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Replace the old ADCE implementation with a new one that more simply solves
the one case that ADCE catches that normal DCE doesn't: non-induction variable loop computations. This implementation handles this problem without using postdominators. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51668 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -1,4 +1,4 @@
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//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
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//===- DCE.cpp - Code to perform dead code elimination --------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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@ -7,481 +7,86 @@
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//
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//===----------------------------------------------------------------------===//
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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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// This file implements the Aggressive Dead Code Elimination pass. This pass
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// optimistically assumes that all instructions are dead until proven otherwise,
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// allowing it to eliminate dead computations that other DCE passes do not
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// catch, particularly involving loop computations.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "adce"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/PostDominators.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/DepthFirstIterator.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/Compiler.h"
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#include <algorithm>
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#include "llvm/Support/InstIterator.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/SmallPtrSet.h"
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using namespace llvm;
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STATISTIC(NumBlockRemoved, "Number of basic blocks removed");
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STATISTIC(NumInstRemoved , "Number of instructions removed");
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STATISTIC(NumCallRemoved , "Number of calls removed");
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STATISTIC(NumRemoved, "Number of instructions removed");
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namespace {
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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 VISIBILITY_HIDDEN 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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static char ID; // Pass identification, replacement for typeid
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ADCE() : FunctionPass((intptr_t)&ID) {}
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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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// We require that all function nodes are unified, because otherwise code
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// can be marked live that wouldn't necessarily be otherwise.
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AU.addRequired<UnifyFunctionExitNodes>();
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AU.addRequired<AliasAnalysis>();
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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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// deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in
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// the specified basic block, deleting ones that are dead according to
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// LiveSet.
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bool deleteDeadInstructionsInLiveBlock(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.insert(I).second) return;
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DOUT << "Insn Live: " << *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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DOUT << "Terminator Live: " << *BB->getTerminator();
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markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
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}
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};
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} // End of anonymous namespace
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struct VISIBILITY_HIDDEN ADCE : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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ADCE() : FunctionPass((intptr_t)&ID) {}
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virtual bool runOnFunction(Function& F);
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virtual void getAnalysisUsage(AnalysisUsage& AU) const {
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AU.setPreservesCFG();
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}
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};
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}
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char ADCE::ID = 0;
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static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
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FunctionPass *llvm::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 dependent 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 dependent on...
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const PostDominanceFrontier::DomSetType &CDB = It->second;
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for (PostDominanceFrontier::DomSetType::const_iterator I =
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CDB.begin(), E = CDB.end(); I != E; ++I)
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markTerminatorLive(*I); // Mark all their terminators as live
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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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// deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in the
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// specified basic block, deleting ones that are dead according to LiveSet.
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bool ADCE::deleteDeadInstructionsInLiveBlock(BasicBlock *BB) {
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bool Changed = false;
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for (BasicBlock::iterator II = BB->begin(), E = --BB->end(); II != E; ) {
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Instruction *I = II++;
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if (!LiveSet.count(I)) { // Is this instruction alive?
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if (!I->use_empty())
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I->replaceAllUsesWith(UndefValue::get(I->getType()));
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// Nope... remove the instruction from it's basic block...
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if (isa<CallInst>(I))
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++NumCallRemoved;
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else
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++NumInstRemoved;
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BB->getInstList().erase(I);
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Changed = true;
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}
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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 = BranchInst::Create(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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AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
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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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std::set<BasicBlock*> ReachableBBs;
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std::vector<BasicBlock*> Stack;
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Stack.push_back(&Func->getEntryBlock());
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bool ADCE::runOnFunction(Function& F) {
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SmallPtrSet<Instruction*, 32> alive;
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std::vector<Instruction*> worklist;
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while (!Stack.empty()) {
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BasicBlock* BB = Stack.back();
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if (ReachableBBs.count(BB)) {
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Stack.pop_back();
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continue;
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} else {
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ReachableBBs.insert(BB);
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// Collect the set of "root" instructions that are known live.
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for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
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if (isa<TerminatorInst>(I.getInstructionIterator()) ||
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I->mayWriteToMemory()) {
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alive.insert(I.getInstructionIterator());
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worklist.push_back(I.getInstructionIterator());
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}
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// Propagate liveness backwards to operands.
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while (!worklist.empty()) {
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Instruction* curr = worklist.back();
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worklist.pop_back();
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for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
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Instruction *I = II++;
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if (CallInst *CI = dyn_cast<CallInst>(I)) {
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if (AA.onlyReadsMemory(CI)) {
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if (CI->use_empty()) {
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BB->getInstList().erase(CI);
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++NumCallRemoved;
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}
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} else {
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markInstructionLive(I);
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}
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} else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
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isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
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// FIXME: Unreachable instructions should not be marked intrinsically
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// live here.
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markInstructionLive(I);
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} else if (isInstructionTriviallyDead(I)) {
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// Remove the instruction from it's basic block...
