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Extend jump threading to support much more general threading

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predicates.  This allows us to jump thread things like:

_ZN12StringSwitchI5ColorE4CaseILj7EEERS1_RAT__KcRKS0_.exit119:
  %tmp1.i24166 = phi i8 [ 1, %bb5.i117 ], [ %tmp1.i24165, %_Z....exit ], [ %tmp1.i24165, %bb4.i114 ] 
  %toBoolnot.i87 = icmp eq i8 %tmp1.i24166, 0     ; <i1> [#uses=1]
  %tmp4.i90 = icmp eq i32 %tmp2.i, 6              ; <i1> [#uses=1]
  %or.cond173 = and i1 %toBoolnot.i87, %tmp4.i90  ; <i1> [#uses=1]
  br i1 %or.cond173, label %bb4.i96, label %_ZN12...

Where it is "obvious" that when coming from %bb5.i117 that the 'and' is always 
false.  This triggers a surprisingly high number of times in the testsuite, 
and gets us closer to generating good code for doug's strswitch testcase.

This also make a bunch of other code in jump threading redundant, I'll rip
out in the next patch.  This survived an enable-checking llvm-gcc bootstrap.

llvm-svn: 86264
This commit is contained in:
Chris Lattner 2009-11-06 18:15:14 +00:00
parent aadf548295
commit b688868418
2 changed files with 356 additions and 27 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

