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llvm-mirror/lib/Transforms/Utils/Local.cpp
Justin Bogner 42f9d0cf93 Transforms: Don't create bad branch weights when folding a switch 
This avoids creating branch weight metadata of length one when we fold
cases into the default of a switch instruction, which was triggering
an assert.

llvm-svn: 196845
2013-12-10 00:13:41 +00:00

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//===-- Local.cpp - Functions to perform local transformations ------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This family of functions perform various local transformations to the
// program.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/DIBuilder.h"
#include "llvm/DebugInfo.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/GlobalAlias.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Operator.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/ValueHandle.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
STATISTIC(NumRemoved, "Number of unreachable basic blocks removed");
//===----------------------------------------------------------------------===//
// Local constant propagation.
//
/// ConstantFoldTerminator - If a terminator instruction is predicated on a
/// constant value, convert it into an unconditional branch to the constant
/// destination. This is a nontrivial operation because the successors of this
/// basic block must have their PHI nodes updated.
/// Also calls RecursivelyDeleteTriviallyDeadInstructions() on any branch/switch
/// conditions and indirectbr addresses this might make dead if
/// DeleteDeadConditions is true.
bool llvm::ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions,
const TargetLibraryInfo *TLI) {
TerminatorInst *T = BB->getTerminator();
IRBuilder<> Builder(T);
// Branch - See if we are conditional jumping on constant
if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
if (BI->isUnconditional()) return false; // Can't optimize uncond branch
BasicBlock *Dest1 = BI->getSuccessor(0);
BasicBlock *Dest2 = BI->getSuccessor(1);
if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
// Are we branching on constant?
// YES. Change to unconditional branch...
BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1;
//cerr << "Function: " << T->getParent()->getParent()
// << "\nRemoving branch from " << T->getParent()
// << "\n\nTo: " << OldDest << endl;
// Let the basic block know that we are letting go of it. Based on this,
// it will adjust it's PHI nodes.
OldDest->removePredecessor(BB);
// Replace the conditional branch with an unconditional one.
Builder.CreateBr(Destination);
BI->eraseFromParent();
return true;
}
if (Dest2 == Dest1) { // Conditional branch to same location?
// This branch matches something like this:
// br bool %cond, label %Dest, label %Dest
// and changes it into: br label %Dest
// Let the basic block know that we are letting go of one copy of it.
assert(BI->getParent() && "Terminator not inserted in block!");
Dest1->removePredecessor(BI->getParent());
// Replace the conditional branch with an unconditional one.
Builder.CreateBr(Dest1);
Value *Cond = BI->getCondition();
BI->eraseFromParent();
if (DeleteDeadConditions)
RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
return true;
}
return false;
}
if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
// If we are switching on a constant, we can convert the switch into a
// single branch instruction!
ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
BasicBlock *TheOnlyDest = SI->getDefaultDest();
BasicBlock *DefaultDest = TheOnlyDest;
// Figure out which case it goes to.
for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
i != e; ++i) {
// Found case matching a constant operand?
if (i.getCaseValue() == CI) {
TheOnlyDest = i.getCaseSuccessor();
break;
}
// Check to see if this branch is going to the same place as the default
// dest. If so, eliminate it as an explicit compare.
if (i.getCaseSuccessor() == DefaultDest) {
MDNode* MD = SI->getMetadata(LLVMContext::MD_prof);
unsigned NCases = SI->getNumCases();
// Fold the case metadata into the default if there will be any branches
// left, unless the metadata doesn't match the switch.
if (NCases > 1 && MD && MD->getNumOperands() == 2 + NCases) {
// Collect branch weights into a vector.
SmallVector<uint32_t, 8> Weights;
for (unsigned MD_i = 1, MD_e = MD->getNumOperands(); MD_i < MD_e;
++MD_i) {
ConstantInt* CI = dyn_cast<ConstantInt>(MD->getOperand(MD_i));
assert(CI);
Weights.push_back(CI->getValue().getZExtValue());
}
// Merge weight of this case to the default weight.
unsigned idx = i.getCaseIndex();
Weights[0] += Weights[idx+1];
// Remove weight for this case.
std::swap(Weights[idx+1], Weights.back());
Weights.pop_back();
SI->setMetadata(LLVMContext::MD_prof,
MDBuilder(BB->getContext()).
createBranchWeights(Weights));
}
// Remove this entry.
DefaultDest->removePredecessor(SI->getParent());
SI->removeCase(i);
--i; --e;
continue;
}
// Otherwise, check to see if the switch only branches to one destination.
// We do this by reseting "TheOnlyDest" to null when we find two non-equal
// destinations.
if (i.getCaseSuccessor() != TheOnlyDest) TheOnlyDest = 0;
}
if (CI && !TheOnlyDest) {
// Branching on a constant, but not any of the cases, go to the default
// successor.
TheOnlyDest = SI->getDefaultDest();
}
// If we found a single destination that we can fold the switch into, do so
// now.
if (TheOnlyDest) {
// Insert the new branch.
Builder.CreateBr(TheOnlyDest);
BasicBlock *BB = SI->getParent();
// Remove entries from PHI nodes which we no longer branch to...
for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
// Found case matching a constant operand?
BasicBlock *Succ = SI->getSuccessor(i);
if (Succ == TheOnlyDest)
TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest
else
Succ->removePredecessor(BB);
}
// Delete the old switch.
