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llvm-project/llvm/lib/Target/AMDGPU/GCNRegBankReassign.cpp

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//===-- GCNRegBankReassign.cpp - Reassign registers after regalloc --------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
/// \file
/// \brief Try to reassign registers on GFX10+ to reduce register bank
/// conflicts.
///
/// On GFX10 registers are organized in banks. VGPRs have 4 banks assigned in
/// a round-robin fashion: v0, v4, v8... belong to bank 0. v1, v5, v9... to
/// bank 1, etc. SGPRs have 8 banks and allocated in pairs, so that s0:s1,
/// s16:s17, s32:s33 are at bank 0. s2:s3, s18:s19, s34:s35 are at bank 1 etc.
///
/// The shader can read one dword from each of these banks once per cycle.
/// If an instruction has to read more register operands from the same bank
/// an additional cycle is needed. HW attempts to pre-load registers through
/// input operand gathering, but a stall cycle may occur if that fails. For
/// example V_FMA_F32 V111 = V0 + V4 * V8 will need 3 cycles to read operands,
/// potentially incuring 2 stall cycles.
///
/// The pass tries to reassign registers to reduce bank conflicts.
///
/// In this pass bank numbers 0-3 are VGPR banks and 4-11 are SGPR banks, so
/// that 4 has to be subtracted from an SGPR bank number to get the real value.
/// This also corresponds to bit numbers in bank masks used in the pass.
///
//===----------------------------------------------------------------------===//
#include "AMDGPU.h"
#include "AMDGPUSubtarget.h"
#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
#include "SIInstrInfo.h"
#include "SIMachineFunctionInfo.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LiveInterval.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/LiveRegMatrix.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/VirtRegMap.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/MathExtras.h"
using namespace llvm;
static cl::opt<unsigned> VerifyStallCycles("amdgpu-verify-regbanks-reassign",
cl::desc("Verify stall cycles in the regbanks reassign pass"),
cl::value_desc("0|1|2"),
cl::init(0), cl::Hidden);
#define DEBUG_TYPE "amdgpu-regbanks-reassign"
#define NUM_VGPR_BANKS 4
#define NUM_SGPR_BANKS 8
#define NUM_BANKS (NUM_VGPR_BANKS + NUM_SGPR_BANKS)
#define SGPR_BANK_OFFSET NUM_VGPR_BANKS
#define VGPR_BANK_MASK 0xf
#define SGPR_BANK_MASK 0xff0
#define SGPR_BANK_SHIFTED_MASK (SGPR_BANK_MASK >> SGPR_BANK_OFFSET)
STATISTIC(NumStallsDetected,
"Number of operand read stalls detected");
STATISTIC(NumStallsRecovered,
"Number of operand read stalls recovered");
namespace {
class GCNRegBankReassign : public MachineFunctionPass {
class OperandMask {
public:
OperandMask(unsigned r, unsigned s, unsigned m)
: Reg(r), SubReg(s), Mask(m) {}
unsigned Reg;
unsigned SubReg;
unsigned Mask;
};
class Candidate {
public:
Candidate(MachineInstr *mi, unsigned reg, unsigned freebanks,
unsigned weight)
: MI(mi), Reg(reg), FreeBanks(freebanks), Weight(weight) {}
bool operator< (const Candidate& RHS) const { return Weight < RHS.Weight; }
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
void dump(const GCNRegBankReassign *P) const {
MI->dump();
dbgs() << P->printReg(Reg) << " to banks ";
dumpFreeBanks(FreeBanks);
dbgs() << " weight " << Weight << '\n';
}
#endif
MachineInstr *MI;
unsigned Reg;
unsigned FreeBanks;
unsigned Weight;
};
class CandidateList : public std::list<Candidate> {
public:
// Speedup subsequent sort.
