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llvm-project/llvm/lib/Transforms/Scalar/JumpThreading.cpp

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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.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "jump-threading"
#include "llvm/Transforms/Scalar.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ValueHandle.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
STATISTIC(NumThreads, "Number of jumps threaded");
STATISTIC(NumFolds, "Number of terminators folded");
static cl::opt<unsigned>
Threshold("jump-threading-threshold",
cl::desc("Max block size to duplicate for jump threading"),
cl::init(6), cl::Hidden);
namespace {
/// 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 VISIBILITY_HIDDEN JumpThreading : public FunctionPass {
TargetData *TD;
#ifdef NDEBUG
SmallPtrSet<BasicBlock*, 16> LoopHeaders;
#else
SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
#endif
public:
static char ID; // Pass identification
JumpThreading() : FunctionPass(&ID) {}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
}
bool runOnFunction(Function &F);
void FindLoopHeaders(Function &F);
bool ProcessBlock(BasicBlock *BB);
bool ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB,
unsigned JumpThreadCost);
BasicBlock *FactorCommonPHIPreds(PHINode *PN, Value *Val);
bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
bool ProcessJumpOnPHI(PHINode *PN);
bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd);
bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB);
bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
};
}
char JumpThreading::ID = 0;
static RegisterPass<JumpThreading>
X("jump-threading", "Jump Threading");
// Public interface to the Jump Threading pass
FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
/// runOnFunction - Top level algorithm.
///
bool JumpThreading::runOnFunction(Function &F) {
DEBUG(errs() << "Jump threading on function '" << F.getName() << "'\n");
TD = getAnalysisIfAvailable<TargetData>();
FindLoopHeaders(F);
bool AnotherIteration = true, EverChanged = false;
while (AnotherIteration) {
AnotherIteration = false;
bool Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
BasicBlock *BB = I;
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(errs() << " JT: Deleting dead block '" << BB->getName()
<< "' with terminator: " << *BB->getTerminator());
LoopHeaders.erase(BB);
DeleteDeadBlock(BB);
Changed = true;
}
}
AnotherIteration = Changed;
EverChanged |= Changed;
}
LoopHeaders.clear();
return EverChanged;
}
/// 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));
}
/// FactorCommonPHIPreds - If there are multiple preds with the same incoming
/// value for the PHI, factor them together so we get one block to thread for
/// the whole group.
/// This is important for things like "phi i1 [true, true, false, true, x]"
/// where we only need to clone the block for the true blocks once.
///
BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Value *Val) {
SmallVector<BasicBlock*, 16> CommonPreds;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == Val)
CommonPreds.push_back(PN->getIncomingBlock(i));
if (CommonPreds.size() == 1)
return CommonPreds[0];
DEBUG(errs() << " Factoring out " << CommonPreds.size()
<< " common predecessors.\n");
return SplitBlockPredecessors(PN->getParent(),
&CommonPreds[0], CommonPreds.size(),
".thr_comm", this);
}
/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
/// thread across it.
static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
/// Ignore PHI nodes, these will be flattened when duplication happens.
BasicBlock::const_iterator I = BB->getFirstNonPHI();
// 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) {
// 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) && isa<PointerType>(I->getType()))
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 (!isa<IntrinsicInst>(CI))
Size += 3;
else if (!isa<VectorType>(CI->getType()))
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;
return Size;
}
/// ProcessBlock - If there are any predecessors whose control can be threaded
/// through to a successor, transform them now.
bool JumpThreading::ProcessBlock(BasicBlock *BB) {
// 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) {
// 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();
MergeBasicBlockIntoOnlyPred(BB);
if (isEntry && BB != &BB->getParent()->getEntryBlock())
BB->moveBefore(&BB->getParent()->getEntryBlock());
return true;
}
// See if this block ends with a branch or switch. If so, see if the
// condition is a phi node. If so, and if an entry of the phi node is a
// constant, we can thread the block.
Value *Condition;
if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
// Can't thread an unconditional jump.
if (BI->isUnconditional()) return false;
Condition = BI->getCondition();
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
Condition = SI->getCondition();
else
return false; // Must be an invoke.
