//===- DCE.cpp - Code to perform dead code elimination --------------------===//
//
-// This file implements dead code elimination and basic block merging.
+// The LLVM Compiler Infrastructure
//
-// Specifically, this:
-// * removes definitions with no uses (including unused constants)
-// * removes basic blocks with no predecessors
-// * merges a basic block into its predecessor if there is only one and the
-// predecessor only has one successor.
-// * Eliminates PHI nodes for basic blocks with a single predecessor
-// * Eliminates a basic block that only contains an unconditional branch
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
//
-// TODO: This should REALLY be recursive instead of iterative. Right now, we
-// scan linearly through values, removing unused ones as we go. The problem is
-// that this may cause other earlier values to become unused. To make sure that
-// we get them all, we iterate until things stop changing. Instead, when
-// removing a value, recheck all of its operands to see if they are now unused.
-// Piece of cake, and more efficient as well.
+//===----------------------------------------------------------------------===//
+//
+// This file implements dead inst elimination and dead code elimination.
//
-// Note, this is not trivial, because we have to worry about invalidating
-// iterators. :(
+// Dead Inst Elimination performs a single pass over the function removing
+// instructions that are obviously dead. Dead Code Elimination is similar, but
+// it rechecks instructions that were used by removed instructions to see if
+// they are newly dead.
//
//===----------------------------------------------------------------------===//
-#include "llvm/Module.h"
-#include "llvm/Method.h"
-#include "llvm/BasicBlock.h"
-#include "llvm/iTerminators.h"
-#include "llvm/iOther.h"
-#include "llvm/Opt/AllOpts.h"
-#include "llvm/Assembly/Writer.h"
-#include "llvm/CFG.h"
-
-using namespace cfg;
-
-struct ConstPoolDCE {
- enum { EndOffs = 0 };
- static bool isDCEable(const Value *) { return true; }
-};
-
-struct BasicBlockDCE {
- enum { EndOffs = 1 };
- static bool isDCEable(const Instruction *I) {
- return !I->hasSideEffects();
- }
-};
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/IR/InstIterator.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/Pass.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Transforms/Utils/Local.h"
+using namespace llvm;
+#define DEBUG_TYPE "dce"
-template<class ValueSubclass, class ItemParentType, class DCEController>
-static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
- DCEController DCEControl) {
- bool Changed = false;
- typedef ValueHolder<ValueSubclass, ItemParentType> Container;
+STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
+STATISTIC(DCEEliminated, "Number of insts removed");
- int Offset = DCEController::EndOffs;
- for (Container::iterator DI = Vals.begin(); DI != Vals.end()-Offset; ) {
- // Look for un"used" definitions...
- if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) {
- // Bye bye
- //cerr << "Removing: " << *DI;
- delete Vals.remove(DI);
- Changed = true;
- } else {
- DI++;
+namespace {
+ //===--------------------------------------------------------------------===//
+ // DeadInstElimination pass implementation
+ //
+ struct DeadInstElimination : public BasicBlockPass {
+ static char ID; // Pass identification, replacement for typeid
+ DeadInstElimination() : BasicBlockPass(ID) {
+ initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
+ }
+ bool runOnBasicBlock(BasicBlock &BB) override {
+ if (skipOptnoneFunction(BB))
+ return false;
+ auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
+ TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
+ bool Changed = false;
+ for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
+ Instruction *Inst = &*DI++;
+ if (isInstructionTriviallyDead(Inst, TLI)) {
+ Inst->eraseFromParent();
+ Changed = true;
+ ++DIEEliminated;
+ }
+ }
+ return Changed;
}
- }
- return Changed;
-}
-
-// RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only
-// a single predecessor. This means that the PHI node must only have a single
-// RHS value and can be eliminated.
-//
-// This routine is very simple because we know that PHI nodes must be the first
-// things in a basic block, if they are present.
-//
-static bool RemoveSingularPHIs(BasicBlock *BB) {
- pred_iterator PI(pred_begin(BB));
- if (PI == pred_end(BB) || ++PI != pred_end(BB))
- return false; // More than one predecessor...