void push(const Candidate&& C) {
if (C.Weight) push_back(C);
else push_front(C);
}
};
public:
static char ID;
public:
GCNRegBankReassign() : MachineFunctionPass(ID) {
initializeGCNRegBankReassignPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &MF) override;
StringRef getPassName() const override { return "GCN RegBank Reassign"; }
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<MachineLoopInfo>();
AU.addRequired<LiveIntervals>();
AU.addRequired<VirtRegMap>();
AU.addRequired<LiveRegMatrix>();
AU.setPreservesAll();
MachineFunctionPass::getAnalysisUsage(AU);
}
private:
const GCNSubtarget *ST;
const MachineRegisterInfo *MRI;
const SIRegisterInfo *TRI;
MachineLoopInfo *MLI;
VirtRegMap *VRM;
LiveRegMatrix *LRM;
LiveIntervals *LIS;
unsigned MaxNumVGPRs;
unsigned MaxNumSGPRs;
BitVector RegsUsed;
SmallVector<OperandMask, 8> OperandMasks;
CandidateList Candidates;
const MCPhysReg *CSRegs;
// Returns bank for a phys reg.
unsigned getPhysRegBank(unsigned Reg) const;
// Return a bit set for each register bank used. 4 banks for VGPRs and
// 8 banks for SGPRs.
// Registers already processed and recorded in RegsUsed are excluded.
// If Bank is not -1 assume Reg:SubReg to belong to that Bank.
uint32_t getRegBankMask(unsigned Reg, unsigned SubReg, int Bank);
// Analyze one instruction returning the number of stalls and a mask of the
// banks used by all operands.
// If Reg and Bank are provided, assume all uses of Reg will be replaced with
// a register chosen from Bank.
std::pair<unsigned, unsigned> analyzeInst(const MachineInstr &MI,
unsigned Reg = AMDGPU::NoRegister,
int Bank = -1);
// Return true if register is regular VGPR or SGPR or their tuples.
// Returns false for special registers like m0, vcc etc.
bool isReassignable(unsigned Reg) const;
// Check if registers' defs are old and may be pre-loaded.
// Returns 0 if both registers are old enough, 1 or 2 if one or both
// registers will not likely be pre-loaded.
unsigned getOperandGatherWeight(const MachineInstr& MI,
unsigned Reg1,
unsigned Reg2,
unsigned StallCycles) const;
// Find all bank bits in UsedBanks where Mask can be relocated to.
unsigned getFreeBanks(unsigned Mask, unsigned UsedBanks) const;
// Find all bank bits in UsedBanks where Mask can be relocated to.
// Bank is relative to the register and not its subregister component.
// Returns 0 is a register is not reassignable.
unsigned getFreeBanks(unsigned Reg, unsigned SubReg, unsigned Mask,
unsigned UsedBanks) const;
// Add cadidate instruction to the work list.
void collectCandidates(MachineInstr& MI, unsigned UsedBanks,
unsigned StallCycles);
// Collect cadidate instructions across function. Returns a number stall
// cycles detected. Only counts stalls if Collect is false.
unsigned collectCandidates(MachineFunction &MF, bool Collect = true);
// Remove all candidates that read specified register.
void removeCandidates(unsigned Reg);
// Compute stalls within the uses of SrcReg replaced by a register from
// Bank. If Bank is -1 does not perform substitution. If Collect is set
// candidates are collected and added to work list.
unsigned computeStallCycles(unsigned SrcReg,
unsigned Reg = AMDGPU::NoRegister,
int Bank = -1, bool Collect = false);
// Search for a register in Bank unused within LI.