// 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 (isa<ConstantInt>(Condition)) {
DEBUG(errs() << " In block '" << BB->getName()
<< "' folding terminator: " << *BB->getTerminator());
++NumFolds;
ConstantFoldTerminator(BB);
return true;
}
// If the terminator is branching on an undef, we can pick any of the
// successors to branch to. Since this is arbitrary, we pick the successor
// with the fewest predecessors. This should reduce the in-degree of the
// others.
if (isa<UndefValue>(Condition)) {
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;
}
// Fold the branch/switch.
for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
if (i == MinSucc) continue;
BBTerm->getSuccessor(i)->removePredecessor(BB);
}
DEBUG(errs() << " In block '" << BB->getName()
<< "' folding undef terminator: " << *BBTerm);
BranchInst::Create(BBTerm->getSuccessor(MinSucc), BBTerm);
BBTerm->eraseFromParent();
return true;
}
Instruction *CondInst = dyn_cast<Instruction>(Condition);
// If the condition is an instruction defined in another block, see if a
// predecessor has the same condition:
// br COND, BBX, BBY
// BBX:
// br COND, BBZ, BBW
if (!Condition->hasOneUse() && // Multiple uses.
(CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
pred_iterator PI = pred_begin(BB), E = pred_end(BB);
if (isa<BranchInst>(BB->getTerminator())) {
for (; PI != E; ++PI)
if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
if (PBI->isConditional() && PBI->getCondition() == Condition &&
ProcessBranchOnDuplicateCond(*PI, BB))
return true;
} else {
assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
for (; PI != E; ++PI)
if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator()))
if (PSI->getCondition() == Condition &&
ProcessSwitchOnDuplicateCond(*PI, BB))
return true;
}
}
// All the rest of our checks depend on the condition being an instruction.
if (CondInst == 0)
return false;
// See if this is a phi node in the current block.
if (PHINode *PN = dyn_cast<PHINode>(CondInst))
if (PN->getParent() == BB)
return ProcessJumpOnPHI(PN);
// If this is a conditional branch whose condition is and/or of a phi, try to
// simplify it.
if ((CondInst->getOpcode() == Instruction::And ||
CondInst->getOpcode() == Instruction::Or) &&
isa<BranchInst>(BB->getTerminator()) &&
ProcessBranchOnLogical(CondInst, BB,
CondInst->getOpcode() == Instruction::And))
return true;
if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
if (isa<PHINode>(CondCmp->getOperand(0))) {
// If we have "br (phi != 42)" and the phi node has any constant values
// as operands, we can thread through this block.
//
// If we have "br (cmp phi, x)" and the phi node contains x such that the
// comparison uniquely identifies the branch target, we can thread
// through this block.
if (ProcessBranchOnCompare(CondCmp, BB))
return true;
}
// If we have a comparison, loop over the predecessors to see if there is
// a condition with the same value.
pred_iterator PI = pred_begin(BB), E = pred_end(BB);
for (; PI != E; ++PI)
if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
if (PBI->isConditional() && *PI != BB) {
if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
if (CI->getOperand(0) == CondCmp->getOperand(0) &&
CI->getOperand(1) == CondCmp->getOperand(1) &&
CI->getPredicate() == CondCmp->getPredicate()) {
// TODO: Could handle things like (x != 4) --> (x == 17)
if (ProcessBranchOnDuplicateCond(*PI, 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.
//
// This is particularly important because reg2mem inserts loads and stores all
// over the place, and this blocks jump threading if we don't zap them.
Value *SimplifyValue = CondInst;
if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
if (isa<Constant>(CondCmp->getOperand(1)))
SimplifyValue = CondCmp->getOperand(0);
if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
if (SimplifyPartiallyRedundantLoad(LI))
return true;
// TODO: If we have: "br (X > 0)" and we have a predecessor where we know
// "(X == 4)" thread through this block.
return false;
}
/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
/// block that jump on exactly the same condition. This means that we almost
/// always know the direction of the edge in the DESTBB:
/// PREDBB:
/// br COND, DESTBB, BBY
/// DESTBB:
/// br COND, BBZ, BBW
///
/// If DESTBB has multiple predecessors, we can't just constant fold the branch
/// in DESTBB, we have to thread over it.
bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
BasicBlock *BB) {
BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
// If both successors of PredBB go to DESTBB, we don't know anything. We can
// fold the branch to an unconditional one, which allows other recursive
// simplifications.
bool BranchDir;
if (PredBI->getSuccessor(1) != BB)
BranchDir = true;
else if (PredBI->getSuccessor(0) != BB)
BranchDir = false;
else {
DEBUG(errs() << " In block '" << PredBB->getName()
<< "' folding terminator: " << *PredBB->getTerminator());
++NumFolds;
ConstantFoldTerminator(PredBB);
return true;
}
BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
// If the dest block has one predecessor, just fix the branch condition to a
// constant and fold it.
if (BB->getSinglePredecessor()) {
DEBUG(errs() << " In block '" << BB->getName()
<< "' folding condition to '" << BranchDir << "': "
<< *BB->getTerminator());
++NumFolds;
DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
BranchDir));
ConstantFoldTerminator(BB);
return true;
}
// Otherwise we need to thread from PredBB to DestBB's successor which
// involves code duplication. Check to see if it is worth it.
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
if (JumpThreadCost > Threshold) {
DEBUG(errs() << " Not threading BB '" << BB->getName()
<< "' - Cost is too high: " << JumpThreadCost << "\n");
return false;
}
// Next, figure out which successor we are threading to.
BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
// Ok, try to thread it!
return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost);
}
/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
/// block that switch on exactly the same condition. This means that we almost
/// always know the direction of the edge in the DESTBB:
/// PREDBB:
/// switch COND [... DESTBB, BBY ... ]
/// DESTBB:
/// switch COND [... BBZ, BBW ]
///
/// Optimizing switches like this is very important, because simplifycfg builds
/// switches out of repeated 'if' conditions.
bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
BasicBlock *DestBB) {
// Can't thread edge to self.
if (PredBB == DestBB)
return false;
SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
// There are a variety of optimizations that we can potentially do on these
// blocks: we order them from most to least preferable.
// If DESTBB *just* contains the switch, then we can forward edges from PREDBB
// directly to their destination. This does not introduce *any* code size
// growth. Skip debug info first.
BasicBlock::iterator BBI = DestBB->begin();
while (isa<DbgInfoIntrinsic>(BBI))
BBI++;
// FIXME: Thread if it just contains a PHI.
if (isa<SwitchInst>(BBI)) {
bool MadeChange = false;
// Ignore the default edge for now.
for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
ConstantInt *DestVal = DestSI->getCaseValue(i);
BasicBlock *DestSucc = DestSI->getSuccessor(i);
// Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if
// PredSI has an explicit case for it. If so, forward. If it is covered
// by the default case, we can't update PredSI.
unsigned PredCase = PredSI->findCaseValue(DestVal);
if (PredCase == 0) continue;
// If PredSI doesn't go to DestBB on this value, then it won't reach the
// case on this condition.
if (PredSI->getSuccessor(PredCase) != DestBB &&
DestSI->getSuccessor(i) != DestBB)
continue;
// Otherwise, we're safe to make the change. Make sure that the edge from
// DestSI to DestSucc is not critical and has no PHI nodes.
DEBUG(errs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI);
DEBUG(errs() << "THROUGH: " << *DestSI);
// If the destination has PHI nodes, just split the edge for updating
// simplicity.
if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
SplitCriticalEdge(DestSI, i, this);
DestSucc = DestSI->getSuccessor(i);
}
FoldSingleEntryPHINodes(DestSucc);
PredSI->setSuccessor(PredCase, DestSucc);
MadeChange = true;
}
if (MadeChange)
return true;
}
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 loads.
if (LI->isVolatile()) 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;
Value *LoadedPtr = LI->getOperand(0);
// If the loaded operand is defined in the LoadBB, it can't be available.
// FIXME: Could do 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;
SmallPtrSet<BasicBlock*, 8> PredsScanned;
typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
AvailablePredsTy AvailablePreds;
BasicBlock *OneUnavailablePred = 0;
// 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();
Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
if (!PredAvailable) {
OneUnavailablePred = PredBB;
continue;
}
// 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 = 0;
// 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)
if (!AvailablePredSet.count(*PI))
PredsToSplit.push_back(*PI);
// Split them out to their own block.
