pop_back();
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}
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// Delete the old terminator instruction...
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BB->getInstList().pop_back();
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const Type *RetTy = Func->getReturnType();
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BB->getInstList().push_back(new ReturnInst(RetTy != Type::VoidTy ?
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Constant::getNullValue(RetTy) : 0));
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}
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}
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// Loop over all of the basic blocks in the function, dropping references of
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// the dead basic blocks. We must do this after the previous step to avoid
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// dropping references to PHIs which still have entries...
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//
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for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
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if (!AliveBlocks.count(BB))
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BB->dropAllReferences();
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// Now loop through all of the blocks and delete the dead ones. We can safely
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// do this now because we know that there are no references to dead blocks
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// (because they have dropped all of their references... we also remove dead
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// instructions from alive blocks.
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//
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for (Function::iterator BI = Func->begin(); BI != Func->end(); )
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if (!AliveBlocks.count(BI)) { // Delete dead blocks...
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BI = Func->getBasicBlockList().erase(BI);
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} else { // Scan alive blocks...
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for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
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if (!LiveSet.count(II)) { // Is this instruction alive?
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// Nope... remove the instruction from it's basic block...
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II = BI->getInstList().erase(II);
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++NumInstRemoved;
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MadeChanges = true;
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} else {
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++II;
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}
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++BI; // Increment iterator...
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}
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return MadeChanges;
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}
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