Value *Cond = SI->getCondition();
SI->eraseFromParent();
if (DeleteDeadConditions)
RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
return true;
}
if (SI->getNumCases() == 1) {
// Otherwise, we can fold this switch into a conditional branch
// instruction if it has only one non-default destination.
SwitchInst::CaseIt FirstCase = SI->case_begin();
Value *Cond = Builder.CreateICmpEQ(SI->getCondition(),
FirstCase.getCaseValue(), "cond");
// Insert the new branch.
BranchInst *NewBr = Builder.CreateCondBr(Cond,
FirstCase.getCaseSuccessor(),
SI->getDefaultDest());
MDNode* MD = SI->getMetadata(LLVMContext::MD_prof);
if (MD && MD->getNumOperands() == 3) {
ConstantInt *SICase = dyn_cast<ConstantInt>(MD->getOperand(2));
ConstantInt *SIDef = dyn_cast<ConstantInt>(MD->getOperand(1));
assert(SICase && SIDef);
// The TrueWeight should be the weight for the single case of SI.
NewBr->setMetadata(LLVMContext::MD_prof,
MDBuilder(BB->getContext()).
createBranchWeights(SICase->getValue().getZExtValue(),
SIDef->getValue().getZExtValue()));
}
// Delete the old switch.
SI->eraseFromParent();
return true;
}
return false;
}
if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(T)) {
// indirectbr blockaddress(@F, @BB) -> br label @BB
if (BlockAddress *BA =
dyn_cast<BlockAddress>(IBI->getAddress()->stripPointerCasts())) {
BasicBlock *TheOnlyDest = BA->getBasicBlock();
// Insert the new branch.
Builder.CreateBr(TheOnlyDest);
for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
if (IBI->getDestination(i) == TheOnlyDest)
TheOnlyDest = 0;
else
IBI->getDestination(i)->removePredecessor(IBI->getParent());
}
Value *Address = IBI->getAddress();
IBI->eraseFromParent();
if (DeleteDeadConditions)
RecursivelyDeleteTriviallyDeadInstructions(Address, TLI);
// If we didn't find our destination in the IBI successor list, then we
// have undefined behavior. Replace the unconditional branch with an
// 'unreachable' instruction.
if (TheOnlyDest) {
BB->getTerminator()->eraseFromParent();
new UnreachableInst(BB->getContext(), BB);
}
return true;
}
}
return false;
}
//===----------------------------------------------------------------------===//
// Local dead code elimination.
//
/// isInstructionTriviallyDead - Return true if the result produced by the
/// instruction is not used, and the instruction has no side effects.
///
bool llvm::isInstructionTriviallyDead(Instruction *I,
const TargetLibraryInfo *TLI) {
if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
// We don't want the landingpad instruction removed by anything this general.
if (isa<LandingPadInst>(I))
return false;
// We don't want debug info removed by anything this general, unless
// debug info is empty.
if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(I)) {
if (DDI->getAddress())
return false;
return true;
}
if (DbgValueInst *DVI = dyn_cast<DbgValueInst>(I)) {
if (DVI->getValue())
return false;
return true;
}
if (!I->mayHaveSideEffects()) return true;
// Special case intrinsics that "may have side effects" but can be deleted
// when dead.
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
// Safe to delete llvm.stacksave if dead.
if (II->getIntrinsicID() == Intrinsic::stacksave)
return true;
// Lifetime intrinsics are dead when their right-hand is undef.
if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
II->getIntrinsicID() == Intrinsic::lifetime_end)
return isa<UndefValue>(II->getArgOperand(1));
}
if (isAllocLikeFn(I, TLI)) return true;
if (CallInst *CI = isFreeCall(I, TLI))
if (Constant *C = dyn_cast<Constant>(CI->getArgOperand(0)))
return C->isNullValue() || isa<UndefValue>(C);
return false;
}
/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a
/// trivially dead instruction, delete it. If that makes any of its operands
/// trivially dead, delete them too, recursively. Return true if any
/// instructions were deleted.
bool
llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V,
const TargetLibraryInfo *TLI) {
Instruction *I = dyn_cast<Instruction>(V);
if (!I || !I->use_empty() || !isInstructionTriviallyDead(I, TLI))
return false;
SmallVector<Instruction*, 16> DeadInsts;
DeadInsts.push_back(I);
do {
I = DeadInsts.pop_back_val();
// 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, 0);
if (!OpV->use_empty()) 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))
DeadInsts.push_back(OpI);
}
I->eraseFromParent();
} while (!DeadInsts.empty());
return true;
}
/// areAllUsesEqual - Check whether the uses of a value are all the same.
