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llvm-6502/lib/Transforms/Scalar/JumpThreading.cpp
Craig Topper 8d7221ccf5 [C++] Use 'nullptr'. Transforms edition. 
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207196 91177308-0d34-0410-b5e6-96231b3b80d8
2014-04-25 05:29:35 +00:00

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//===- JumpThreading.cpp - Thread control through conditional blocks ------===//
//
// 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 Jump Threading pass.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LazyValueInfo.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
using namespace llvm;
#define DEBUG_TYPE "jump-threading"
STATISTIC(NumThreads, "Number of jumps threaded");
STATISTIC(NumFolds, "Number of terminators folded");
STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
static cl::opt<unsigned>
Threshold("jump-threading-threshold",
cl::desc("Max block size to duplicate for jump threading"),
cl::init(6), cl::Hidden);
namespace {
// These are at global scope so static functions can use them too.
typedef SmallVectorImpl<std::pair<Constant*, BasicBlock*> > PredValueInfo;
typedef SmallVector<std::pair<Constant*, BasicBlock*>, 8> PredValueInfoTy;
// This is used to keep track of what kind of constant we're currently hoping
// to find.
enum ConstantPreference {
WantInteger,
WantBlockAddress
};
/// This pass performs 'jump threading', which looks at blocks that have
/// multiple predecessors and multiple successors. If one or more of the
/// predecessors of the block can be proven to always jump to one of the
/// successors, we forward the edge from the predecessor to the successor by
/// duplicating the contents of this block.
///
/// An example of when this can occur is code like this:
///
/// if () { ...
/// X = 4;
/// }
/// if (X < 3) {
///
/// In this case, the unconditional branch at the end of the first if can be
/// revectored to the false side of the second if.
///
class JumpThreading : public FunctionPass {
const DataLayout *DL;
TargetLibraryInfo *TLI;
LazyValueInfo *LVI;
#ifdef NDEBUG
SmallPtrSet<BasicBlock*, 16> LoopHeaders;
#else
SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
#endif
DenseSet<std::pair<Value*, BasicBlock*> > RecursionSet;
// RAII helper for updating the recursion stack.
struct RecursionSetRemover {
DenseSet<std::pair<Value*, BasicBlock*> > &TheSet;
std::pair<Value*, BasicBlock*> ThePair;
RecursionSetRemover(DenseSet<std::pair<Value*, BasicBlock*> > &S,
std::pair<Value*, BasicBlock*> P)
: TheSet(S), ThePair(P) { }
~RecursionSetRemover() {
TheSet.erase(ThePair);
}
};
public:
static char ID; // Pass identification
JumpThreading() : FunctionPass(ID) {
initializeJumpThreadingPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<LazyValueInfo>();
AU.addPreserved<LazyValueInfo>();
AU.addRequired<TargetLibraryInfo>();
}
void FindLoopHeaders(Function &F);
bool ProcessBlock(BasicBlock *BB);
bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
BasicBlock *SuccBB);
bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
const SmallVectorImpl<BasicBlock *> &PredBBs);
bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
PredValueInfo &Result,
ConstantPreference Preference);
bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
ConstantPreference Preference);
bool ProcessBranchOnPHI(PHINode *PN);
bool ProcessBranchOnXOR(BinaryOperator *BO);
bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
};
}
char JumpThreading::ID = 0;
INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading",
"Jump Threading", false, false)
INITIALIZE_PASS_DEPENDENCY(LazyValueInfo)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
INITIALIZE_PASS_END(JumpThreading, "jump-threading",
"Jump Threading", false, false)
// Public interface to the Jump Threading pass
FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
/// runOnFunction - Top level algorithm.
///
bool JumpThreading::runOnFunction(Function &F) {
if (skipOptnoneFunction(F))
return false;
DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
DL = DLP ? &DLP->getDataLayout() : nullptr;
TLI = &getAnalysis<TargetLibraryInfo>();
LVI = &getAnalysis<LazyValueInfo>();
FindLoopHeaders(F);
bool Changed, EverChanged = false;
do {
Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
BasicBlock *BB = I;
// Thread all of the branches we can over this block.
while (ProcessBlock(BB))
Changed = true;
++I;
// If the block is trivially dead, zap it. This eliminates the successor
// edges which simplifies the CFG.
if (pred_begin(BB) == pred_end(BB) &&
BB != &BB->getParent()->getEntryBlock()) {
DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName()
<< "' with terminator: " << *BB->getTerminator() << '\n');
LoopHeaders.erase(BB);
LVI->eraseBlock(BB);
DeleteDeadBlock(BB);
Changed = true;
continue;
}
BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
// Can't thread an unconditional jump, but if the block is "almost
// empty", we can replace uses of it with uses of the successor and make
// this dead.
if (BI && BI->isUnconditional() &&
BB != &BB->getParent()->getEntryBlock() &&
// If the terminator is the only non-phi instruction, try to nuke it.
BB->getFirstNonPHIOrDbg()->isTerminator()) {
// Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
// block, we have to make sure it isn't in the LoopHeaders set. We
// reinsert afterward if needed.
bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
BasicBlock *Succ = BI->getSuccessor(0);
// FIXME: It is always conservatively correct to drop the info
// for a block even if it doesn't get erased. This isn't totally
// awesome, but it allows us to use AssertingVH to prevent nasty
// dangling pointer issues within LazyValueInfo.
LVI->eraseBlock(BB);
if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
Changed = true;
// If we deleted BB and BB was the header of a loop, then the
// successor is now the header of the loop.
BB = Succ;
}
if (ErasedFromLoopHeaders)
LoopHeaders.insert(BB);
}
}
EverChanged |= Changed;
} while (Changed);
LoopHeaders.clear();
return EverChanged;
}
/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
/// thread across it. Stop scanning the block when passing the threshold.
static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB,
unsigned Threshold) {
/// Ignore PHI nodes, these will be flattened when duplication happens.
BasicBlock::const_iterator I = BB->getFirstNonPHI();
// FIXME: THREADING will delete values that are just used to compute the
// branch, so they shouldn't count against the duplication cost.
