nils.hoelscher
/
llvm-for-llvmta
1
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity
llvm-for-llvmta/lib/CodeGen/CriticalAntiDepBreaker.cpp

708 lines
28 KiB
C++
Raw Permalink Blame History

//===- CriticalAntiDepBreaker.cpp - Anti-dep breaker ----------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file implements the CriticalAntiDepBreaker class, which
// implements register anti-dependence breaking along a blocks
// critical path during post-RA scheduler.
//
//===----------------------------------------------------------------------===//
#include "CriticalAntiDepBreaker.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/RegisterClassInfo.h"
#include "llvm/CodeGen/ScheduleDAG.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "post-RA-sched"
CriticalAntiDepBreaker::CriticalAntiDepBreaker(MachineFunction &MFi,
const RegisterClassInfo &RCI)
: AntiDepBreaker(), MF(MFi), MRI(MF.getRegInfo()),
TII(MF.getSubtarget().getInstrInfo()),
TRI(MF.getSubtarget().getRegisterInfo()), RegClassInfo(RCI),
Classes(TRI->getNumRegs(), nullptr), KillIndices(TRI->getNumRegs(), 0),
DefIndices(TRI->getNumRegs(), 0), KeepRegs(TRI->getNumRegs(), false) {}
CriticalAntiDepBreaker::~CriticalAntiDepBreaker() = default;
void CriticalAntiDepBreaker::StartBlock(MachineBasicBlock *BB) {
const unsigned BBSize = BB->size();
for (unsigned i = 0, e = TRI->getNumRegs(); i != e; ++i) {
// Clear out the register class data.
Classes[i] = nullptr;
// Initialize the indices to indicate that no registers are live.
KillIndices[i] = ~0u;
DefIndices[i] = BBSize;
}
// Clear "do not change" set.
KeepRegs.reset();
bool IsReturnBlock = BB->isReturnBlock();
// Examine the live-in regs of all successors.
for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
SE = BB->succ_end(); SI != SE; ++SI)
for (const auto &LI : (*SI)->liveins()) {
for (MCRegAliasIterator AI(LI.PhysReg, TRI, true); AI.isValid(); ++AI) {
unsigned Reg = *AI;
Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
KillIndices[Reg] = BBSize;
DefIndices[Reg] = ~0u;
}
}
// Mark live-out callee-saved registers. In a return block this is
// all callee-saved registers. In non-return this is any
// callee-saved register that is not saved in the prolog.
const MachineFrameInfo &MFI = MF.getFrameInfo();
BitVector Pristine = MFI.getPristineRegs(MF);
for (const MCPhysReg *I = MF.getRegInfo().getCalleeSavedRegs(); *I;
++I) {
unsigned Reg = *I;
if (!IsReturnBlock && !Pristine.test(Reg))
continue;
for (MCRegAliasIterator AI(*I, TRI, true); AI.isValid(); ++AI) {
unsigned Reg = *AI;
Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
KillIndices[Reg] = BBSize;
DefIndices[Reg] = ~0u;
}
}
}
void CriticalAntiDepBreaker::FinishBlock() {
RegRefs.clear();
KeepRegs.reset();
}
void CriticalAntiDepBreaker::Observe(MachineInstr &MI, unsigned Count,
unsigned InsertPosIndex) {
// Kill instructions can define registers but are really nops, and there might
// be a real definition earlier that needs to be paired with uses dominated by
// this kill.
// FIXME: It may be possible to remove the isKill() restriction once PR18663
// has been properly fixed. There can be value in processing kills as seen in
// the AggressiveAntiDepBreaker class.
if (MI.isDebugInstr() || MI.isKill())
return;
assert(Count < InsertPosIndex && "Instruction index out of expected range!");
for (unsigned Reg = 0; Reg != TRI->getNumRegs(); ++Reg) {
if (KillIndices[Reg] != ~0u) {
// If Reg is currently live, then mark that it can't be renamed as
// we don't know the extent of its live-range anymore (now that it
// has been scheduled).
Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
KillIndices[Reg] = Count;
} else if (DefIndices[Reg] < InsertPosIndex && DefIndices[Reg] >= Count) {
// Any register which was defined within the previous scheduling region
// may have been rescheduled and its lifetime may overlap with registers
// in ways not reflected in our current liveness state. For each such
// register, adjust the liveness state to be conservatively correct.
Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
// Move the def index to the end of the previous region, to reflect
// that the def could theoretically have been scheduled at the end.
