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Split local live ranges.
A local live range is live in a single basic block. If such a range fails to allocate, try to find a sub-range that would get a larger spill weight than its interference. llvm-svn: 125764
This commit is contained in:
Jakob Stoklund Olesen
parent
71117af2db
commit
65f52c387c
@ -72,6 +72,10 @@ class RAGreedy : public MachineFunctionPass, public RegAllocBase {
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/// All basic blocks where the current register is live.
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SmallVector<SpillPlacement::BlockConstraint, 8> SpillConstraints;
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/// For every instruction in SA->UseSlots, store the previous non-copy
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/// instruction.
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SmallVector<SlotIndex, 8> PrevSlot;
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public:
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RAGreedy();
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@ -106,11 +110,17 @@ private:
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float calcGlobalSplitCost(const BitVector&);
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void splitAroundRegion(LiveInterval&, unsigned, const BitVector&,
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SmallVectorImpl<LiveInterval*>&);
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void calcGapWeights(unsigned, SmallVectorImpl<float>&);
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SlotIndex getPrevMappedIndex(const MachineInstr*);
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void calcPrevSlots();
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unsigned nextSplitPoint(unsigned);
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unsigned tryReassignOrEvict(LiveInterval&, AllocationOrder&,
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SmallVectorImpl<LiveInterval*>&);
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unsigned tryRegionSplit(LiveInterval&, AllocationOrder&,
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SmallVectorImpl<LiveInterval*>&);
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unsigned tryLocalSplit(LiveInterval&, AllocationOrder&,
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SmallVectorImpl<LiveInterval*>&);
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unsigned trySplit(LiveInterval&, AllocationOrder&,
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SmallVectorImpl<LiveInterval*>&);
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unsigned trySpillInterferences(LiveInterval&, AllocationOrder&,
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@ -823,6 +833,271 @@ unsigned RAGreedy::tryRegionSplit(LiveInterval &VirtReg, AllocationOrder &Order,
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}
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//===----------------------------------------------------------------------===//
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// Local Splitting
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//===----------------------------------------------------------------------===//
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/// calcGapWeights - Compute the maximum spill weight that needs to be evicted
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/// in order to use PhysReg between two entries in SA->UseSlots.
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///
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/// GapWeight[i] represents the gap between UseSlots[i] and UseSlots[i+1].
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///
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void RAGreedy::calcGapWeights(unsigned PhysReg,
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SmallVectorImpl<float> &GapWeight) {
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assert(SA->LiveBlocks.size() == 1 && "Not a local interval");
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const SplitAnalysis::BlockInfo &BI = SA->LiveBlocks.front();
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const SmallVectorImpl<SlotIndex> &Uses = SA->UseSlots;
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const unsigned NumGaps = Uses.size()-1;
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// Start and end points for the interference check.
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SlotIndex StartIdx = BI.LiveIn ? BI.FirstUse.getBaseIndex() : BI.FirstUse;
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SlotIndex StopIdx = BI.LiveOut ? BI.LastUse.getBoundaryIndex() : BI.LastUse;
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GapWeight.assign(NumGaps, 0.0f);
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// Add interference from each overlapping register.
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for (const unsigned *AI = TRI->getOverlaps(PhysReg); *AI; ++AI) {
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if (!query(const_cast<LiveInterval&>(SA->getParent()), *AI)
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.checkInterference())
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continue;
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// We know that VirtReg is a continuous interval from FirstUse to LastUse,
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// so we don't need InterferenceQuery.
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//
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// Interference that overlaps an instruction is counted in both gaps
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// surrounding the instruction. The exception is interference before
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// StartIdx and after StopIdx.
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//
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LiveIntervalUnion::SegmentIter IntI = PhysReg2LiveUnion[*AI].find(StartIdx);
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for (unsigned Gap = 0; IntI.valid() && IntI.start() < StopIdx; ++IntI) {
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// Skip the gaps before IntI.
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while (Uses[Gap+1].getBoundaryIndex() < IntI.start())
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if (++Gap == NumGaps)
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break;
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if (Gap == NumGaps)
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break;
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// Update the gaps covered by IntI.
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const float weight = IntI.value()->weight;
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for (; Gap != NumGaps; ++Gap) {
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GapWeight[Gap] = std::max(GapWeight[Gap], weight);
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if (Uses[Gap+1].getBaseIndex() >= IntI.stop())
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break;
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}
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if (Gap == NumGaps)
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break;
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}
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}
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}
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/// getPrevMappedIndex - Return the slot index of the last non-copy instruction
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/// before MI that has a slot index. If MI is the first mapped instruction in
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/// its block, return the block start index instead.