// Sum up the cost of each instruction until we get to the terminator. Don't
// include the terminator because the copy won't include it.
unsigned Size = 0;
for (; !isa<TerminatorInst>(I); ++I) {
// Stop scanning the block if we've reached the threshold.
if (Size > Threshold)
return Size;
// Debugger intrinsics don't incur code size.
if (isa<DbgInfoIntrinsic>(I)) continue;
// If this is a pointer->pointer bitcast, it is free.
if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
continue;
// All other instructions count for at least one unit.
++Size;
// Calls are more expensive. If they are non-intrinsic calls, we model them
// as having cost of 4. If they are a non-vector intrinsic, we model them
// as having cost of 2 total, and if they are a vector intrinsic, we model
// them as having cost 1.
if (const CallInst *CI = dyn_cast<CallInst>(I)) {
if (CI->cannotDuplicate())
// Blocks with NoDuplicate are modelled as having infinite cost, so they
// are never duplicated.
return ~0U;
else if (!isa<IntrinsicInst>(CI))
Size += 3;
else if (!CI->getType()->isVectorTy())
Size += 1;
}
}
// Threading through a switch statement is particularly profitable. If this
// block ends in a switch, decrease its cost to make it more likely to happen.
if (isa<SwitchInst>(I))
Size = Size > 6 ? Size-6 : 0;
// The same holds for indirect branches, but slightly more so.
if (isa<IndirectBrInst>(I))
Size = Size > 8 ? Size-8 : 0;
return Size;
}
/// FindLoopHeaders - We do not want jump threading to turn proper loop
/// structures into irreducible loops. Doing this breaks up the loop nesting
/// hierarchy and pessimizes later transformations. To prevent this from
/// happening, we first have to find the loop headers. Here we approximate this
/// by finding targets of backedges in the CFG.
///
/// Note that there definitely are cases when we want to allow threading of
/// edges across a loop header. For example, threading a jump from outside the
/// loop (the preheader) to an exit block of the loop is definitely profitable.
/// It is also almost always profitable to thread backedges from within the loop
/// to exit blocks, and is often profitable to thread backedges to other blocks
/// within the loop (forming a nested loop). This simple analysis is not rich
/// enough to track all of these properties and keep it up-to-date as the CFG
/// mutates, so we don't allow any of these transformations.
///
void JumpThreading::FindLoopHeaders(Function &F) {
SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
FindFunctionBackedges(F, Edges);
for (unsigned i = 0, e = Edges.size(); i != e; ++i)
LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
}
/// getKnownConstant - Helper method to determine if we can thread over a
/// terminator with the given value as its condition, and if so what value to
/// use for that. What kind of value this is depends on whether we want an
/// integer or a block address, but an undef is always accepted.
/// Returns null if Val is null or not an appropriate constant.
static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
if (!Val)
return nullptr;
// Undef is "known" enough.
if (UndefValue *U = dyn_cast<UndefValue>(Val))
return U;
if (Preference == WantBlockAddress)
return dyn_cast<BlockAddress>(Val->stripPointerCasts());
return dyn_cast<ConstantInt>(Val);
}
/// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
/// if we can infer that the value is a known ConstantInt/BlockAddress or undef
/// in any of our predecessors. If so, return the known list of value and pred
/// BB in the result vector.
///
/// This returns true if there were any known values.
///
bool JumpThreading::
ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
ConstantPreference Preference) {
// This method walks up use-def chains recursively. Because of this, we could
// get into an infinite loop going around loops in the use-def chain. To
// prevent this, keep track of what (value, block) pairs we've already visited
// and terminate the search if we loop back to them
if (!RecursionSet.insert(std::make_pair(V, BB)).second)
return false;
// An RAII help to remove this pair from the recursion set once the recursion
// stack pops back out again.
RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB));
// If V is a constant, then it is known in all predecessors.
if (Constant *KC = getKnownConstant(V, Preference)) {
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
Result.push_back(std::make_pair(KC, *PI));
return true;
}
// If V is a non-instruction value, or an instruction in a different block,
// then it can't be derived from a PHI.
Instruction *I = dyn_cast<Instruction>(V);
if (!I || I->getParent() != BB) {
// Okay, if this is a live-in value, see if it has a known value at the end
// of any of our predecessors.
//
// FIXME: This should be an edge property, not a block end property.
/// TODO: Per PR2563, we could infer value range information about a
/// predecessor based on its terminator.
//
// FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
// "I" is a non-local compare-with-a-constant instruction. This would be
// able to handle value inequalities better, for example if the compare is
// "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
// Perhaps getConstantOnEdge should be smart enough to do this?
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
BasicBlock *P = *PI;
// If the value is known by LazyValueInfo to be a constant in a
// predecessor, use that information to try to thread this block.
Constant *PredCst = LVI->getConstantOnEdge(V, P, BB);
if (Constant *KC = getKnownConstant(PredCst, Preference))
Result.push_back(std::make_pair(KC, P));
}
return !Result.empty();
}
/// If I is a PHI node, then we know the incoming values for any constants.
if (PHINode *PN = dyn_cast<PHINode>(I)) {
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *InVal = PN->getIncomingValue(i);
if (Constant *KC = getKnownConstant(InVal, Preference)) {
Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
} else {
Constant *CI = LVI->getConstantOnEdge(InVal,
PN->getIncomingBlock(i), BB);
if (Constant *KC = getKnownConstant(CI, Preference))
Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
}
}
return !Result.empty();
}
PredValueInfoTy LHSVals, RHSVals;
// Handle some boolean conditions.
if (I->getType()->getPrimitiveSizeInBits() == 1) {
assert(Preference == WantInteger && "One-bit non-integer type?");
// X | true -> true
// X & false -> false
if (I->getOpcode() == Instruction::Or ||
I->getOpcode() == Instruction::And) {
ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
WantInteger);
ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals,
WantInteger);
if (LHSVals.empty() && RHSVals.empty())
return false;
ConstantInt *InterestingVal;
if (I->getOpcode() == Instruction::Or)
InterestingVal = ConstantInt::getTrue(I->getContext());
else
InterestingVal = ConstantInt::getFalse(I->getContext());
SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
// Scan for the sentinel. If we find an undef, force it to the
// interesting value: x|undef -> true and x&undef -> false.
for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
if (LHSVals[i].first == InterestingVal ||
isa<UndefValue>(LHSVals[i].first)) {
Result.push_back(LHSVals[i]);
Result.back().first = InterestingVal;
LHSKnownBBs.insert(LHSVals[i].second);
}
for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
if (RHSVals[i].first == InterestingVal ||
isa<UndefValue>(RHSVals[i].first)) {
// If we already inferred a value for this block on the LHS, don't
// re-add it.
if (!LHSKnownBBs.count(RHSVals[i].second)) {
Result.push_back(RHSVals[i]);
Result.back().first = InterestingVal;
}
}
return !Result.empty();
}
// Handle the NOT form of XOR.
if (I->getOpcode() == Instruction::Xor &&
isa<ConstantInt>(I->getOperand(1)) &&
cast<ConstantInt>(I->getOperand(1))->isOne()) {
ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result,
WantInteger);
if (Result.empty())
return false;
// Invert the known values.
for (unsigned i = 0, e = Result.size(); i != e; ++i)
Result[i].first = ConstantExpr::getNot(Result[i].first);
return true;
}
// Try to simplify some other binary operator values.