DefIndices[Reg] = InsertPosIndex;
}
}
PrescanInstruction(MI);
ScanInstruction(MI, Count);
}
/// CriticalPathStep - Return the next SUnit after SU on the bottom-up
/// critical path.
static const SDep *CriticalPathStep(const SUnit *SU) {
const SDep *Next = nullptr;
unsigned NextDepth = 0;
// Find the predecessor edge with the greatest depth.
for (SUnit::const_pred_iterator P = SU->Preds.begin(), PE = SU->Preds.end();
P != PE; ++P) {
const SUnit *PredSU = P->getSUnit();
unsigned PredLatency = P->getLatency();
unsigned PredTotalLatency = PredSU->getDepth() + PredLatency;
// In the case of a latency tie, prefer an anti-dependency edge over
// other types of edges.
if (NextDepth < PredTotalLatency ||
(NextDepth == PredTotalLatency && P->getKind() == SDep::Anti)) {
NextDepth = PredTotalLatency;
Next = &*P;
}
}
return Next;
}
void CriticalAntiDepBreaker::PrescanInstruction(MachineInstr &MI) {
// It's not safe to change register allocation for source operands of
// instructions that have special allocation requirements. Also assume all
// registers used in a call must not be changed (ABI).
// FIXME: The issue with predicated instruction is more complex. We are being
// conservative here because the kill markers cannot be trusted after
// if-conversion:
// %r6 = LDR %sp, %reg0, 92, 14, %reg0; mem:LD4[FixedStack14]
// ...
// STR %r0, killed %r6, %reg0, 0, 0, %cpsr; mem:ST4[%395]
// %r6 = LDR %sp, %reg0, 100, 0, %cpsr; mem:LD4[FixedStack12]
// STR %r0, killed %r6, %reg0, 0, 14, %reg0; mem:ST4[%396](align=8)
//
// The first R6 kill is not really a kill since it's killed by a predicated
// instruction which may not be executed. The second R6 def may or may not
// re-define R6 so it's not safe to change it since the last R6 use cannot be
// changed.
bool Special =
MI.isCall() || MI.hasExtraSrcRegAllocReq() || TII->isPredicated(MI);
// Scan the register operands for this instruction and update
// Classes and RegRefs.
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI.getOperand(i);
if (!MO.isReg()) continue;
Register Reg = MO.getReg();
if (Reg == 0) continue;
const TargetRegisterClass *NewRC = nullptr;
if (i < MI.getDesc().getNumOperands())
NewRC = TII->getRegClass(MI.getDesc(), i, TRI, MF);
// For now, only allow the register to be changed if its register
// class is consistent across all uses.
if (!Classes[Reg] && NewRC)
Classes[Reg] = NewRC;
else if (!NewRC || Classes[Reg] != NewRC)
Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
// Now check for aliases.
for (MCRegAliasIterator AI(Reg, TRI, false); AI.isValid(); ++AI) {
// If an alias of the reg is used during the live range, give up.
// Note that this allows us to skip checking if AntiDepReg
// overlaps with any of the aliases, among other things.
unsigned AliasReg = *AI;
if (Classes[AliasReg]) {
Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1);
Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
}
}
// If we're still willing to consider this register, note the reference.
if (Classes[Reg] != reinterpret_cast<TargetRegisterClass *>(-1))
RegRefs.insert(std::make_pair(Reg, &MO));
// If this reg is tied and live (Classes[Reg] is set to -1), we can't change
// it or any of its sub or super regs. We need to use KeepRegs to mark the
// reg because not all uses of the same reg within an instruction are
// necessarily tagged as tied.
// Example: an x86 "xor %eax, %eax" will have one source operand tied to the
// def register but not the second (see PR20020 for details).
// FIXME: can this check be relaxed to account for undef uses
// of a register? In the above 'xor' example, the uses of %eax are undef, so
// earlier instructions could still replace %eax even though the 'xor'
// itself can't be changed.
if (MI.isRegTiedToUseOperand(i) &&
Classes[Reg] == reinterpret_cast<TargetRegisterClass *>(-1)) {
for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
SubRegs.isValid(); ++SubRegs) {
KeepRegs.set(*SubRegs);
}
for (MCSuperRegIterator SuperRegs(Reg, TRI);
SuperRegs.isValid(); ++SuperRegs) {
KeepRegs.set(*SuperRegs);
}
}
if (MO.isUse() && Special) {
if (!KeepRegs.test(Reg)) {
for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
SubRegs.isValid(); ++SubRegs)
KeepRegs.set(*SubRegs);
}
}
}
}
void CriticalAntiDepBreaker::ScanInstruction(MachineInstr &MI, unsigned Count) {
// Update liveness.