/// This is similar to Instruction::hasOneUse() except this will also return
/// true when there are no uses or multiple uses that all refer to the same
/// value.
static bool areAllUsesEqual(Instruction *I) {
Value::use_iterator UI = I->use_begin();
Value::use_iterator UE = I->use_end();
if (UI == UE)
return true;
User *TheUse = *UI;
for (++UI; UI != UE; ++UI) {
if (*UI != TheUse)
return false;
}
return true;
}
/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively
/// dead PHI node, due to being a def-use chain of single-use nodes that
/// either forms a cycle or is terminated by a trivially dead instruction,
/// delete it. If that makes any of its operands trivially dead, delete them
/// too, recursively. Return true if a change was made.
bool llvm::RecursivelyDeleteDeadPHINode(PHINode *PN,
const TargetLibraryInfo *TLI) {
SmallPtrSet<Instruction*, 4> Visited;
for (Instruction *I = PN; areAllUsesEqual(I) && !I->mayHaveSideEffects();
I = cast<Instruction>(*I->use_begin())) {
if (I->use_empty())
return RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
// If we find an instruction more than once, we're on a cycle that
// won't prove fruitful.
if (!Visited.insert(I)) {
// Break the cycle and delete the instruction and its operands.
I->replaceAllUsesWith(UndefValue::get(I->getType()));
(void)RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
return true;
}
}
return false;
}
/// SimplifyInstructionsInBlock - Scan the specified basic block and try to
/// simplify any instructions in it and recursively delete dead instructions.
///
/// This returns true if it changed the code, note that it can delete
/// instructions in other blocks as well in this block.
bool llvm::SimplifyInstructionsInBlock(BasicBlock *BB, const DataLayout *TD,
const TargetLibraryInfo *TLI) {
bool MadeChange = false;
#ifndef NDEBUG
// In debug builds, ensure that the terminator of the block is never replaced
// or deleted by these simplifications. The idea of simplification is that it
// cannot introduce new instructions, and there is no way to replace the
// terminator of a block without introducing a new instruction.
AssertingVH<Instruction> TerminatorVH(--BB->end());
#endif
for (BasicBlock::iterator BI = BB->begin(), E = --BB->end(); BI != E; ) {
assert(!BI->isTerminator());
Instruction *Inst = BI++;
WeakVH BIHandle(BI);
if (recursivelySimplifyInstruction(Inst, TD, TLI)) {
MadeChange = true;
if (BIHandle != BI)
BI = BB->begin();
continue;
}
MadeChange |= RecursivelyDeleteTriviallyDeadInstructions(Inst, TLI);
if (BIHandle != BI)
BI = BB->begin();
}
return MadeChange;
}
//===----------------------------------------------------------------------===//
// Control Flow Graph Restructuring.
//
/// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this
/// method is called when we're about to delete Pred as a predecessor of BB. If
/// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred.
///
/// Unlike the removePredecessor method, this attempts to simplify uses of PHI
/// nodes that collapse into identity values. For example, if we have:
/// x = phi(1, 0, 0, 0)
/// y = and x, z
///
/// .. and delete the predecessor corresponding to the '1', this will attempt to
/// recursively fold the and to 0.
void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred,
DataLayout *TD) {
// This only adjusts blocks with PHI nodes.
if (!isa<PHINode>(BB->begin()))
return;
// Remove the entries for Pred from the PHI nodes in BB, but do not simplify
// them down. This will leave us with single entry phi nodes and other phis
// that can be removed.
BB->removePredecessor(Pred, true);
WeakVH PhiIt = &BB->front();
while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) {
PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt));
Value *OldPhiIt = PhiIt;
if (!recursivelySimplifyInstruction(PN, TD))
continue;
// If recursive simplification ended up deleting the next PHI node we would
// iterate to, then our iterator is invalid, restart scanning from the top
// of the block.
if (PhiIt != OldPhiIt) PhiIt = &BB->front();
}
}
/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
/// predecessor is known to have one successor (DestBB!). Eliminate the edge
/// between them, moving the instructions in the predecessor into DestBB and
/// deleting the predecessor block.
///
void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) {
// If BB has single-entry PHI nodes, fold them.
while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
Value *NewVal = PN->getIncomingValue(0);
// Replace self referencing PHI with undef, it must be dead.
if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
PN->replaceAllUsesWith(NewVal);
PN->eraseFromParent();
}
BasicBlock *PredBB = DestBB->getSinglePredecessor();
assert(PredBB && "Block doesn't have a single predecessor!");
// Zap anything that took the address of DestBB. Not doing this will give the
// address an invalid value.
if (DestBB->hasAddressTaken()) {
BlockAddress *BA = BlockAddress::get(DestBB);
Constant *Replacement =
ConstantInt::get(llvm::Type::getInt32Ty(BA->getContext()), 1);
BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(Replacement,
BA->getType()));
BA->destroyConstant();
}
// Anything that branched to PredBB now branches to DestBB.
PredBB->replaceAllUsesWith(DestBB);
// Splice all the instructions from PredBB to DestBB.
PredBB->getTerminator()->eraseFromParent();
DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
if (P) {
DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>();
if (DT) {
BasicBlock *PredBBIDom = DT->getNode(PredBB)->getIDom()->getBlock();
DT->changeImmediateDominator(DestBB, PredBBIDom);
DT->eraseNode(PredBB);
}
}
// Nuke BB.
PredBB->eraseFromParent();
}
/// CanMergeValues - Return true if we can choose one of these values to use
/// in place of the other. Note that we will always choose the non-undef
/// value to keep.
static bool CanMergeValues(Value *First, Value *Second) {
return First == Second || isa<UndefValue>(First) || isa<UndefValue>(Second);
}
/// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
/// almost-empty BB ending in an unconditional branch to Succ, into Succ.
///
/// Assumption: Succ is the single successor for BB.