352
lib/Transforms/Scalar/JumpThreading.cpp
View File

@ -75,8 +75,16 @@ namespace {
bool ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB);
bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
BasicBlock *PredBB);
BasicBlock *FactorCommonPHIPreds(PHINode *PN, Value *Val);
typedef SmallVectorImpl<std::pair<ConstantInt*,
BasicBlock*> > PredValueInfo;
bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
PredValueInfo &Result);
bool ProcessThreadableEdges(Instruction *CondInst, BasicBlock *BB);
bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
@ -220,7 +228,133 @@ BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Value *Val) {
&CommonPreds[0], CommonPreds.size(),
".thr_comm", this);
}
/// 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) {
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()))
if (ICmpInst::isTrueWhenEqual(pred))
return ConstantInt::getTrue(LHS->getContext());
else
return ConstantInt::getFalse(LHS->getContext());
return 0;
}
/// 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 the list of value and pred BB in the
/// result vector. If a value is known to be undef, it is returned as null.
///
/// The BB basic block is known to start with a PHI node.
///
/// This returns true if there were any known values.
///
///
/// TODO: Per PR2563, we could infer value range information about a predecessor
/// based on its terminator.
bool JumpThreading::
ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
PHINode *TheFirstPHI = cast<PHINode>(BB->begin());
// If V is a constantint, then it is known in all predecessors.
if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
ConstantInt *CI = dyn_cast<ConstantInt>(V);
Result.resize(TheFirstPHI->getNumIncomingValues());
for (unsigned i = 0, e = Result.size(); i != e; ++i)
Result.push_back(std::make_pair(CI, TheFirstPHI->getIncomingBlock(i)));
return true;
}
// If V is a non-instruction value, or an instruction in a different block,
// then it can't be derived from a PHI.
Instruction *I = dyn_cast<Instruction>(V);
if (I == 0 || I->getParent() != BB)
return false;
/// If I is a PHI node, then we know the incoming values for any constants.
if (PHINode *PN = dyn_cast<PHINode>(I)) {
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *InVal = PN->getIncomingValue(i);
if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
}
}
return !Result.empty();
}
SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
// Handle some boolean conditions.
if (I->getType()->getPrimitiveSizeInBits() == 1) {
// 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);
if (LHSVals.empty() && RHSVals.empty())
return false;
ConstantInt *InterestingVal;
if (I->getOpcode() == Instruction::Or)
InterestingVal = ConstantInt::getTrue(I->getContext());
else
InterestingVal = ConstantInt::getFalse(I->getContext());
// Scan for the sentinel.
for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0)
Result.push_back(LHSVals[i]);
for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0)
Result.push_back(RHSVals[i]);
return !Result.empty();
}
// TODO: Should handle the NOT form of XOR.
}
// Handle compare with phi operand, where the PHI is defined in this block.
if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
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.
// See if any do.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *PredBB = PN->getIncomingBlock(i);
Value *LHS = PN->getIncomingValue(i);
Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
Constant *Res = GetResultOfComparison(Cmp->getPredicate(), LHS, RHS);
if (Res == 0) continue;
if (isa<UndefValue>(Res))
Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
Result.push_back(std::make_pair(CI, PredBB));
}
return !Result.empty();
}
// TODO: We could also recurse to see if we can determine constants another
// way.
}
return false;
}
/// GetBestDestForBranchOnUndef - If we determine that the specified block ends
/// in an undefined jump, decide which block is best to revector to.
@ -251,7 +385,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
// 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 (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
SinglePred != BB) {
// If SinglePred was a loop header, BB becomes one.
@ -267,10 +401,10 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
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.
}
// Look to see if the terminator is a branch of switch, if not we can't thread
// it.
Value *Condition;
if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
// Can't thread an unconditional jump.
@ -369,7 +503,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
}
// If we have a comparison, loop over the predecessors to see if there is
// a condition with the same value.
// a condition with a lexically identical value.
pred_iterator PI = pred_begin(BB), E = pred_end(BB);
for (; PI != E; ++PI)
if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
@ -402,6 +536,19 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
if (SimplifyPartiallyRedundantLoad(LI))
return true;
// Handle a variety of cases where we are branching on something derived from
// 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.
//
// We only bother doing this if the current block has a PHI node and if the
// conditional instruction lives in the current block. If either condition
// fail, this won't be a computable value anyway.
if (CondInst->getParent() == BB && isa<PHINode>(BB->front()))
if (ProcessThreadableEdges(CondInst, BB))
return true;
// TODO: If we have: "br (X > 0)" and we have a predecessor where we know
// "(X == 4)" thread through this block.
@ -690,6 +837,176 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
return true;
}
/// FindMostPopularDest - The specified list contains multiple possible
/// threadable destinations. Pick the one that occurs the most frequently in
/// the list.
static BasicBlock *
FindMostPopularDest(BasicBlock *BB,
const SmallVectorImpl<std::pair<BasicBlock*,
BasicBlock*> > &PredToDestList) {
assert(!PredToDestList.empty());
// Determine popularity. If there are multiple possible destinations, we
// explicitly choose to ignore 'undef' destinations. We prefer to thread
// blocks with known and real destinations to threading undef. We'll handle
// them later if interesting.
DenseMap<BasicBlock*, unsigned> DestPopularity;