// Proceeding upwards, registers that are defed but not used in this
// instruction are now dead.
assert(!MI.isKill() && "Attempting to scan a kill instruction");
if (!TII->isPredicated(MI)) {
// Predicated defs are modeled as read + write, i.e. similar to two
// address updates.
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI.getOperand(i);
if (MO.isRegMask()) {
auto ClobbersPhysRegAndSubRegs = [&](unsigned PhysReg) {
for (MCSubRegIterator SRI(PhysReg, TRI, true); SRI.isValid(); ++SRI)
if (!MO.clobbersPhysReg(*SRI))
return false;
return true;
};
for (unsigned i = 0, e = TRI->getNumRegs(); i != e; ++i) {
if (ClobbersPhysRegAndSubRegs(i)) {
DefIndices[i] = Count;
KillIndices[i] = ~0u;
KeepRegs.reset(i);
Classes[i] = nullptr;
RegRefs.erase(i);
}
}
}
if (!MO.isReg()) continue;
Register Reg = MO.getReg();
if (Reg == 0) continue;
if (!MO.isDef()) continue;
// Ignore two-addr defs.
if (MI.isRegTiedToUseOperand(i))
continue;
// If we've already marked this reg as unchangeable, don't remove
// it or any of its subregs from KeepRegs.
bool Keep = KeepRegs.test(Reg);
// For the reg itself and all subregs: update the def to current;
// reset the kill state, any restrictions, and references.
for (MCSubRegIterator SRI(Reg, TRI, true); SRI.isValid(); ++SRI) {
unsigned SubregReg = *SRI;
DefIndices[SubregReg] = Count;
KillIndices[SubregReg] = ~0u;
Classes[SubregReg] = nullptr;
RegRefs.erase(SubregReg);
if (!Keep)
KeepRegs.reset(SubregReg);
}
// Conservatively mark super-registers as unusable.
for (MCSuperRegIterator SR(Reg, TRI); SR.isValid(); ++SR)
Classes[*SR] = reinterpret_cast<TargetRegisterClass *>(-1);
}
}
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI.getOperand(i);
if (!MO.isReg()) continue;
Register Reg = MO.getReg();
if (Reg == 0) continue;
if (!MO.isUse()) continue;
const TargetRegisterClass *NewRC = nullptr;
if (i < MI.getDesc().getNumOperands())
NewRC = TII->getRegClass(MI.getDesc(), i, TRI, MF);
// For now, only allow the register to be changed if its register
// class is consistent across all uses.
if (!Classes[Reg] && NewRC)
Classes[Reg] = NewRC;
else if (!NewRC || Classes[Reg] != NewRC)
Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
RegRefs.insert(std::make_pair(Reg, &MO));
// It wasn't previously live but now it is, this is a kill.
// Repeat for all aliases.
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
unsigned AliasReg = *AI;
if (KillIndices[AliasReg] == ~0u) {
KillIndices[AliasReg] = Count;
DefIndices[AliasReg] = ~0u;
}
}
}
}
// Check all machine operands that reference the antidependent register and must
// be replaced by NewReg. Return true if any of their parent instructions may
// clobber the new register.
//
// Note: AntiDepReg may be referenced by a two-address instruction such that
// it's use operand is tied to a def operand. We guard against the case in which
// the two-address instruction also defines NewReg, as may happen with
// pre/postincrement loads. In this case, both the use and def operands are in
// RegRefs because the def is inserted by PrescanInstruction and not erased
// during ScanInstruction. So checking for an instruction with definitions of
// both NewReg and AntiDepReg covers it.
bool
CriticalAntiDepBreaker::isNewRegClobberedByRefs(RegRefIter RegRefBegin,
RegRefIter RegRefEnd,
unsigned NewReg) {
for (RegRefIter I = RegRefBegin; I != RegRefEnd; ++I ) {
MachineOperand *RefOper = I->second;
// Don't allow the instruction defining AntiDepReg to earlyclobber its
// operands, in case they may be assigned to NewReg. In this case antidep
// breaking must fail, but it's too rare to bother optimizing.
if (RefOper->isDef() && RefOper->isEarlyClobber())
return true;
// Handle cases in which this instruction defines NewReg.