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BB->getInstList().erase(I);
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++NumInstRemoved;
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}
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for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
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OI != OE; ++OI)
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if (Instruction* Inst = dyn_cast<Instruction>(OI))
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if (alive.insert(Inst))
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worklist.push_back(Inst);
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}
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// The inverse of the live set is the dead set. These are those instructions
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// which have no side effects and do not influence the control flow or return
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// value of the function, and may therefore be deleted safely.
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SmallPtrSet<Instruction*, 32> dead;
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for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
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if (!alive.count(I.getInstructionIterator())) {
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dead.insert(I.getInstructionIterator());
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I->dropAllReferences();
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}
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI) {
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// Back edges (as opposed to cross edges) indicate loops, so implicitly
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// mark them live.
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if (std::find(Stack.begin(), Stack.end(), *SI) != Stack.end())
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markInstructionLive(BB->getTerminator());
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if (!ReachableBBs.count(*SI))
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Stack.push_back(*SI);
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}
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for (SmallPtrSet<Instruction*, 32>::iterator I = dead.begin(),
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E = dead.end(); I != E; ++I) {
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NumRemoved++;
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(*I)->eraseFromParent();
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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->getEntryBlock()] == 0) {
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WorkList.clear();
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return MadeChanges;
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}
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// Scan the function marking blocks without post-dominance information as
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// live. Blocks without post-dominance information occur when there is an
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// infinite loop in the program. Because the infinite loop could contain a
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// function which unwinds, exits or has side-effects, we don't want to delete
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// the infinite loop or those blocks leading up to it.
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for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
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if (DT[I] == 0 && ReachableBBs.count(I))
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for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
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markInstructionLive((*PI)->getTerminator());
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DOUT << "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 necessary 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 (!ReachableBBs.count(BB)) continue;
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if (AliveBlocks.insert(BB).second) // Basic block not alive yet.
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markBlockAlive(BB); // Make it so now!
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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 (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
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// If the incoming edge is clearly dead, it won't have control
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// dependence information. Do not mark it live.
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BasicBlock *PredBB = PN->getIncomingBlock(i);
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if (ReachableBBs.count(PredBB)) {
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// FIXME: This should mark the control dependent edge as live, not
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// necessarily the predecessor itself!
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if (AliveBlocks.insert(PredBB).second)
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markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE!
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if (Instruction *Op = dyn_cast<Instruction>(PN->getIncomingValue(i)))
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markInstructionLive(Op);
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}
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}
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} else {
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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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}
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DEBUG(
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DOUT << "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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DOUT << 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)) DOUT << "X ";
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DOUT << *BI;
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}
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});
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// All blocks being live is a common case, handle it specially.
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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 deleting instructions
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// to drop their references.
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deleteDeadInstructionsInLiveBlock(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 an unconditional 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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return MadeChanges;
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}
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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 = BasicBlock::Create();
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BranchInst::Create(&Func->front(), NewEntry);
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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 on
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// a condition that doesn't matter. Make it an unconditional branch to
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// ONE of the successors. This has the side effect of dropping a use of
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// 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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DomTreeNode *LastNode = DT[TI->getSuccessor(i)];
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DomTreeNode *NextNode = 0;
|
||||
|
||||
if (LastNode) {
|
||||
NextNode = LastNode->getIDom();
|
||||
while (!AliveBlocks.count(NextNode->getBlock())) {
|
||||
LastNode = NextNode;
|
||||
NextNode = NextNode->getIDom();
|
||||
if (NextNode == 0) {
|
||||
LastNode = 0;
|
||||
break;
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
// There is a special case here... if there IS no post-dominator for
|
||||
// the block we have nowhere to point our branch to. Instead, convert
|
||||
// it to a return. This can only happen if the code branched into an
|
||||
// infinite loop. Note that this may not be desirable, because we
|
||||
// _are_ altering the behavior of the code. This is a well known
|
||||
// drawback of ADCE, so in the future if we choose to revisit the
|
||||
// decision, this is where it should be.
|
||||
//
|
||||
if (LastNode == 0) { // No postdominator!
|
||||
if (!isa<InvokeInst>(TI)) {
|
||||
// Call RemoveSuccessor to transmogrify the terminator instruction
|
||||
// to not contain the outgoing branch, or to create a new
|
||||
// terminator if the form fundamentally changes (i.e.,
|
||||
// unconditional branch to return). Note that this will change a
|
||||
// branch into an infinite loop into a return instruction!
|
||||
//
|
||||
RemoveSuccessor(TI, i);
|
||||
|
||||
// RemoveSuccessor may replace TI... make sure we have a fresh
|
||||
// pointer.
|
||||
//
|
||||
TI = BB->getTerminator();
|
||||
|
||||
// Rescan this successor...
|
||||
--i;
|
||||
} else {
|
||||
|
||||
}
|
||||
} else {
|
||||
// Get the basic blocks that we need...