} else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
assert(Preference != WantBlockAddress
&& "A binary operator creating a block address?");
if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
PredValueInfoTy LHSVals;
ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals,
WantInteger);
// Try to use constant folding to simplify the binary operator.
for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
Constant *V = LHSVals[i].first;
Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI);
if (Constant *KC = getKnownConstant(Folded, WantInteger))
Result.push_back(std::make_pair(KC, LHSVals[i].second));
}
}
return !Result.empty();
}
// Handle compare with phi operand, where the PHI is defined in this block.
if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
assert(Preference == WantInteger && "Compares only produce integers");
PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
if (PN && PN->getParent() == BB) {
// We can do this simplification if any comparisons fold to true or false.
// See if any do.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *PredBB = PN->getIncomingBlock(i);
Value *LHS = PN->getIncomingValue(i);
Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, DL);
if (!Res) {
if (!isa<Constant>(RHS))
continue;
LazyValueInfo::Tristate
ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
cast<Constant>(RHS), PredBB, BB);
if (ResT == LazyValueInfo::Unknown)
continue;
Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
}
if (Constant *KC = getKnownConstant(Res, WantInteger))
Result.push_back(std::make_pair(KC, PredBB));
}
return !Result.empty();
}
// If comparing a live-in value against a constant, see if we know the
// live-in value on any predecessors.
if (isa<Constant>(Cmp->getOperand(1)) && Cmp->getType()->isIntegerTy()) {
if (!isa<Instruction>(Cmp->getOperand(0)) ||
cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) {
Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB);PI != E; ++PI){
BasicBlock *P = *PI;
// If the value is known by LazyValueInfo to be a constant in a
// predecessor, use that information to try to thread this block.
LazyValueInfo::Tristate Res =
LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
RHSCst, P, BB);
if (Res == LazyValueInfo::Unknown)
continue;
Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
Result.push_back(std::make_pair(ResC, P));
}
return !Result.empty();
}
// Try to find a constant value for the LHS of a comparison,
// and evaluate it statically if we can.
if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
PredValueInfoTy LHSVals;
ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
WantInteger);
for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
Constant *V = LHSVals[i].first;
Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(),
V, CmpConst);
if (Constant *KC = getKnownConstant(Folded, WantInteger))
Result.push_back(std::make_pair(KC, LHSVals[i].second));
}
return !Result.empty();
}
}
}
if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
// Handle select instructions where at least one operand is a known constant
// and we can figure out the condition value for any predecessor block.
Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
PredValueInfoTy Conds;
if ((TrueVal || FalseVal) &&
ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
WantInteger)) {
for (unsigned i = 0, e = Conds.size(); i != e; ++i) {
Constant *Cond = Conds[i].first;
// Figure out what value to use for the condition.
bool KnownCond;
if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
// A known boolean.
KnownCond = CI->isOne();
} else {
assert(isa<UndefValue>(Cond) && "Unexpected condition value");
// Either operand will do, so be sure to pick the one that's a known
// constant.
// FIXME: Do this more cleverly if both values are known constants?
KnownCond = (TrueVal != nullptr);
}
// See if the select has a known constant value for this predecessor.
if (Constant *Val = KnownCond ? TrueVal : FalseVal)
Result.push_back(std::make_pair(Val, Conds[i].second));
}
return !Result.empty();
}
}
// If all else fails, see if LVI can figure out a constant value for us.
Constant *CI = LVI->getConstant(V, BB);
if (Constant *KC = getKnownConstant(CI, Preference)) {
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
Result.push_back(std::make_pair(KC, *PI));
}
return !Result.empty();
}
/// GetBestDestForBranchOnUndef - If we determine that the specified block ends
/// in an undefined jump, decide which block is best to revector to.
///
/// Since we can pick an arbitrary destination, we pick the successor with the
/// fewest predecessors. This should reduce the in-degree of the others.
///
static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
TerminatorInst *BBTerm = BB->getTerminator();
unsigned MinSucc = 0;
BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
// Compute the successor with the minimum number of predecessors.
unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
TestBB = BBTerm->getSuccessor(i);
unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
if (NumPreds < MinNumPreds) {
MinSucc = i;
MinNumPreds = NumPreds;
}
}
return MinSucc;
}
static bool hasAddressTakenAndUsed(BasicBlock *BB) {
if (!BB->hasAddressTaken()) return false;
// If the block has its address taken, it may be a tree of dead constants
// hanging off of it. These shouldn't keep the block alive.
BlockAddress *BA = BlockAddress::get(BB);
BA->removeDeadConstantUsers();
return !BA->use_empty();
}
/// ProcessBlock - If there are any predecessors whose control can be threaded
/// through to a successor, transform them now.
bool JumpThreading::ProcessBlock(BasicBlock *BB) {
// If the block is trivially dead, just return and let the caller nuke it.
// This simplifies other transformations.
if (pred_begin(BB) == pred_end(BB) &&
BB != &BB->getParent()->getEntryBlock())
return false;
// If this block has a single predecessor, and if that pred has a single
// successor, merge the blocks. This encourages recursive jump threading
// because now the condition in this block can be threaded through
// predecessors of our predecessor block.
if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
SinglePred != BB && !hasAddressTakenAndUsed(BB)) {
// If SinglePred was a loop header, BB becomes one.
if (LoopHeaders.erase(SinglePred))
LoopHeaders.insert(BB);
// Remember if SinglePred was the entry block of the function. If so, we
// will need to move BB back to the entry position.
bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
LVI->eraseBlock(SinglePred);
MergeBasicBlockIntoOnlyPred(BB);
if (isEntry && BB != &BB->getParent()->getEntryBlock())
BB->moveBefore(&BB->getParent()->getEntryBlock());
return true;
}
}
// What kind of constant we're looking for.
