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//===---- BDCE.cpp - Bit-tracking dead code elimination -------------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file implements the Bit-Tracking Dead Code Elimination pass. Some
// instructions (shifts, some ands, ors, etc.) kill some of their input bits.
// We track these dead bits and remove instructions that compute only these
// dead bits.
#include "llvm/Transforms/Scalar/BDCE.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/DemandedBits.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
using namespace llvm;
#define DEBUG_TYPE "bdce"
STATISTIC(NumRemoved, "Number of instructions removed (unused)");
STATISTIC(NumSimplified, "Number of instructions trivialized (dead bits)");
/// If an instruction is trivialized (dead), then the chain of users of that
/// instruction may need to be cleared of assumptions that can no longer be
/// guaranteed correct.
static void clearAssumptionsOfUsers(Instruction *I, DemandedBits &DB) {
assert(I->getType()->isIntegerTy() && "Trivializing a non-integer value?");
// Initialize the worklist with eligible direct users.
SmallVector<Instruction *, 16> WorkList;
for (User *JU : I->users()) {
// If all bits of a user are demanded, then we know that nothing below that
// in the def-use chain needs to be changed.
auto *J = dyn_cast<Instruction>(JU);
if (J && J->getType()->isSized() &&
// Note that we need to check for unsized types above before asking for
// demanded bits. Normally, the only way to reach an instruction with an
// unsized type is via an instruction that has side effects (or otherwise
// will demand its input bits). However, if we have a readnone function
// that returns an unsized type (e.g., void), we must avoid asking for the
// demanded bits of the function call's return value. A void-returning
// readnone function is always dead (and so we can stop walking the use/def
// chain here), but the check is necessary to avoid asserting.
// DFS through subsequent users while tracking visits to avoid cycles.
SmallPtrSet<Instruction *, 16> Visited;
while (!WorkList.empty()) {
Instruction *J = WorkList.pop_back_val();
// NSW, NUW, and exact are based on operands that might have changed.
// We do not have to worry about llvm.assume or range metadata:
// 1. llvm.assume demands its operand, so trivializing can't change it.
// 2. range metadata only applies to memory accesses which demand all bits.
for (User *KU : J->users()) {
// If all bits of a user are demanded, then we know that nothing below
// that in the def-use chain needs to be changed.
auto *K = dyn_cast<Instruction>(KU);
if (K && !Visited.count(K) && K->getType()->isSized() &&
static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
SmallVector<Instruction*, 128> Worklist;
bool Changed = false;
for (Instruction &I : instructions(F)) {
// If the instruction has side effects and no non-dbg uses,
// skip it. This way we avoid computing known bits on an instruction
// that will not help us.
if (I.mayHaveSideEffects() && I.use_empty())
if (I.getType()->isIntegerTy() &&
!DB.getDemandedBits(&I).getBoolValue()) {
// For live instructions that have all dead bits, first make them dead by
// replacing all uses with something else. Then, if they don't need to
// remain live (because they have side effects, etc.) we can remove them.
DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
clearAssumptionsOfUsers(&I, DB);
// FIXME: In theory we could substitute undef here instead of zero.
// This should be reconsidered once we settle on the semantics of
// undef, poison, etc.
Value *Zero = ConstantInt::get(I.getType(), 0);
Changed = true;
if (!DB.isInstructionDead(&I))
Changed = true;
for (Instruction *&I : Worklist) {
return Changed;
PreservedAnalyses BDCEPass::run(Function &F, FunctionAnalysisManager &AM) {
auto &DB = AM.getResult<DemandedBitsAnalysis>(F);
if (!bitTrackingDCE(F, DB))
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
namespace {
struct BDCELegacyPass : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
BDCELegacyPass() : FunctionPass(ID) {
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
auto &DB = getAnalysis<DemandedBitsWrapperPass>().getDemandedBits();
return bitTrackingDCE(F, DB);
void getAnalysisUsage(AnalysisUsage &AU) const override {
char BDCELegacyPass::ID = 0;
"Bit-Tracking Dead Code Elimination", false, false)
"Bit-Tracking Dead Code Elimination", false, false)
FunctionPass *llvm::createBitTrackingDCEPass() { return new BDCELegacyPass(); }