diff --git a/llvm/lib/CodeGen/RegAllocLinearScan.cpp b/llvm/lib/CodeGen/RegAllocLinearScan.cpp index fff50da947c1..be1fa08cfa00 100644 --- a/llvm/lib/CodeGen/RegAllocLinearScan.cpp +++ b/llvm/lib/CodeGen/RegAllocLinearScan.cpp @@ -64,9 +64,30 @@ linearscanRegAlloc("linearscan", "linear scan register allocator", createLinearScanRegisterAllocator); namespace { + // When we allocate a register, add it to a fixed-size queue of + // registers to skip in subsequent allocations. This trades a small + // amount of register pressure and increased spills for flexibility in + // the post-pass scheduler. + // + // Note that in a the number of registers used for reloading spills + // will be one greater than the value of this option. + // + // One big limitation of this is that it doesn't differentiate between + // different register classes. So on x86-64, if there is xmm register + // pressure, it can caused fewer GPRs to be held in the queue. + static cl::opt + NumRecentlyUsedRegs("linearscan-skip-count", + cl::desc("Number of registers for linearscan to remember to skip."), + cl::init(0), + cl::Hidden); + struct RALinScan : public MachineFunctionPass { static char ID; - RALinScan() : MachineFunctionPass(&ID) {} + RALinScan() : MachineFunctionPass(&ID) { + // Initialize the queue to record recently-used registers. + if (NumRecentlyUsedRegs > 0) + RecentRegs.resize(NumRecentlyUsedRegs, 0); + } typedef std::pair IntervalPtr; typedef SmallVector IntervalPtrs; @@ -132,6 +153,18 @@ namespace { std::auto_ptr spiller_; + // The queue of recently-used registers. + SmallVector RecentRegs; + + // Record that we just picked this register. + void recordRecentlyUsed(unsigned reg) { + assert(reg != 0 && "Recently used register is NOREG!"); + if (!RecentRegs.empty()) { + std::copy(RecentRegs.begin() + 1, RecentRegs.end(), RecentRegs.begin()); + RecentRegs.back() = reg; + } + } + public: virtual const char* getPassName() const { return "Linear Scan Register Allocator"; @@ -161,6 +194,12 @@ namespace { /// runOnMachineFunction - register allocate the whole function bool runOnMachineFunction(MachineFunction&); + // Determine if we skip this register due to its being recently used. + bool isRecentlyUsed(unsigned reg) const { + return std::find(RecentRegs.begin(), RecentRegs.end(), reg) != + RecentRegs.end(); + } + private: /// linearScan - the linear scan algorithm void linearScan(); @@ -833,9 +872,15 @@ void RALinScan::findIntervalsToSpill(LiveInterval *cur, namespace { struct WeightCompare { + private: + const RALinScan &Allocator; + + public: + WeightCompare(const RALinScan &Alloc) : Allocator(Alloc) {}; + typedef std::pair RegWeightPair; bool operator()(const RegWeightPair &LHS, const RegWeightPair &RHS) const { - return LHS.second < RHS.second; + return LHS.second < RHS.second && !Allocator.isRecentlyUsed(LHS.first); } }; } @@ -1079,7 +1124,8 @@ void RALinScan::assignRegOrStackSlotAtInterval(LiveInterval* cur) { e = RC->allocation_order_end(*mf_); i != e; ++i) { unsigned reg = *i; float regWeight = SpillWeights[reg]; - if (minWeight > regWeight) + // Skip recently allocated registers. + if (minWeight > regWeight && !isRecentlyUsed(reg)) Found = true; RegsWeights.push_back(std::make_pair(reg, regWeight)); } @@ -1097,7 +1143,7 @@ void RALinScan::assignRegOrStackSlotAtInterval(LiveInterval* cur) { } // Sort all potential spill candidates by weight. - std::sort(RegsWeights.begin(), RegsWeights.end(), WeightCompare()); + std::sort(RegsWeights.begin(), RegsWeights.end(), WeightCompare(*this)); minReg = RegsWeights[0].first; minWeight = RegsWeights[0].second; if (minWeight == HUGE_VALF) { @@ -1360,7 +1406,8 @@ unsigned RALinScan::getFreePhysReg(LiveInterval* cur, // Ignore "downgraded" registers. if (SkipDGRegs && DowngradedRegs.count(Reg)) continue; - if (isRegAvail(Reg)) { + // Skip recently allocated registers. + if (isRegAvail(Reg) && !isRecentlyUsed(Reg)) { FreeReg = Reg; if (FreeReg < inactiveCounts.size()) FreeRegInactiveCount = inactiveCounts[FreeReg]; @@ -1372,9 +1419,12 @@ unsigned RALinScan::getFreePhysReg(LiveInterval* cur, // If there are no free regs, or if this reg has the max inactive count, // return this register. - if (FreeReg == 0 || FreeRegInactiveCount == MaxInactiveCount) + if (FreeReg == 0 || FreeRegInactiveCount == MaxInactiveCount) { + // Remember what register we picked so we can skip it next time. + if (FreeReg != 0) recordRecentlyUsed(FreeReg); return FreeReg; - + } + // Continue scanning the registers, looking for the one with the highest // inactive count. Alkis found that this reduced register pressure very // slightly on X86 (in rev 1.94 of this file), though this should probably be @@ -1393,6 +1443,9 @@ unsigned RALinScan::getFreePhysReg(LiveInterval* cur, } } + // Remember what register we picked so we can skip it next time. + recordRecentlyUsed(FreeReg); + return FreeReg; }