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add the ability to get a rewritten value from the middle of a block,

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not just at the end.  Add a big comment explaining when this could
be useful (which never happens for jump threading).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83741 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2009-10-10 23:00:11 +00:00
parent 5fb107287f
commit 1a8d4de397
2 changed files with 119 additions and 6 deletions
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30
include/llvm/Transforms/Utils/SSAUpdater.h
View File

@ -48,16 +48,40 @@ public:
/// updates. ProtoValue is the value used to name PHI nodes.
void Initialize(Value *ProtoValue);
/// AddAvailableValue - Indicate that a rewritten value is available in the
/// specified block with the specified value.
/// AddAvailableValue - Indicate that a rewritten value is available at the
/// end of the specified block with the specified value.
void AddAvailableValue(BasicBlock *BB, Value *V);
/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
/// live at the end of the specified block.
Value *GetValueAtEndOfBlock(BasicBlock *BB);
/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
/// is live in the middle of the specified block.
///
/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
/// important case: if there is a definition of the rewritten value after the
/// 'use' in BB. Consider code like this:
///
/// X1 = ...
/// SomeBB:
/// use(X)
/// X2 = ...
/// br Cond, SomeBB, OutBB
///
/// In this case, there are two values (X1 and X2) added to the AvailableVals
/// set by the client of the rewriter, and those values are both live out of
/// their respective blocks. However, the use of X happens in the *middle* of
/// a block. Because of this, we need to insert a new PHI node in SomeBB to
/// merge the appropriate values, and this value isn't live out of the block.
///
Value *GetValueInMiddleOfBlock(BasicBlock *BB);
/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
/// which use their value in the corresponding predecessor.
/// which use their value in the corresponding predecessor. Note that this
/// will not work if the use is supposed to be rewritten to a value defined in
/// the same block as the use, but above it. Any 'AddAvailableValue's added
/// for the use's block will be considered to be below it.
void RewriteUse(Use &U);
private:

95
lib/Transforms/Utils/SSAUpdater.cpp
View File

@ -64,8 +64,8 @@ void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
getAvailableVals(AV)[BB] = V;
}
/// GetValueAtEndOfBlock - Construct SSA form, materializing a value in the
/// specified block.
/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
/// live at the end of the specified block.
Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
Value *Res = GetValueAtEndOfBlockInternal(BB);
@ -73,6 +73,95 @@ Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
return Res;
}
/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
/// is live in the middle of the specified block.
///
/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
/// important case: if there is a definition of the rewritten value after the
/// 'use' in BB. Consider code like this:
///
/// X1 = ...
/// SomeBB:
/// use(X)
/// X2 = ...
/// br Cond, SomeBB, OutBB
///
/// In this case, there are two values (X1 and X2) added to the AvailableVals
/// set by the client of the rewriter, and those values are both live out of
/// their respective blocks. However, the use of X happens in the *middle* of
/// a block. Because of this, we need to insert a new PHI node in SomeBB to
/// merge the appropriate values, and this value isn't live out of the block.
///
Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
// If there is no definition of the renamed variable in this block, just use
// GetValueAtEndOfBlock to do our work.
if (!getAvailableVals(AV).count(BB))
return GetValueAtEndOfBlock(BB);
// Otherwise, we have the hard case. Get the live-in values for each
// predecessor.
SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues;
Value *SingularValue = 0;
// We can get our predecessor info by walking the pred_iterator list, but it
// is relatively slow. If we already have PHI nodes in this block, walk one
// of them to get the predecessor list instead.
if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
Value *PredVal = GetValueAtEndOfBlock(PredBB);
PredValues.push_back(std::make_pair(PredBB, PredVal));
// Compute SingularValue.
if (i == 0)
SingularValue = PredVal;
else if (PredVal != SingularValue)
SingularValue = 0;
}
} else {
bool isFirstPred = true;
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
BasicBlock *PredBB = *PI;
Value *PredVal = GetValueAtEndOfBlock(PredBB);
PredValues.push_back(std::make_pair(PredBB, PredVal));
// Compute SingularValue.
if (isFirstPred) {
SingularValue = PredVal;
isFirstPred = false;
} else if (PredVal != SingularValue)
SingularValue = 0;
}
}
// If there are no predecessors, just return undef.
if (PredValues.empty())
return UndefValue::get(PrototypeValue->getType());
// Otherwise, if all the merged values are the same, just use it.
if (SingularValue != 0)
return SingularValue;
// Otherwise, we do need a PHI: insert one now.
PHINode *InsertedPHI = PHINode::Create(PrototypeValue->getType(),
PrototypeValue->getName(),
&BB->front());
InsertedPHI->reserveOperandSpace(PredValues.size());
// Fill in all the predecessors of the PHI.
for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first);
// See if the PHI node can be merged to a single value. This can happen in
// loop cases when we get a PHI of itself and one other value.
if (Value *ConstVal = InsertedPHI->hasConstantValue()) {
InsertedPHI->eraseFromParent();
return ConstVal;
}
DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n");
return InsertedPHI;
}
/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
/// which use their value in the corresponding predecessor.
void SSAUpdater::RewriteUse(Use &U) {
@ -81,7 +170,7 @@ void SSAUpdater::RewriteUse(Use &U) {
if (PHINode *UserPN = dyn_cast<PHINode>(User))
UseBB = UserPN->getIncomingBlock(U);
U.set(GetValueAtEndOfBlock(UseBB));
U.set(GetValueInMiddleOfBlock(UseBB));
}