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

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//===-- ARMLowOverheadLoops.cpp - CodeGen Low-overhead Loops ---*- C++ -*-===//
//
// 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
/// Finalize v8.1-m low-overhead loops by converting the associated pseudo
/// instructions into machine operations.
/// The expectation is that the loop contains three pseudo instructions:
/// - t2*LoopStart - placed in the preheader or pre-preheader. The do-loop
/// form should be in the preheader, whereas the while form should be in the
/// preheaders only predecessor.
/// - t2LoopDec - placed within in the loop body.
/// - t2LoopEnd - the loop latch terminator.
///
/// In addition to this, we also look for the presence of the VCTP instruction,
/// which determines whether we can generated the tail-predicated low-overhead
/// loop form.
///
/// Assumptions and Dependencies:
/// Low-overhead loops are constructed and executed using a setup instruction:
/// DLS, WLS, DLSTP or WLSTP and an instruction that loops back: LE or LETP.
/// WLS(TP) and LE(TP) are branching instructions with a (large) limited range
/// but fixed polarity: WLS can only branch forwards and LE can only branch
/// backwards. These restrictions mean that this pass is dependent upon block
/// layout and block sizes, which is why it's the last pass to run. The same is
/// true for ConstantIslands, but this pass does not increase the size of the
/// basic blocks, nor does it change the CFG. Instructions are mainly removed
/// during the transform and pseudo instructions are replaced by real ones. In
/// some cases, when we have to revert to a 'normal' loop, we have to introduce
/// multiple instructions for a single pseudo (see RevertWhile and
/// RevertLoopEnd). To handle this situation, t2WhileLoopStart and t2LoopEnd
/// are defined to be as large as this maximum sequence of replacement
/// instructions.
///
//===----------------------------------------------------------------------===//
#include "ARM.h"
#include "ARMBaseInstrInfo.h"
#include "ARMBaseRegisterInfo.h"
#include "ARMBasicBlockInfo.h"
#include "ARMSubtarget.h"
#include "Thumb2InstrInfo.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineLoopUtils.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/ReachingDefAnalysis.h"
#include "llvm/MC/MCInstrDesc.h"
using namespace llvm;
#define DEBUG_TYPE "arm-low-overhead-loops"
#define ARM_LOW_OVERHEAD_LOOPS_NAME "ARM Low Overhead Loops pass"
namespace {
class PostOrderLoopTraversal {
MachineLoop &ML;
MachineLoopInfo &MLI;
SmallPtrSet<MachineBasicBlock*, 4> Visited;
SmallVector<MachineBasicBlock*, 4> Order;
public:
PostOrderLoopTraversal(MachineLoop &ML, MachineLoopInfo &MLI)
: ML(ML), MLI(MLI) { }
const SmallVectorImpl<MachineBasicBlock*> &getOrder() const {
return Order;
}
// Visit all the blocks within the loop, as well as exit blocks and any
// blocks properly dominating the header.
void ProcessLoop() {
std::function<void(MachineBasicBlock*)> Search = [this, &Search]
(MachineBasicBlock *MBB) -> void {
if (Visited.count(MBB))
return;
Visited.insert(MBB);
for (auto *Succ : MBB->successors()) {
if (!ML.contains(Succ))
continue;
Search(Succ);
}
Order.push_back(MBB);
};
// Insert exit blocks.
SmallVector<MachineBasicBlock*, 2> ExitBlocks;
ML.getExitBlocks(ExitBlocks);
for (auto *MBB : ExitBlocks)
Order.push_back(MBB);
// Then add the loop body.
Search(ML.getHeader());
// Then try the preheader and its predecessors.
std::function<void(MachineBasicBlock*)> GetPredecessor =
[this, &GetPredecessor] (MachineBasicBlock *MBB) -> void {
Order.push_back(MBB);
if (MBB->pred_size() == 1)
GetPredecessor(*MBB->pred_begin());
};
if (auto *Preheader = ML.getLoopPreheader())
GetPredecessor(Preheader);
else if (auto *Preheader = MLI.findLoopPreheader(&ML, true))
GetPredecessor(Preheader);
}
};
struct PredicatedMI {
MachineInstr *MI = nullptr;
SetVector<MachineInstr*> Predicates;
public:
PredicatedMI(MachineInstr *I, SetVector<MachineInstr*> &Preds) :
MI(I) { Predicates.insert(Preds.begin(), Preds.end()); }
};
// Represent a VPT block, a list of instructions that begins with a VPST and
// has a maximum of four proceeding instructions. All instructions within the
// block are predicated upon the vpr and we allow instructions to define the
// vpr within in the block too.
