llvm-6502/lib/Transforms/Scalar/JumpThreading.cpp
Dan Gohman 844731a7f1 Clean up the use of static and anonymous namespaces. This turned up
several things that were neither in an anonymous namespace nor static
but not intended to be global.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51017 91177308-0d34-0410-b5e6-96231b3b80d8
2008-05-13 00:00:25 +00:00

517 lines
19 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((intptr_t)&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) {
BasicBlock::const_iterator I = BB->begin();
/// Ignore PHI nodes, these will be flattened when duplication happens.
while (isa<PHINode>(*I)) ++I;
// 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 of 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.
if (!isa<StructType>(I->getType())) {
DemoteRegToStack(*I);
continue;
}
// Alternatively, I must be a call or invoke that returns multiple retvals.
// We can't use 'DemoteRegToStack' because that will create loads and
// stores of aggregates which is not valid yet. If I is a call, we can just
// pull all the getresult instructions up to this block. If I is an invoke,
// we are out of luck.
BasicBlock::iterator IP = I; ++IP;
for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
UI != E; ++UI)
cast<GetResultInst>(UI)->moveBefore(IP);
}
// 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);
}
}