// Returns phys reg or NoRegister.
unsigned scavengeReg(LiveInterval& LI, unsigned Bank) const;
// Try to reassign candidate. Returns number or stall cycles saved.
unsigned tryReassign(Candidate &C);
bool verifyCycles(MachineFunction &MF,
unsigned OriginalCycles, unsigned CyclesSaved);
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
public:
Printable printReg(unsigned Reg, unsigned SubReg = 0) const {
return Printable([Reg, SubReg, this](raw_ostream &OS) {
if (Register::isPhysicalRegister(Reg)) {
OS << llvm::printReg(Reg, TRI);
return;
}
if (!VRM->isAssignedReg(Reg))
OS << "<unassigned> " << llvm::printReg(Reg, TRI);
else
OS << llvm::printReg(Reg, TRI) << '('
<< llvm::printReg(VRM->getPhys(Reg), TRI) << ')';
if (SubReg)
OS << ':' << TRI->getSubRegIndexName(SubReg);
});
}
static Printable printBank(unsigned Bank) {
return Printable([Bank](raw_ostream &OS) {
OS << ((Bank >= SGPR_BANK_OFFSET) ? Bank - SGPR_BANK_OFFSET : Bank);
});
}
static void dumpFreeBanks(unsigned FreeBanks) {
for (unsigned L = 0; L < NUM_BANKS; ++L)
if (FreeBanks & (1 << L))
dbgs() << printBank(L) << ' ';
}
#endif
};
} // End anonymous namespace.
INITIALIZE_PASS_BEGIN(GCNRegBankReassign, DEBUG_TYPE, "GCN RegBank Reassign",
false, false)
INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_PASS_DEPENDENCY(VirtRegMap)
INITIALIZE_PASS_DEPENDENCY(LiveRegMatrix)
INITIALIZE_PASS_END(GCNRegBankReassign, DEBUG_TYPE, "GCN RegBank Reassign",
false, false)
char GCNRegBankReassign::ID = 0;
char &llvm::GCNRegBankReassignID = GCNRegBankReassign::ID;
unsigned GCNRegBankReassign::getPhysRegBank(unsigned Reg) const {
assert(Register::isPhysicalRegister(Reg));
const TargetRegisterClass *RC = TRI->getMinimalPhysRegClass(Reg);
unsigned Size = TRI->getRegSizeInBits(*RC);
if (Size == 16)
Reg = TRI->get32BitRegister(Reg);
else if (Size > 32)
Reg = TRI->getSubReg(Reg, AMDGPU::sub0);
if (TRI->hasVGPRs(RC)) {
Reg -= AMDGPU::VGPR0;
return Reg % NUM_VGPR_BANKS;
}
Reg = TRI->getEncodingValue(Reg) / 2;
return Reg % NUM_SGPR_BANKS + SGPR_BANK_OFFSET;
}
uint32_t GCNRegBankReassign::getRegBankMask(unsigned Reg, unsigned SubReg,
int Bank) {
if (Register::isVirtualRegister(Reg)) {
if (!VRM->isAssignedReg(Reg))
return 0;
Reg = VRM->getPhys(Reg);
if (!Reg)
return 0;
if (SubReg)
Reg = TRI->getSubReg(Reg, SubReg);
}
const TargetRegisterClass *RC = TRI->getMinimalPhysRegClass(Reg);
unsigned Size = TRI->getRegSizeInBits(*RC);
if (Size == 16) {
Reg = TRI->get32BitRegister(Reg);
Size = 1;
} else {
Size /= 32;
if (Size > 1)
Reg = TRI->getSubReg(Reg, AMDGPU::sub0);
}
if (TRI->hasVGPRs(RC)) {
// VGPRs have 4 banks assigned in a round-robin fashion.
Reg -= AMDGPU::VGPR0;
uint32_t Mask = maskTrailingOnes<uint32_t>(Size);
unsigned Used = 0;
// Bitmask lacks an extract method
for (unsigned I = 0; I < Size; ++I)
if (RegsUsed.test(Reg + I))
Used |= 1 << I;
RegsUsed.set(Reg, Reg + Size);
Mask &= ~Used;
Mask <<= (Bank == -1) ? Reg % NUM_VGPR_BANKS : uint32_t(Bank);
return (Mask | (Mask >> NUM_VGPR_BANKS)) & VGPR_BANK_MASK;
}
// SGPRs have 8 banks holding 2 consequitive registers each.
