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llvm-project/llvm/lib/Target/ARM/MVEVPTOptimisationsPass.cpp

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//===-- MVEVPTOptimisationsPass.cpp ---------------------------------------===//
//
// 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 This pass does a few optimisations related to MVE VPT blocks before
/// register allocation is performed. The goal is to maximize the sizes of the
/// blocks that will be created by the MVE VPT Block Insertion pass (which runs
/// after register allocation). The first optimisation done by this pass is the
/// replacement of "opposite" VCMPs with VPNOTs, so the Block Insertion pass
/// can delete them later to create larger VPT blocks.
/// The second optimisation replaces re-uses of old VCCR values with VPNOTs when
/// inside a block of predicated instructions. This is done to avoid
/// spill/reloads of VPR in the middle of a block, which prevents the Block
/// Insertion pass from creating large blocks.
//
//===----------------------------------------------------------------------===//
#include "ARM.h"
#include "ARMSubtarget.h"
#include "MCTargetDesc/ARMBaseInfo.h"
#include "Thumb2InstrInfo.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/Support/Debug.h"
#include <cassert>
using namespace llvm;
#define DEBUG_TYPE "arm-mve-vpt-opts"
namespace {
class MVEVPTOptimisations : public MachineFunctionPass {
public:
static char ID;
const Thumb2InstrInfo *TII;
MachineRegisterInfo *MRI;
MVEVPTOptimisations() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &Fn) override;
StringRef getPassName() const override {
return "ARM MVE VPT Optimisation Pass";
}
private:
MachineInstr &ReplaceRegisterUseWithVPNOT(MachineBasicBlock &MBB,
MachineInstr &Instr,
MachineOperand &User,
Register Target);
bool ReduceOldVCCRValueUses(MachineBasicBlock &MBB);
bool ReplaceVCMPsByVPNOTs(MachineBasicBlock &MBB);
bool ConvertVPSEL(MachineBasicBlock &MBB);
};
char MVEVPTOptimisations::ID = 0;
} // end anonymous namespace
INITIALIZE_PASS(MVEVPTOptimisations, DEBUG_TYPE,
"ARM MVE VPT Optimisations pass", false, false)
// Returns true if Opcode is any VCMP Opcode.
static bool IsVCMP(unsigned Opcode) { return VCMPOpcodeToVPT(Opcode) != 0; }
// Returns true if a VCMP with this Opcode can have its operands swapped.
// There is 2 kind of VCMP that can't have their operands swapped: Float VCMPs,
// and VCMPr instructions (since the r is always on the right).
static bool CanHaveSwappedOperands(unsigned Opcode) {
switch (Opcode) {
default:
return true;
case ARM::MVE_VCMPf32:
case ARM::MVE_VCMPf16:
case ARM::MVE_VCMPf32r:
case ARM::MVE_VCMPf16r:
case ARM::MVE_VCMPi8r:
case ARM::MVE_VCMPi16r:
case ARM::MVE_VCMPi32r:
case ARM::MVE_VCMPu8r:
case ARM::MVE_VCMPu16r:
case ARM::MVE_VCMPu32r:
case ARM::MVE_VCMPs8r:
case ARM::MVE_VCMPs16r:
case ARM::MVE_VCMPs32r:
return false;
}
}
// Returns the CondCode of a VCMP Instruction.
static ARMCC::CondCodes GetCondCode(MachineInstr &Instr) {
assert(IsVCMP(Instr.getOpcode()) && "Inst must be a VCMP");
return ARMCC::CondCodes(Instr.getOperand(3).getImm());
}
// Returns true if Cond is equivalent to a VPNOT instruction on the result of
// Prev. Cond and Prev must be VCMPs.
static bool IsVPNOTEquivalent(MachineInstr &Cond, MachineInstr &Prev) {
assert(IsVCMP(Cond.getOpcode()) && IsVCMP(Prev.getOpcode()));
// Opcodes must match.
