//===- 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.
+// 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.
+//
+// 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/Opt/AllOpts.h"
-
-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();
- }
-};
-
-template<class ValueSubclass, class ItemParentType, class DCEController>
-static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
- DCEController DCEControl) {
- bool Changed = false;
- typedef ValueHolder<ValueSubclass, ItemParentType> Container;
-
- 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
- delete Vals.remove(DI);
- Changed = true;
- } else {
- DI++;
- }
- }
- return Changed;
-}
-
+#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;
-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;
-}
+#define DEBUG_TYPE "dce"
+STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
+STATISTIC(DCEEliminated, "Number of insts removed");
-static void ReplaceUsesWithConstant(Instruction *I) {
- // Get the method level constant pool
- ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
+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;
+ }
- 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();
- }
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.setPreservesCFG();
+ }
+ };
+}
- if (CPV == 0) { // We don't have an existing constant to reuse. Just add one.
- CPV = ConstPoolVal::getNullConstant(I->getType()); // Create a new constant
+char DeadInstElimination::ID = 0;
+INITIALIZE_PASS(DeadInstElimination, "die",
+ "Dead Instruction Elimination", false, false)
- // 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();
}
-static bool DoDCEPass(Method *M) {
- Method::BasicBlocksType::iterator BBIt;
- Method::BasicBlocksType &BBs = M->getBasicBlocks();
- bool Changed = false;
-
- // Loop through now and remove instructions that have no uses...
- for (BBIt = BBs.begin(); BBIt != BBs.end(); BBIt++)
- Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
- // Scan through and remove basic blocks that have no predecessors (except,
- // of course, the first one. :) (so skip first block)
+namespace {
+ //===--------------------------------------------------------------------===//
+ // DeadCodeElimination pass implementation
//
- for (BBIt = BBs.begin(), ++BBIt; BBIt != BBs.end(); BBIt++) {
- BasicBlock *BB = *BBIt;
- assert(BB->getTerminator() &&
- "Degenerate basic block encountered!"); // Empty bb???
-
- if (BB->pred_begin() == BB->pred_end() &&
- !BB->hasConstantPoolReferences()) {
-
- 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
- BasicBlock::InstListType::iterator f = BB->getInstList().begin();
- delete BB->getInstList().remove(f);
- }
-
- delete BBs.remove(BBIt);
- ++BBIt; // remove puts use on the previous block, we want the next one
- Changed = true;
+ struct DCE : public FunctionPass {
+ static char ID; // Pass identification, replacement for typeid
+ DCE() : FunctionPass(ID) {
+ initializeDCEPass(*PassRegistry::getPassRegistry());
}
- }
-
- // Loop through an merge basic blocks into their predecessor if there is only
- // one, and if there is only one successor of the predecessor.
- //
- for (BBIt = BBs.begin(); BBIt != BBs.end(); BBIt++) {
- BasicBlock *BB = *BBIt;
-
- // Is there exactly one predecessor to this block?
- BasicBlock::pred_iterator PI(BB->pred_begin());
- if (PI != BB->pred_end() && ++PI == BB->pred_end() &&
- !BB->hasConstantPoolReferences()) {
- BasicBlock *Pred = *BB->pred_begin();
- TerminatorInst *Term = Pred->getTerminator();
- if (Term == 0) continue; // Err... malformed basic block!
-
- // Is it an unconditional branch?
- if (Term->getInstType() != Instruction::Br ||
- !((BranchInst*)Term)->isUnconditional())
- continue; // Nope, maybe next time...
-
- Changed = true;
-
- // Make all branches to the predecessor now point to the successor...
- Pred->replaceAllUsesWith(BB);
-
- // Move all definitions in the predecessor to the successor...
- BasicBlock::InstListType::iterator DI = Pred->getInstList().end();
- assert(Pred->getTerminator() &&
- "Degenerate basic block encountered!"); // Empty bb???
- delete Pred->getInstList().remove(--DI); // Remove terminator
-
- while (Pred->getInstList().begin() != (DI = Pred->getInstList().end())) {
- Instruction *Def = Pred->getInstList().remove(--DI); // Remove from end
- BB->getInstList().push_front(Def); // Add to front...
- }
- // Remove basic block from the method...
- BBs.remove(Pred);
+ bool runOnFunction(Function &F) override;
- // Always inherit predecessors name if it exists...
- if (Pred->hasName()) BB->setName(Pred->getName());
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.setPreservesCFG();
+ }
+ };
+}
- // So long you waste of a basic block you...
- delete Pred;
+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);
}
- }
- // Remove unused constants
- Changed |= DoRemoveUnusedConstants(M);
- return Changed;
+ I->eraseFromParent();
+ ++DCEEliminated;
+ return true;
+ }
+ return false;
}
+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);
+ }
-// 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;
+ while (!WorkList.empty()) {
+ Instruction *I = WorkList.pop_back_val();
+ MadeChange |= DCEInstruction(I, WorkList, TLI);
+ }
+ return MadeChange;
}
-bool DoDeadCodeElimination(Module *C) {
- bool Val = ApplyOptToAllMethods(C, DoDeadCodeElimination);
- while (DoRemoveUnusedConstants(C)) Val = true;
- return Val;
+FunctionPass *llvm::createDeadCodeEliminationPass() {
+ return new DCE();
}
+
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