MachineInstr *MI = RefOper->getParent();
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &CheckOper = MI->getOperand(i);
if (CheckOper.isRegMask() && CheckOper.clobbersPhysReg(NewReg))
return true;
if (!CheckOper.isReg() || !CheckOper.isDef() ||
CheckOper.getReg() != NewReg)
continue;
// Don't allow the instruction to define NewReg and AntiDepReg.
// When AntiDepReg is renamed it will be an illegal op.
if (RefOper->isDef())
return true;
// Don't allow an instruction using AntiDepReg to be earlyclobbered by
// NewReg.
if (CheckOper.isEarlyClobber())
return true;
// Don't allow inline asm to define NewReg at all. Who knows what it's
// doing with it.
if (MI->isInlineAsm())
return true;
}
}
return false;
}
unsigned CriticalAntiDepBreaker::
findSuitableFreeRegister(RegRefIter RegRefBegin,
RegRefIter RegRefEnd,
unsigned AntiDepReg,
unsigned LastNewReg,
const TargetRegisterClass *RC,
SmallVectorImpl<unsigned> &Forbid) {
ArrayRef<MCPhysReg> Order = RegClassInfo.getOrder(RC);
for (unsigned i = 0; i != Order.size(); ++i) {
unsigned NewReg = Order[i];
// Don't replace a register with itself.
if (NewReg == AntiDepReg) continue;
// Don't replace a register with one that was recently used to repair
// an anti-dependence with this AntiDepReg, because that would
// re-introduce that anti-dependence.
if (NewReg == LastNewReg) continue;
// If any instructions that define AntiDepReg also define the NewReg, it's
// not suitable. For example, Instruction with multiple definitions can
// result in this condition.
if (isNewRegClobberedByRefs(RegRefBegin, RegRefEnd, NewReg)) continue;
// If NewReg is dead and NewReg's most recent def is not before
// AntiDepReg's kill, it's safe to replace AntiDepReg with NewReg.
assert(((KillIndices[AntiDepReg] == ~0u) != (DefIndices[AntiDepReg] == ~0u))
&& "Kill and Def maps aren't consistent for AntiDepReg!");
assert(((KillIndices[NewReg] == ~0u) != (DefIndices[NewReg] == ~0u))
&& "Kill and Def maps aren't consistent for NewReg!");
if (KillIndices[NewReg] != ~0u ||
Classes[NewReg] == reinterpret_cast<TargetRegisterClass *>(-1) ||
KillIndices[AntiDepReg] > DefIndices[NewReg])
continue;
// If NewReg overlaps any of the forbidden registers, we can't use it.
bool Forbidden = false;
for (SmallVectorImpl<unsigned>::iterator it = Forbid.begin(),
ite = Forbid.end(); it != ite; ++it)
if (TRI->regsOverlap(NewReg, *it)) {
Forbidden = true;
break;
}
if (Forbidden) continue;
return NewReg;
}
// No registers are free and available!
return 0;
}
unsigned CriticalAntiDepBreaker::
BreakAntiDependencies(const std::vector<SUnit> &SUnits,
MachineBasicBlock::iterator Begin,
MachineBasicBlock::iterator End,
unsigned InsertPosIndex,
DbgValueVector &DbgValues) {
// The code below assumes that there is at least one instruction,
// so just duck out immediately if the block is empty.
if (SUnits.empty()) return 0;
// Keep a map of the MachineInstr*'s back to the SUnit representing them.
// This is used for updating debug information.
//
// FIXME: Replace this with the existing map in ScheduleDAGInstrs::MISUnitMap
DenseMap<MachineInstr *, const SUnit *> MISUnitMap;
// Find the node at the bottom of the critical path.
const SUnit *Max = nullptr;
for (unsigned i = 0, e = SUnits.size(); i != e; ++i) {
const SUnit *SU = &SUnits[i];
MISUnitMap[SU->getInstr()] = SU;
if (!Max || SU->getDepth() + SU->Latency > Max->getDepth() + Max->Latency)
Max = SU;
}
assert(Max && "Failed to find bottom of the critical path");
#ifndef NDEBUG
{
LLVM_DEBUG(dbgs() << "Critical path has total latency "
<< (Max->getDepth() + Max->Latency) << "\n");
LLVM_DEBUG(dbgs() << "Available regs:");
for (unsigned Reg = 0; Reg < TRI->getNumRegs(); ++Reg) {
if (KillIndices[Reg] == ~0u)
LLVM_DEBUG(dbgs() << " " << printReg(Reg, TRI));
}
LLVM_DEBUG(dbgs() << '\n');
}
#endif
// Track progress along the critical path through the SUnit graph as we walk
// the instructions.
const SUnit *CriticalPathSU = Max;
MachineInstr *CriticalPathMI = CriticalPathSU->getInstr();
// Consider this pattern:
// A = ...