ConstantPreference Preference = WantInteger;
// Look to see if the terminator is a conditional branch, switch or indirect
// branch, if not we can't thread it.
Value *Condition;
Instruction *Terminator = BB->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) {
// Can't thread an unconditional jump.
if (BI->isUnconditional()) return false;
Condition = BI->getCondition();
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
Condition = SI->getCondition();
} else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
// Can't thread indirect branch with no successors.
if (IB->getNumSuccessors() == 0) return false;
Condition = IB->getAddress()->stripPointerCasts();
Preference = WantBlockAddress;
} else {
return false; // Must be an invoke.
}
// Run constant folding to see if we can reduce the condition to a simple
// constant.
if (Instruction *I = dyn_cast<Instruction>(Condition)) {
Value *SimpleVal = ConstantFoldInstruction(I, DL, TLI);
if (SimpleVal) {
I->replaceAllUsesWith(SimpleVal);
I->eraseFromParent();
Condition = SimpleVal;
}
}
// If the terminator is branching on an undef, we can pick any of the
// successors to branch to. Let GetBestDestForJumpOnUndef decide.
if (isa<UndefValue>(Condition)) {
unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
// Fold the branch/switch.
TerminatorInst *BBTerm = BB->getTerminator();
for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
if (i == BestSucc) continue;
BBTerm->getSuccessor(i)->removePredecessor(BB, true);
}
DEBUG(dbgs() << " In block '" << BB->getName()
<< "' folding undef terminator: " << *BBTerm << '\n');
BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
BBTerm->eraseFromParent();
return true;
}
// If the terminator of this block is branching on a constant, simplify the
// terminator to an unconditional branch. This can occur due to threading in
// other blocks.
if (getKnownConstant(Condition, Preference)) {
DEBUG(dbgs() << " In block '" << BB->getName()
<< "' folding terminator: " << *BB->getTerminator() << '\n');
++NumFolds;
ConstantFoldTerminator(BB, true);
return true;
}
Instruction *CondInst = dyn_cast<Instruction>(Condition);
// All the rest of our checks depend on the condition being an instruction.
if (!CondInst) {
// FIXME: Unify this with code below.
if (ProcessThreadableEdges(Condition, BB, Preference))
return true;
return false;
}
if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
// For a comparison where the LHS is outside this block, it's possible
// that we've branched on it before. Used LVI to see if we can simplify
// the branch based on that.
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
if (CondBr && CondConst && CondBr->isConditional() && PI != PE &&
(!isa<Instruction>(CondCmp->getOperand(0)) ||
cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) {
// For predecessor edge, determine if the comparison is true or false
// on that edge. If they're all true or all false, we can simplify the
// branch.
// FIXME: We could handle mixed true/false by duplicating code.
LazyValueInfo::Tristate Baseline =
LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0),
CondConst, *PI, BB);
if (Baseline != LazyValueInfo::Unknown) {
// Check that all remaining incoming values match the first one.
while (++PI != PE) {
LazyValueInfo::Tristate Ret =
LVI->getPredicateOnEdge(CondCmp->getPredicate(),
CondCmp->getOperand(0), CondConst, *PI, BB);
if (Ret != Baseline) break;
}
// If we terminated early, then one of the values didn't match.
if (PI == PE) {
unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0;
unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1;
CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
CondBr->eraseFromParent();
return true;
}
}
}
if (CondBr && CondConst && TryToUnfoldSelect(CondCmp, BB))
return true;
}
// Check for some cases that are worth simplifying. Right now we want to look
// for loads that are used by a switch or by the condition for the branch. If
// we see one, check to see if it's partially redundant. If so, insert a PHI
// which can then be used to thread the values.
//
Value *SimplifyValue = CondInst;
if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
if (isa<Constant>(CondCmp->getOperand(1)))
SimplifyValue = CondCmp->getOperand(0);
// TODO: There are other places where load PRE would be profitable, such as
// more complex comparisons.
if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
if (SimplifyPartiallyRedundantLoad(LI))
return true;
// Handle a variety of cases where we are branching on something derived from
// a PHI node in the current block. If we can prove that any predecessors
// compute a predictable value based on a PHI node, thread those predecessors.
//
if (ProcessThreadableEdges(CondInst, BB, Preference))
return true;
// If this is an otherwise-unfoldable branch on a phi node in the current
// block, see if we can simplify.
if (PHINode *PN = dyn_cast<PHINode>(CondInst))
if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
return ProcessBranchOnPHI(PN);
// If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
if (CondInst->getOpcode() == Instruction::Xor &&
CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
// TODO: If we have: "br (X > 0)" and we have a predecessor where we know
// "(X == 4)", thread through this block.
return false;
}
/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
/// load instruction, eliminate it by replacing it with a PHI node. This is an
/// important optimization that encourages jump threading, and needs to be run
/// interlaced with other jump threading tasks.
bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
// Don't hack volatile/atomic loads.
if (!LI->isSimple()) return false;
// If the load is defined in a block with exactly one predecessor, it can't be
// partially redundant.
BasicBlock *LoadBB = LI->getParent();
if (LoadBB->getSinglePredecessor())
return false;
// If the load is defined in a landing pad, it can't be partially redundant,
// because the edges between the invoke and the landing pad cannot have other
// instructions between them.
if (LoadBB->isLandingPad())
return false;
Value *LoadedPtr = LI->getOperand(0);
// If the loaded operand is defined in the LoadBB, it can't be available.
// TODO: Could do simple PHI translation, that would be fun :)
if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
if (PtrOp->getParent() == LoadBB)
return false;
// Scan a few instructions up from the load, to see if it is obviously live at
// the entry to its block.
BasicBlock::iterator BBIt = LI;
if (Value *AvailableVal =
FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
// If the value if the load is locally available within the block, just use
// it. This frequently occurs for reg2mem'd allocas.
//cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
// If the returned value is the load itself, replace with an undef. This can
// only happen in dead loops.
if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
LI->replaceAllUsesWith(AvailableVal);
LI->eraseFromParent();
return true;
}
// Otherwise, if we scanned the whole block and got to the top of the block,
// we know the block is locally transparent to the load. If not, something
// might clobber its value.
if (BBIt != LoadBB->begin())
return false;
// If all of the loads and stores that feed the value have the same TBAA tag,
// then we can propagate it onto any newly inserted loads.
MDNode *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa);
SmallPtrSet<BasicBlock*, 8> PredsScanned;
typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
AvailablePredsTy AvailablePreds;
BasicBlock *OneUnavailablePred = nullptr;
// If we got here, the loaded value is transparent through to the start of the
// block. Check to see if it is available in any of the predecessor blocks.
for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
PI != PE; ++PI) {
BasicBlock *PredBB = *PI;
// If we already scanned this predecessor, skip it.
if (!PredsScanned.insert(PredBB))
continue;
// Scan the predecessor to see if the value is available in the pred.
BBIt = PredBB->end();
MDNode *ThisTBAATag = nullptr;
Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6,
nullptr, &ThisTBAATag);
if (!PredAvailable) {
OneUnavailablePred = PredBB;
continue;
}
// If tbaa tags disagree or are not present, forget about them.
if (TBAATag != ThisTBAATag) TBAATag = nullptr;
// If so, this load is partially redundant. Remember this info so that we
// can create a PHI node.
AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
}
// If the loaded value isn't available in any predecessor, it isn't partially
// redundant.
if (AvailablePreds.empty()) return false;
// Okay, the loaded value is available in at least one (and maybe all!)
// predecessors. If the value is unavailable in more than one unique
// predecessor, we want to insert a merge block for those common predecessors.
// This ensures that we only have to insert one reload, thus not increasing
// code size.
BasicBlock *UnavailablePred = nullptr;
// If there is exactly one predecessor where the value is unavailable, the
// already computed 'OneUnavailablePred' block is it. If it ends in an
// unconditional branch, we know that it isn't a critical edge.
if (PredsScanned.size() == AvailablePreds.size()+1 &&
OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
UnavailablePred = OneUnavailablePred;
} else if (PredsScanned.size() != AvailablePreds.size()) {
// Otherwise, we had multiple unavailable predecessors or we had a critical
// edge from the one.
SmallVector<BasicBlock*, 8> PredsToSplit;
SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
AvailablePredSet.insert(AvailablePreds[i].first);
// Add all the unavailable predecessors to the PredsToSplit list.
for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
PI != PE; ++PI) {
BasicBlock *P = *PI;
// If the predecessor is an indirect goto, we can't split the edge.
if (isa<IndirectBrInst>(P->getTerminator()))
return false;
if (!AvailablePredSet.count(P))
PredsToSplit.push_back(P);
}
// Split them out to their own block.
UnavailablePred =
SplitBlockPredecessors(LoadBB, PredsToSplit, "thread-pre-split", this);
}
// If the value isn't available in all predecessors, then there will be
// exactly one where it isn't available. Insert a load on that edge and add
// it to the AvailablePreds list.
if (UnavailablePred) {
assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
"Can't handle critical edge here!");
LoadInst *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
LI->getAlignment(),
UnavailablePred->getTerminator());
NewVal->setDebugLoc(LI->getDebugLoc());
if (TBAATag)
NewVal->setMetadata(LLVMContext::MD_tbaa, TBAATag);
AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
}
// Now we know that each predecessor of this block has a value in
// AvailablePreds, sort them for efficient access as we're walking the preds.
array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
// Create a PHI node at the start of the block for the PRE'd load value.
pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB);
PHINode *PN = PHINode::Create(LI->getType(), std::distance(PB, PE), "",
LoadBB->begin());
PN->takeName(LI);
PN->setDebugLoc(LI->getDebugLoc());
// Insert new entries into the PHI for each predecessor. A single block may
// have multiple entries here.
for (pred_iterator PI = PB; PI != PE; ++PI) {
BasicBlock *P = *PI;
AvailablePredsTy::iterator I =
std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
std::make_pair(P, (Value*)nullptr));
assert(I != AvailablePreds.end() && I->first == P &&
"Didn't find entry for predecessor!");
PN->addIncoming(I->second, I->first);
}
//cerr << "PRE: " << *LI << *PN << "\n";
LI->replaceAllUsesWith(PN);
LI->eraseFromParent();
return true;
}
/// FindMostPopularDest - The specified list contains multiple possible
/// threadable destinations. Pick the one that occurs the most frequently in
/// the list.
static BasicBlock *
FindMostPopularDest(BasicBlock *BB,
const SmallVectorImpl<std::pair<BasicBlock*,
BasicBlock*> > &PredToDestList) {
assert(!PredToDestList.empty());
// Determine popularity. If there are multiple possible destinations, we
// explicitly choose to ignore 'undef' destinations. We prefer to thread
// blocks with known and real destinations to threading undef. We'll handle
// them later if interesting.
DenseMap<BasicBlock*, unsigned> DestPopularity;
for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
if (PredToDestList[i].second)
DestPopularity[PredToDestList[i].second]++;
// Find the most popular dest.
DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
BasicBlock *MostPopularDest = DPI->first;
unsigned Popularity = DPI->second;
SmallVector<BasicBlock*, 4> SamePopularity;
for (++DPI; DPI != DestPopularity.end(); ++DPI) {
// If the popularity of this entry isn't higher than the popularity we've
// seen so far, ignore it.
if (DPI->second < Popularity)
; // ignore.
else if (DPI->second == Popularity) {
// If it is the same as what we've seen so far, keep track of it.
SamePopularity.push_back(DPI->first);
} else {
// If it is more popular, remember it.