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///
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SlotIndex RAGreedy::getPrevMappedIndex(const MachineInstr *MI) {
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assert(MI && "Missing MachineInstr");
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const MachineBasicBlock *MBB = MI->getParent();
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MachineBasicBlock::const_iterator B = MBB->begin(), I = MI;
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while (I != B)
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if (!(--I)->isDebugValue() && !I->isCopy())
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return Indexes->getInstructionIndex(I);
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return Indexes->getMBBStartIdx(MBB);
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}
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/// calcPrevSlots - Fill in the PrevSlot array with the index of the previous
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/// real non-copy instruction for each instruction in SA->UseSlots.
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///
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void RAGreedy::calcPrevSlots() {
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const SmallVectorImpl<SlotIndex> &Uses = SA->UseSlots;
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PrevSlot.clear();
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PrevSlot.reserve(Uses.size());
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for (unsigned i = 0, e = Uses.size(); i != e; ++i) {
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const MachineInstr *MI = Indexes->getInstructionFromIndex(Uses[i]);
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PrevSlot.push_back(getPrevMappedIndex(MI).getDefIndex());
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}
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}
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/// nextSplitPoint - Find the next index into SA->UseSlots > i such that it may
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/// be beneficial to split before UseSlots[i].
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///
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/// 0 is always a valid split point
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unsigned RAGreedy::nextSplitPoint(unsigned i) {
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const SmallVectorImpl<SlotIndex> &Uses = SA->UseSlots;
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const unsigned Size = Uses.size();
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assert(i != Size && "No split points after the end");
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// Allow split before i when Uses[i] is not adjacent to the previous use.
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while (++i != Size && PrevSlot[i].getBaseIndex() <= Uses[i-1].getBaseIndex())
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;
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return i;
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}
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/// tryLocalSplit - Try to split VirtReg into smaller intervals inside its only
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/// basic block.
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///
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unsigned RAGreedy::tryLocalSplit(LiveInterval &VirtReg, AllocationOrder &Order,
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SmallVectorImpl<LiveInterval*> &NewVRegs) {
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assert(SA->LiveBlocks.size() == 1 && "Not a local interval");
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const SplitAnalysis::BlockInfo &BI = SA->LiveBlocks.front();
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// Note that it is possible to have an interval that is live-in or live-out
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// while only covering a single block - A phi-def can use undef values from
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// predecessors, and the block could be a single-block loop.
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// We don't bother doing anything clever about such a case, we simply assume
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// that the interval is continuous from FirstUse to LastUse. We should make
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// sure that we don't do anything illegal to such an interval, though.
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const SmallVectorImpl<SlotIndex> &Uses = SA->UseSlots;
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if (Uses.size() <= 2)
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return 0;
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const unsigned NumGaps = Uses.size()-1;
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DEBUG({
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dbgs() << "tryLocalSplit: ";
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for (unsigned i = 0, e = Uses.size(); i != e; ++i)
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dbgs() << ' ' << SA->UseSlots[i];
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dbgs() << '\n';
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});
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// For every use, find the previous mapped non-copy instruction.
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// We use this to detect valid split points, and to estimate new interval
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// sizes.
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calcPrevSlots();
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unsigned BestBefore = NumGaps;
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unsigned BestAfter = 0;
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float BestDiff = 0;
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const float blockFreq = SpillPlacer->getBlockFrequency(BI.MBB);
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SmallVector<float, 8> GapWeight;
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Order.rewind();
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while (unsigned PhysReg = Order.next()) {
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// Keep track of the largest spill weight that would need to be evicted in
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// order to make use of PhysReg between UseSlots[i] and UseSlots[i+1].
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calcGapWeights(PhysReg, GapWeight);
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// Try to find the best sequence of gaps to close.
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// The new spill weight must be larger than any gap interference.
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// We will split before Uses[SplitBefore] and after Uses[SplitAfter].
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unsigned SplitBefore = 0, SplitAfter = nextSplitPoint(1) - 1;
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// MaxGap should always be max(GapWeight[SplitBefore..SplitAfter-1]).
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// It is the spill weight that needs to be evicted.
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float MaxGap = GapWeight[0];
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for (unsigned i = 1; i != SplitAfter; ++i)
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MaxGap = std::max(MaxGap, GapWeight[i]);
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for (;;) {
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// Live before/after split?
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const bool LiveBefore = SplitBefore != 0 || BI.LiveIn;
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const bool LiveAfter = SplitAfter != NumGaps || BI.LiveOut;
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DEBUG(dbgs() << PrintReg(PhysReg, TRI) << ' '
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<< Uses[SplitBefore] << '-' << Uses[SplitAfter]
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<< " i=" << MaxGap);
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// Stop before the interval gets so big we wouldn't be making progress.
