#include "llvm/CodeGen/RegAllocRegistry.h"
#include "llvm/CodeGen/RegisterClassInfo.h"
#include "llvm/CodeGen/VirtRegMap.h"
+#include "llvm/IR/LLVMContext.h"
#include "llvm/PassAnalysisSupport.h"
+#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
" interference at a time"),
cl::init(8));
+static cl::opt<bool>
+ExhaustiveSearch("exhaustive-register-search", cl::NotHidden,
+ cl::desc("Exhaustive Search for registers bypassing the depth "
+ "and interference cutoffs of last chance recoloring"));
+
+// FIXME: Find a good default for this flag and remove the flag.
+static cl::opt<unsigned>
+CSRFirstTimeCost("regalloc-csr-first-time-cost",
+ cl::desc("Cost for first time use of callee-saved register."),
+ cl::init(0), cl::Hidden);
+
static RegisterRegAlloc greedyRegAlloc("greedy", "greedy register allocator",
createGreedyRegisterAllocator);
RS_Done
};
+ // Enum CutOffStage to keep a track whether the register allocation failed
+ // because of the cutoffs encountered in last chance recoloring.
+ // Note: This is used as bitmask. New value should be next power of 2.
+ enum CutOffStage {
+ // No cutoffs encountered
+ CO_None = 0,
+
+ // lcr-max-depth cutoff encountered
+ CO_Depth = 1,
+
+ // lcr-max-interf cutoff encountered
+ CO_Interf = 2
+ };
+
+ uint8_t CutOffInfo;
+
#ifndef NDEBUG
static const char *const StageName[];
#endif
/// NoCand which indicates the stack interval.
SmallVector<unsigned, 32> BundleCand;
+ /// Callee-save register cost, calculated once per machine function.
+ BlockFrequency CSRCost;
+
public:
RAGreedy();
unsigned calculateRegionSplitCost(LiveInterval &VirtReg,
AllocationOrder &Order,
BlockFrequency &BestCost,
- unsigned &NumCands);
+ unsigned &NumCands, bool IgnoreCSR);
/// Perform region splitting.
unsigned doRegionSplit(LiveInterval &VirtReg, unsigned BestCand,
bool HasCompact,
SmallVectorImpl<unsigned> &NewVRegs);
+ /// Check other options before using a callee-saved register for the first
+ /// time.
+ unsigned tryAssignCSRFirstTime(LiveInterval &VirtReg, AllocationOrder &Order,
+ unsigned PhysReg, unsigned &CostPerUseLimit,
+ SmallVectorImpl<unsigned> &NewVRegs);
+ void initializeCSRCost();
unsigned tryBlockSplit(LiveInterval&, AllocationOrder&,
SmallVectorImpl<unsigned>&);
unsigned tryInstructionSplit(LiveInterval&, AllocationOrder&,
}
void RAGreedy::releaseMemory() {
- SpillerInstance.reset(0);
+ SpillerInstance.reset(nullptr);
ExtraRegInfo.clear();
GlobalCand.clear();
}
LiveInterval *RAGreedy::dequeue(PQueue &CurQueue) {
if (CurQueue.empty())
- return 0;
+ return nullptr;
LiveInterval *LI = &LIS->getInterval(~CurQueue.top().second);
CurQueue.pop();
return LI;
}
unsigned BestCand =
- calculateRegionSplitCost(VirtReg, Order, BestCost, NumCands);
+ calculateRegionSplitCost(VirtReg, Order, BestCost, NumCands,
+ false/*IgnoreCSR*/);
// No solutions found, fall back to single block splitting.
if (!HasCompact && BestCand == NoCand)
unsigned RAGreedy::calculateRegionSplitCost(LiveInterval &VirtReg,
AllocationOrder &Order,
BlockFrequency &BestCost,
- unsigned &NumCands) {
+ unsigned &NumCands,
+ bool IgnoreCSR) {
unsigned BestCand = NoCand;
Order.rewind();
while (unsigned PhysReg = Order.next()) {
+ if (unsigned CSR = RegClassInfo.getLastCalleeSavedAlias(PhysReg))
+ if (IgnoreCSR && !MRI->isPhysRegUsed(CSR))
+ continue;
+
// Discard bad candidates before we run out of interference cache cursors.