///
static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
DEBUG(dbgs() << "Looking to fold " << BB->getName() << " into "
<< Succ->getName() << "\n");
// Shortcut, if there is only a single predecessor it must be BB and merging
// is always safe
if (Succ->getSinglePredecessor()) return true;
// Make a list of the predecessors of BB
SmallPtrSet<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
// Look at all the phi nodes in Succ, to see if they present a conflict when
// merging these blocks
for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
// If the incoming value from BB is again a PHINode in
// BB which has the same incoming value for *PI as PN does, we can
// merge the phi nodes and then the blocks can still be merged
PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB));
if (BBPN && BBPN->getParent() == BB) {
for (unsigned PI = 0, PE = PN->getNumIncomingValues(); PI != PE; ++PI) {
BasicBlock *IBB = PN->getIncomingBlock(PI);
if (BBPreds.count(IBB) &&
!CanMergeValues(BBPN->getIncomingValueForBlock(IBB),
PN->getIncomingValue(PI))) {
DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in "
<< Succ->getName() << " is conflicting with "
<< BBPN->getName() << " with regard to common predecessor "
<< IBB->getName() << "\n");
return false;
}
}
} else {
Value* Val = PN->getIncomingValueForBlock(BB);
for (unsigned PI = 0, PE = PN->getNumIncomingValues(); PI != PE; ++PI) {
// See if the incoming value for the common predecessor is equal to the
// one for BB, in which case this phi node will not prevent the merging
// of the block.
BasicBlock *IBB = PN->getIncomingBlock(PI);
if (BBPreds.count(IBB) &&
!CanMergeValues(Val, PN->getIncomingValue(PI))) {
DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in "
<< Succ->getName() << " is conflicting with regard to common "
<< "predecessor " << IBB->getName() << "\n");
return false;
}
}
}
}
return true;
}
typedef SmallVector<BasicBlock *, 16> PredBlockVector;
typedef DenseMap<BasicBlock *, Value *> IncomingValueMap;
/// \brief Determines the value to use as the phi node input for a block.
///
/// Select between \p OldVal any value that we know flows from \p BB
/// to a particular phi on the basis of which one (if either) is not
/// undef. Update IncomingValues based on the selected value.
///
/// \param OldVal The value we are considering selecting.
/// \param BB The block that the value flows in from.
/// \param IncomingValues A map from block-to-value for other phi inputs
/// that we have examined.
///
/// \returns the selected value.
static Value *selectIncomingValueForBlock(Value *OldVal, BasicBlock *BB,
IncomingValueMap &IncomingValues) {
if (!isa<UndefValue>(OldVal)) {
assert((!IncomingValues.count(BB) ||
IncomingValues.find(BB)->second == OldVal) &&
"Expected OldVal to match incoming value from BB!");
IncomingValues.insert(std::make_pair(BB, OldVal));
return OldVal;
}
IncomingValueMap::const_iterator It = IncomingValues.find(BB);
if (It != IncomingValues.end()) return It->second;
return OldVal;
}
/// \brief Create a map from block to value for the operands of a
/// given phi.
///
/// Create a map from block to value for each non-undef value flowing
/// into \p PN.
///
/// \param PN The phi we are collecting the map for.
/// \param IncomingValues [out] The map from block to value for this phi.
static void gatherIncomingValuesToPhi(PHINode *PN,
IncomingValueMap &IncomingValues) {
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *BB = PN->getIncomingBlock(i);
Value *V = PN->getIncomingValue(i);
if (!isa<UndefValue>(V))
IncomingValues.insert(std::make_pair(BB, V));
}
}
/// \brief Replace the incoming undef values to a phi with the values
/// from a block-to-value map.
///
/// \param PN The phi we are replacing the undefs in.
/// \param IncomingValues A map from block to value.
static void replaceUndefValuesInPhi(PHINode *PN,
const IncomingValueMap &IncomingValues) {
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *V = PN->getIncomingValue(i);
if (!isa<UndefValue>(V)) continue;
BasicBlock *BB = PN->getIncomingBlock(i);
IncomingValueMap::const_iterator It = IncomingValues.find(BB);
if (It == IncomingValues.end()) continue;
PN->setIncomingValue(i, It->second);
}
}
/// \brief Replace a value flowing from a block to a phi with
/// potentially multiple instances of that value flowing from the
/// block's predecessors to the phi.
///
/// \param BB The block with the value flowing into the phi.
/// \param BBPreds The predecessors of BB.
/// \param PN The phi that we are updating.
static void redirectValuesFromPredecessorsToPhi(BasicBlock *BB,
const PredBlockVector &BBPreds,
PHINode *PN) {
Value *OldVal = PN->removeIncomingValue(BB, false);
assert(OldVal && "No entry in PHI for Pred BB!");
IncomingValueMap IncomingValues;
// We are merging two blocks - BB, and the block containing PN - and
// as a result we need to redirect edges from the predecessors of BB
// to go to the block containing PN, and update PN
// accordingly. Since we allow merging blocks in the case where the
// predecessor and successor blocks both share some predecessors,
// and where some of those common predecessors might have undef
// values flowing into PN, we want to rewrite those values to be
// consistent with the non-undef values.
gatherIncomingValuesToPhi(PN, IncomingValues);
// If this incoming value is one of the PHI nodes in BB, the new entries
// in the PHI node are the entries from the old PHI.
if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
PHINode *OldValPN = cast<PHINode>(OldVal);
for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) {
// Note that, since we are merging phi nodes and BB and Succ might
// have common predecessors, we could end up with a phi node with
// identical incoming branches. This will be cleaned up later (and
// will trigger asserts if we try to clean it up now, without also
// simplifying the corresponding conditional branch).