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if (!LiveBefore && !LiveAfter) {
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DEBUG(dbgs() << " all\n");
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break;
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}
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// Should the interval be extended or shrunk?
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bool Shrink = true;
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if (MaxGap < HUGE_VALF) {
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// Estimate the new spill weight.
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//
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// Each instruction reads and writes the register, except the first
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// instr doesn't read when !FirstLive, and the last instr doesn't write
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// when !LastLive.
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//
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// We will be inserting copies before and after, so the total number of
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// reads and writes is 2 * EstUses.
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//
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const unsigned EstUses = 2*(SplitAfter - SplitBefore) +
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2*(LiveBefore + LiveAfter);
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// Try to guess the size of the new interval. This should be trivial,
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// but the slot index of an inserted copy can be a lot smaller than the
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// instruction it is inserted before if there are many dead indexes
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// between them.
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//
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// We measure the distance from the instruction before SplitBefore to
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// get a conservative estimate.
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//
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// The final distance can still be different if inserting copies
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// triggers a slot index renumbering.
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//
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const float EstWeight = normalizeSpillWeight(blockFreq * EstUses,
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PrevSlot[SplitBefore].distance(Uses[SplitAfter]));
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// Would this split be possible to allocate?
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// Never allocate all gaps, we wouldn't be making progress.
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float Diff = EstWeight - MaxGap;
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DEBUG(dbgs() << " w=" << EstWeight << " d=" << Diff);
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if (Diff > 0) {
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Shrink = false;
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if (Diff > BestDiff) {
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DEBUG(dbgs() << " (best)");
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BestDiff = Diff;
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BestBefore = SplitBefore;
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BestAfter = SplitAfter;
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}
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}
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}
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// Try to shrink.
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if (Shrink) {
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SplitBefore = nextSplitPoint(SplitBefore);
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if (SplitBefore < SplitAfter) {
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DEBUG(dbgs() << " shrink\n");
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// Recompute the max when necessary.
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if (GapWeight[SplitBefore - 1] >= MaxGap) {
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MaxGap = GapWeight[SplitBefore];
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for (unsigned i = SplitBefore + 1; i != SplitAfter; ++i)
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MaxGap = std::max(MaxGap, GapWeight[i]);
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}
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continue;
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}
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MaxGap = 0;
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}
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// Try to extend the interval.
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if (SplitAfter >= NumGaps) {
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DEBUG(dbgs() << " end\n");
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break;
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}
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DEBUG(dbgs() << " extend\n");
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for (unsigned e = nextSplitPoint(SplitAfter + 1) - 1;
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SplitAfter != e; ++SplitAfter)
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MaxGap = std::max(MaxGap, GapWeight[SplitAfter]);
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continue;
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}
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}
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// Didn't find any candidates?
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if (BestBefore == NumGaps)
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return 0;
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DEBUG(dbgs() << "Best local split range: " << Uses[BestBefore]
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<< '-' << Uses[BestAfter] << ", " << BestDiff
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<< ", " << (BestAfter - BestBefore + 1) << " instrs\n");
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SmallVector<LiveInterval*, 4> SpillRegs;
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LiveRangeEdit LREdit(VirtReg, NewVRegs, SpillRegs);
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SplitEditor SE(*SA, *LIS, *VRM, *DomTree, LREdit);
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SE.openIntv();
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SlotIndex SegStart = SE.enterIntvBefore(Uses[BestBefore]);
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SlotIndex SegStop = SE.leaveIntvAfter(Uses[BestAfter]);
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SE.useIntv(SegStart, SegStop);
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SE.closeIntv();
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SE.finish();
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return 0;
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}
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//===----------------------------------------------------------------------===//
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// Live Range Splitting
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//===----------------------------------------------------------------------===//
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@ -835,9 +1110,9 @@ unsigned RAGreedy::trySplit(LiveInterval &VirtReg, AllocationOrder &Order,
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NamedRegionTimer T("Splitter", TimerGroupName, TimePassesIsEnabled);
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SA->analyze(&VirtReg);
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// Don't attempt splitting on local intervals for now. TBD.
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// Local intervals are handled separately.
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if (LIS->intervalIsInOneMBB(VirtReg))
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return 0;
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return tryLocalSplit(VirtReg, Order, NewVRegs);
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// First try to split around a region spanning multiple blocks.
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unsigned PhysReg = tryRegionSplit(VirtReg, Order, NewVRegs);
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@ -116,6 +116,9 @@ public:
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/// new interval.
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void clear();
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/// getParent - Return the last analyzed interval.
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const LiveInterval &getParent() const { return *CurLI; }
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/// hasUses - Return true if MBB has any uses of CurLI.
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bool hasUses(const MachineBasicBlock *MBB) const {
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return UsingBlocks.lookup(MBB);
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