SamePopularity.clear();
MostPopularDest = DPI->first;
Popularity = DPI->second;
}
}
// Okay, now we know the most popular destination. If there is more than one
// destination, we need to determine one. This is arbitrary, but we need
// to make a deterministic decision. Pick the first one that appears in the
// successor list.
if (!SamePopularity.empty()) {
SamePopularity.push_back(MostPopularDest);
TerminatorInst *TI = BB->getTerminator();
for (unsigned i = 0; ; ++i) {
assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
if (std::find(SamePopularity.begin(), SamePopularity.end(),
TI->getSuccessor(i)) == SamePopularity.end())
continue;
MostPopularDest = TI->getSuccessor(i);
break;
}
}
// Okay, we have finally picked the most popular destination.
return MostPopularDest;
}
bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
ConstantPreference Preference) {
// If threading this would thread across a loop header, don't even try to
// thread the edge.
if (LoopHeaders.count(BB))
return false;
PredValueInfoTy PredValues;
if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference))
return false;
assert(!PredValues.empty() &&
"ComputeValueKnownInPredecessors returned true with no values");
DEBUG(dbgs() << "IN BB: " << *BB;
for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
dbgs() << " BB '" << BB->getName() << "': FOUND condition = "
<< *PredValues[i].first
<< " for pred '" << PredValues[i].second->getName() << "'.\n";
});
// Decide what we want to thread through. Convert our list of known values to
// a list of known destinations for each pred. This also discards duplicate
// predecessors and keeps track of the undefined inputs (which are represented
// as a null dest in the PredToDestList).
SmallPtrSet<BasicBlock*, 16> SeenPreds;
SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
BasicBlock *OnlyDest = nullptr;
BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
BasicBlock *Pred = PredValues[i].second;
if (!SeenPreds.insert(Pred))
continue; // Duplicate predecessor entry.
// If the predecessor ends with an indirect goto, we can't change its
// destination.
if (isa<IndirectBrInst>(Pred->getTerminator()))
continue;
Constant *Val = PredValues[i].first;
BasicBlock *DestBB;
if (isa<UndefValue>(Val))
DestBB = nullptr;
else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
DestBB = SI->findCaseValue(cast<ConstantInt>(Val)).getCaseSuccessor();
} else {
assert(isa<IndirectBrInst>(BB->getTerminator())
&& "Unexpected terminator");
DestBB = cast<BlockAddress>(Val)->getBasicBlock();
}
// If we have exactly one destination, remember it for efficiency below.
if (PredToDestList.empty())
OnlyDest = DestBB;
else if (OnlyDest != DestBB)
OnlyDest = MultipleDestSentinel;
PredToDestList.push_back(std::make_pair(Pred, DestBB));
}
// If all edges were unthreadable, we fail.
if (PredToDestList.empty())
return false;
// Determine which is the most common successor. If we have many inputs and
// this block is a switch, we want to start by threading the batch that goes
// to the most popular destination first. If we only know about one
// threadable destination (the common case) we can avoid this.
BasicBlock *MostPopularDest = OnlyDest;
if (MostPopularDest == MultipleDestSentinel)
MostPopularDest = FindMostPopularDest(BB, PredToDestList);
// Now that we know what the most popular destination is, factor all
// predecessors that will jump to it into a single predecessor.
SmallVector<BasicBlock*, 16> PredsToFactor;
for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
if (PredToDestList[i].second == MostPopularDest) {
BasicBlock *Pred = PredToDestList[i].first;
// This predecessor may be a switch or something else that has multiple
// edges to the block. Factor each of these edges by listing them
// according to # occurrences in PredsToFactor.
TerminatorInst *PredTI = Pred->getTerminator();
for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
if (PredTI->getSuccessor(i) == BB)
PredsToFactor.push_back(Pred);
}
// If the threadable edges are branching on an undefined value, we get to pick
// the destination that these predecessors should get to.
if (!MostPopularDest)
MostPopularDest = BB->getTerminator()->
getSuccessor(GetBestDestForJumpOnUndef(BB));
// Ok, try to thread it!
return ThreadEdge(BB, PredsToFactor, MostPopularDest);
}
/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
/// a PHI node in the current block. See if there are any simplifications we
/// can do based on inputs to the phi node.
///
bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
BasicBlock *BB = PN->getParent();
// TODO: We could make use of this to do it once for blocks with common PHI
// values.
SmallVector<BasicBlock*, 1> PredBBs;
PredBBs.resize(1);
// If any of the predecessor blocks end in an unconditional branch, we can
// *duplicate* the conditional branch into that block in order to further
// encourage jump threading and to eliminate cases where we have branch on a
// phi of an icmp (branch on icmp is much better).
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *PredBB = PN->getIncomingBlock(i);
if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
if (PredBr->isUnconditional()) {
PredBBs[0] = PredBB;
// Try to duplicate BB into PredBB.
if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
return true;
}
}
return false;
}
/// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
/// a xor instruction in the current block. See if there are any
/// simplifications we can do based on inputs to the xor.
///
bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
BasicBlock *BB = BO->getParent();
// If either the LHS or RHS of the xor is a constant, don't do this
// optimization.
if (isa<ConstantInt>(BO->getOperand(0)) ||
isa<ConstantInt>(BO->getOperand(1)))
return false;
// If the first instruction in BB isn't a phi, we won't be able to infer
// anything special about any particular predecessor.
if (!isa<PHINode>(BB->front()))
return false;
// If we have a xor as the branch input to this block, and we know that the
// LHS or RHS of the xor in any predecessor is true/false, then we can clone
// the condition into the predecessor and fix that value to true, saving some
// logical ops on that path and encouraging other paths to simplify.
//
// This copies something like this:
//
// BB:
// %X = phi i1 [1], [%X']
// %Y = icmp eq i32 %A, %B
// %Z = xor i1 %X, %Y
// br i1 %Z, ...
//
// Into:
// BB':
// %Y = icmp ne i32 %A, %B
// br i1 %Z, ...
PredValueInfoTy XorOpValues;
bool isLHS = true;
if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
WantInteger)) {
assert(XorOpValues.empty());
if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
WantInteger))
return false;
isLHS = false;
}
assert(!XorOpValues.empty() &&
"ComputeValueKnownInPredecessors returned true with no values");
// Scan the information to see which is most popular: true or false. The
// predecessors can be of the set true, false, or undef.
unsigned NumTrue = 0, NumFalse = 0;
for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
if (isa<UndefValue>(XorOpValues[i].first))
// Ignore undefs for the count.
continue;
if (cast<ConstantInt>(XorOpValues[i].first)->isZero())
++NumFalse;
else
++NumTrue;
}
// Determine which value to split on, true, false, or undef if neither.
ConstantInt *SplitVal = nullptr;
if (NumTrue > NumFalse)
SplitVal = ConstantInt::getTrue(BB->getContext());
else if (NumTrue != 0 || NumFalse != 0)
SplitVal = ConstantInt::getFalse(BB->getContext());
// Collect all of the blocks that this can be folded into so that we can
// factor this once and clone it once.