if (Cond.getOpcode() != Prev.getOpcode())
return false;
MachineOperand &CondOP1 = Cond.getOperand(1), &CondOP2 = Cond.getOperand(2);
MachineOperand &PrevOP1 = Prev.getOperand(1), &PrevOP2 = Prev.getOperand(2);
// If the VCMP has the opposite condition with the same operands, we can
// replace it with a VPNOT
ARMCC::CondCodes ExpectedCode = GetCondCode(Cond);
ExpectedCode = ARMCC::getOppositeCondition(ExpectedCode);
if (ExpectedCode == GetCondCode(Prev))
if (CondOP1.isIdenticalTo(PrevOP1) && CondOP2.isIdenticalTo(PrevOP2))
return true;
// Check again with operands swapped if possible
if (!CanHaveSwappedOperands(Cond.getOpcode()))
return false;
ExpectedCode = ARMCC::getSwappedCondition(ExpectedCode);
return ExpectedCode == GetCondCode(Prev) && CondOP1.isIdenticalTo(PrevOP2) &&
CondOP2.isIdenticalTo(PrevOP1);
}
// Returns true if Instr writes to VCCR.
static bool IsWritingToVCCR(MachineInstr &Instr) {
if (Instr.getNumOperands() == 0)
return false;
MachineOperand &Dst = Instr.getOperand(0);
if (!Dst.isReg())
return false;
Register DstReg = Dst.getReg();
if (!DstReg.isVirtual())
return false;
MachineRegisterInfo &RegInfo = Instr.getMF()->getRegInfo();
const TargetRegisterClass *RegClass = RegInfo.getRegClassOrNull(DstReg);
return RegClass && (RegClass->getID() == ARM::VCCRRegClassID);
}
// Transforms
// <Instr that uses %A ('User' Operand)>
// Into
// %K = VPNOT %Target
// <Instr that uses %K ('User' Operand)>
// And returns the newly inserted VPNOT.
// This optimization is done in the hopes of preventing spills/reloads of VPR by
// reducing the number of VCCR values with overlapping lifetimes.
MachineInstr &MVEVPTOptimisations::ReplaceRegisterUseWithVPNOT(
MachineBasicBlock &MBB, MachineInstr &Instr, MachineOperand &User,
Register Target) {
Register NewResult = MRI->createVirtualRegister(MRI->getRegClass(Target));
MachineInstrBuilder MIBuilder =
BuildMI(MBB, &Instr, Instr.getDebugLoc(), TII->get(ARM::MVE_VPNOT))
.addDef(NewResult)
.addReg(Target);
addUnpredicatedMveVpredNOp(MIBuilder);
// Make the user use NewResult instead, and clear its kill flag.
User.setReg(NewResult);
User.setIsKill(false);
LLVM_DEBUG(dbgs() << " Inserting VPNOT (for spill prevention): ";
MIBuilder.getInstr()->dump());
return *MIBuilder.getInstr();
}
// Moves a VPNOT before its first user if an instruction that uses Reg is found
// in-between the VPNOT and its user.
// Returns true if there is at least one user of the VPNOT in the block.
static bool MoveVPNOTBeforeFirstUser(MachineBasicBlock &MBB,
MachineBasicBlock::iterator Iter,
Register Reg) {
assert(Iter->getOpcode() == ARM::MVE_VPNOT && "Not a VPNOT!");
assert(getVPTInstrPredicate(*Iter) == ARMVCC::None &&
"The VPNOT cannot be predicated");
MachineInstr &VPNOT = *Iter;
Register VPNOTResult = VPNOT.getOperand(0).getReg();
Register VPNOTOperand = VPNOT.getOperand(1).getReg();
// Whether the VPNOT will need to be moved, and whether we found a user of the
// VPNOT.
bool MustMove = false, HasUser = false;
MachineOperand *VPNOTOperandKiller = nullptr;
for (; Iter != MBB.end(); ++Iter) {
if (MachineOperand *MO =
Iter->findRegisterUseOperand(VPNOTOperand, /*isKill*/ true)) {
// If we find the operand that kills the VPNOTOperand's result, save it.