BasicBlock *PredBB = OldValPN->getIncomingBlock(i);
Value *PredVal = OldValPN->getIncomingValue(i);
Value *Selected = selectIncomingValueForBlock(PredVal, PredBB,
IncomingValues);
// And add a new incoming value for this predecessor for the
// newly retargeted branch.
PN->addIncoming(Selected, PredBB);
}
} else {
for (unsigned i = 0, e = BBPreds.size(); i != e; ++i) {
// Update existing incoming values in PN for this
// predecessor of BB.
BasicBlock *PredBB = BBPreds[i];
Value *Selected = selectIncomingValueForBlock(OldVal, PredBB,
IncomingValues);
// And add a new incoming value for this predecessor for the
// newly retargeted branch.
PN->addIncoming(Selected, PredBB);
}
}
replaceUndefValuesInPhi(PN, IncomingValues);
}
/// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an
/// unconditional branch, and contains no instructions other than PHI nodes,
/// potential side-effect free intrinsics and the branch. If possible,
/// eliminate BB by rewriting all the predecessors to branch to the successor
/// block and return true. If we can't transform, return false.
bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) {
assert(BB != &BB->getParent()->getEntryBlock() &&
"TryToSimplifyUncondBranchFromEmptyBlock called on entry block!");
// We can't eliminate infinite loops.
BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0);
if (BB == Succ) return false;
// Check to see if merging these blocks would cause conflicts for any of the
// phi nodes in BB or Succ. If not, we can safely merge.
if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
// Check for cases where Succ has multiple predecessors and a PHI node in BB
// has uses which will not disappear when the PHI nodes are merged. It is
// possible to handle such cases, but difficult: it requires checking whether
// BB dominates Succ, which is non-trivial to calculate in the case where
// Succ has multiple predecessors. Also, it requires checking whether
// constructing the necessary self-referential PHI node doesn't introduce any
// conflicts; this isn't too difficult, but the previous code for doing this
// was incorrect.
//
// Note that if this check finds a live use, BB dominates Succ, so BB is
// something like a loop pre-header (or rarely, a part of an irreducible CFG);
// folding the branch isn't profitable in that case anyway.
if (!Succ->getSinglePredecessor()) {
BasicBlock::iterator BBI = BB->begin();
while (isa<PHINode>(*BBI)) {
for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
UI != E; ++UI) {
if (PHINode* PN = dyn_cast<PHINode>(*UI)) {
if (PN->getIncomingBlock(UI) != BB)
return false;
} else {
return false;
}
}
++BBI;
}
}
DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB);
if (isa<PHINode>(Succ->begin())) {
// If there is more than one pred of succ, and there are PHI nodes in
// the successor, then we need to add incoming edges for the PHI nodes
//
const PredBlockVector BBPreds(pred_begin(BB), pred_end(BB));
// Loop over all of the PHI nodes in the successor of BB.
for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
redirectValuesFromPredecessorsToPhi(BB, BBPreds, PN);
}
}
if (Succ->getSinglePredecessor()) {
// BB is the only predecessor of Succ, so Succ will end up with exactly
// the same predecessors BB had.
// Copy over any phi, debug or lifetime instruction.
BB->getTerminator()->eraseFromParent();
Succ->getInstList().splice(Succ->getFirstNonPHI(), BB->getInstList());
} else {
while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
// We explicitly check for such uses in CanPropagatePredecessorsForPHIs.
assert(PN->use_empty() && "There shouldn't be any uses here!");
PN->eraseFromParent();
}
}
// Everything that jumped to BB now goes to Succ.
BB->replaceAllUsesWith(Succ);
if (!Succ->hasName()) Succ->takeName(BB);
BB->eraseFromParent(); // Delete the old basic block.
return true;
}
/// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI
/// nodes in this block. This doesn't try to be clever about PHI nodes
/// which differ only in the order of the incoming values, but instcombine
/// orders them so it usually won't matter.
///
bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
bool Changed = false;
// This implementation doesn't currently consider undef operands
// specially. Theoretically, two phis which are identical except for
// one having an undef where the other doesn't could be collapsed.
// Map from PHI hash values to PHI nodes. If multiple PHIs have
// the same hash value, the element is the first PHI in the
// linked list in CollisionMap.
DenseMap<uintptr_t, PHINode *> HashMap;
// Maintain linked lists of PHI nodes with common hash values.
DenseMap<PHINode *, PHINode *> CollisionMap;
// Examine each PHI.
for (BasicBlock::iterator I = BB->begin();
PHINode *PN = dyn_cast<PHINode>(I++); ) {
// Compute a hash value on the operands. Instcombine will likely have sorted
// them, which helps expose duplicates, but we have to check all the
// operands to be safe in case instcombine hasn't run.
uintptr_t Hash = 0;
// This hash algorithm is quite weak as hash functions go, but it seems
// to do a good enough job for this particular purpose, and is very quick.
for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) {
Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I));
Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7));
}
for (PHINode::block_iterator I = PN->block_begin(), E = PN->block_end();
I != E; ++I) {
Hash ^= reinterpret_cast<uintptr_t>(static_cast<BasicBlock *>(*I));
Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7));
}
// Avoid colliding with the DenseMap sentinels ~0 and ~0-1.
