From 829dd81377abfd0c02257ebd10d31bb0b1377815 Mon Sep 17 00:00:00 2001 From: "Arnaud A. de Grandmaison" Date: Tue, 4 Nov 2014 20:51:24 +0000 Subject: [PATCH] [PBQP] Tweak spill costs and coalescing benefits This patch improves how the different costs (register, interference, spill and coalescing) relates together. The assumption is now that: - coalescing (or any other "side effect" of reg alloc) is negative, and instead of being derived from a spill cost, they use the block frequency info. - spill costs are in the [MinSpillCost:+inf( range - register or interference costs are in [0.0:MinSpillCost( or +inf The current MinSpillCost is set to 10.0, which is a random value high enough that the current constraint builders do not need to worry about when settings costs. It would however be worth adding a normalization step for register and interference costs as the last step in the constraint builder chain to ensure they are not greater than SpillMinCost (unless this has some sense for some architectures). This would work well with the current builder pipeline, where all costs are tweaked relatively to each others, but could grow above MinSpillCost if the pipeline is deep enough. The current heuristic is tuned to depend rather on the number of uses of a live interval rather than a density of uses, as used by the greedy allocator. This heuristic provides a few percent improvement on a number of benchmarks (eembc, spec, ...) and will definitely need to change once spill placement is implemented: the current spill placement is really ineficient, so making the cost proportionnal to the number of use is a clear win. llvm-svn: 221292 --- llvm/include/llvm/CodeGen/CalcSpillWeights.h | 6 ++++-- llvm/lib/CodeGen/CalcSpillWeights.cpp | 4 +++- llvm/lib/CodeGen/RegAllocGreedy.cpp | 8 +++++--- llvm/lib/CodeGen/RegAllocPBQP.cpp | 23 +++++++++++++++++------ 4 files changed, 29 insertions(+), 12 deletions(-) diff --git a/llvm/include/llvm/CodeGen/CalcSpillWeights.h b/llvm/include/llvm/CodeGen/CalcSpillWeights.h index 0d79b1d..91fb0a9 100644 --- a/llvm/include/llvm/CodeGen/CalcSpillWeights.h +++ b/llvm/include/llvm/CodeGen/CalcSpillWeights.h @@ -30,8 +30,10 @@ namespace llvm { /// @param UseDefFreq Expected number of executed use and def instructions /// per function call. Derived from block frequencies. /// @param Size Size of live interval as returnexd by getSize() + /// @param NumInstr Number of instructions using this live interval /// - static inline float normalizeSpillWeight(float UseDefFreq, unsigned Size) { + static inline float normalizeSpillWeight(float UseDefFreq, unsigned Size, + unsigned NumInstr) { // The constant 25 instructions is added to avoid depending too much on // accidental SlotIndex gaps for small intervals. The effect is that small // intervals have a spill weight that is mostly proportional to the number @@ -44,7 +46,7 @@ namespace llvm { /// spill weight and allocation hint. class VirtRegAuxInfo { public: - typedef float (*NormalizingFn)(float, unsigned); + typedef float (*NormalizingFn)(float, unsigned, unsigned); private: MachineFunction &MF; diff --git a/llvm/lib/CodeGen/CalcSpillWeights.cpp b/llvm/lib/CodeGen/CalcSpillWeights.cpp index bf3882a..0b0c176 100644 --- a/llvm/lib/CodeGen/CalcSpillWeights.cpp +++ b/llvm/lib/CodeGen/CalcSpillWeights.cpp @@ -100,6 +100,7 @@ VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) { MachineLoop *loop = nullptr; bool isExiting = false; float totalWeight = 0; + unsigned numInstr = 0; // Number of instructions using li SmallPtrSet visited; // Find the best