-
- Instruction *I = BB->getInstList().front();
- if (I->getInstType() != Instruction::PHINode) return false; // No PHI nodes
-
- //cerr << "Killing PHIs from " << BB;
- //cerr << "Pred #0 = " << *pred_begin(BB);
-
- //cerr << "Method == " << BB->getParent();
-
- do {
- PHINode *PN = (PHINode*)I;
- assert(PN->getOperand(2) == 0 && "PHI node should only have one value!");
- Value *V = PN->getOperand(0);
-
- PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
- delete BB->getInstList().remove(BB->getInstList().begin());
-
- I = BB->getInstList().front();
- } while (I->getInstType() == Instruction::PHINode);
-
- return true; // Yes, we nuked at least one phi node
-}
-bool DoRemoveUnusedConstants(SymTabValue *S) {
- bool Changed = false;
- ConstantPool &CP = S->getConstantPool();
- for (ConstantPool::plane_iterator PI = CP.begin(); PI != CP.end(); ++PI)
- Changed |= RemoveUnusedDefs(**PI, ConstPoolDCE());
- return Changed;
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.setPreservesCFG();
+ }
+ };
}
-static void ReplaceUsesWithConstant(Instruction *I) {
- // Get the method level constant pool
- ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
+char DeadInstElimination::ID = 0;
+INITIALIZE_PASS(DeadInstElimination, "die",
+ "Dead Instruction Elimination", false, false)
- ConstPoolVal *CPV = 0;
- ConstantPool::PlaneType *P;
- if (!CP.getPlane(I->getType(), P)) { // Does plane exist?
- // Yes, is it empty?
- if (!P->empty()) CPV = P->front();
- }
-
- if (CPV == 0) { // We don't have an existing constant to reuse. Just add one.
- CPV = ConstPoolVal::getNullConstant(I->getType()); // Create a new constant
-
- // Add the new value to the constant pool...
- CP.insert(CPV);
- }
-
- // Make all users of this instruction reference the constant instead
- I->replaceAllUsesWith(CPV);
+Pass *llvm::createDeadInstEliminationPass() {
+ return new DeadInstElimination();
}
-// RemovePredecessorFromBlock - This function is called when we are about
-// to remove a predecessor from a basic block. This function takes care of
-// removing the predecessor from the PHI nodes in BB so that after the pred
-// is removed, the number of PHI slots per bb is equal to the number of
-// predecessors.
-//
-static void RemovePredecessorFromBlock(BasicBlock *BB, BasicBlock *Pred) {
- pred_iterator PI(pred_begin(BB)), EI(pred_end(BB));
- unsigned max_idx;
- //cerr << "RPFB: " << Pred << "From Block: " << BB;
-
- // Loop over the rest of the predecssors until we run out, or until we find
- // out that there are more than 2 predecessors.
- for (max_idx = 0; PI != EI && max_idx < 3; ++PI, ++max_idx) /*empty*/;
-
- // If there are exactly two predecessors, then we want to nuke the PHI nodes
- // altogether.
- bool NukePHIs = max_idx == 2;
- assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
-
- // Okay, now we know that we need to remove predecessor #pred_idx from all
- // PHI nodes. Iterate over each PHI node fixing them up
- BasicBlock::InstListType::iterator II(BB->getInstList().begin());
- for (; (*II)->getInstType() == Instruction::PHINode; ++II) {
- PHINode *PN = (PHINode*)*II;
- PN->removeIncomingValue(BB);
+namespace {
+ //===--------------------------------------------------------------------===//
+ // DeadCodeElimination pass implementation
+ //
+ struct DCE : public FunctionPass {
+ static char ID; // Pass identification, replacement for typeid
+ DCE() : FunctionPass(ID) {
+ initializeDCEPass(*PassRegistry::getPassRegistry());
+ }
- if (NukePHIs) { // Destroy the PHI altogether??
- assert(PN->getOperand(1) == 0 && "PHI node should only have one value!");
- Value *V = PN->getOperand(0);
+ bool runOnFunction(Function &F) override;
- PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
- delete BB->getInstList().remove(II);
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.setPreservesCFG();
}
- }
+ };
}
-// PropogatePredecessors - This gets "Succ" ready to have the predecessors from
-// "BB". This is a little tricky because "Succ" has PHI nodes, which need to
-// have extra slots added to them to hold the merge edges from BB's
-// predecessors.