UnavailablePred =
SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
"thread-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!");
Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr",
UnavailablePred->getTerminator());
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.
PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
PN->takeName(LI);
// Insert new entries into the PHI for each predecessor. A single block may
// have multiple entries here.
for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
++PI) {
AvailablePredsTy::iterator I =
std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
std::make_pair(*PI, (Value*)0));
assert(I != AvailablePreds.end() && I->first == *PI &&
"Didn't find entry for predecessor!");
PN->addIncoming(I->second, I->first);
}
//cerr << "PRE: " << *LI << *PN << "\n";
LI->replaceAllUsesWith(PN);
LI->eraseFromParent();
return true;
}
/// ProcessJumpOnPHI - We have a conditional branch of switch 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::ProcessJumpOnPHI(PHINode *PN) {
// See if the phi node has any constant values. If so, we can determine where
// the corresponding predecessor will branch.
ConstantInt *PredCst = 0;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i))))
break;
// If no incoming value has a constant, we don't know the destination of any
// predecessors.
if (PredCst == 0)
return false;
// See if the cost of duplicating this block is low enough.
BasicBlock *BB = PN->getParent();
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
if (JumpThreadCost > Threshold) {
DEBUG(errs() << " Not threading BB '" << BB->getName()
<< "' - Cost is too high: " << JumpThreadCost << "\n");
return false;
}
// If so, we can actually do this threading. Merge any common predecessors
// that will act the same.
BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
// Next, figure out which successor we are threading to.
BasicBlock *SuccBB;
if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
SuccBB = BI->getSuccessor(PredCst ==
ConstantInt::getFalse(PredBB->getContext()));
else {
SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst));
}
// Ok, try to thread it!
return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost);
}
/// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch
/// whose condition is an AND/OR where one side is PN. If PN has constant
/// operands that permit us to evaluate the condition for some operand, thread
/// through the block. For example with:
/// br (and X, phi(Y, Z, false))
/// the predecessor corresponding to the 'false' will always jump to the false
/// destination of the branch.
///
bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB,
bool isAnd) {
// If this is a binary operator tree of the same AND/OR opcode, check the
// LHS/RHS.
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V))
if ((isAnd && BO->getOpcode() == Instruction::And) ||
(!isAnd && BO->getOpcode() == Instruction::Or)) {
if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd))
return true;
if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd))
return true;
}
// If this isn't a PHI node, we can't handle it.
PHINode *PN = dyn_cast<PHINode>(V);
if (!PN || PN->getParent() != BB) return false;
// We can only do the simplification for phi nodes of 'false' with AND or
// 'true' with OR. See if we have any entries in the phi for this.
unsigned PredNo = ~0U;
ConstantInt *PredCst = ConstantInt::get(Type::getInt1Ty(BB->getContext()),
!isAnd);
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
if (PN->getIncomingValue(i) == PredCst) {
PredNo = i;
break;
}
}
// If no match, bail out.
if (PredNo == ~0U)
return false;
// See if the cost of duplicating this block is low enough.
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
if (JumpThreadCost > Threshold) {
DEBUG(errs() << " Not threading BB '" << BB->getName()
<< "' - Cost is too high: " << JumpThreadCost << "\n");
return false;
}
// If so, we can actually do this threading. Merge any common predecessors
// that will act the same.
BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
// Next, figure out which successor we are threading to. If this was an AND,
// the constant must be FALSE, and we must be targeting the 'false' block.
// If this is an OR, the constant must be TRUE, and we must be targeting the
// 'true' block.
BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd);
// Ok, try to thread it!
return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost);
}
/// GetResultOfComparison - Given an icmp/fcmp predicate and the left and right
/// hand sides of the compare instruction, try to determine the result. If the
/// result can not be determined, a null pointer is returned.
static Constant *GetResultOfComparison(CmpInst::Predicate pred,
Value *LHS, Value *RHS,
LLVMContext &Context) {
if (Constant *CLHS = dyn_cast<Constant>(LHS))
if (Constant *CRHS = dyn_cast<Constant>(RHS))
return ConstantExpr::getCompare(pred, CLHS, CRHS);
if (LHS == RHS)
if (isa<IntegerType>(LHS->getType()) || isa<PointerType>(LHS->getType()))
return ICmpInst::isTrueWhenEqual(pred) ?