physreg hint and the best virtreg hint. @@ -116,6 +117,7 @@ VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) { I = mri.reg_instr_begin(li.reg), E = mri.reg_instr_end(); I != E; ) { MachineInstr *mi = &*(I++); + numInstr++; if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue()) continue; if (!visited.insert(mi)) @@ -189,5 +191,5 @@ VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) { if (isRematerializable(li, LIS, *MF.getSubtarget().getInstrInfo())) totalWeight *= 0.5F; - li.weight = normalize(totalWeight, li.getSize()); + li.weight = normalize(totalWeight, li.getSize(), numInstr); } diff --git a/llvm/lib/CodeGen/RegAllocGreedy.cpp b/llvm/lib/CodeGen/RegAllocGreedy.cpp index 1d081c7..dfeb9c0 100644 --- a/llvm/lib/CodeGen/RegAllocGreedy.cpp +++ b/llvm/lib/CodeGen/RegAllocGreedy.cpp @@ -1789,9 +1789,11 @@ unsigned RAGreedy::tryLocalSplit(LiveInterval &VirtReg, AllocationOrder &Order, // instructions. // // Try to guess the size of the new interval. - const float EstWeight = normalizeSpillWeight(blockFreq * (NewGaps + 1), - Uses[SplitBefore].distance(Uses[SplitAfter]) + - (LiveBefore + LiveAfter)*SlotIndex::InstrDist); + const float EstWeight = normalizeSpillWeight( + blockFreq * (NewGaps + 1), + Uses[SplitBefore].distance(Uses[SplitAfter]) + + (LiveBefore + LiveAfter) * SlotIndex::InstrDist, + 1); // Would this split be possible to allocate? // Never allocate all gaps, we wouldn't be making progress. DEBUG(dbgs() << " w=" << EstWeight); diff --git a/llvm/lib/CodeGen/RegAllocPBQP.cpp b/llvm/lib/CodeGen/RegAllocPBQP.cpp index 768e650..fa45dd2 100644 --- a/llvm/lib/CodeGen/RegAllocPBQP.cpp +++ b/llvm/lib/CodeGen/RegAllocPBQP.cpp @@ -150,11 +150,17 @@ public: void apply(PBQPRAGraph &G) override { LiveIntervals &LIS = G.getMetadata().LIS; + // A minimum spill costs, so that register constraints can can be set + // without normalization in the [0.0:MinSpillCost( interval. + const PBQP::PBQPNum MinSpillCost = 10.0; + for (auto NId : G.nodeIds()) { PBQP::PBQPNum SpillCost = LIS.getInterval(G.getNodeMetadata(NId).getVReg()).weight; if (SpillCost == 0.0) SpillCost = std::numeric_limits::min(); + else + SpillCost += MinSpillCost; PBQPRAGraph::RawVector NodeCosts(G.getNodeCosts(NId)); NodeCosts[PBQP::RegAlloc::getSpillOptionIdx()] = SpillCost; G.setNodeCosts(NId, std::move(NodeCosts)); @@ -350,11 +356,8 @@ public: unsigned DstReg = CP.getDstReg(); unsigned SrcReg = CP.getSrcReg(); - const float CopyFactor = 0.5; // Cost of copy relative to load. Current - // value plucked randomly out of the air. - - PBQP::PBQPNum CBenefit = - CopyFactor * LiveIntervals::getSpillWeight(false, true, &MBFI, &MI); + const float Scale = 1.0f / MBFI.getEntryFreq(); + PBQP::PBQPNum CBenefit = MBFI.getBlockFreq(&MBB).getFrequency() * Scale; if (CP.isPhys()) { if (!MF.getRegInfo().isAllocatable(DstReg)) @@ -607,12 +610,20 @@ void RegAllocPBQP::finalizeAlloc(MachineFunction &MF, } } +static inline float normalizePBQPSpillWeight(float UseDefFreq, unsigned Size, + unsigned NumInstr) { + // All intervals have a spill weight that is mostly proportional to the number + // of uses, with uses in loops having a bigger weight. + return NumInstr * normalizeSpillWeight(UseDefFreq, Size, 1); +} + bool RegAllocPBQP::runOnMachineFunction(MachineFunction &MF) { LiveIntervals &LIS = getAnalysis(); MachineBlockFrequencyInfo &MBFI = getAnalysis(); - calculateSpillWeightsAndHints(LIS, MF, getAnalysis(), MBFI); + calculateSpillWeightsAndHints(LIS, MF, getAnalysis(), MBFI, + normalizePBQPSpillWeight); VirtRegMap &VRM = getAnalysis(); -- 2.7.4