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Always compare the cost of region splitting with the cost of per-block splitting.

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Sometimes it is better to split per block, and we missed those cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130025 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jakob Stoklund Olesen 2011-04-22 22:47:40 +00:00
parent 3335a99fe8
commit 200729882a
1 changed files with 45 additions and 6 deletions
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51
lib/CodeGen/RegAllocGreedy.cpp
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@ -175,6 +175,7 @@ private:
void LRE_WillShrinkVirtReg(unsigned);
void LRE_DidCloneVirtReg(unsigned, unsigned);
float calcSpillCost();
bool addSplitConstraints(InterferenceCache::Cursor, float&);
void addThroughConstraints(InterferenceCache::Cursor, ArrayRef<unsigned>);
void growRegion(GlobalSplitCandidate &Cand, InterferenceCache::Cursor);
@ -202,6 +203,11 @@ private:
char RAGreedy::ID = 0;
// Hysteresis to use when comparing floats.
// This helps stabilize decisions based on float comparisons.
const float Hysteresis = 0.98f;
FunctionPass* llvm::createGreedyRegisterAllocator() {
return new RAGreedy();
}
@ -615,6 +621,33 @@ void RAGreedy::growRegion(GlobalSplitCandidate &Cand,
DEBUG(dbgs() << ", v=" << Visited);
}
/// calcSpillCost - Compute how expensive it would be to split the live range in
/// SA around all use blocks instead of forming bundle regions.
float RAGreedy::calcSpillCost() {
float Cost = 0;
const LiveInterval &LI = SA->getParent();
ArrayRef<SplitAnalysis::BlockInfo> UseBlocks = SA->getUseBlocks();
for (unsigned i = 0; i != UseBlocks.size(); ++i) {
const SplitAnalysis::BlockInfo &BI = UseBlocks[i];
unsigned Number = BI.MBB->getNumber();
// We normally only need one spill instruction - a load or a store.
Cost += SpillPlacer->getBlockFrequency(Number);
// Unless the value is redefined in the block.
if (BI.LiveIn && BI.LiveOut) {
SlotIndex Start, Stop;
tie(Start, Stop) = Indexes->getMBBRange(Number);
LiveInterval::const_iterator I = LI.find(Start);
assert(I != LI.end() && "Expected live-in value");
// Is there a different live-out value? If so, we need an extra spill
// instruction.
if (I->end < Stop)
Cost += SpillPlacer->getBlockFrequency(Number);
}
}
return Cost;
}
/// calcGlobalSplitCost - Return the global split cost of following the split
/// pattern in LiveBundles. This cost should be added to the local cost of the
/// interference pattern in SplitConstraints.
@ -919,7 +952,8 @@ void RAGreedy::splitAroundRegion(LiveInterval &VirtReg,
unsigned RAGreedy::tryRegionSplit(LiveInterval &VirtReg, AllocationOrder &Order,
SmallVectorImpl<LiveInterval*> &NewVRegs) {
float BestCost = 0;
float BestCost = Hysteresis * calcSpillCost();
DEBUG(dbgs() << "Cost of isolating all blocks = " << BestCost << '\n');
const unsigned NoCand = ~0u;
unsigned BestCand = NoCand;
@ -937,9 +971,14 @@ unsigned RAGreedy::tryRegionSplit(LiveInterval &VirtReg, AllocationOrder &Order,
continue;
}
DEBUG(dbgs() << PrintReg(PhysReg, TRI) << "\tstatic = " << Cost);
if (BestCand != NoCand && Cost >= BestCost) {
DEBUG(dbgs() << " worse than "
<< PrintReg(GlobalCand[BestCand].PhysReg, TRI) << '\n');
if (Cost >= BestCost) {
DEBUG({
if (BestCand == NoCand)
dbgs() << " worse than no bundles\n";
else
dbgs() << " worse than "
<< PrintReg(GlobalCand[BestCand].PhysReg, TRI) << '\n';
});
continue;
}
growRegion(GlobalCand[Cand], Intf);
@ -960,9 +999,9 @@ unsigned RAGreedy::tryRegionSplit(LiveInterval &VirtReg, AllocationOrder &Order,
dbgs() << " EB#" << i;
dbgs() << ".\n";
});
if (BestCand == NoCand || Cost < BestCost) {
if (Cost < BestCost) {
BestCand = Cand;
BestCost = 0.98f * Cost; // Prevent rounding effects.
BestCost = Hysteresis * Cost; // Prevent rounding effects.
}
}