class VPTBlock {
std::unique_ptr<PredicatedMI> VPST;
PredicatedMI *Divergent = nullptr;
SmallVector<PredicatedMI, 4> Insts;
public:
VPTBlock(MachineInstr *MI, SetVector<MachineInstr*> &Preds) {
VPST = std::make_unique<PredicatedMI>(MI, Preds);
}
void addInst(MachineInstr *MI, SetVector<MachineInstr*> &Preds) {
LLVM_DEBUG(dbgs() << "ARM Loops: Adding predicated MI: " << *MI);
if (!Divergent && !set_difference(Preds, VPST->Predicates).empty()) {
Divergent = &Insts.back();
LLVM_DEBUG(dbgs() << " - has divergent predicate: " << *Divergent->MI);
}
Insts.emplace_back(MI, Preds);
assert(Insts.size() <= 4 && "Too many instructions in VPT block!");
}
// Have we found an instruction within the block which defines the vpr? If
// so, not all the instructions in the block will have the same predicate.
bool HasNonUniformPredicate() const {
return Divergent != nullptr;
}
// Is the given instruction part of the predicate set controlling the entry
// to the block.
bool IsPredicatedOn(MachineInstr *MI) const {
return VPST->Predicates.count(MI);
}
// Is the given instruction the only predicate which controls the entry to
// the block.
bool IsOnlyPredicatedOn(MachineInstr *MI) const {
return IsPredicatedOn(MI) && VPST->Predicates.size() == 1;
}
unsigned size() const { return Insts.size(); }
SmallVectorImpl<PredicatedMI> &getInsts() { return Insts; }
MachineInstr *getVPST() const { return VPST->MI; }
PredicatedMI *getDivergent() const { return Divergent; }
};
struct LowOverheadLoop {
MachineLoop &ML;
MachineLoopInfo &MLI;
ReachingDefAnalysis &RDA;
const TargetRegisterInfo &TRI;
MachineFunction *MF = nullptr;
MachineInstr *InsertPt = nullptr;
MachineInstr *Start = nullptr;
MachineInstr *Dec = nullptr;
MachineInstr *End = nullptr;
MachineInstr *VCTP = nullptr;
VPTBlock *CurrentBlock = nullptr;
SetVector<MachineInstr*> CurrentPredicate;
SmallVector<VPTBlock, 4> VPTBlocks;
SmallPtrSet<MachineInstr*, 4> ToRemove;
bool Revert = false;
bool CannotTailPredicate = false;
LowOverheadLoop(MachineLoop &ML, MachineLoopInfo &MLI,
ReachingDefAnalysis &RDA, const TargetRegisterInfo &TRI)
: ML(ML), MLI(MLI), RDA(RDA), TRI(TRI) {
MF = ML.getHeader()->getParent();
}
// If this is an MVE instruction, check that we know how to use tail
// predication with it. Record VPT blocks and return whether the
// instruction is valid for tail predication.
bool ValidateMVEInst(MachineInstr *MI);
void AnalyseMVEInst(MachineInstr *MI) {
CannotTailPredicate = !ValidateMVEInst(MI);
}
bool IsTailPredicationLegal() const {
// For now, let's keep things really simple and only support a single
// block for tail predication.
return !Revert && FoundAllComponents() && VCTP &&
!CannotTailPredicate && ML.getNumBlocks() == 1;
}
// Check that the predication in the loop will be equivalent once we
// perform the conversion. Also ensure that we can provide the number
// of elements to the loop start instruction.
bool ValidateTailPredicate(MachineInstr *StartInsertPt);
// Check that any values available outside of the loop will be the same
// after tail predication conversion.
bool ValidateLiveOuts() const;
// Is it safe to define LR with DLS/WLS?
// LR can be defined if it is the operand to start, because it's the same
// value, or if it's going to be equivalent to the operand to Start.
MachineInstr *isSafeToDefineLR();
// Check the branch targets are within range and we satisfy our
// restrictions.
void CheckLegality(ARMBasicBlockUtils *BBUtils);
bool FoundAllComponents() const {
return Start && Dec && End;
}
SmallVectorImpl<VPTBlock> &getVPTBlocks() { return VPTBlocks; }
// Return the loop iteration count, or the number of elements if we're tail
// predicating.