Reg = TRI->getEncodingValue(Reg) / 2;
unsigned StartBit = AMDGPU::VGPR_32RegClass.getNumRegs();
if (Reg + StartBit >= RegsUsed.size())
return 0;
if (Size > 1)
Size /= 2;
unsigned Mask = (1 << Size) - 1;
unsigned Used = 0;
for (unsigned I = 0; I < Size; ++I)
if (RegsUsed.test(StartBit + Reg + I))
Used |= 1 << I;
RegsUsed.set(StartBit + Reg, StartBit + Reg + Size);
Mask &= ~Used;
Mask <<= (Bank == -1) ? Reg % NUM_SGPR_BANKS
: unsigned(Bank - SGPR_BANK_OFFSET);
Mask = (Mask | (Mask >> NUM_SGPR_BANKS)) & SGPR_BANK_SHIFTED_MASK;
// Reserve 4 bank ids for VGPRs.
return Mask << SGPR_BANK_OFFSET;
}
std::pair<unsigned, unsigned>
GCNRegBankReassign::analyzeInst(const MachineInstr &MI, unsigned Reg,
int Bank) {
unsigned StallCycles = 0;
unsigned UsedBanks = 0;
if (MI.isDebugValue())
return std::make_pair(StallCycles, UsedBanks);
RegsUsed.reset();
OperandMasks.clear();
for (const auto& Op : MI.explicit_uses()) {
// Undef can be assigned to any register, so two vregs can be assigned
// the same phys reg within the same instruction.
if (!Op.isReg() || Op.isUndef())
continue;
Register R = Op.getReg();
if (TRI->hasAGPRs(TRI->getRegClassForReg(*MRI, R)))
continue;
unsigned ShiftedBank = Bank;
if (Bank != -1 && R == Reg && Op.getSubReg()) {
unsigned Offset = TRI->getChannelFromSubReg(Op.getSubReg());
LaneBitmask LM = TRI->getSubRegIndexLaneMask(Op.getSubReg());
if (Offset && Bank < NUM_VGPR_BANKS) {
// If a register spans all banks we cannot shift it to avoid conflict.
if (TRI->getNumCoveredRegs(LM) >= NUM_VGPR_BANKS)
continue;
ShiftedBank = (Bank + Offset) % NUM_VGPR_BANKS;
} else if (Offset > 1 && Bank >= SGPR_BANK_OFFSET) {
// If a register spans all banks we cannot shift it to avoid conflict.
if (TRI->getNumCoveredRegs(LM) / 2 >= NUM_SGPR_BANKS)
continue;
ShiftedBank = SGPR_BANK_OFFSET +
(Bank - SGPR_BANK_OFFSET + (Offset >> 1)) % NUM_SGPR_BANKS;
}
}
uint32_t Mask = getRegBankMask(R, Op.getSubReg(),
(Reg == R) ? ShiftedBank : -1);
StallCycles += countPopulation(UsedBanks & Mask);
UsedBanks |= Mask;
OperandMasks.push_back(OperandMask(Op.getReg(), Op.getSubReg(), Mask));
}
return std::make_pair(StallCycles, UsedBanks);
}
unsigned GCNRegBankReassign::getOperandGatherWeight(const MachineInstr& MI,
unsigned Reg1,
unsigned Reg2,
unsigned StallCycles) const
{
unsigned Defs = 0;
MachineBasicBlock::const_instr_iterator Def(MI.getIterator());
MachineBasicBlock::const_instr_iterator B(MI.getParent()->instr_begin());
for (unsigned S = StallCycles; S && Def != B && Defs != 3; --S) {
if (MI.isDebugInstr())
continue;
--Def;
if (Def->getOpcode() == TargetOpcode::IMPLICIT_DEF)
continue;
if (Def->modifiesRegister(Reg1, TRI))
Defs |= 1;
if (Def->modifiesRegister(Reg2, TRI))
Defs |= 2;
}
return countPopulation(Defs);
}
bool GCNRegBankReassign::isReassignable(unsigned Reg) const {
if (Register::isPhysicalRegister(Reg) || !VRM->isAssignedReg(Reg))
return false;
const MachineInstr *Def = MRI->getUniqueVRegDef(Reg);
Register PhysReg = VRM->getPhys(Reg);
if (Def && Def->isCopy() && Def->getOperand(1).getReg() == PhysReg)
return false;
for (auto U : MRI->use_nodbg_operands(Reg)) {
if (U.isImplicit())
return false;
const MachineInstr *UseInst = U.getParent();
if (UseInst->isCopy() && UseInst->getOperand(0).getReg() == PhysReg)
return false;
}
const TargetRegisterClass *RC = TRI->getMinimalPhysRegClass(PhysReg);
unsigned Size = TRI->getRegSizeInBits(*RC);
// TODO: Support 16 bit registers. Those needs to be moved with their
// parent VGPR_32 and potentially a sibling 16 bit sub-register.
