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Implement jump threading of 'indirectbr' by keeping track of whether we're looking for ConstantInt*s or BlockAddress*s. 

llvm-svn: 121066
This commit is contained in:
Frits van Bommel 2010-12-06 23:36:56 +00:00
parent 9847abacb1
commit d9df6eaa9c
2 changed files with 141 additions and 46 deletions
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126
llvm/lib/Transforms/Scalar/JumpThreading.cpp
View File

@ -49,6 +49,13 @@ namespace {
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
@ -109,8 +116,10 @@ namespace {
const SmallVectorImpl<BasicBlock *> &PredBBs);
bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
PredValueInfo &Result);
bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB);
PredValueInfo &Result,
ConstantPreference Preference);
bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
ConstantPreference Preference);
bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
@ -247,6 +256,10 @@ static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
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;
}
@ -275,9 +288,10 @@ void JumpThreading::FindLoopHeaders(Function &F) {
/// 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.
/// 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) {
static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
if (!Val)
return 0;
@ -285,26 +299,22 @@ static Constant *getKnownConstant(Value *Val) {
if (UndefValue *U = dyn_cast<UndefValue>(Val))
return U;
if (Preference == WantBlockAddress)
return dyn_cast<BlockAddress>(Val->stripPointerCasts());
return dyn_cast<ConstantInt>(Val);
}
// Helper method for ComputeValueKnownInPredecessors. If Value is a
// ConstantInt or undef, push it. Otherwise, do nothing.
static void PushKnownConstantOrUndef(PredValueInfo &Result, Constant *Value,
BasicBlock *BB) {
if (Constant *KC = getKnownConstant(Value))
Result.push_back(std::make_pair(KC, BB));
}
/// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
/// if we can infer that the value is a known ConstantInt in any of our
/// predecessors. If so, return the known list of value and pred BB in the
/// result vector. If a value is known to be undef, it is returned as null.
/// 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){
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
@ -317,7 +327,7 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
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)) {
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));
@ -347,7 +357,7 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
// 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))
if (Constant *KC = getKnownConstant(PredCst, Preference))
Result.push_back(std::make_pair(KC, P));
}
@ -358,14 +368,13 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
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)) {
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);
// LVI returns null is no value could be determined.
if (!CI) continue;
PushKnownConstantOrUndef(Result, CI, PN->getIncomingBlock(i));
if (Constant *KC = getKnownConstant(CI, Preference))
Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
}
}
@ -376,12 +385,15 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
// 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);
ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
WantInteger);
ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals,
WantInteger);
if (LHSVals.empty() && RHSVals.empty())
return false;
@ -421,7 +433,8 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
if (I->getOpcode() == Instruction::Xor &&
isa<ConstantInt>(I->getOperand(1)) &&
cast<ConstantInt>(I->getOperand(1))->isOne()) {
ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result);
ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result,
WantInteger);
if (Result.empty())
return false;
@ -434,16 +447,20 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
// 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);
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);
PushKnownConstantOrUndef(Result, Folded, LHSVals[i].second);
if (Constant *KC = getKnownConstant(Folded, WantInteger))
Result.push_back(std::make_pair(KC, LHSVals[i].second));
}
}
@ -452,6 +469,7 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
// 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.
@ -474,8 +492,8 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
}
if (Constant *ConstRes = dyn_cast<Constant>(Res))
PushKnownConstantOrUndef(Result, ConstRes, PredBB);
if (Constant *KC = getKnownConstant(Res, WantInteger))
Result.push_back(std::make_pair(KC, PredBB));
}
return !Result.empty();
@ -510,13 +528,15 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
// and evaluate it statically if we can.
if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
PredValueInfoTy LHSVals;
ComputeValueKnownInPredecessors(I->getOperand(0), BB, 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);
PushKnownConstantOrUndef(Result, Folded, LHSVals[i].second);
if (Constant *KC = getKnownConstant(Folded, WantInteger))
Result.push_back(std::make_pair(KC, LHSVals[i].second));
}
return !Result.empty();
@ -526,7 +546,7 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
// 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)) {
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));
}
@ -590,17 +610,25 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
}
}
// Look to see if the terminator is a branch of switch, if not we can't thread
// it.
// 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;
if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
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>(BB->getTerminator()))
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
Condition = SI->getCondition();
else
} else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
Condition = IB->getAddress()->stripPointerCasts();
Preference = WantBlockAddress;
} else {
return false; // Must be an invoke.
}
// If the terminator is branching on an undef, we can pick any of the
// successors to branch to. Let GetBestDestForJumpOnUndef decide.
@ -624,7 +652,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
// 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)) {
if (getKnownConstant(Condition, Preference)) {
DEBUG(dbgs() << " In block '" << BB->getName()
<< "' folding terminator: " << *BB->getTerminator() << '\n');
++NumFolds;
@ -637,7 +665,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
// All the rest of our checks depend on the condition being an instruction.
if (CondInst == 0) {
// FIXME: Unify this with code below.
if (ProcessThreadableEdges(Condition, BB))
if (ProcessThreadableEdges(Condition, BB, Preference))
return true;
return false;
}
@ -703,7 +731,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
// 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))
if (ProcessThreadableEdges(CondInst, BB, Preference))
return true;
// If this is an otherwise-unfoldable branch on a phi node in the current
@ -1088,14 +1116,15 @@ FindMostPopularDest(BasicBlock *BB,
return MostPopularDest;
}
bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) {
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))
if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference))
return false;
assert(!PredValues.empty() &&
@ -1135,9 +1164,12 @@ bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) {
DestBB = 0;
else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
else {
SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
DestBB = SI->getSuccessor(SI->findCaseValue(cast<ConstantInt>(Val)));
else {
assert(isa<IndirectBrInst>(BB->getTerminator())
&& "Unexpected terminator");
DestBB = cast<BlockAddress>(Val)->getBasicBlock();
}
// If we have exactly one destination, remember it for efficiency below.
@ -1256,9 +1288,11 @@ bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
PredValueInfoTy XorOpValues;
bool isLHS = true;
if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) {
if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
WantInteger)) {
assert(XorOpValues.empty());
if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues))
if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
WantInteger))
return false;
isLHS = false;
}