VPNOTOperandKiller = MO;
}
if (Iter->findRegisterUseOperandIdx(Reg) != -1) {
MustMove = true;
continue;
}
if (Iter->findRegisterUseOperandIdx(VPNOTResult) == -1)
continue;
HasUser = true;
if (!MustMove)
break;
// Move the VPNOT right before Iter
LLVM_DEBUG(dbgs() << "Moving: "; VPNOT.dump(); dbgs() << " Before: ";
Iter->dump());
MBB.splice(Iter, &MBB, VPNOT.getIterator());
// If we move the instr, and its operand was killed earlier, remove the kill
// flag.
if (VPNOTOperandKiller)
VPNOTOperandKiller->setIsKill(false);
break;
}
return HasUser;
}
// This optimisation attempts to reduce the number of overlapping lifetimes of
// VCCR values by replacing uses of old VCCR values with VPNOTs. For example,
// this replaces
// %A:vccr = (something)
// %B:vccr = VPNOT %A
// %Foo = (some op that uses %B)
// %Bar = (some op that uses %A)
// With
// %A:vccr = (something)
// %B:vccr = VPNOT %A
// %Foo = (some op that uses %B)
// %TMP2:vccr = VPNOT %B
// %Bar = (some op that uses %A)
bool MVEVPTOptimisations::ReduceOldVCCRValueUses(MachineBasicBlock &MBB) {
MachineBasicBlock::iterator Iter = MBB.begin(), End = MBB.end();
SmallVector<MachineInstr *, 4> DeadInstructions;
bool Modified = false;
while (Iter != End) {
Register VCCRValue, OppositeVCCRValue;
// The first loop looks for 2 unpredicated instructions:
// %A:vccr = (instr) ; A is stored in VCCRValue
// %B:vccr = VPNOT %A ; B is stored in OppositeVCCRValue
for (; Iter != End; ++Iter) {
// We're only interested in unpredicated instructions that write to VCCR.
if (!IsWritingToVCCR(*Iter) ||
getVPTInstrPredicate(*Iter) != ARMVCC::None)
continue;
Register Dst = Iter->getOperand(0).getReg();
// If we already have a VCCRValue, and this is a VPNOT on VCCRValue, we've
// found what we were looking for.
if (VCCRValue && Iter->getOpcode() == ARM::MVE_VPNOT &&
Iter->findRegisterUseOperandIdx(VCCRValue) != -1) {
// Move the VPNOT closer to its first user if needed, and ignore if it
// has no users.
if (!MoveVPNOTBeforeFirstUser(MBB, Iter, VCCRValue))
continue;
OppositeVCCRValue = Dst;
++Iter;
break;
}
// Else, just set VCCRValue.
VCCRValue = Dst;
}
// If the first inner loop didn't find anything, stop here.
if (Iter == End)
break;
assert(VCCRValue && OppositeVCCRValue &&
"VCCRValue and OppositeVCCRValue shouldn't be empty if the loop "
"stopped before the end of the block!");
assert(VCCRValue != OppositeVCCRValue &&
"VCCRValue should not be equal to OppositeVCCRValue!");
// LastVPNOTResult always contains the same value as OppositeVCCRValue.
Register LastVPNOTResult = OppositeVCCRValue;
// This second loop tries to optimize the remaining instructions.
for (; Iter != End; ++Iter) {
bool IsInteresting = false;
if (MachineOperand *MO = Iter->findRegisterUseOperand(VCCRValue)) {
IsInteresting = true;
// - If the instruction is a VPNOT, it can be removed, and we can just
// replace its uses with LastVPNOTResult.