for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
if (PredToDestList[i].second)
DestPopularity[PredToDestList[i].second]++;
// Find the most popular dest.
DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
BasicBlock *MostPopularDest = DPI->first;
unsigned Popularity = DPI->second;
SmallVector<BasicBlock*, 4> SamePopularity;
for (++DPI; DPI != DestPopularity.end(); ++DPI) {
// If the popularity of this entry isn't higher than the popularity we've
// seen so far, ignore it.
if (DPI->second < Popularity)
; // ignore.
else if (DPI->second == Popularity) {
// If it is the same as what we've seen so far, keep track of it.
SamePopularity.push_back(DPI->first);
} else {
// If it is more popular, remember it.
SamePopularity.clear();
MostPopularDest = DPI->first;
Popularity = DPI->second;
}
}
// Okay, now we know the most popular destination. If there is more than
// destination, we need to determine one. This is arbitrary, but we need
// to make a deterministic decision. Pick the first one that appears in the
// successor list.
if (!SamePopularity.empty()) {
SamePopularity.push_back(MostPopularDest);
TerminatorInst *TI = BB->getTerminator();
for (unsigned i = 0; ; ++i) {
assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
if (std::find(SamePopularity.begin(), SamePopularity.end(),
TI->getSuccessor(i)) == SamePopularity.end())
continue;
MostPopularDest = TI->getSuccessor(i);
break;
}
}
// Okay, we have finally picked the most popular destination.
return MostPopularDest;
}
bool JumpThreading::ProcessThreadableEdges(Instruction *CondInst,
BasicBlock *BB) {
// If threading this would thread across a loop header, don't even try to
// thread the edge.
if (LoopHeaders.count(BB))
return false;
SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
if (!ComputeValueKnownInPredecessors(CondInst, BB, PredValues))
return false;
assert(!PredValues.empty() &&
"ComputeValueKnownInPredecessors returned true with no values");
DEBUG(errs() << "IN BB: " << *BB;
for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
errs() << " BB '" << BB->getName() << "': FOUND condition = ";
if (PredValues[i].first)
errs() << *PredValues[i].first;
else
errs() << "UNDEF";
errs() << " for pred '" << PredValues[i].second->getName()
<< "'.\n";
});
// Decide what we want to thread through. Convert our list of known values to
// a list of known destinations for each pred. This also discards duplicate
// predecessors and keeps track of the undefined inputs (which are represented
// as a null dest in the PredToDestList.
SmallPtrSet<BasicBlock*, 16> SeenPreds;
SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
BasicBlock *OnlyDest = 0;
BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
BasicBlock *Pred = PredValues[i].second;
if (!SeenPreds.insert(Pred))
continue; // Duplicate predecessor entry.
// If the predecessor ends with an indirect goto, we can't change its
// destination.
if (isa<IndirectBrInst>(Pred->getTerminator()))
continue;
ConstantInt *Val = PredValues[i].first;
BasicBlock *DestBB;
if (Val == 0) // Undef.
DestBB = 0;
else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
DestBB = BI->getSuccessor(Val->isZero());
else {
SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
DestBB = SI->getSuccessor(SI->findCaseValue(Val));
}
// If we have exactly one destination, remember it for efficiency below.
if (i == 0)
OnlyDest = DestBB;
else if (OnlyDest != DestBB)
OnlyDest = MultipleDestSentinel;
PredToDestList.push_back(std::make_pair(Pred, DestBB));
}
// If all edges were unthreadable, we fail.
if (PredToDestList.empty())
return false;
// Determine which is the most common successor. If we have many inputs and
// this block is a switch, we want to start by threading the batch that goes
// to the most popular destination first. If we only know about one
// threadable destination (the common case) we can avoid this.
BasicBlock *MostPopularDest = OnlyDest;
if (MostPopularDest == MultipleDestSentinel)
MostPopularDest = FindMostPopularDest(BB, PredToDestList);
// Now that we know what the most popular destination is, factor all
// predecessors that will jump to it into a single predecessor.
SmallVector<BasicBlock*, 16> PredsToFactor;
for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
if (PredToDestList[i].second == MostPopularDest)
PredsToFactor.push_back(PredToDestList[i].first);
BasicBlock *PredToThread;
if (PredsToFactor.size() == 1)
PredToThread = PredsToFactor[0];
else {
DEBUG(errs() << " Factoring out " << PredsToFactor.size()
<< " common predecessors.\n");
PredToThread = SplitBlockPredecessors(BB, &PredsToFactor[0],
PredsToFactor.size(),
".thr_comm", this);
}
// If the threadable edges are branching on an undefined value, we get to pick
// the destination that these predecessors should get to.
if (MostPopularDest == 0)
MostPopularDest = BB->getTerminator()->
getSuccessor(GetBestDestForJumpOnUndef(BB));
// Ok, try to thread it!
return ThreadEdge(BB, PredToThread, MostPopularDest);
}
/// ProcessJumpOnPHI - We have a conditional branch or switch on a PHI node in
/// the current block. See if there are any simplifications we can do based on
@ -814,24 +1131,6 @@ bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB,
return ThreadEdge(BB, PredBB, SuccBB);
}
/// 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
@ -852,8 +1151,7 @@ bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) {
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
PredVal = PN->getIncomingValue(i);
Constant *Res = GetResultOfComparison(Cmp->getPredicate(), PredVal,
RHS, Cmp->getContext());
Constant *Res = GetResultOfComparison(Cmp->getPredicate(), PredVal, RHS);
if (!Res) {
PredVal = 0;
continue;

31
test/Transforms/JumpThreading/basic.ll
View File

@ -170,5 +170,36 @@ BB4:
}
;; This tests that the branch in 'merge' can be cloned up into T1.
;; rdar://7367025
define i32 @test7(i1 %cond, i1 %cond2) {
Entry:
; CHECK: @test7
%v1 = call i32 @f1()
br i1 %cond, label %Merge, label %F1
F1:
%v2 = call i32 @f2()
br label %Merge
Merge:
%B = phi i32 [%v1, %Entry], [%v2, %F1]
%M = icmp ne i32 %B, %v1
%N = icmp eq i32 %B, 47
%O = and i1 %M, %N
br i1 %O, label %T2, label %F2
; CHECK: Merge:
; CHECK-NOT: phi
; CHECK-NEXT: %v2 = call i32 @f2()
T2:
call void @f3()
ret i32 %B
F2:
ret i32 %B
; CHECK: F2:
; CHECK-NEXT: phi i32
}