Hash >>= 1;
// If we've never seen this hash value before, it's a unique PHI.
std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair =
HashMap.insert(std::make_pair(Hash, PN));
if (Pair.second) continue;
// Otherwise it's either a duplicate or a hash collision.
for (PHINode *OtherPN = Pair.first->second; ; ) {
if (OtherPN->isIdenticalTo(PN)) {
// A duplicate. Replace this PHI with its duplicate.
PN->replaceAllUsesWith(OtherPN);
PN->eraseFromParent();
Changed = true;
break;
}
// A non-duplicate hash collision.
DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN);
if (I == CollisionMap.end()) {
// Set this PHI to be the head of the linked list of colliding PHIs.
PHINode *Old = Pair.first->second;
Pair.first->second = PN;
CollisionMap[PN] = Old;
break;
}
// Proceed to the next PHI in the list.
OtherPN = I->second;
}
}
return Changed;
}
/// enforceKnownAlignment - If the specified pointer points to an object that
/// we control, modify the object's alignment to PrefAlign. This isn't
/// often possible though. If alignment is important, a more reliable approach
/// is to simply align all global variables and allocation instructions to
/// their preferred alignment from the beginning.
///
static unsigned enforceKnownAlignment(Value *V, unsigned Align,
unsigned PrefAlign, const DataLayout *TD) {
V = V->stripPointerCasts();
if (AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
// If the preferred alignment is greater than the natural stack alignment
// then don't round up. This avoids dynamic stack realignment.
if (TD && TD->exceedsNaturalStackAlignment(PrefAlign))
return Align;
// If there is a requested alignment and if this is an alloca, round up.
if (AI->getAlignment() >= PrefAlign)
return AI->getAlignment();
AI->setAlignment(PrefAlign);
return PrefAlign;
}
if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
// If there is a large requested alignment and we can, bump up the alignment
// of the global.
if (GV->isDeclaration()) return Align;
// If the memory we set aside for the global may not be the memory used by
// the final program then it is impossible for us to reliably enforce the
// preferred alignment.
if (GV->isWeakForLinker()) return Align;
if (GV->getAlignment() >= PrefAlign)
return GV->getAlignment();
// We can only increase the alignment of the global if it has no alignment
// specified or if it is not assigned a section. If it is assigned a
// section, the global could be densely packed with other objects in the
// section, increasing the alignment could cause padding issues.
if (!GV->hasSection() || GV->getAlignment() == 0)
GV->setAlignment(PrefAlign);
return GV->getAlignment();
}
return Align;
}
/// getOrEnforceKnownAlignment - If the specified pointer has an alignment that
/// we can determine, return it, otherwise return 0. If PrefAlign is specified,
/// and it is more than the alignment of the ultimate object, see if we can
/// increase the alignment of the ultimate object, making this check succeed.
unsigned llvm::getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign,
const DataLayout *DL) {
assert(V->getType()->isPointerTy() &&
"getOrEnforceKnownAlignment expects a pointer!");
unsigned BitWidth = DL ? DL->getPointerTypeSizeInBits(V->getType()) : 64;
APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
ComputeMaskedBits(V, KnownZero, KnownOne, DL);
unsigned TrailZ = KnownZero.countTrailingOnes();
// Avoid trouble with ridiculously large TrailZ values, such as
// those computed from a null pointer.
TrailZ = std::min(TrailZ, unsigned(sizeof(unsigned) * CHAR_BIT - 1));
unsigned Align = 1u << std::min(BitWidth - 1, TrailZ);
// LLVM doesn't support alignments larger than this currently.
Align = std::min(Align, +Value::MaximumAlignment);
if (PrefAlign > Align)
Align = enforceKnownAlignment(V, Align, PrefAlign, DL);
// We don't need to make any adjustment.
return Align;
}
///===---------------------------------------------------------------------===//
/// Dbg Intrinsic utilities
///
/// See if there is a dbg.value intrinsic for DIVar before I.
static bool LdStHasDebugValue(DIVariable &DIVar, Instruction *I) {
// Since we can't guarantee that the original dbg.declare instrinsic
// is removed by LowerDbgDeclare(), we need to make sure that we are
// not inserting the same dbg.value intrinsic over and over.
llvm::BasicBlock::InstListType::iterator PrevI(I);
if (PrevI != I->getParent()->getInstList().begin()) {
--PrevI;
if (DbgValueInst *DVI = dyn_cast<DbgValueInst>(PrevI))
if (DVI->getValue() == I->getOperand(0) &&
DVI->getOffset() == 0 &&
DVI->getVariable() == DIVar)
return true;
}
return false;
}
/// Inserts a llvm.dbg.value intrinsic before a store to an alloca'd value
/// that has an associated llvm.dbg.decl intrinsic.
bool llvm::ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI,
StoreInst *SI, DIBuilder &Builder) {
DIVariable DIVar(DDI->getVariable());
assert((!DIVar || DIVar.isVariable()) &&
"Variable in DbgDeclareInst should be either null or a DIVariable.");
if (!DIVar)
return false;
if (LdStHasDebugValue(DIVar, SI))
return true;
Instruction *DbgVal = NULL;
// If an argument is zero extended then use argument directly. The ZExt
// may be zapped by an optimization pass in future.