// - Else, insert a new VPNOT on LastVPNOTResult to recompute VCCRValue.
if (Iter->getOpcode() == ARM::MVE_VPNOT) {
Register Result = Iter->getOperand(0).getReg();
MRI->replaceRegWith(Result, LastVPNOTResult);
DeadInstructions.push_back(&*Iter);
Modified = true;
LLVM_DEBUG(dbgs()
<< "Replacing all uses of '" << printReg(Result)
<< "' with '" << printReg(LastVPNOTResult) << "'\n");
} else {
MachineInstr &VPNOT =
ReplaceRegisterUseWithVPNOT(MBB, *Iter, *MO, LastVPNOTResult);
Modified = true;
LastVPNOTResult = VPNOT.getOperand(0).getReg();
std::swap(VCCRValue, OppositeVCCRValue);
LLVM_DEBUG(dbgs() << "Replacing use of '" << printReg(VCCRValue)
<< "' with '" << printReg(LastVPNOTResult)
<< "' in instr: " << *Iter);
}
} else {
// If the instr uses OppositeVCCRValue, make it use LastVPNOTResult
// instead as they contain the same value.
if (MachineOperand *MO =
Iter->findRegisterUseOperand(OppositeVCCRValue)) {
IsInteresting = true;
// This is pointless if LastVPNOTResult == OppositeVCCRValue.
if (LastVPNOTResult != OppositeVCCRValue) {
LLVM_DEBUG(dbgs() << "Replacing usage of '"
<< printReg(OppositeVCCRValue) << "' with '"
<< printReg(LastVPNOTResult) << " for instr: ";
Iter->dump());
MO->setReg(LastVPNOTResult);
Modified = true;
}
MO->setIsKill(false);
}
// If this is an unpredicated VPNOT on
// LastVPNOTResult/OppositeVCCRValue, we can act like we inserted it.
if (Iter->getOpcode() == ARM::MVE_VPNOT &&
getVPTInstrPredicate(*Iter) == ARMVCC::None) {
Register VPNOTOperand = Iter->getOperand(1).getReg();
if (VPNOTOperand == LastVPNOTResult ||
VPNOTOperand == OppositeVCCRValue) {
IsInteresting = true;
std::swap(VCCRValue, OppositeVCCRValue);
LastVPNOTResult = Iter->getOperand(0).getReg();
}
}
}
// If this instruction was not interesting, and it writes to VCCR, stop.
if (!IsInteresting && IsWritingToVCCR(*Iter))
break;
}
}
for (MachineInstr *DeadInstruction : DeadInstructions)
DeadInstruction->eraseFromParent();
return Modified;
}
// This optimisation replaces VCMPs with VPNOTs when they are equivalent.
bool MVEVPTOptimisations::ReplaceVCMPsByVPNOTs(MachineBasicBlock &MBB) {
SmallVector<MachineInstr *, 4> DeadInstructions;
// The last VCMP that we have seen and that couldn't be replaced.
// This is reset when an instruction that writes to VCCR/VPR is found, or when
// a VCMP is replaced with a VPNOT.
// We'll only replace VCMPs with VPNOTs when this is not null, and when the
// current VCMP is the opposite of PrevVCMP.
MachineInstr *PrevVCMP = nullptr;
// If we find an instruction that kills the result of PrevVCMP, we save the
// operand here to remove the kill flag in case we need to use PrevVCMP's
// result.
MachineOperand *PrevVCMPResultKiller = nullptr;
for (MachineInstr &Instr : MBB.instrs()) {
if (PrevVCMP) {
if (MachineOperand *MO = Instr.findRegisterUseOperand(
PrevVCMP->getOperand(0).getReg(), /*isKill*/ true)) {
// If we come accross the instr that kills PrevVCMP's result, record it
// so we can remove the kill flag later if we need to.
PrevVCMPResultKiller = MO;
}
}
// Ignore predicated instructions.
if (getVPTInstrPredicate(Instr) != ARMVCC::None)
continue;
// Only look at VCMPs
if (!IsVCMP(Instr.getOpcode())) {
// If the instruction writes to VCCR, forget the previous VCMP.
if (IsWritingToVCCR(Instr))
PrevVCMP = nullptr;
continue;
}
if (!PrevVCMP || !IsVPNOTEquivalent(Instr, *PrevVCMP)) {
PrevVCMP = &Instr;
continue;
}
// The register containing the result of the VCMP that we're going to
// replace.