MachineOperand &getCount() {
return IsTailPredicationLegal() ?
VCTP->getOperand(1) : Start->getOperand(0);
}
unsigned getStartOpcode() const {
bool IsDo = Start->getOpcode() == ARM::t2DoLoopStart;
if (!IsTailPredicationLegal())
return IsDo ? ARM::t2DLS : ARM::t2WLS;
return VCTPOpcodeToLSTP(VCTP->getOpcode(), IsDo);
}
void dump() const {
if (Start) dbgs() << "ARM Loops: Found Loop Start: " << *Start;
if (Dec) dbgs() << "ARM Loops: Found Loop Dec: " << *Dec;
if (End) dbgs() << "ARM Loops: Found Loop End: " << *End;
if (VCTP) dbgs() << "ARM Loops: Found VCTP: " << *VCTP;
if (!FoundAllComponents())
dbgs() << "ARM Loops: Not a low-overhead loop.\n";
else if (!(Start && Dec && End))
dbgs() << "ARM Loops: Failed to find all loop components.\n";
}
};
class ARMLowOverheadLoops : public MachineFunctionPass {
MachineFunction *MF = nullptr;
MachineLoopInfo *MLI = nullptr;
ReachingDefAnalysis *RDA = nullptr;
const ARMBaseInstrInfo *TII = nullptr;
MachineRegisterInfo *MRI = nullptr;
const TargetRegisterInfo *TRI = nullptr;
std::unique_ptr<ARMBasicBlockUtils> BBUtils = nullptr;
public:
static char ID;
ARMLowOverheadLoops() : MachineFunctionPass(ID) { }
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<MachineLoopInfo>();
AU.addRequired<ReachingDefAnalysis>();
MachineFunctionPass::getAnalysisUsage(AU);
}
bool runOnMachineFunction(MachineFunction &MF) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
MachineFunctionProperties::Property::NoVRegs).set(
MachineFunctionProperties::Property::TracksLiveness);
}
StringRef getPassName() const override {
return ARM_LOW_OVERHEAD_LOOPS_NAME;
}
private:
bool ProcessLoop(MachineLoop *ML);
bool RevertNonLoops();
void RevertWhile(MachineInstr *MI) const;
bool RevertLoopDec(MachineInstr *MI) const;
void RevertLoopEnd(MachineInstr *MI, bool SkipCmp = false) const;
void ConvertVPTBlocks(LowOverheadLoop &LoLoop);
MachineInstr *ExpandLoopStart(LowOverheadLoop &LoLoop);
void Expand(LowOverheadLoop &LoLoop);
void IterationCountDCE(LowOverheadLoop &LoLoop);
};
}
char ARMLowOverheadLoops::ID = 0;
INITIALIZE_PASS(ARMLowOverheadLoops, DEBUG_TYPE, ARM_LOW_OVERHEAD_LOOPS_NAME,
false, false)
MachineInstr *LowOverheadLoop::isSafeToDefineLR() {
// We can define LR because LR already contains the same value.
if (Start->getOperand(0).getReg() == ARM::LR)
return Start;
unsigned CountReg = Start->getOperand(0).getReg();
auto IsMoveLR = [&CountReg](MachineInstr *MI) {
return MI->getOpcode() == ARM::tMOVr &&
MI->getOperand(0).getReg() == ARM::LR &&
MI->getOperand(1).getReg() == CountReg &&
MI->getOperand(2).getImm() == ARMCC::AL;
};
MachineBasicBlock *MBB = Start->getParent();
// Find an insertion point:
// - Is there a (mov lr, Count) before Start? If so, and nothing else writes
// to Count before Start, we can insert at that mov.
if (auto *LRDef = RDA.getReachingMIDef(Start, ARM::LR))
if (IsMoveLR(LRDef) && RDA.hasSameReachingDef(Start, LRDef, CountReg))
return LRDef;
// - Is there a (mov lr, Count) after Start? If so, and nothing else writes
// to Count after Start, we can insert at that mov.
if (auto *LRDef = RDA.getLocalLiveOutMIDef(MBB, ARM::LR))
if (IsMoveLR(LRDef) && RDA.hasSameReachingDef(Start, LRDef, CountReg))
return LRDef;
// We've found no suitable LR def and Start doesn't use LR directly. Can we
// just define LR anyway?