if (Size < 32)
return false;
if (TRI->hasVGPRs(RC))
return true;
if (Size == 16)
return AMDGPU::SGPR_LO16RegClass.contains(PhysReg);
if (Size > 32)
PhysReg = TRI->getSubReg(PhysReg, AMDGPU::sub0);
return AMDGPU::SGPR_32RegClass.contains(PhysReg);
}
unsigned GCNRegBankReassign::getFreeBanks(unsigned Mask,
unsigned UsedBanks) const {
unsigned Size = countPopulation(Mask);
unsigned FreeBanks = 0;
unsigned Bank = findFirstSet(Mask);
UsedBanks &= ~Mask;
// Find free VGPR banks
if ((Mask & VGPR_BANK_MASK) && (Size < NUM_VGPR_BANKS)) {
for (unsigned I = 0; I < NUM_VGPR_BANKS; ++I) {
if (Bank == I)
continue;
unsigned NewMask = ((1 << Size) - 1) << I;
NewMask = (NewMask | (NewMask >> NUM_VGPR_BANKS)) & VGPR_BANK_MASK;
if (!(UsedBanks & NewMask))
FreeBanks |= 1 << I;
}
return FreeBanks;
}
// Find free SGPR banks
// SGPR tuples must be aligned, so step is size in banks it
// crosses.
Bank -= SGPR_BANK_OFFSET;
for (unsigned I = 0; I < NUM_SGPR_BANKS; I += Size) {
if (Bank == I)
continue;
unsigned NewMask = ((1 << Size) - 1) << I;
NewMask = (NewMask | (NewMask >> NUM_SGPR_BANKS)) & SGPR_BANK_SHIFTED_MASK;
if (!(UsedBanks & (NewMask << SGPR_BANK_OFFSET)))
FreeBanks |= (1 << SGPR_BANK_OFFSET) << I;
}
return FreeBanks;
}
unsigned GCNRegBankReassign::getFreeBanks(unsigned Reg,
unsigned SubReg,
unsigned Mask,
unsigned UsedBanks) const {
if (!isReassignable(Reg))
return 0;
unsigned FreeBanks = getFreeBanks(Mask, UsedBanks);
unsigned Offset = TRI->getChannelFromSubReg(SubReg);
if (Offset && (Mask & VGPR_BANK_MASK)) {
unsigned Shift = Offset;
if (Shift >= NUM_VGPR_BANKS)
return 0;
unsigned VB = FreeBanks & VGPR_BANK_MASK;
FreeBanks = ((VB >> Shift) | (VB << (NUM_VGPR_BANKS - Shift))) &
VGPR_BANK_MASK;
} else if (Offset > 1 && (Mask & SGPR_BANK_MASK)) {
unsigned Shift = Offset >> 1;
if (Shift >= NUM_SGPR_BANKS)
return 0;
unsigned SB = FreeBanks >> SGPR_BANK_OFFSET;
FreeBanks = ((SB >> Shift) | (SB << (NUM_SGPR_BANKS - Shift))) &
SGPR_BANK_SHIFTED_MASK;
FreeBanks <<= SGPR_BANK_OFFSET;
}
LLVM_DEBUG(if (FreeBanks) {
dbgs() << "Potential reassignments of " << printReg(Reg, SubReg)
<< " to banks: "; dumpFreeBanks(FreeBanks);
dbgs() << '\n'; });
return FreeBanks;
}
void GCNRegBankReassign::collectCandidates(MachineInstr& MI,