ConstantInt::getTrue(Context) : ConstantInt::getFalse(Context);
return 0;
}
/// ProcessBranchOnCompare - We found a branch on a comparison between a phi
/// node and a value. If we can identify when the comparison is true between
/// the phi inputs and the value, we can fold the compare for that edge and
/// thread through it.
bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) {
PHINode *PN = cast<PHINode>(Cmp->getOperand(0));
Value *RHS = Cmp->getOperand(1);
// If the phi isn't in the current block, an incoming edge to this block
// doesn't control the destination.
if (PN->getParent() != BB)
return false;
// We can do this simplification if any comparisons fold to true or false.
// See if any do.
Value *PredVal = 0;
bool TrueDirection = false;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
PredVal = PN->getIncomingValue(i);
Constant *Res = GetResultOfComparison(Cmp->getPredicate(), PredVal,
RHS, Cmp->getContext());
if (!Res) {
PredVal = 0;
continue;
}
// If this folded to a constant expr, we can't do anything.
if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) {
TrueDirection = ResC->getZExtValue();
break;
}
// If this folded to undef, just go the false way.
if (isa<UndefValue>(Res)) {
TrueDirection = false;
break;
}
// Otherwise, we can't fold this input.
PredVal = 0;
}
// If no match, bail out.
if (PredVal == 0)
return false;
// See if the cost of duplicating this block is low enough.
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
if (JumpThreadCost > Threshold) {
DEBUG(errs() << " Not threading BB '" << BB->getName()
<< "' - Cost is too high: " << JumpThreadCost << "\n");
return false;
}
// If so, we can actually do this threading. Merge any common predecessors
// that will act the same.
BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredVal);
// Next, get our successor.
BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection);
// Ok, try to thread it!
return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost);
}
/// ThreadEdge - We have decided that it is safe and profitable to thread an
/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this
/// change.
bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
BasicBlock *SuccBB, unsigned JumpThreadCost) {
// If threading to the same block as we come from, we would infinite loop.
if (SuccBB == BB) {
DEBUG(errs() << " 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(errs() << " Not threading from '" << PredBB->getName()
<< "' across loop header BB '" << BB->getName()
<< "' to dest BB '" << SuccBB->getName()
<< "' - it might create an irreducible loop!\n");
return false;
}
// And finally, do it!
DEBUG(errs() << " Threading edge from '" << PredBB->getName() << "' to '"
<< SuccBB->getName() << "' with cost: " << JumpThreadCost
<< ", across block:\n "
<< *BB << "\n");
// Jump Threading can not update SSA properties correctly if the values
// defined in the duplicated block are used outside of the block itself. For
// this reason, we spill all values that are used outside of BB to the stack.
for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
if (!I->isUsedOutsideOfBlock(BB))
continue;
// We found a use of I outside of BB. Create a new stack slot to
// break this inter-block usage pattern.
DemoteRegToStack(*I);
}
// 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(BI->getContext());
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::Create(SuccBB, NewBB);
// Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
// PHI nodes for NewBB now.
for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) {
PHINode *PN = cast<PHINode>(PNI);
// Ok, we have a PHI node. Figure out what the incoming value was for the
// DestBlock.
Value *IV = PN->getIncomingValueForBlock(BB);
// Remap the value if necessary.
if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
if (I != ValueMapping.end())
IV = I->second;
}
PN->addIncoming(IV, NewBB);
}
// 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);
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.
BI = NewBB->begin();
for (BasicBlock::iterator E = NewBB->end(); BI != E; ) {
Instruction *Inst = BI++;
if (Constant *C = ConstantFoldInstruction(Inst, BB->getContext(), TD)) {
Inst->replaceAllUsesWith(C);
Inst->eraseFromParent();
continue;
}
RecursivelyDeleteTriviallyDeadInstructions(Inst);
}
// Threaded an edge!
++NumThreads;
return true;
}
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