return RDA.isSafeToDefRegAt(Start, ARM::LR) ? Start : nullptr;
}
bool LowOverheadLoop::ValidateTailPredicate(MachineInstr *StartInsertPt) {
assert(VCTP && "VCTP instruction expected but is not set");
// All predication within the loop should be based on vctp. If the block
// isn't predicated on entry, check whether the vctp is within the block
// and that all other instructions are then predicated on it.
for (auto &Block : VPTBlocks) {
if (Block.IsPredicatedOn(VCTP))
continue;
if (!Block.HasNonUniformPredicate() || !isVCTP(Block.getDivergent()->MI)) {
LLVM_DEBUG(dbgs() << "ARM Loops: Found unsupported diverging predicate: "
<< *Block.getDivergent()->MI);
return false;
}
SmallVectorImpl<PredicatedMI> &Insts = Block.getInsts();
for (auto &PredMI : Insts) {
if (PredMI.Predicates.count(VCTP) || isVCTP(PredMI.MI))
continue;
LLVM_DEBUG(dbgs() << "ARM Loops: Can't convert: " << *PredMI.MI
<< " - which is predicated on:\n";
for (auto *MI : PredMI.Predicates)
dbgs() << " - " << *MI);
return false;
}
}
if (!ValidateLiveOuts())
return false;
// For tail predication, we need to provide the number of elements, instead
// of the iteration count, to the loop start instruction. The number of
// elements is provided to the vctp instruction, so we need to check that
// we can use this register at InsertPt.
Register NumElements = VCTP->getOperand(1).getReg();
// If the register is defined within loop, then we can't perform TP.
// TODO: Check whether this is just a mov of a register that would be
// available.
if (RDA.hasLocalDefBefore(VCTP, NumElements)) {
LLVM_DEBUG(dbgs() << "ARM Loops: VCTP operand is defined in the loop.\n");
return false;
}
// The element count register maybe defined after InsertPt, in which case we
// need to try to move either InsertPt or the def so that the [w|d]lstp can
// use the value.
// TODO: On failing to move an instruction, check if the count is provided by
// a mov and whether we can use the mov operand directly.
MachineBasicBlock *InsertBB = StartInsertPt->getParent();
if (!RDA.isReachingDefLiveOut(StartInsertPt, NumElements)) {
if (auto *ElemDef = RDA.getLocalLiveOutMIDef(InsertBB, NumElements)) {
if (RDA.isSafeToMoveForwards(ElemDef, StartInsertPt)) {
ElemDef->removeFromParent();
InsertBB->insert(MachineBasicBlock::iterator(StartInsertPt), ElemDef);
LLVM_DEBUG(dbgs() << "ARM Loops: Moved element count def: "
<< *ElemDef);
} else if (RDA.isSafeToMoveBackwards(StartInsertPt, ElemDef)) {
StartInsertPt->removeFromParent();
InsertBB->insertAfter(MachineBasicBlock::iterator(ElemDef),
StartInsertPt);
LLVM_DEBUG(dbgs() << "ARM Loops: Moved start past: " << *ElemDef);
} else {
LLVM_DEBUG(dbgs() << "ARM Loops: Unable to move element count to loop "
<< "start instruction.\n");
return false;
}
}
}
// Especially in the case of while loops, InsertBB may not be the
// preheader, so we need to check that the register isn't redefined
// before entering the loop.
auto CannotProvideElements = [this](MachineBasicBlock *MBB,
Register NumElements) {
// NumElements is redefined in this block.
if (RDA.hasLocalDefBefore(&MBB->back(), NumElements))
return true;
// Don't continue searching up through multiple predecessors.
if (MBB->pred_size() > 1)
return true;
return false;
};
// First, find the block that looks like the preheader.