unsigned UsedBanks,
unsigned StallCycles) {
LLVM_DEBUG(MI.dump());
if (!StallCycles)
return;
LLVM_DEBUG(dbgs() << "Stall cycles = " << StallCycles << '\n');
for (unsigned I = 0, E = OperandMasks.size(); I + 1 < E; ++I) {
for (unsigned J = I + 1; J != E; ++J) {
if (!(OperandMasks[I].Mask & OperandMasks[J].Mask))
continue;
unsigned Reg1 = OperandMasks[I].Reg;
unsigned Reg2 = OperandMasks[J].Reg;
unsigned SubReg1 = OperandMasks[I].SubReg;
unsigned SubReg2 = OperandMasks[J].SubReg;
unsigned Mask1 = OperandMasks[I].Mask;
unsigned Mask2 = OperandMasks[J].Mask;
unsigned Size1 = countPopulation(Mask1);
unsigned Size2 = countPopulation(Mask2);
LLVM_DEBUG(dbgs() << "Conflicting operands: " << printReg(Reg1, SubReg1) <<
" and " << printReg(Reg2, SubReg2) << '\n');
unsigned Weight = getOperandGatherWeight(MI, Reg1, Reg2, StallCycles);
Weight += MLI->getLoopDepth(MI.getParent()) * 10;
LLVM_DEBUG(dbgs() << "Stall weight = " << Weight << '\n');
unsigned FreeBanks1 = getFreeBanks(Reg1, SubReg1, Mask1, UsedBanks);
unsigned FreeBanks2 = getFreeBanks(Reg2, SubReg2, Mask2, UsedBanks);
if (FreeBanks1)
Candidates.push(Candidate(&MI, Reg1, FreeBanks1, Weight
+ ((Size2 > Size1) ? 1 : 0)));
if (FreeBanks2)
Candidates.push(Candidate(&MI, Reg2, FreeBanks2, Weight
+ ((Size1 > Size2) ? 1 : 0)));
}
}
}
unsigned GCNRegBankReassign::computeStallCycles(unsigned SrcReg,
unsigned Reg, int Bank,
bool Collect) {
unsigned TotalStallCycles = 0;
SmallSet<const MachineInstr *, 16> Visited;
for (auto &MI : MRI->use_nodbg_instructions(SrcReg)) {
if (MI.isBundle())
continue;
if (!Visited.insert(&MI).second)
continue;
unsigned StallCycles;
unsigned UsedBanks;
std::tie(StallCycles, UsedBanks) = analyzeInst(MI, Reg, Bank);
TotalStallCycles += StallCycles;
if (Collect)
collectCandidates(MI, UsedBanks, StallCycles);
}
return TotalStallCycles;
}
unsigned GCNRegBankReassign::scavengeReg(LiveInterval& LI,
unsigned Bank) const {
const TargetRegisterClass *RC = MRI->getRegClass(LI.reg);
unsigned MaxNumRegs = (Bank < NUM_VGPR_BANKS) ? MaxNumVGPRs
: MaxNumSGPRs;
unsigned MaxReg = MaxNumRegs + (Bank < NUM_VGPR_BANKS ? AMDGPU::VGPR0
: AMDGPU::SGPR0);
for (unsigned Reg : RC->getRegisters()) {
// Check occupancy limit.