// ... = A
// A = ...
// ... = A
// A = ...
// ... = A
// A = ...
// ... = A
// There are three anti-dependencies here, and without special care,
// we'd break all of them using the same register:
// A = ...
// ... = A
// B = ...
// ... = B
// B = ...
// ... = B
// B = ...
// ... = B
// because at each anti-dependence, B is the first register that
// isn't A which is free. This re-introduces anti-dependencies
// at all but one of the original anti-dependencies that we were
// trying to break. To avoid this, keep track of the most recent
// register that each register was replaced with, avoid
// using it to repair an anti-dependence on the same register.
// This lets us produce this:
// A = ...
// ... = A
// B = ...
// ... = B
// C = ...
// ... = C
// B = ...
// ... = B
// This still has an anti-dependence on B, but at least it isn't on the
// original critical path.
//
// TODO: If we tracked more than one register here, we could potentially
// fix that remaining critical edge too. This is a little more involved,
// because unlike the most recent register, less recent registers should
// still be considered, though only if no other registers are available.
std::vector<unsigned> LastNewReg(TRI->getNumRegs(), 0);
// Attempt to break anti-dependence edges on the critical path. Walk the
// instructions from the bottom up, tracking information about liveness
// as we go to help determine which registers are available.
unsigned Broken = 0;
unsigned Count = InsertPosIndex - 1;
for (MachineBasicBlock::iterator I = End, E = Begin; I != E; --Count) {
MachineInstr &MI = *--I;
// Kill instructions can define registers but are really nops, and there
// might be a real definition earlier that needs to be paired with uses
// dominated by this kill.
// FIXME: It may be possible to remove the isKill() restriction once PR18663
// has been properly fixed. There can be value in processing kills as seen
// in the AggressiveAntiDepBreaker class.
if (MI.isDebugInstr() || MI.isKill())
continue;
// Check if this instruction has a dependence on the critical path that
// is an anti-dependence that we may be able to break. If it is, set
// AntiDepReg to the non-zero register associated with the anti-dependence.
//
// We limit our attention to the critical path as a heuristic to avoid
// breaking anti-dependence edges that aren't going to significantly
// impact the overall schedule. There are a limited number of registers
// and we want to save them for the important edges.
//
// TODO: Instructions with multiple defs could have multiple
// anti-dependencies. The current code here only knows how to break one
// edge per instruction. Note that we'd have to be able to break all of
// the anti-dependencies in an instruction in order to be effective.
unsigned AntiDepReg = 0;
if (&MI == CriticalPathMI) {
if (const SDep *Edge = CriticalPathStep(CriticalPathSU)) {
const SUnit *NextSU = Edge->getSUnit();
// Only consider anti-dependence edges.
if (Edge->getKind() == SDep::Anti) {
AntiDepReg = Edge->getReg();
assert(AntiDepReg != 0 && "Anti-dependence on reg0?");
if (!MRI.isAllocatable(AntiDepReg))
// Don't break anti-dependencies on non-allocatable registers.
AntiDepReg = 0;
else if (KeepRegs.test(AntiDepReg))
// Don't break anti-dependencies if a use down below requires
// this exact register.
AntiDepReg = 0;
else {
// If the SUnit has other dependencies on the SUnit that it
// anti-depends on, don't bother breaking the anti-dependency
// since those edges would prevent such units from being
// scheduled past each other regardless.
//
// Also, if there are dependencies on other SUnits with the
// same register as the anti-dependency, don't attempt to
// break it.
for (SUnit::const_pred_iterator P = CriticalPathSU->Preds.begin(),
PE = CriticalPathSU->Preds.end(); P != PE; ++P)
if (P->getSUnit() == NextSU ?