MachineBasicBlock *MBB = MLI.findLoopPreheader(&ML, true);
if (!MBB) {
LLVM_DEBUG(dbgs() << "ARM Loops: Didn't find preheader.\n");
return false;
}
// Then search backwards for a def, until we get to InsertBB.
while (MBB != InsertBB) {
if (CannotProvideElements(MBB, NumElements)) {
LLVM_DEBUG(dbgs() << "ARM Loops: Unable to provide element count.\n");
return false;
}
MBB = *MBB->pred_begin();
}
// Check that the value change of the element count is what we expect and
// that the predication will be equivalent. For this we need:
// NumElements = NumElements - VectorWidth. The sub will be a sub immediate
// and we can also allow register copies within the chain too.
auto IsValidSub = [](MachineInstr *MI, unsigned ExpectedVecWidth) {
unsigned ImmOpIdx = 0;
switch (MI->getOpcode()) {
default:
llvm_unreachable("unhandled sub opcode");
case ARM::tSUBi3:
case ARM::tSUBi8:
ImmOpIdx = 3;
break;
case ARM::t2SUBri:
case ARM::t2SUBri12:
ImmOpIdx = 2;
break;
}
return MI->getOperand(ImmOpIdx).getImm() == ExpectedVecWidth;
};
MBB = VCTP->getParent();
if (MachineInstr *Def = RDA.getReachingMIDef(&MBB->back(), NumElements)) {
SmallPtrSet<MachineInstr*, 2> ElementChain;
SmallPtrSet<MachineInstr*, 2> Ignore = { VCTP };
unsigned ExpectedVectorWidth = getTailPredVectorWidth(VCTP->getOpcode());
if (RDA.isSafeToRemove(Def, ElementChain, Ignore)) {
bool FoundSub = false;
for (auto *MI : ElementChain) {
if (isMovRegOpcode(MI->getOpcode()))
continue;
if (isSubImmOpcode(MI->getOpcode())) {
if (FoundSub || !IsValidSub(MI, ExpectedVectorWidth))
return false;
FoundSub = true;
} else
return false;
}
LLVM_DEBUG(dbgs() << "ARM Loops: Will remove element count chain:\n";
for (auto *MI : ElementChain)
dbgs() << " - " << *MI);
ToRemove.insert(ElementChain.begin(), ElementChain.end());
}
}
return true;
}
bool LowOverheadLoop::ValidateLiveOuts() const {
// Collect Q-regs that are live in the exit blocks. We don't collect scalars
// because they won't be affected by lane predication.
const TargetRegisterClass *QPRs = TRI.getRegClass(ARM::MQPRRegClassID);
SmallSet<Register, 2> LiveOuts;
SmallVector<MachineBasicBlock*, 2> ExitBlocks;
ML.getExitBlocks(ExitBlocks);
for (auto *MBB : ExitBlocks)
for (const MachineBasicBlock::RegisterMaskPair &RegMask : MBB->liveins())
if (QPRs->contains(RegMask.PhysReg))
LiveOuts.insert(RegMask.PhysReg);
// Collect the instructions in the loop body that define the live-out values.
SmallPtrSet<MachineInstr*, 2> LiveMIs;
MachineBasicBlock *MBB = ML.getHeader();
for (auto Reg : LiveOuts)
if (auto *MI = RDA.getLocalLiveOutMIDef(MBB, Reg))
LiveMIs.insert(MI);
LLVM_DEBUG(dbgs() << "ARM Loops: Found loop live-outs:\n";
for (auto *MI : LiveMIs)
dbgs() << " - " << *MI);
// We've already validated that any VPT predication within the loop will be
// equivalent when we perform the predication transformation; so we know that
// any VPT predicated instruction is predicated upon VCTP. Any live-out
// instruction needs to be predicated, so check this here.
for (auto *MI : LiveMIs) {
int PIdx = llvm::findFirstVPTPredOperandIdx(*MI);
if (PIdx == -1 || MI->getOperand(PIdx+1).getReg() != ARM::VPR)
return false;
}
return true;
}
void LowOverheadLoop::CheckLegality(ARMBasicBlockUtils *BBUtils) {
if (Revert)
return;
if (!End->getOperand(1).isMBB())
report_fatal_error("Expected LoopEnd to target basic block");
// TODO Maybe there's cases where the target doesn't have to be the header,
// but for now be safe and revert.
if (End->getOperand(1).getMBB() != ML.getHeader()) {
LLVM_DEBUG(dbgs() << "ARM Loops: LoopEnd is not targetting header.\n");
Revert = true;
return;
}
// The WLS and LE instructions have 12-bits for the label offset. WLS
// requires a positive offset, while LE uses negative.