// This will only affect register classes with a lot of registers (>32).
if (NumCands == IntfCache.getMaxCursors()) {
// If there is LastChanceRecoloringMaxInterference or more interferences,
// chances are one would not be recolorable.
if (Q.collectInterferingVRegs(LastChanceRecoloringMaxInterference) >=
- LastChanceRecoloringMaxInterference) {
+ LastChanceRecoloringMaxInterference && !ExhaustiveSearch) {
DEBUG(dbgs() << "Early abort: too many interferences.\n");
+ CutOffInfo |= CO_Interf;
return false;
}
for (unsigned i = Q.interferingVRegs().size(); i; --i) {
// We may want to reconsider that if we end up with a too large search space
// for target with hundreds of registers.
// Indeed, in that case we may want to cut the search space earlier.
- if (Depth >= LastChanceRecoloringMaxDepth) {
+ if (Depth >= LastChanceRecoloringMaxDepth && !ExhaustiveSearch) {
DEBUG(dbgs() << "Abort because max depth has been reached.\n");
+ CutOffInfo |= CO_Depth;
return ~0u;
}
unsigned RAGreedy::selectOrSplit(LiveInterval &VirtReg,
SmallVectorImpl<unsigned> &NewVRegs) {
+ CutOffInfo = CO_None;
+ LLVMContext &Ctx = MF->getFunction()->getContext();
SmallVirtRegSet FixedRegisters;
- return selectOrSplitImpl(VirtReg, NewVRegs, FixedRegisters);
+ unsigned Reg = selectOrSplitImpl(VirtReg, NewVRegs, FixedRegisters);
+ if (Reg == ~0U && (CutOffInfo != CO_None)) {
+ uint8_t CutOffEncountered = CutOffInfo & (CO_Depth | CO_Interf);
+ if (CutOffEncountered == CO_Depth)
+ Ctx.emitError("register allocation failed: maximum depth for recoloring "
+ "reached. Use -fexhaustive-register-search to skip "
+ "cutoffs");
+ else if (CutOffEncountered == CO_Interf)
+ Ctx.emitError("register allocation failed: maximum interference for "
+ "recoloring reached. Use -fexhaustive-register-search "
+ "to skip cutoffs");
+ else if (CutOffEncountered == (CO_Depth | CO_Interf))
+ Ctx.emitError("register allocation failed: maximum interference and "
+ "depth for recoloring reached. Use "
+ "-fexhaustive-register-search to skip cutoffs");
+ }
+ return Reg;
+}
+
+/// Using a CSR for the first time has a cost because it causes push|pop
+/// to be added to prologue|epilogue. Splitting a cold section of the live
+/// range can have lower cost than using the CSR for the first time;
+/// Spilling a live range in the cold path can have lower cost than using
+/// the CSR for the first time. Returns the physical register if we decide
+/// to use the CSR; otherwise return 0.
+unsigned RAGreedy::tryAssignCSRFirstTime(LiveInterval &VirtReg,
+ AllocationOrder &Order,
+ unsigned PhysReg,
+ unsigned &CostPerUseLimit,
+ SmallVectorImpl<unsigned> &NewVRegs) {
+ if (getStage(VirtReg) == RS_Spill && VirtReg.isSpillable()) {
+ // We choose spill over using the CSR for the first time if the spill cost
+ // is lower than CSRCost.
+ SA->analyze(&VirtReg);
+ if (calcSpillCost() >= CSRCost)
+ return PhysReg;
+
+ // We are going to spill, set CostPerUseLimit to 1 to make sure that
+ // we will not use a callee-saved register in tryEvict.
+ CostPerUseLimit = 1;
+ return 0;
+ }
+ if (getStage(VirtReg) < RS_Split) {
+ // We choose pre-splitting over using the CSR for the first time if
+ // the cost of splitting is lower than CSRCost.