Argument *ExtendedArg = NULL;
if (ZExtInst *ZExt = dyn_cast<ZExtInst>(SI->getOperand(0)))
ExtendedArg = dyn_cast<Argument>(ZExt->getOperand(0));
if (SExtInst *SExt = dyn_cast<SExtInst>(SI->getOperand(0)))
ExtendedArg = dyn_cast<Argument>(SExt->getOperand(0));
if (ExtendedArg)
DbgVal = Builder.insertDbgValueIntrinsic(ExtendedArg, 0, DIVar, SI);
else
DbgVal = Builder.insertDbgValueIntrinsic(SI->getOperand(0), 0, DIVar, SI);
// Propagate any debug metadata from the store onto the dbg.value.
DebugLoc SIDL = SI->getDebugLoc();
if (!SIDL.isUnknown())
DbgVal->setDebugLoc(SIDL);
// Otherwise propagate debug metadata from dbg.declare.
else
DbgVal->setDebugLoc(DDI->getDebugLoc());
return true;
}
/// Inserts a llvm.dbg.value intrinsic before a load of an alloca'd value
/// that has an associated llvm.dbg.decl intrinsic.
bool llvm::ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI,
LoadInst *LI, DIBuilder &Builder) {
DIVariable DIVar(DDI->getVariable());
assert((!DIVar || DIVar.isVariable()) &&
"Variable in DbgDeclareInst should be either null or a DIVariable.");
if (!DIVar)
return false;
if (LdStHasDebugValue(DIVar, LI))
return true;
Instruction *DbgVal =
Builder.insertDbgValueIntrinsic(LI->getOperand(0), 0,
DIVar, LI);
// Propagate any debug metadata from the store onto the dbg.value.
DebugLoc LIDL = LI->getDebugLoc();
if (!LIDL.isUnknown())
DbgVal->setDebugLoc(LIDL);
// Otherwise propagate debug metadata from dbg.declare.
else
DbgVal->setDebugLoc(DDI->getDebugLoc());
return true;
}
/// LowerDbgDeclare - Lowers llvm.dbg.declare intrinsics into appropriate set
/// of llvm.dbg.value intrinsics.
bool llvm::LowerDbgDeclare(Function &F) {
DIBuilder DIB(*F.getParent());
SmallVector<DbgDeclareInst *, 4> Dbgs;
for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ++BI) {
if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(BI))
Dbgs.push_back(DDI);
}
if (Dbgs.empty())
return false;
for (SmallVectorImpl<DbgDeclareInst *>::iterator I = Dbgs.begin(),
E = Dbgs.end(); I != E; ++I) {
DbgDeclareInst *DDI = *I;
AllocaInst *AI = dyn_cast_or_null<AllocaInst>(DDI->getAddress());
// If this is an alloca for a scalar variable, insert a dbg.value
// at each load and store to the alloca and erase the dbg.declare.
if (AI && !AI->isArrayAllocation()) {
// We only remove the dbg.declare intrinsic if all uses are
// converted to dbg.value intrinsics.
bool RemoveDDI = true;
for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end();
UI != E; ++UI)
if (StoreInst *SI = dyn_cast<StoreInst>(*UI))
ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
else if (LoadInst *LI = dyn_cast<LoadInst>(*UI))
ConvertDebugDeclareToDebugValue(DDI, LI, DIB);
else
RemoveDDI = false;
if (RemoveDDI)
DDI->eraseFromParent();
}
}
return true;
}
/// FindAllocaDbgDeclare - Finds the llvm.dbg.declare intrinsic describing the
/// alloca 'V', if any.
DbgDeclareInst *llvm::FindAllocaDbgDeclare(Value *V) {
if (MDNode *DebugNode = MDNode::getIfExists(V->getContext(), V))
for (Value::use_iterator UI = DebugNode->use_begin(),
E = DebugNode->use_end(); UI != E; ++UI)
if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(*UI))
return DDI;
return 0;
}
bool llvm::replaceDbgDeclareForAlloca(AllocaInst *AI, Value *NewAllocaAddress,
DIBuilder &Builder) {
DbgDeclareInst *DDI = FindAllocaDbgDeclare(AI);
if (!DDI)
return false;
DIVariable DIVar(DDI->getVariable());
assert((!DIVar || DIVar.isVariable()) &&
"Variable in DbgDeclareInst should be either null or a DIVariable.");
if (!DIVar)
return false;
// Create a copy of the original DIDescriptor for user variable, appending
// "deref" operation to a list of address elements, as new llvm.dbg.declare
// will take a value storing address of the memory for variable, not
// alloca itself.
