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Teach the SLP Vectorizer that keeping some values live over a callsite can have a cost.

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Some types, such as 128-bit vector types on AArch64, don't have any callee-saved registers. So if a value needs to stay live over a callsite, it must be spilled and refilled. This cost is now taken into account.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214859 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
James Molloy
2014-08-05 12:30:34 +00:00
parent ff8028c8da
commit 72035e9a8e
5 changed files with 146 additions and 0 deletions
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68
lib/Transforms/Vectorize/SLPVectorizer.cpp
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@ -361,6 +361,10 @@ public:
/// Returns the vectorized root.
Value *vectorizeTree();
/// \returns the cost incurred by unwanted spills and fills, caused by
/// holding live values over call sites.
int getSpillCost();
/// \returns the vectorization cost of the subtree that starts at \p VL.
/// A negative number means that this is profitable.
int getTreeCost();
@ -1543,6 +1547,68 @@ bool BoUpSLP::isFullyVectorizableTinyTree() {
return true;
}
int BoUpSLP::getSpillCost() {
// Walk from the bottom of the tree to the top, tracking which values are
// live. When we see a call instruction that is not part of our tree,
// query TTI to see if there is a cost to keeping values live over it
// (for example, if spills and fills are required).
unsigned BundleWidth = VectorizableTree.front().Scalars.size();
int Cost = 0;
SmallPtrSet<Instruction*, 4> LiveValues;
Instruction *PrevInst = nullptr;
for (unsigned N = 0; N < VectorizableTree.size(); ++N) {
Instruction *Inst = dyn_cast<Instruction>(VectorizableTree[N].Scalars[0]);
if (!Inst)
continue;
if (!PrevInst) {
PrevInst = Inst;
continue;
}
DEBUG(
dbgs() << "SLP: #LV: " << LiveValues.size();
for (auto *X : LiveValues)
dbgs() << " " << X->getName();
dbgs() << ", Looking at ";
Inst->dump();
);
// Update LiveValues.
LiveValues.erase(PrevInst);
for (auto &J : PrevInst->operands()) {
if (isa<Instruction>(&*J) && ScalarToTreeEntry.count(&*J))
LiveValues.insert(cast<Instruction>(&*J));
}
// Now find the sequence of instructions between PrevInst and Inst.
BasicBlock::reverse_iterator InstIt(Inst), PrevInstIt(PrevInst);
--PrevInstIt;
while (InstIt != PrevInstIt) {
if (PrevInstIt == PrevInst->getParent()->rend()) {
PrevInstIt = Inst->getParent()->rbegin();
continue;
}
if (isa<CallInst>(&*PrevInstIt) && &*PrevInstIt != PrevInst) {
SmallVector<Type*, 4> V;
for (auto *II : LiveValues)
V.push_back(VectorType::get(II->getType(), BundleWidth));
Cost += TTI->getCostOfKeepingLiveOverCall(V);
}
++PrevInstIt;
}
PrevInst = Inst;
}
DEBUG(dbgs() << "SLP: SpillCost=" << Cost << "\n");
return Cost;
}
int BoUpSLP::getTreeCost() {
int Cost = 0;
DEBUG(dbgs() << "SLP: Calculating cost for tree of size " <<
@ -1578,6 +1644,8 @@ int BoUpSLP::getTreeCost() {
I->Lane);
}
Cost += getSpillCost();
DEBUG(dbgs() << "SLP: Total Cost " << Cost + ExtractCost<< ".\n");
return Cost + ExtractCost;
}