Register PrevVCMPResultReg = PrevVCMP->getOperand(0).getReg();
// Build a VPNOT to replace the VCMP, reusing its operands.
MachineInstrBuilder MIBuilder =
BuildMI(MBB, &Instr, Instr.getDebugLoc(), TII->get(ARM::MVE_VPNOT))
.add(Instr.getOperand(0))
.addReg(PrevVCMPResultReg);
addUnpredicatedMveVpredNOp(MIBuilder);
LLVM_DEBUG(dbgs() << "Inserting VPNOT (to replace VCMP): ";
MIBuilder.getInstr()->dump(); dbgs() << " Removed VCMP: ";
Instr.dump());
// If we found an instruction that uses, and kills PrevVCMP's result,
// remove the kill flag.
if (PrevVCMPResultKiller)
PrevVCMPResultKiller->setIsKill(false);
// Finally, mark the old VCMP for removal and reset
// PrevVCMP/PrevVCMPResultKiller.
DeadInstructions.push_back(&Instr);
PrevVCMP = nullptr;
PrevVCMPResultKiller = nullptr;
}
for (MachineInstr *DeadInstruction : DeadInstructions)
DeadInstruction->eraseFromParent();
return !DeadInstructions.empty();
}
// Replace VPSEL with a predicated VMOV in blocks with a VCTP. This is a
// somewhat blunt approximation to allow tail predicated with vpsel
// instructions. We turn a vselect into a VPSEL in ISEL, but they have slightly
// different semantics under tail predication. Until that is modelled we just
// convert to a VMOVT (via a predicated VORR) instead.
bool MVEVPTOptimisations::ConvertVPSEL(MachineBasicBlock &MBB) {
bool HasVCTP = false;
SmallVector<MachineInstr *, 4> DeadInstructions;
for (MachineInstr &MI : MBB.instrs()) {
if (isVCTP(&MI)) {
HasVCTP = true;
continue;
}
if (!HasVCTP || MI.getOpcode() != ARM::MVE_VPSEL)
continue;
MachineInstrBuilder MIBuilder =
BuildMI(MBB, &MI, MI.getDebugLoc(), TII->get(ARM::MVE_VORR))
.add(MI.getOperand(0))
.add(MI.getOperand(1))
.add(MI.getOperand(1))
.addImm(ARMVCC::Then)
.add(MI.getOperand(4))
.add(MI.getOperand(2));
// Silence unused variable warning in release builds.
(void)MIBuilder;
LLVM_DEBUG(dbgs() << "Replacing VPSEL: "; MI.dump();
dbgs() << " with VMOVT: "; MIBuilder.getInstr()->dump());
DeadInstructions.push_back(&MI);
}
for (MachineInstr *DeadInstruction : DeadInstructions)
DeadInstruction->eraseFromParent();
return !DeadInstructions.empty();
}
bool MVEVPTOptimisations::runOnMachineFunction(MachineFunction &Fn) {
const ARMSubtarget &STI =
static_cast<const ARMSubtarget &>(Fn.getSubtarget());
if (!STI.isThumb2() || !STI.hasMVEIntegerOps())
return false;
TII = static_cast<const Thumb2InstrInfo *>(STI.getInstrInfo());
MRI = &Fn.getRegInfo();
LLVM_DEBUG(dbgs() << "********** ARM MVE VPT Optimisations **********\n"
<< "********** Function: " << Fn.getName() << '\n');
bool Modified = false;
for (MachineBasicBlock &MBB : Fn) {
Modified |= ReplaceVCMPsByVPNOTs(MBB);
Modified |= ReduceOldVCCRValueUses(MBB);
Modified |= ConvertVPSEL(MBB);
}
LLVM_DEBUG(dbgs() << "**************************************\n");
return Modified;
}
/// createMVEVPTOptimisationsPass
FunctionPass *llvm::createMVEVPTOptimisationsPass() {
return new MVEVPTOptimisations();
}
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