Type *Int64Ty = Type::getInt64Ty(AI->getContext());
SmallVector<Value*, 4> NewDIVarAddress;
if (DIVar.hasComplexAddress()) {
for (unsigned i = 0, n = DIVar.getNumAddrElements(); i < n; ++i) {
NewDIVarAddress.push_back(
ConstantInt::get(Int64Ty, DIVar.getAddrElement(i)));
}
}
NewDIVarAddress.push_back(ConstantInt::get(Int64Ty, DIBuilder::OpDeref));
DIVariable NewDIVar = Builder.createComplexVariable(
DIVar.getTag(), DIVar.getContext(), DIVar.getName(),
DIVar.getFile(), DIVar.getLineNumber(), DIVar.getType(),
NewDIVarAddress, DIVar.getArgNumber());
// Insert llvm.dbg.declare in the same basic block as the original alloca,
// and remove old llvm.dbg.declare.
BasicBlock *BB = AI->getParent();
Builder.insertDeclare(NewAllocaAddress, NewDIVar, BB);
DDI->eraseFromParent();
return true;
}
/// changeToUnreachable - Insert an unreachable instruction before the specified
/// instruction, making it and the rest of the code in the block dead.
static void changeToUnreachable(Instruction *I, bool UseLLVMTrap) {
BasicBlock *BB = I->getParent();
// Loop over all of the successors, removing BB's entry from any PHI
// nodes.
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
(*SI)->removePredecessor(BB);
// Insert a call to llvm.trap right before this. This turns the undefined
// behavior into a hard fail instead of falling through into random code.
if (UseLLVMTrap) {
Function *TrapFn =
Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
CallInst *CallTrap = CallInst::Create(TrapFn, "", I);
CallTrap->setDebugLoc(I->getDebugLoc());
}
new UnreachableInst(I->getContext(), I);
// All instructions after this are dead.
BasicBlock::iterator BBI = I, BBE = BB->end();
while (BBI != BBE) {
if (!BBI->use_empty())
BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
BB->getInstList().erase(BBI++);
}
}
/// changeToCall - Convert the specified invoke into a normal call.
static void changeToCall(InvokeInst *II) {
SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args, "", II);
NewCall->takeName(II);
NewCall->setCallingConv(II->getCallingConv());
NewCall->setAttributes(II->getAttributes());
NewCall->setDebugLoc(II->getDebugLoc());
II->replaceAllUsesWith(NewCall);
// Follow the call by a branch to the normal destination.
BranchInst::Create(II->getNormalDest(), II);
// Update PHI nodes in the unwind destination
II->getUnwindDest()->removePredecessor(II->getParent());
II->eraseFromParent();
}
static bool markAliveBlocks(BasicBlock *BB,
SmallPtrSet<BasicBlock*, 128> &Reachable) {
SmallVector<BasicBlock*, 128> Worklist;
Worklist.push_back(BB);
Reachable.insert(BB);
bool Changed = false;
do {
BB = Worklist.pop_back_val();
// Do a quick scan of the basic block, turning any obviously unreachable
// instructions into LLVM unreachable insts. The instruction combining pass
// canonicalizes unreachable insts into stores to null or undef.
for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
if (CI->doesNotReturn()) {
// If we found a call to a no-return function, insert an unreachable
// instruction after it. Make sure there isn't *already* one there
// though.
++BBI;
if (!isa<UnreachableInst>(BBI)) {
// Don't insert a call to llvm.trap right before the unreachable.
changeToUnreachable(BBI, false);
Changed = true;
}
break;
}
}
// Store to undef and store to null are undefined and used to signal that
// they should be changed to unreachable by passes that can't modify the
// CFG.
if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
// Don't touch volatile stores.
if (SI->isVolatile()) continue;
Value *Ptr = SI->getOperand(1);
if (isa<UndefValue>(Ptr) ||
(isa<ConstantPointerNull>(Ptr) &&
SI->getPointerAddressSpace() == 0)) {
changeToUnreachable(SI, true);
Changed = true;
break;
}
}
}
// Turn invokes that call 'nounwind' functions into ordinary calls.
if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) {
Value *Callee = II->getCalledValue();
if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
changeToUnreachable(II, true);
Changed = true;
} else if (II->doesNotThrow()) {
if (II->use_empty() && II->onlyReadsMemory()) {
// jump to the normal destination branch.
BranchInst::Create(II->getNormalDest(), II);
II->getUnwindDest()->removePredecessor(II->getParent());
II->eraseFromParent();
} else
changeToCall(II);
Changed = true;
}
}
Changed |= ConstantFoldTerminator(BB, true);
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
if (Reachable.insert(*SI))
Worklist.push_back(*SI);
} while (!Worklist.empty());
return Changed;
}
/// removeUnreachableBlocksFromFn - Remove blocks that are not reachable, even
/// if they are in a dead cycle. Return true if a change was made, false
/// otherwise.
bool llvm::removeUnreachableBlocks(Function &F) {
SmallPtrSet<BasicBlock*, 128> Reachable;
bool Changed = markAliveBlocks(F.begin(), Reachable);
// If there are unreachable blocks in the CFG...
if (Reachable.size() == F.size())
return Changed;
assert(Reachable.size() < F.size());
NumRemoved += F.size()-Reachable.size();
// Loop over all of the basic blocks that are not reachable, dropping all of
// their internal references...
for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
if (Reachable.count(BB))
continue;
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
if (Reachable.count(*SI))
(*SI)->removePredecessor(BB);
BB->dropAllReferences();
}
for (Function::iterator I = ++F.begin(); I != F.end();)
if (!Reachable.count(I))
I = F.getBasicBlockList().erase(I);
else
++I;
return true;
}
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