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

503 lines
18 KiB
C++

//===- 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/Pass.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.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 {
public:
static char ID; // Pass identification
JumpThreading() : FunctionPass(&ID) {}
bool runOnFunction(Function &F);
bool ThreadBlock(BasicBlock *BB);
void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB);
BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal);
bool ProcessJumpOnPHI(PHINode *PN);
bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd);
bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB);
};
}
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) {
DOUT << "Jump threading on function '" << F.getNameStart() << "'\n";
bool AnotherIteration = true, EverChanged = false;
while (AnotherIteration) {
AnotherIteration = false;
bool Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
while (ThreadBlock(I))
Changed = true;
AnotherIteration = Changed;
EverChanged |= Changed;
}
return EverChanged;
}
/// 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, Constant *CstVal) {
SmallVector<BasicBlock*, 16> CommonPreds;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == CstVal)
CommonPreds.push_back(PN->getIncomingBlock(i));
if (CommonPreds.size() == 1)
return CommonPreds[0];
DOUT << " 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;
}
/// ThreadBlock - If there are any predecessors whose control can be threaded
/// through to a successor, transform them now.
bool JumpThreading::ThreadBlock(BasicBlock *BB) {
// 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)) {
DOUT << " In block '" << BB->getNameStart()
<< "' folding terminator: " << *BB->getTerminator();
++NumFolds;
ConstantFoldTerminator(BB);
return true;
}
// If there is only a single predecessor of this block, nothing to fold.
if (BB->getSinglePredecessor())
return false;
// See if this is a phi node in the current block.
PHINode *PN = dyn_cast<PHINode>(Condition);
if (PN && 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 (BinaryOperator *CondI = dyn_cast<BinaryOperator>(Condition)) {
if ((CondI->getOpcode() == Instruction::And ||
CondI->getOpcode() == Instruction::Or) &&
isa<BranchInst>(BB->getTerminator()) &&
ProcessBranchOnLogical(CondI, BB,
CondI->getOpcode() == Instruction::And))
return true;
}
// If we have "br (phi != 42)" and the phi node has any constant values as
// operands, we can thread through this block.
if (CmpInst *CondCmp = dyn_cast<CmpInst>(Condition))
if (isa<PHINode>(CondCmp->getOperand(0)) &&
isa<Constant>(CondCmp->getOperand(1)) &&
ProcessBranchOnCompare(CondCmp, BB))
return true;
return false;
}
/// 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) {
DOUT << " Not threading BB '" << BB->getNameStart()
<< "' - 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());
else {
SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst));
}
// If threading to the same block as we come from, we would infinite loop.
if (SuccBB == BB) {
DOUT << " Not threading BB '" << BB->getNameStart()
<< "' - would thread to self!\n";
return false;
}
// And finally, do it!
DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '"
<< SuccBB->getNameStart() << "' with cost: " << JumpThreadCost
<< ", across block:\n "
<< *BB << "\n";
ThreadEdge(BB, PredBB, SuccBB);
++NumThreads;
return true;
}
/// 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::Int1Ty, !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) {
DOUT << " Not threading BB '" << BB->getNameStart()
<< "' - 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);
// If threading to the same block as we come from, we would infinite loop.
if (SuccBB == BB) {
DOUT << " Not threading BB '" << BB->getNameStart()
<< "' - would thread to self!\n";
return false;
}
// And finally, do it!
DOUT << " Threading edge through bool from '" << PredBB->getNameStart()
<< "' to '" << SuccBB->getNameStart() << "' with cost: "
<< JumpThreadCost << ", across block:\n "
<< *BB << "\n";
ThreadEdge(BB, PredBB, SuccBB);
++NumThreads;
return true;
}
/// ProcessBranchOnCompare - We found a branch on a comparison between a phi
/// node and a constant. If the PHI node contains any constants as inputs, 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));
Constant *RHS = cast<Constant>(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.
Constant *PredCst = 0;
bool TrueDirection = false;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
PredCst = dyn_cast<Constant>(PN->getIncomingValue(i));
if (PredCst == 0) continue;
Constant *Res;
if (ICmpInst *ICI = dyn_cast<ICmpInst>(Cmp))
Res = ConstantExpr::getICmp(ICI->getPredicate(), PredCst, RHS);
else
Res = ConstantExpr::getFCmp(cast<FCmpInst>(Cmp)->getPredicate(),
PredCst, RHS);
// 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.
PredCst = 0;
}
// If no match, bail out.
if (PredCst == 0)
return false;
// See if the cost of duplicating this block is low enough.
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
if (JumpThreadCost > Threshold) {
DOUT << " Not threading BB '" << BB->getNameStart()
<< "' - 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, get our successor.
BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection);
// If threading to the same block as we come from, we would infinite loop.
if (SuccBB == BB) {
DOUT << " Not threading BB '" << BB->getNameStart()
<< "' - would thread to self!\n";
return false;
}
// And finally, do it!
DOUT << " Threading edge through bool from '" << PredBB->getNameStart()
<< "' to '" << SuccBB->getNameStart() << "' with cost: "
<< JumpThreadCost << ", across block:\n "
<< *BB << "\n";
ThreadEdge(BB, PredBB, SuccBB);
++NumThreads;
return true;
}
/// 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.
void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
BasicBlock *SuccBB) {
// 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->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->getNameStart());
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)))
if (Value *Remapped = ValueMapping[Inst])
New->setOperand(i, Remapped);
}
// 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))
if (Value *MappedIV = ValueMapping[Inst])
IV = MappedIV;
PN->addIncoming(IV, NewBB);
}
// Finally, 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);
}
}