-//
-// Assumption: BB is the single predecessor of Succ.
-//
-static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
- assert(BB && Succ && *pred_begin(Succ) == BB && "BB is only pred of Succ" &&
- ++pred_begin(Succ) == pred_end(Succ));
+char DCE::ID = 0;
+INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
+
+static bool DCEInstruction(Instruction *I,
+ SmallSetVector<Instruction *, 16> &WorkList,
+ const TargetLibraryInfo *TLI) {
+ if (isInstructionTriviallyDead(I, TLI)) {
+ // Null out all of the instruction's operands to see if any operand becomes
+ // dead as we go.
+ for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
+ Value *OpV = I->getOperand(i);
+ I->setOperand(i, nullptr);
+
+ if (!OpV->use_empty() || I == OpV)
+ continue;
+
+ // If the operand is an instruction that became dead as we nulled out the
+ // operand, and if it is 'trivially' dead, delete it in a future loop
+ // iteration.
+ if (Instruction *OpI = dyn_cast<Instruction>(OpV))
+ if (isInstructionTriviallyDead(OpI, TLI))
+ WorkList.insert(OpI);
+ }
- // If there is more than one predecessor, and there are PHI nodes in
- // the successor, then we need to add incoming edges for the PHI nodes
- pred_iterator PI(pred_begin(BB));
- for (; PI != pred_end(BB); ++PI) {
- // TODO:
+ I->eraseFromParent();
+ ++DCEEliminated;
+ return true;
}
+ return false;
}
-static bool DoDCEPass(Method *M) {
- Method::BasicBlocksType &BBs = M->getBasicBlocks();
- Method::BasicBlocksType::iterator BBIt, BBEnd = BBs.end();
- if (BBs.begin() == BBEnd) return false; // Nothing to do
- bool Changed = false;
-
- // Loop through now and remove instructions that have no uses...
- for (BBIt = BBs.begin(); BBIt != BBEnd; BBIt++) {
- Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
- Changed |= RemoveSingularPHIs(*BBIt);
+bool DCE::runOnFunction(Function &F) {
+ if (skipOptnoneFunction(F))
+ return false;
+
+ auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
+ TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
+
+ bool MadeChange = false;
+ SmallSetVector<Instruction *, 16> WorkList;
+ // Iterate over the original function, only adding insts to the worklist
+ // if they actually need to be revisited. This avoids having to pre-init
+ // the worklist with the entire function's worth of instructions.
+ for (inst_iterator FI = inst_begin(F), FE = inst_end(F); FI != FE;) {
+ Instruction *I = &*FI;
+ ++FI;
+
+ // We're visiting this instruction now, so make sure it's not in the
+ // worklist from an earlier visit.
+ if (!WorkList.count(I))
+ MadeChange |= DCEInstruction(I, WorkList, TLI);
}
- // Loop over all of the basic blocks (except the first one) and remove them
- // if they are unneeded...
- //
- for (BBIt = BBs.begin(), ++BBIt; BBIt != BBs.end(); ++BBIt) {
- BasicBlock *BB = *BBIt;
- assert(BB->getTerminator() && "Degenerate basic block encountered!");
-
-#if 0
- // Remove basic blocks that have no predecessors... which are unreachable.
- if (pred_begin(BB) == pred_end(BB) &&
- !BB->hasConstantPoolReferences() && 0) {
- cerr << "Removing BB: \n" << BB;
-
- // Loop through all of our successors and make sure they know that one
- // of their predecessors is going away.
- for (succ_iterator SI = succ_begin(BB), EI = succ_end(BB); SI != EI; ++SI)
- RemovePredecessorFromBlock(*SI, BB);
-
- while (!BB->getInstList().empty()) {
- Instruction *I = BB->getInstList().front();
- // If this instruction is used, replace uses with an arbitrary
- // constant value. Because control flow can't get here, we don't care
- // what we replace the value with.