61
llvm/test/Transforms/JumpThreading/indirectbr.ll Normal file
View File

@ -0,0 +1,61 @@
; RUN: opt -S < %s -jump-threading | FileCheck %s
; Keep block addresses alive.
@addresses = constant [4 x i8*] [
i8* blockaddress(@test1, %L1), i8* blockaddress(@test1, %L2),
i8* blockaddress(@test2, %L1), i8* blockaddress(@test2, %L2)
]
declare void @bar()
declare void @baz()
; Check basic jump threading for indirectbr instructions.
; CHECK: void @test1
; CHECK: br i1 %tobool, label %L1, label %indirectgoto
; CHECK-NOT: if.else:
; CHECK: L1:
; CHECK: indirectbr i8* %address, [label %L1, label %L2]
define void @test1(i32 %i, i8* %address) nounwind {
entry:
%rem = srem i32 %i, 2
%tobool = icmp ne i32 %rem, 0
br i1 %tobool, label %indirectgoto, label %if.else
if.else: ; preds = %entry
br label %indirectgoto
L1: ; preds = %indirectgoto
call void @bar()
ret void
L2: ; preds = %indirectgoto
call void @baz()
ret void
indirectgoto: ; preds = %if.else, %entry
%indirect.goto.dest = phi i8* [ %address, %if.else ], [ blockaddress(@test1, %L1), %entry ]
indirectbr i8* %indirect.goto.dest, [label %L1, label %L2]
}
; Check constant folding of indirectbr
; CHECK: void @test2
; CHECK-NEXT: :
; CHECK-NEXT: call void @bar
; CHECK-NEXT: ret void
define void @test2() nounwind {
entry:
indirectbr i8* blockaddress(@test2, %L1), [label %L1, label %L2]
L1: ; preds = %indirectgoto
call void @bar()
ret void
L2: ; preds = %indirectgoto
call void @baz()
ret void
}