if (BBUtils->getOffsetOf(End) < BBUtils->getOffsetOf(ML.getHeader()) ||
!BBUtils->isBBInRange(End, ML.getHeader(), 4094)) {
LLVM_DEBUG(dbgs() << "ARM Loops: LE offset is out-of-range\n");
Revert = true;
return;
}
if (Start->getOpcode() == ARM::t2WhileLoopStart &&
(BBUtils->getOffsetOf(Start) >
BBUtils->getOffsetOf(Start->getOperand(1).getMBB()) ||
!BBUtils->isBBInRange(Start, Start->getOperand(1).getMBB(), 4094))) {
LLVM_DEBUG(dbgs() << "ARM Loops: WLS offset is out-of-range!\n");
Revert = true;
return;
}
InsertPt = Revert ? nullptr : isSafeToDefineLR();
if (!InsertPt) {
LLVM_DEBUG(dbgs() << "ARM Loops: Unable to find safe insertion point.\n");
Revert = true;
return;
} else
LLVM_DEBUG(dbgs() << "ARM Loops: Start insertion point: " << *InsertPt);
if (!IsTailPredicationLegal()) {
LLVM_DEBUG(if (!VCTP)
dbgs() << "ARM Loops: Didn't find a VCTP instruction.\n";
dbgs() << "ARM Loops: Tail-predication is not valid.\n");
return;
}
assert(ML.getBlocks().size() == 1 &&
"Shouldn't be processing a loop with more than one block");
CannotTailPredicate = !ValidateTailPredicate(InsertPt);
LLVM_DEBUG(if (CannotTailPredicate)
dbgs() << "ARM Loops: Couldn't validate tail predicate.\n");
}
bool LowOverheadLoop::ValidateMVEInst(MachineInstr* MI) {
if (CannotTailPredicate)
return false;
// Only support a single vctp.
if (isVCTP(MI) && VCTP)
return false;
// Start a new vpt block when we discover a vpt.
if (MI->getOpcode() == ARM::MVE_VPST) {
VPTBlocks.emplace_back(MI, CurrentPredicate);
CurrentBlock = &VPTBlocks.back();
return true;
} else if (isVCTP(MI))
VCTP = MI;
else if (MI->getOpcode() == ARM::MVE_VPSEL ||
MI->getOpcode() == ARM::MVE_VPNOT)
return false;
// TODO: Allow VPSEL and VPNOT, we currently cannot because:
// 1) It will use the VPR as a predicate operand, but doesn't have to be
// instead a VPT block, which means we can assert while building up
// the VPT block because we don't find another VPST to being a new
// one.
// 2) VPSEL still requires a VPR operand even after tail predicating,
// which means we can't remove it unless there is another
// instruction, such as vcmp, that can provide the VPR def.
bool IsUse = false;
bool IsDef = false;
const MCInstrDesc &MCID = MI->getDesc();
for (int i = MI->getNumOperands() - 1; i >= 0; --i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || MO.getReg() != ARM::VPR)
continue;
if (MO.isDef()) {
CurrentPredicate.insert(MI);
IsDef = true;
} else if (ARM::isVpred(MCID.OpInfo[i].OperandType)) {
CurrentBlock->addInst(MI, CurrentPredicate);
IsUse = true;
} else {
LLVM_DEBUG(dbgs() << "ARM Loops: Found instruction using vpr: " << *MI);
return false;
}
}
// If we find a vpr def that is not already predicated on the vctp, we've
// got disjoint predicates that may not be equivalent when we do the
// conversion.
if (IsDef && !IsUse && VCTP && !isVCTP(MI)) {
LLVM_DEBUG(dbgs() << "ARM Loops: Found disjoint vpr def: " << *MI);
return false;
}
uint64_t Flags = MCID.TSFlags;
if ((Flags & ARMII::DomainMask) != ARMII::DomainMVE)
return true;
// If we find an instruction that has been marked as not valid for tail
// predication, only allow the instruction if it's contained within a valid
// VPT block.
if ((Flags & ARMII::ValidForTailPredication) == 0 && !IsUse) {
LLVM_DEBUG(dbgs() << "ARM Loops: Can't tail predicate: " << *MI);
return false;
}
// If the instruction is already explicitly predicated, then the conversion
// will be fine, but ensure that all memory operations are predicated.