SmallVector<BasicBlock*, 8> BlocksToFoldInto;
for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
if (XorOpValues[i].first != SplitVal &&
!isa<UndefValue>(XorOpValues[i].first))
continue;
BlocksToFoldInto.push_back(XorOpValues[i].second);
}
// If we inferred a value for all of the predecessors, then duplication won't
// help us. However, we can just replace the LHS or RHS with the constant.
if (BlocksToFoldInto.size() ==
cast<PHINode>(BB->front()).getNumIncomingValues()) {
if (!SplitVal) {
// If all preds provide undef, just nuke the xor, because it is undef too.
BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
BO->eraseFromParent();
} else if (SplitVal->isZero()) {
// If all preds provide 0, replace the xor with the other input.
BO->replaceAllUsesWith(BO->getOperand(isLHS));
BO->eraseFromParent();
} else {
// If all preds provide 1, set the computed value to 1.
BO->setOperand(!isLHS, SplitVal);
}
return true;
}
// Try to duplicate BB into PredBB.
return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
}
/// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
/// predecessor to the PHIBB block. If it has PHI nodes, add entries for
/// NewPred using the entries from OldPred (suitably mapped).
static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
BasicBlock *OldPred,
BasicBlock *NewPred,
DenseMap<Instruction*, Value*> &ValueMap) {
for (BasicBlock::iterator PNI = PHIBB->begin();
PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
// Ok, we have a PHI node. Figure out what the incoming value was for the
// DestBlock.
Value *IV = PN->getIncomingValueForBlock(OldPred);
// Remap the value if necessary.
if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
if (I != ValueMap.end())
IV = I->second;
}
PN->addIncoming(IV, NewPred);
}
}
/// ThreadEdge - We have decided that it is safe and profitable to factor the
/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
/// across BB. Transform the IR to reflect this change.
bool JumpThreading::ThreadEdge(BasicBlock *BB,
const SmallVectorImpl<BasicBlock*> &PredBBs,
BasicBlock *SuccBB) {
// If threading to the same block as we come from, we would infinite loop.
if (SuccBB == BB) {
DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
<< "' - would thread to self!\n");
return false;
}
// If threading this would thread across a loop header, don't thread the edge.
// See the comments above FindLoopHeaders for justifications and caveats.
if (LoopHeaders.count(BB)) {
DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName()
<< "' to dest BB '" << SuccBB->getName()
<< "' - it might create an irreducible loop!\n");
return false;
}
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB, Threshold);
if (JumpThreadCost > Threshold) {
DEBUG(dbgs() << " Not threading BB '" << BB->getName()
<< "' - Cost is too high: " << JumpThreadCost << "\n");
return false;
}
// And finally, do it! Start by factoring the predecessors is needed.
BasicBlock *PredBB;
if (PredBBs.size() == 1)
PredBB = PredBBs[0];
else {
DEBUG(dbgs() << " Factoring out " << PredBBs.size()
<< " common predecessors.\n");
PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm", this);
}
// And finally, do it!
DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '"
<< SuccBB->getName() << "' with cost: " << JumpThreadCost
<< ", across block:\n "
<< *BB << "\n");
LVI->threadEdge(PredBB, BB, SuccBB);
// We are going to have to map operands from the original BB block to the new
// copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
// account for entry from PredBB.
DenseMap<Instruction*, Value*> ValueMapping;
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
BB->getName()+".thread",
BB->getParent(), BB);
NewBB->moveAfter(PredBB);
BasicBlock::iterator BI = BB->begin();
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
// Clone the non-phi instructions of BB into NewBB, keeping track of the
// mapping and using it to remap operands in the cloned instructions.
for (; !isa<TerminatorInst>(BI); ++BI) {
Instruction *New = BI->clone();
New->setName(BI->getName());
NewBB->getInstList().push_back(New);
ValueMapping[BI] = New;
// Remap operands to patch up intra-block references.
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
if (I != ValueMapping.end())
New->setOperand(i, I->second);
}
}
// We didn't copy the terminator from BB over to NewBB, because there is now
// an unconditional jump to SuccBB. Insert the unconditional jump.
BranchInst *NewBI =BranchInst::Create(SuccBB, NewBB);
NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
// Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
// PHI nodes for NewBB now.
AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
// If there were values defined in BB that are used outside the block, then we
// now have to update all uses of the value to use either the original value,
// the cloned value, or some PHI derived value. This can require arbitrary
// PHI insertion, of which we are prepared to do, clean these up now.
SSAUpdater SSAUpdate;
SmallVector<Use*, 16> UsesToRename;
for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
// Scan all uses of this instruction to see if it is used outside of its
// block, and if so, record them in UsesToRename.
for (Use &U : I->uses()) {
Instruction *User = cast<Instruction>(U.getUser());
if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
if (UserPN->getIncomingBlock(U) == BB)
continue;
} else if (User->getParent() == BB)
continue;
UsesToRename.push_back(&U);
}
// If there are no uses outside the block, we're done with this instruction.
if (UsesToRename.empty())
continue;
DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
// We found a use of I outside of BB. Rename all uses of I that are outside
// its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
// with the two values we know.
SSAUpdate.Initialize(I->getType(), I->getName());
SSAUpdate.AddAvailableValue(BB, I);
SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
while (!UsesToRename.empty())
SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
DEBUG(dbgs() << "\n");
}
// Ok, NewBB is good to go. Update the terminator of PredBB to jump to
// NewBB instead of BB. This eliminates predecessors from BB, which requires
// us to simplify any PHI nodes in BB.
TerminatorInst *PredTerm = PredBB->getTerminator();
for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
if (PredTerm->getSuccessor(i) == BB) {
BB->removePredecessor(PredBB, true);
PredTerm->setSuccessor(i, NewBB);
}
// At this point, the IR is fully up to date and consistent. Do a quick scan
// over the new instructions and zap any that are constants or dead. This
// frequently happens because of phi translation.
SimplifyInstructionsInBlock(NewBB, DL, TLI);
// Threaded an edge!
++NumThreads;
return true;
}
/// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
/// to BB which contains an i1 PHI node and a conditional branch on that PHI.