if (TRI->isSubRegisterEq(Reg, MaxReg))
break;
if (!MRI->isAllocatable(Reg) || getPhysRegBank(Reg) != Bank)
continue;
for (unsigned I = 0; CSRegs[I]; ++I)
if (TRI->isSubRegisterEq(Reg, CSRegs[I]) &&
!LRM->isPhysRegUsed(CSRegs[I]))
return AMDGPU::NoRegister;
LLVM_DEBUG(dbgs() << "Trying register " << printReg(Reg) << '\n');
if (!LRM->checkInterference(LI, Reg))
return Reg;
}
return AMDGPU::NoRegister;
}
unsigned GCNRegBankReassign::tryReassign(Candidate &C) {
if (!LIS->hasInterval(C.Reg))
return 0;
LiveInterval &LI = LIS->getInterval(C.Reg);
LLVM_DEBUG(dbgs() << "Try reassign " << printReg(C.Reg) << " in "; C.MI->dump();
LI.dump());
// For each candidate bank walk all instructions in the range of live
// interval and check if replacing the register with one belonging to
// the candidate bank reduces conflicts.
unsigned OrigStalls = computeStallCycles(C.Reg);
LLVM_DEBUG(dbgs() << "--- Stall cycles in range = " << OrigStalls << '\n');
if (!OrigStalls)
return 0;
struct BankStall {
BankStall(unsigned b, unsigned s) : Bank(b), Stalls(s) {};
bool operator<(const BankStall &RHS) const {
if (Stalls == RHS.Stalls)
return Bank < RHS.Bank;
return Stalls > RHS.Stalls;
}
unsigned Bank;
unsigned Stalls;
};
SmallVector<BankStall, 8> BankStalls;
for (int Bank = 0; Bank < NUM_BANKS; ++Bank) {
if (C.FreeBanks & (1 << Bank)) {
LLVM_DEBUG(dbgs() << "Trying bank " << printBank(Bank) << '\n');
unsigned Stalls = computeStallCycles(C.Reg, C.Reg, Bank);
if (Stalls < OrigStalls) {
LLVM_DEBUG(dbgs() << "With bank " << printBank(Bank) << " -> "
<< Stalls << '\n');
BankStalls.push_back(BankStall((unsigned)Bank, Stalls));
}
}
}
llvm::sort(BankStalls);
Register OrigReg = VRM->getPhys(C.Reg);
LRM->unassign(LI);
while (!BankStalls.empty()) {
BankStall BS = BankStalls.pop_back_val();
unsigned Reg = scavengeReg(LI, BS.Bank);
if (Reg == AMDGPU::NoRegister) {
LLVM_DEBUG(dbgs() << "No free registers in bank " << printBank(BS.Bank)
<< '\n');
continue;
}
LLVM_DEBUG(dbgs() << "Found free register " << printReg(Reg)
<< (LRM->isPhysRegUsed(Reg) ? "" : " (new)")
<< " in bank " << printBank(BS.Bank) << '\n');
LRM->assign(LI, Reg);
LLVM_DEBUG(dbgs() << "--- Cycles saved: " << OrigStalls - BS.Stalls << '\n');
return OrigStalls - BS.Stalls;
}
LRM->assign(LI, OrigReg);
return 0;
}
unsigned GCNRegBankReassign::collectCandidates(MachineFunction &MF,
bool Collect) {
unsigned TotalStallCycles = 0;
for (MachineBasicBlock &MBB : MF) {
LLVM_DEBUG(if (Collect) {
if (MBB.getName().empty()) dbgs() << "bb." << MBB.getNumber();
else dbgs() << MBB.getName(); dbgs() << ":\n";
});
for (MachineInstr &MI : MBB.instrs()) {
if (MI.isBundle())
continue; // we analyze the instructions inside the bundle individually
unsigned StallCycles;
unsigned UsedBanks;
std::tie(StallCycles, UsedBanks) = analyzeInst(MI);
if (Collect)
collectCandidates(MI, UsedBanks, StallCycles);