+ SA->analyze(&VirtReg);
+ unsigned NumCands = 0;
+ BlockFrequency BestCost = CSRCost; // Don't modify CSRCost.
+ unsigned BestCand = calculateRegionSplitCost(VirtReg, Order, BestCost,
+ NumCands, true /*IgnoreCSR*/);
+ if (BestCand == NoCand)
+ // Use the CSR if we can't find a region split below CSRCost.
+ return PhysReg;
+
+ // Perform the actual pre-splitting.
+ doRegionSplit(VirtReg, BestCand, false/*HasCompact*/, NewVRegs);
+ return 0;
+ }
+ return PhysReg;
+}
+
+void RAGreedy::initializeCSRCost() {
+ // We use the larger one out of the command-line option and the value report
+ // by TRI.
+ CSRCost = BlockFrequency(
+ std::max((unsigned)CSRFirstTimeCost, TRI->getCSRFirstUseCost()));
+ if (!CSRCost.getFrequency())
+ return;
+
+ // Raw cost is relative to Entry == 2^14; scale it appropriately.
+ uint64_t ActualEntry = MBFI->getEntryFreq();
+ if (!ActualEntry) {
+ CSRCost = 0;
+ return;
+ }
+ uint64_t FixedEntry = 1 << 14;
+ if (ActualEntry < FixedEntry)
+ CSRCost *= BranchProbability(ActualEntry, FixedEntry);
+ else if (ActualEntry <= UINT32_MAX)
+ // Invert the fraction and divide.
+ CSRCost /= BranchProbability(FixedEntry, ActualEntry);
+ else
+ // Can't use BranchProbability in general, since it takes 32-bit numbers.
+ CSRCost = CSRCost.getFrequency() * (ActualEntry / FixedEntry);
}
unsigned RAGreedy::selectOrSplitImpl(LiveInterval &VirtReg,
SmallVectorImpl<unsigned> &NewVRegs,
SmallVirtRegSet &FixedRegisters,
unsigned Depth) {
+ unsigned CostPerUseLimit = ~0u;
// First try assigning a free register.
AllocationOrder Order(VirtReg.reg, *VRM, RegClassInfo);
- if (unsigned PhysReg = tryAssign(VirtReg, Order, NewVRegs))
- return PhysReg;
+ if (unsigned PhysReg = tryAssign(VirtReg, Order, NewVRegs)) {
+ // We check other options if we are using a CSR for the first time.
+ bool CSRFirstUse = false;
+ if (unsigned CSR = RegClassInfo.getLastCalleeSavedAlias(PhysReg))
+ if (!MRI->isPhysRegUsed(CSR))
+ CSRFirstUse = true;
+
+ // When NewVRegs is not empty, we may have made decisions such as evicting
+ // a virtual register, go with the earlier decisions and use the physical
+ // register.
+ if (CSRCost.getFrequency() && CSRFirstUse && NewVRegs.empty()) {
+ unsigned CSRReg = tryAssignCSRFirstTime(VirtReg, Order, PhysReg,
+ CostPerUseLimit, NewVRegs);
+ if (CSRReg || !NewVRegs.empty())
+ // Return now if we decide to use a CSR or create new vregs due to
+ // pre-splitting.
+ return CSRReg;
+ } else
+ return PhysReg;
+ }
LiveRangeStage Stage = getStage(VirtReg);
DEBUG(dbgs() << StageName[Stage]
// queue. The RS_Split ranges already failed to do this, and they should not
// get a second chance until they have been split.
if (Stage != RS_Split)
- if (unsigned PhysReg = tryEvict(VirtReg, Order, NewVRegs))
+ if (unsigned PhysReg = tryEvict(VirtReg, Order, NewVRegs, CostPerUseLimit))
return PhysReg;
assert(NewVRegs.empty() && "Cannot append to existing NewVRegs");
SpillPlacer = &getAnalysis<SpillPlacement>();
DebugVars = &getAnalysis<LiveDebugVariables>();
+ initializeCSRCost();
+
calculateSpillWeightsAndHints(*LIS, mf, *Loops, *MBFI);
DEBUG(LIS->dump());
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