- if (!I->use_empty()) ReplaceUsesWithConstant(I);
-
- // Remove the instruction from the basic block
- delete BB->getInstList().remove(BB->getInstList().begin());
- }
- delete BBs.remove(BBIt);
- --BBIt; // remove puts use on the next block, we want the previous one
- Changed = true;
- continue;
- }
-
- // Check to see if this block has no instructions and only a single
- // successor. If so, replace block references with successor.
- succ_iterator SI(succ_begin(BB));
- if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
- Instruction *I = BB->getInstList().front();
- if (I->isTerminator()) { // Terminator is the only instruction!
-
- if (Succ->getInstList().front()->getInstType() == Instruction::PHINode){
- // Add entries to the PHI nodes so that the PHI nodes have the right
- // number of entries...
- PropogatePredecessorsForPHIs(BB, Succ);
- }
-
- BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
- BB->replaceAllUsesWith(Succ);
- cerr << "Killing Trivial BB: \n" << BB;
-
- BB = BBs.remove(BBIt);
- --BBIt; // remove puts use on the next block, we want the previous one
-
- if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
- Succ->setName(BB->getName());
- delete BB; // Delete basic block
-
- cerr << "Method after removal: \n" << M;
- Changed = true;
- continue;
- }
- }
-#endif
-
- // Merge basic blocks into their predecessor if there is only one pred,
- // and if there is only one successor of the predecessor.
- pred_iterator PI(pred_begin(BB));
- if (PI != pred_end(BB) && *PI != BB && // Not empty? Not same BB?
- ++PI == pred_end(BB) && !BB->hasConstantPoolReferences()) {
- BasicBlock *Pred = *pred_begin(BB);
- TerminatorInst *Term = Pred->getTerminator();
- assert(Term != 0 && "malformed basic block without terminator!");
-
- // Does the predecessor block only have a single successor?
- succ_iterator SI(succ_begin(Pred));
- if (++SI == succ_end(Pred)) {
- //cerr << "Merging: " << BB << "into: " << Pred;
-
- // Delete the unconditianal branch from the predecessor...
- BasicBlock::InstListType::iterator DI = Pred->getInstList().end();
- assert(Pred->getTerminator() &&
- "Degenerate basic block encountered!"); // Empty bb???
- delete Pred->getInstList().remove(--DI); // Destroy uncond branch
-
- // Move all definitions in the succecessor to the predecessor...
- while (!BB->getInstList().empty()) {
- DI = BB->getInstList().begin();
- Instruction *Def = BB->getInstList().remove(DI); // Remove from front
- Pred->getInstList().push_back(Def); // Add to end...
- }
-
- // Remove basic block from the method... and advance iterator to the
- // next valid block...
- BB = BBs.remove(BBIt);
- --BBIt; // remove puts us on the NEXT bb. We want the prev BB
- Changed = true;
-
- // Make all PHI nodes that refered to BB now refer to Pred as their
- // source...
- BB->replaceAllUsesWith(Pred);
-
- // Inherit predecessors name if it exists...
- if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
-
- // You ARE the weakest link... goodbye
- delete BB;
-
- WriteToVCG(M, "MergedInto");
- }
- }
+ while (!WorkList.empty()) {
+ Instruction *I = WorkList.pop_back_val();
+ MadeChange |= DCEInstruction(I, WorkList, TLI);
}
-
- // Remove unused constants
- Changed |= DoRemoveUnusedConstants(M);
- return Changed;
+ return MadeChange;
}
-
-// It is possible that we may require multiple passes over the code to fully
-// eliminate dead code. Iterate until we are done.
-//
-bool DoDeadCodeElimination(Method *M) {
- bool Changed = false;
- while (DoDCEPass(M)) Changed = true;
- return Changed;
+FunctionPass *llvm::createDeadCodeEliminationPass() {
+ return new DCE();
}
-bool DoDeadCodeElimination(Module *C) {
- bool Val = ApplyOptToAllMethods(C, DoDeadCodeElimination);
- while (DoRemoveUnusedConstants(C)) Val = true;
- return Val;
-}
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