return !IsUse && MI->mayLoadOrStore() ? false : true;
}
bool ARMLowOverheadLoops::runOnMachineFunction(MachineFunction &mf) {
const ARMSubtarget &ST = static_cast<const ARMSubtarget&>(mf.getSubtarget());
if (!ST.hasLOB())
return false;
MF = &mf;
LLVM_DEBUG(dbgs() << "ARM Loops on " << MF->getName() << " ------------- \n");
MLI = &getAnalysis<MachineLoopInfo>();
RDA = &getAnalysis<ReachingDefAnalysis>();
MF->getProperties().set(MachineFunctionProperties::Property::TracksLiveness);
MRI = &MF->getRegInfo();
TII = static_cast<const ARMBaseInstrInfo*>(ST.getInstrInfo());
TRI = ST.getRegisterInfo();
BBUtils = std::unique_ptr<ARMBasicBlockUtils>(new ARMBasicBlockUtils(*MF));
BBUtils->computeAllBlockSizes();
BBUtils->adjustBBOffsetsAfter(&MF->front());
bool Changed = false;
for (auto ML : *MLI) {
if (!ML->getParentLoop())
Changed |= ProcessLoop(ML);
}
Changed |= RevertNonLoops();
return Changed;
}
bool ARMLowOverheadLoops::ProcessLoop(MachineLoop *ML) {
bool Changed = false;
// Process inner loops first.
for (auto I = ML->begin(), E = ML->end(); I != E; ++I)
Changed |= ProcessLoop(*I);
LLVM_DEBUG(dbgs() << "ARM Loops: Processing loop containing:\n";
if (auto *Preheader = ML->getLoopPreheader())
dbgs() << " - " << Preheader->getName() << "\n";
else if (auto *Preheader = MLI->findLoopPreheader(ML))
dbgs() << " - " << Preheader->getName() << "\n";
else if (auto *Preheader = MLI->findLoopPreheader(ML, true))
dbgs() << " - " << Preheader->getName() << "\n";
for (auto *MBB : ML->getBlocks())
dbgs() << " - " << MBB->getName() << "\n";
);
// Search the given block for a loop start instruction. If one isn't found,
// and there's only one predecessor block, search that one too.
std::function<MachineInstr*(MachineBasicBlock*)> SearchForStart =
[&SearchForStart](MachineBasicBlock *MBB) -> MachineInstr* {
for (auto &MI : *MBB) {
if (isLoopStart(MI))
return &MI;
}
if (MBB->pred_size() == 1)
return SearchForStart(*MBB->pred_begin());
return nullptr;
};
LowOverheadLoop LoLoop(*ML, *MLI, *RDA, *TRI);
// Search the preheader for the start intrinsic.
// FIXME: I don't see why we shouldn't be supporting multiple predecessors
// with potentially multiple set.loop.iterations, so we need to enable this.
if (auto *Preheader = ML->getLoopPreheader())
LoLoop.Start = SearchForStart(Preheader);
else if (auto *Preheader = MLI->findLoopPreheader(ML, true))
LoLoop.Start = SearchForStart(Preheader);
else
return false;
// Find the low-overhead loop components and decide whether or not to fall
// back to a normal loop. Also look for a vctp instructions and decide
// whether we can convert that predicate using tail predication.
for (auto *MBB : reverse(ML->getBlocks())) {
for (auto &MI : *MBB) {
if (MI.isDebugValue())
continue;
else if (MI.getOpcode() == ARM::t2LoopDec)
LoLoop.Dec = &MI;
else if (MI.getOpcode() == ARM::t2LoopEnd)
LoLoop.End = &MI;
else if (isLoopStart(MI))
LoLoop.Start = &MI;
else if (MI.getDesc().isCall()) {
// TODO: Though the call will require LE to execute again, does this
// mean we should revert? Always executing LE hopefully should be
// faster than performing a sub,cmp,br or even subs,br.
LoLoop.Revert = true;
LLVM_DEBUG(dbgs() << "ARM Loops: Found call.\n");
} else {
// Record VPR defs and build up their corresponding vpt blocks.
// Check we know how to tail predicate any mve instructions.
LoLoop.AnalyseMVEInst(&MI);
}
}
}
LLVM_DEBUG(LoLoop.dump());
if (!LoLoop.FoundAllComponents()) {
LLVM_DEBUG(dbgs() << "ARM Loops: Didn't find loop start, update, end\n");
return false;
}
// Check that the only instruction using LoopDec is LoopEnd.
// TODO: Check for copy chains that really have no effect.