TotalStallCycles += StallCycles;
}
LLVM_DEBUG(if (Collect) { dbgs() << '\n'; });
}
return TotalStallCycles;
}
void GCNRegBankReassign::removeCandidates(unsigned Reg) {
Candidates.remove_if([Reg, this](const Candidate& C) {
return C.MI->readsRegister(Reg, TRI);
});
}
bool GCNRegBankReassign::verifyCycles(MachineFunction &MF,
unsigned OriginalCycles,
unsigned CyclesSaved) {
unsigned StallCycles = collectCandidates(MF, false);
LLVM_DEBUG(dbgs() << "=== After the pass " << StallCycles
<< " stall cycles left\n");
return StallCycles + CyclesSaved == OriginalCycles;
}
bool GCNRegBankReassign::runOnMachineFunction(MachineFunction &MF) {
ST = &MF.getSubtarget<GCNSubtarget>();
if (!ST->hasRegisterBanking() || skipFunction(MF.getFunction()))
return false;
MRI = &MF.getRegInfo();
TRI = ST->getRegisterInfo();
MLI = &getAnalysis<MachineLoopInfo>();
VRM = &getAnalysis<VirtRegMap>();
LRM = &getAnalysis<LiveRegMatrix>();
LIS = &getAnalysis<LiveIntervals>();
const SIMachineFunctionInfo *MFI = MF.getInfo<SIMachineFunctionInfo>();
unsigned Occupancy = MFI->getOccupancy();
MaxNumVGPRs = ST->getMaxNumVGPRs(MF);
MaxNumSGPRs = ST->getMaxNumSGPRs(MF);
MaxNumVGPRs = std::min(ST->getMaxNumVGPRs(Occupancy), MaxNumVGPRs);
MaxNumSGPRs = std::min(ST->getMaxNumSGPRs(Occupancy, true), MaxNumSGPRs);
CSRegs = MRI->getCalleeSavedRegs();
RegsUsed.resize(AMDGPU::VGPR_32RegClass.getNumRegs() +
TRI->getEncodingValue(AMDGPU::SGPR_NULL) / 2 + 1);
LLVM_DEBUG(dbgs() << "=== RegBanks reassign analysis on function " << MF.getName()
<< '\n');
unsigned StallCycles = collectCandidates(MF);
NumStallsDetected += StallCycles;
LLVM_DEBUG(dbgs() << "=== " << StallCycles << " stall cycles detected in "
"function " << MF.getName() << '\n');
Candidates.sort();
LLVM_DEBUG(dbgs() << "\nCandidates:\n\n";
for (auto C : Candidates) C.dump(this);
dbgs() << "\n\n");
unsigned CyclesSaved = 0;
while (!Candidates.empty()) {
Candidate C = Candidates.back();
unsigned LocalCyclesSaved = tryReassign(C);
CyclesSaved += LocalCyclesSaved;
if (VerifyStallCycles > 1 && !verifyCycles(MF, StallCycles, CyclesSaved))
report_fatal_error("RegBank reassign stall cycles verification failed.");
Candidates.pop_back();
if (LocalCyclesSaved) {
removeCandidates(C.Reg);
computeStallCycles(C.Reg, AMDGPU::NoRegister, -1, true);
Candidates.sort();
LLVM_DEBUG(dbgs() << "\nCandidates:\n\n";
for (auto C : Candidates)
C.dump(this);
dbgs() << "\n\n");
}
}
NumStallsRecovered += CyclesSaved;
LLVM_DEBUG(dbgs() << "=== After the pass " << CyclesSaved
<< " cycles saved in function " << MF.getName() << '\n');
Candidates.clear();
if (VerifyStallCycles == 1 && !verifyCycles(MF, StallCycles, CyclesSaved))
report_fatal_error("RegBank reassign stall cycles verification failed.");
RegsUsed.clear();
return CyclesSaved > 0;
}
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