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implement a transformation in jump threading that is currently 

done by condprop, but do it in a much more general form.  The
basic idea is that we can do a limited form of tail duplication
in the case when we have a branch on a phi.  Moving the branch
up in to the predecessor block makes instruction selection
much easier and encourages chained jump threadings.

llvm-svn: 83759
This commit is contained in:
Chris Lattner 2009-10-11 07:24:57 +00:00
parent 4140d8bd5c
commit 97b1405207
2 changed files with 255 additions and 67 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

288
llvm/lib/Transforms/Scalar/JumpThreading.cpp
View File

@ -33,6 +33,7 @@ using namespace llvm;
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",
@ -75,6 +76,9 @@ namespace {
bool ProcessBlock(BasicBlock *BB);
bool ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB);
bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
BasicBlock *PredBB);
BasicBlock *FactorCommonPHIPreds(PHINode *PN, Value *Val);
bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
@ -118,7 +122,7 @@ bool JumpThreading::runOnFunction(Function &F) {
if (pred_begin(BB) == pred_end(BB) &&
BB != &BB->getParent()->getEntryBlock()) {
DEBUG(errs() << " JT: Deleting dead block '" << BB->getName()
<< "' with terminator: " << *BB->getTerminator());
<< "' with terminator: " << *BB->getTerminator() << '\n');
LoopHeaders.erase(BB);
DeleteDeadBlock(BB);
Changed = true;
@ -132,6 +136,48 @@ bool JumpThreading::runOnFunction(Function &F) {
return EverChanged;
}
/// 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;
}
/// 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
@ -243,7 +289,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
// other blocks.
if (isa<ConstantInt>(Condition)) {
DEBUG(errs() << " In block '" << BB->getName()
<< "' folding terminator: " << *BB->getTerminator());
<< "' folding terminator: " << *BB->getTerminator() << '\n');
++NumFolds;
ConstantFoldTerminator(BB);
return true;
@ -262,7 +308,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
}
DEBUG(errs() << " In block '" << BB->getName()
<< "' folding undef terminator: " << *BBTerm);
<< "' folding undef terminator: " << *BBTerm << '\n');
BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
BBTerm->eraseFromParent();
return true;
@ -389,7 +435,7 @@ bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
BranchDir = false;
else {
DEBUG(errs() << " In block '" << PredBB->getName()
<< "' folding terminator: " << *PredBB->getTerminator());
<< "' folding terminator: " << *PredBB->getTerminator() << '\n');
++NumFolds;
ConstantFoldTerminator(PredBB);
return true;
@ -402,7 +448,7 @@ bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
if (BB->getSinglePredecessor()) {
DEBUG(errs() << " In block '" << BB->getName()
<< "' folding condition to '" << BranchDir << "': "
<< *BB->getTerminator());
<< *BB->getTerminator() << '\n');
++NumFolds;
DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
BranchDir));
@ -689,11 +735,31 @@ bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) {
}
}
// If no incoming value has a constant, we don't know the destination of any
// predecessors.
// If the incoming values are all variables, we don't know the destination of
// any predecessors. However, if any of the predecessor blocks end in an
// unconditional branch, we can *duplicate* the jump 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).
// We don't want to do this tranformation for switches, because we don't
// really want to duplicate a switch.
if (isa<SwitchInst>(BB->getTerminator()))
return false;
// Look for unconditional branch predecessors.
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() &&
// Try to duplicate BB into PredBB.
DuplicateCondBranchOnPHIIntoPred(BB, PredBB))
return true;
}
return false;
}
/// 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
@ -824,59 +890,36 @@ bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) {
return ThreadEdge(BB, PredBB, SuccBB);
}
/// 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;
}
/// 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 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 = getJumpThreadDuplicationCost(BB);
if (JumpThreadCost > Threshold) {
DEBUG(errs() << " Not threading BB '" << BB->getName()
<< "' - Cost is too high: " << JumpThreadCost << "\n");
return false;
}
// 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()
@ -894,6 +937,13 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
return false;
}
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
if (JumpThreadCost > Threshold) {
DEBUG(errs() << " Not threading BB '" << BB->getName()
<< "' - Cost is too high: " << JumpThreadCost << "\n");
return false;
}
// And finally, do it!
DEBUG(errs() << " Threading edge from '" << PredBB->getName() << "' to '"
<< SuccBB->getName() << "' with cost: " << JumpThreadCost
@ -937,20 +987,7 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
// 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();
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(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);
}
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,
@ -1021,3 +1058,120 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
++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,
BasicBlock *PredBB) {
// 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(errs() << " Not duplicating loop header '" << BB->getName()
<< "' into predecessor block '" << PredBB->getName()
<< "' - it might create an irreducible loop!\n");
return false;
}
unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
if (DuplicationCost > Threshold) {
DEBUG(errs() << " Not duplicating BB '" << BB->getName()
<< "' - Cost is too high: " << DuplicationCost << "\n");
return false;
}
// Okay, we decided to do this! Clone all the instructions in BB onto the end
// of PredBB.
DEBUG(errs() << " Duplicating block '" << BB->getName() << "' into end of '"
<< PredBB->getName() << "' to eliminate branch on phi. Cost: "
<< DuplicationCost << " block is:" << *BB << "\n");
// 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);
BranchInst *OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
// 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();
New->setName(BI->getName());
PredBB->getInstList().insert(OldPredBranch, 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);
}
}
// 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 (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
++UI) {
Instruction *User = cast<Instruction>(*UI);
if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
if (UserPN->getIncomingBlock(UI) == BB)
continue;
} else if (User->getParent() == BB)
continue;
UsesToRename.push_back(&UI.getUse());
}
// If there are no uses outside the block, we're done with this instruction.
if (UsesToRename.empty())
continue;
DEBUG(errs() << "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);
SSAUpdate.AddAvailableValue(BB, I);
SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
while (!UsesToRename.empty())
SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
DEBUG(errs() << "\n");
}
// PredBB no longer jumps to BB, remove entries in the PHI node for the edge
// that we nuked.
BB->removePredecessor(PredBB);
// Remove the unconditional branch at the end of the PredBB block.
OldPredBranch->eraseFromParent();
++NumDupes;
return true;
}

34
llvm/test/Transforms/JumpThreading/basic.ll
View File

@ -105,3 +105,37 @@ F2:
ret i32 %B
}
;; This tests that the branch in 'merge' can be cloned up into T1.
define i32 @test5(i1 %cond, i1 %cond2) {
; CHECK: @test5
br i1 %cond, label %T1, label %F1
T1:
; CHECK: T1:
; CHECK-NEXT: %v1 = call i32 @f1()
; CHECK-NEXT: %cond3 = icmp eq i32 %v1, 412
; CHECK-NEXT: br i1 %cond3, label %T2, label %F2
%v1 = call i32 @f1()
%cond3 = icmp eq i32 %v1, 412
br label %Merge
F1:
%v2 = call i32 @f2()
br label %Merge
Merge:
%A = phi i1 [%cond3, %T1], [%cond2, %F1]
%B = phi i32 [%v1, %T1], [%v2, %F1]
br i1 %A, label %T2, label %F2
T2:
call void @f3()
ret i32 %B
F2:
ret i32 %B
}