/// If we can duplicate the contents of BB up into PredBB do so now, this
/// improves the odds that the branch will be on an analyzable instruction like
/// a compare.
bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
const SmallVectorImpl<BasicBlock *> &PredBBs) {
assert(!PredBBs.empty() && "Can't handle an empty set");
// If BB is a loop header, then duplicating this block outside the loop would
// cause us to transform this into an irreducible loop, don't do this.
// See the comments above FindLoopHeaders for justifications and caveats.
if (LoopHeaders.count(BB)) {
DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
<< "' into predecessor block '" << PredBBs[0]->getName()
<< "' - it might create an irreducible loop!\n");
return false;
}
unsigned DuplicationCost = getJumpThreadDuplicationCost(BB, Threshold);
if (DuplicationCost > Threshold) {
DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
<< "' - Cost is too high: " << DuplicationCost << "\n");
return false;
}
// And finally, do it! Start by factoring the predecessors is needed.
BasicBlock *PredBB;
if (PredBBs.size() == 1)
PredBB = PredBBs[0];
else {
DEBUG(dbgs() << " Factoring out " << PredBBs.size()
<< " common predecessors.\n");
PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm", this);
}
// Okay, we decided to do this! Clone all the instructions in BB onto the end
// of PredBB.
DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '"
<< PredBB->getName() << "' to eliminate branch on phi. Cost: "
<< DuplicationCost << " block is:" << *BB << "\n");
// Unless PredBB ends with an unconditional branch, split the edge so that we
// can just clone the bits from BB into the end of the new PredBB.
BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
if (!OldPredBranch || !OldPredBranch->isUnconditional()) {
PredBB = SplitEdge(PredBB, BB, this);
OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
}
// We are going to have to map operands from the original BB block into the
// PredBB block. Evaluate PHI nodes in BB.
DenseMap<Instruction*, Value*> ValueMapping;
BasicBlock::iterator BI = BB->begin();
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
// Clone the non-phi instructions of BB into PredBB, keeping track of the
// mapping and using it to remap operands in the cloned instructions.
for (; BI != BB->end(); ++BI) {
Instruction *New = BI->clone();
// Remap operands to patch up intra-block references.
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
if (I != ValueMapping.end())
New->setOperand(i, I->second);
}
// If this instruction can be simplified after the operands are updated,
// just use the simplified value instead. This frequently happens due to
// phi translation.
if (Value *IV = SimplifyInstruction(New, DL)) {
delete New;
ValueMapping[BI] = IV;
} else {
// Otherwise, insert the new instruction into the block.
New->setName(BI->getName());
PredBB->getInstList().insert(OldPredBranch, New);
ValueMapping[BI] = New;
}
}
// Check to see if the targets of the branch had PHI nodes. If so, we need to
// add entries to the PHI nodes for branch from PredBB now.
BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
ValueMapping);
AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
ValueMapping);
// If there were values defined in BB that are used outside the block, then we
// now have to update all uses of the value to use either the original value,
// the cloned value, or some PHI derived value. This can require arbitrary
// PHI insertion, of which we are prepared to do, clean these up now.
SSAUpdater SSAUpdate;
SmallVector<Use*, 16> UsesToRename;
for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
// Scan all uses of this instruction to see if it is used outside of its
// block, and if so, record them in UsesToRename.
for (Use &U : I->uses()) {
Instruction *User = cast<Instruction>(U.getUser());
if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
if (UserPN->getIncomingBlock(U) == BB)
continue;
} else if (User->getParent() == BB)
continue;
UsesToRename.push_back(&U);
}
// If there are no uses outside the block, we're done with this instruction.
if (UsesToRename.empty())
continue;
DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
// We found a use of I outside of BB. Rename all uses of I that are outside
// its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
// with the two values we know.
SSAUpdate.Initialize(I->getType(), I->getName());
SSAUpdate.AddAvailableValue(BB, I);
SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
while (!UsesToRename.empty())
SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
DEBUG(dbgs() << "\n");
}
// PredBB no longer jumps to BB, remove entries in the PHI node for the edge
// that we nuked.
BB->removePredecessor(PredBB, true);
// Remove the unconditional branch at the end of the PredBB block.
OldPredBranch->eraseFromParent();
++NumDupes;
return true;
}
/// TryToUnfoldSelect - Look for blocks of the form
/// bb1:
/// %a = select
/// br bb
///
/// bb2:
/// %p = phi [%a, %bb] ...
/// %c = icmp %p
/// br i1 %c
///
/// And expand the select into a branch structure if one of its arms allows %c
/// to be folded. This later enables threading from bb1 over bb2.
bool JumpThreading::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0));
Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1));
if (!CondBr || !CondBr->isConditional() || !CondLHS ||
CondLHS->getParent() != BB)
return false;
for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) {
BasicBlock *Pred = CondLHS->getIncomingBlock(I);
SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I));
// Look if one of the incoming values is a select in the corresponding
// predecessor.
if (!SI || SI->getParent() != Pred || !SI->hasOneUse())
continue;
BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
if (!PredTerm || !PredTerm->isUnconditional())
continue;
// Now check if one of the select values would allow us to constant fold the
// terminator in BB. We don't do the transform if both sides fold, those
// cases will be threaded in any case.
LazyValueInfo::Tristate LHSFolds =
LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1),
CondRHS, Pred, BB);
LazyValueInfo::Tristate RHSFolds =
LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2),
CondRHS, Pred, BB);
if ((LHSFolds != LazyValueInfo::Unknown ||
RHSFolds != LazyValueInfo::Unknown) &&
LHSFolds != RHSFolds) {
// Expand the select.
//
// Pred --
// | v
// | NewBB
// | |
// |-----
// v
// BB
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold",
BB->getParent(), BB);
// Move the unconditional branch to NewBB.
PredTerm->removeFromParent();
NewBB->getInstList().insert(NewBB->end(), PredTerm);
// Create a conditional branch and update PHI nodes.
BranchInst::Create(NewBB, BB, SI->getCondition(), Pred);
CondLHS->setIncomingValue(I, SI->getFalseValue());
CondLHS->addIncoming(SI->getTrueValue(), NewBB);
// The select is now dead.
SI->eraseFromParent();
// Update any other PHI nodes in BB.
for (BasicBlock::iterator BI = BB->begin();
PHINode *Phi = dyn_cast<PHINode>(BI); ++BI)
if (Phi != CondLHS)
Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB);
return true;
}
}
return false;
}
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