(P->getKind() != SDep::Anti || P->getReg() != AntiDepReg) :
(P->getKind() == SDep::Data && P->getReg() == AntiDepReg)) {
AntiDepReg = 0;
break;
}
}
}
CriticalPathSU = NextSU;
CriticalPathMI = CriticalPathSU->getInstr();
} else {
// We've reached the end of the critical path.
CriticalPathSU = nullptr;
CriticalPathMI = nullptr;
}
}
PrescanInstruction(MI);
SmallVector<unsigned, 2> ForbidRegs;
// If MI's defs have a special allocation requirement, don't allow
// any def registers to be changed. Also assume all registers
// defined in a call must not be changed (ABI).
if (MI.isCall() || MI.hasExtraDefRegAllocReq() || TII->isPredicated(MI))
// If this instruction's defs have special allocation requirement, don't
// break this anti-dependency.
AntiDepReg = 0;
else if (AntiDepReg) {
// If this instruction has a use of AntiDepReg, breaking it
// is invalid. If the instruction defines other registers,
// save a list of them so that we don't pick a new register
// that overlaps any of them.
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI.getOperand(i);
if (!MO.isReg()) continue;
Register Reg = MO.getReg();
if (Reg == 0) continue;
if (MO.isUse() && TRI->regsOverlap(AntiDepReg, Reg)) {
AntiDepReg = 0;
break;
}
if (MO.isDef() && Reg != AntiDepReg)
ForbidRegs.push_back(Reg);
}
}
// Determine AntiDepReg's register class, if it is live and is
// consistently used within a single class.
const TargetRegisterClass *RC = AntiDepReg != 0 ? Classes[AntiDepReg]
: nullptr;
assert((AntiDepReg == 0 || RC != nullptr) &&
"Register should be live if it's causing an anti-dependence!");
if (RC == reinterpret_cast<TargetRegisterClass *>(-1))
AntiDepReg = 0;
// Look for a suitable register to use to break the anti-dependence.
//
// TODO: Instead of picking the first free register, consider which might
// be the best.
if (AntiDepReg != 0) {
std::pair<std::multimap<unsigned, MachineOperand *>::iterator,
std::multimap<unsigned, MachineOperand *>::iterator>
Range = RegRefs.equal_range(AntiDepReg);
if (unsigned NewReg = findSuitableFreeRegister(Range.first, Range.second,
AntiDepReg,
LastNewReg[AntiDepReg],
RC, ForbidRegs)) {
LLVM_DEBUG(dbgs() << "Breaking anti-dependence edge on "
<< printReg(AntiDepReg, TRI) << " with "
<< RegRefs.count(AntiDepReg) << " references"
<< " using " << printReg(NewReg, TRI) << "!\n");
// Update the references to the old register to refer to the new
// register.
for (std::multimap<unsigned, MachineOperand *>::iterator
Q = Range.first, QE = Range.second; Q != QE; ++Q) {
Q->second->setReg(NewReg);
// If the SU for the instruction being updated has debug information
// related to the anti-dependency register, make sure to update that
// as well.
const SUnit *SU = MISUnitMap[Q->second->getParent()];
if (!SU) continue;
UpdateDbgValues(DbgValues, Q->second->getParent(),
AntiDepReg, NewReg);
}
// We just went back in time and modified history; the
// liveness information for the anti-dependence reg is now
// inconsistent. Set the state as if it were dead.
Classes[NewReg] = Classes[AntiDepReg];
DefIndices[NewReg] = DefIndices[AntiDepReg];
KillIndices[NewReg] = KillIndices[AntiDepReg];
assert(((KillIndices[NewReg] == ~0u) !=
(DefIndices[NewReg] == ~0u)) &&
"Kill and Def maps aren't consistent for NewReg!");
Classes[AntiDepReg] = nullptr;
DefIndices[AntiDepReg] = KillIndices[AntiDepReg];
KillIndices[AntiDepReg] = ~0u;
assert(((KillIndices[AntiDepReg] == ~0u) !=
(DefIndices[AntiDepReg] == ~0u)) &&
"Kill and Def maps aren't consistent for AntiDepReg!");
RegRefs.erase(AntiDepReg);
LastNewReg[AntiDepReg] = NewReg;
++Broken;
}
}
ScanInstruction(MI, Count);
}
return Broken;
}
AntiDepBreaker *
llvm::createCriticalAntiDepBreaker(MachineFunction &MFi,
const RegisterClassInfo &RCI) {
return new CriticalAntiDepBreaker(MFi, RCI);
}
Reference in New Issue View Git Blame Copy Permalink