|
||||
BasicBlock *LastDead = LastNode->getBlock();
|
||||
BasicBlock *NextAlive = NextNode->getBlock();
|
||||
|
||||
// Make the conditional branch now go to the next alive block...
|
||||
TI->getSuccessor(i)->removePredecessor(BB);
|
||||
TI->setSuccessor(i, NextAlive);
|
||||
|
||||
// If there are PHI nodes in NextAlive, we need to add entries to
|
||||
// the PHI nodes for the new incoming edge. The incoming values
|
||||
// should be identical to the incoming values for LastDead.
|
||||
//
|
||||
for (BasicBlock::iterator II = NextAlive->begin();
|
||||
isa<PHINode>(II); ++II) {
|
||||
PHINode *PN = cast<PHINode>(II);
|
||||
if (LiveSet.count(PN)) { // Only modify live phi nodes
|
||||
// Get the incoming value for LastDead...
|
||||
int OldIdx = PN->getBasicBlockIndex(LastDead);
|
||||
assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
|
||||
Value *InVal = PN->getIncomingValue(OldIdx);
|
||||
|
||||
// Add an incoming value for BB now...
|
||||
PN->addIncoming(InVal, BB);
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
// Now loop over all of the instructions in the basic block, deleting
|
||||
// dead instructions. This is so that the next sweep over the program
|
||||
// can safely delete dead instructions without other dead instructions
|
||||
// still referring to them.
|
||||
//
|
||||
deleteDeadInstructionsInLiveBlock(BB);
|
||||
}
|
||||
|
||||
// Loop over all of the basic blocks in the function, dropping references of
|
||||
// the dead basic blocks. We must do this after the previous step to avoid
|
||||
// dropping references to PHIs which still have entries...
|
||||
//
|
||||
std::vector<BasicBlock*> DeadBlocks;
|
||||
for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
|
||||
if (!AliveBlocks.count(BB)) {
|
||||
// Remove PHI node entries for this block in live successor blocks.
|
||||
for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
|
||||
if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI))
|
||||
(*SI)->removePredecessor(BB);
|
||||
|
||||
BB->dropAllReferences();
|
||||
MadeChanges = true;
|
||||
DeadBlocks.push_back(BB);
|
||||
}
|
||||
|
||||
NumBlockRemoved += DeadBlocks.size();
|
||||
|
||||
// Now loop through all of the blocks and delete the dead ones. We can safely
|
||||
// do this now because we know that there are no references to dead blocks
|
||||
// (because they have dropped all of their references).
|
||||
for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(),
|
||||
E = DeadBlocks.end(); I != E; ++I)
|
||||
Func->getBasicBlockList().erase(*I);
|
||||
|
||||
return MadeChanges;
|
||||
|
||||
return !dead.empty();
|
||||
}
|
||||
|
||||
FunctionPass *llvm::createAggressiveDCEPass() {
|
||||
return new ADCE();
|
||||
}
|
@ -1,27 +0,0 @@
|
||||
; This testcase was failing because without merging the return blocks, ADCE
|
||||
; didn't know that it could get rid of the then.0 block.
|
||||
|
||||
; RUN: llvm-as < %s | opt -adce | llvm-dis | not grep load
|
||||
|
||||
|
||||
define void @main(i32 %argc, i8** %argv) {
|
||||
entry:
|
||||
call void @__main( )
|
||||
%tmp.1 = icmp ule i32 %argc, 5 ; <i1> [#uses=1]
|
||||
br i1 %tmp.1, label %then.0, label %return
|
||||
|
||||
then.0: ; preds = %entry
|
||||
%tmp.8 = load i8** %argv ; <i8*> [#uses=1]
|
||||
%tmp.10 = load i8* %tmp.8 ; <i8> [#uses=1]
|
||||
%tmp.11 = icmp eq i8 %tmp.10, 98 ; <i1> [#uses=1]
|
||||
br i1 %tmp.11, label %then.1, label %return
|
||||
|
||||
then.1: ; preds = %then.0
|
||||
ret void
|
||||
|
||||
return: ; preds = %then.0, %entry
|
||||
ret void
|
||||
}
|
||||
|
||||
declare void @__main()
|
||||
|
@ -1,17 +0,0 @@
|
||||
; RUN: llvm-as < %s | opt -adce | llvm-dis | not grep call
|
||||
|
||||
; The call is not live just because the PHI uses the call retval!
|
||||
|
||||
define i32 @test(i32 %X) {
|
||||
; <label>:0
|
||||
br label %Done
|
||||
|
||||
DeadBlock: ; No predecessors!
|
||||
%Y = call i32 @test( i32 0 ) ; <i32> [#uses=1]
|
||||
br label %Done
|
||||
|
||||
Done: ; preds = %DeadBlock, %0
|
||||
%Z = phi i32 [ %X, %0 ], [ %Y, %DeadBlock ] ; <i32> [#uses=1]
|
||||
ret i32 %Z
|
||||
}
|
||||
|
Loading…
Reference in New Issue
Block a user