SmallPtrSet<MachineInstr*, 2> Uses;
RDA->getReachingLocalUses(LoLoop.Dec, ARM::LR, Uses);
if (Uses.size() > 1 || !Uses.count(LoLoop.End)) {
LLVM_DEBUG(dbgs() << "ARM Loops: Unable to remove LoopDec.\n");
LoLoop.Revert = true;
}
LoLoop.CheckLegality(BBUtils.get());
Expand(LoLoop);
return true;
}
// WhileLoopStart holds the exit block, so produce a cmp lr, 0 and then a
// beq that branches to the exit branch.
// TODO: We could also try to generate a cbz if the value in LR is also in
// another low register.
void ARMLowOverheadLoops::RevertWhile(MachineInstr *MI) const {
LLVM_DEBUG(dbgs() << "ARM Loops: Reverting to cmp: " << *MI);
MachineBasicBlock *MBB = MI->getParent();
MachineInstrBuilder MIB = BuildMI(*MBB, MI, MI->getDebugLoc(),
TII->get(ARM::t2CMPri));
MIB.add(MI->getOperand(0));
MIB.addImm(0);
MIB.addImm(ARMCC::AL);
MIB.addReg(ARM::NoRegister);
MachineBasicBlock *DestBB = MI->getOperand(1).getMBB();
unsigned BrOpc = BBUtils->isBBInRange(MI, DestBB, 254) ?
ARM::tBcc : ARM::t2Bcc;
MIB = BuildMI(*MBB, MI, MI->getDebugLoc(), TII->get(BrOpc));
MIB.add(MI->getOperand(1)); // branch target
MIB.addImm(ARMCC::EQ); // condition code
MIB.addReg(ARM::CPSR);
MI->eraseFromParent();
}
bool ARMLowOverheadLoops::RevertLoopDec(MachineInstr *MI) const {
LLVM_DEBUG(dbgs() << "ARM Loops: Reverting to sub: " << *MI);
MachineBasicBlock *MBB = MI->getParent();
MachineInstr *Last = &MBB->back();
SmallPtrSet<MachineInstr*, 1> Ignore;
if (Last->getOpcode() == ARM::t2LoopEnd)
Ignore.insert(Last);
// If nothing defines CPSR between LoopDec and LoopEnd, use a t2SUBS.
bool SetFlags = RDA->isSafeToDefRegAt(MI, ARM::CPSR, Ignore);
MachineInstrBuilder MIB = BuildMI(*MBB, MI, MI->getDebugLoc(),
TII->get(ARM::t2SUBri));
MIB.addDef(ARM::LR);
MIB.add(MI->getOperand(1));
MIB.add(MI->getOperand(2));
MIB.addImm(ARMCC::AL);
MIB.addReg(0);
if (SetFlags) {
MIB.addReg(ARM::CPSR);
MIB->getOperand(5).setIsDef(true);
} else
MIB.addReg(0);
MI->eraseFromParent();
return SetFlags;
}
// Generate a subs, or sub and cmp, and a branch instead of an LE.
void ARMLowOverheadLoops::RevertLoopEnd(MachineInstr *MI, bool SkipCmp) const {
LLVM_DEBUG(dbgs() << "ARM Loops: Reverting to cmp, br: " << *MI);
MachineBasicBlock *MBB = MI->getParent();
// Create cmp
if (!SkipCmp) {
MachineInstrBuilder MIB = BuildMI(*MBB, MI, MI->getDebugLoc(),
TII->get(ARM::t2CMPri));
MIB.addReg(ARM::LR);
MIB.addImm(0);
MIB.addImm(ARMCC::AL);
MIB.addReg(ARM::NoRegister);
}
MachineBasicBlock *DestBB = MI->getOperand(1).getMBB();
unsigned BrOpc = BBUtils->isBBInRange(MI, DestBB, 254) ?
ARM::tBcc : ARM::t2Bcc;
// Create bne
MachineInstrBuilder MIB =
BuildMI(*MBB, MI, MI->getDebugLoc(), TII->get(BrOpc));
MIB.add(MI->getOperand(1)); // branch target
MIB.addImm(ARMCC::NE); // condition code
MIB.addReg(ARM::CPSR);
MI->eraseFromParent();
}
// Perform dead code elimation on the loop iteration count setup expression.
// If we are tail-predicating, the number of elements to be processed is the
// operand of the VCTP instruction in the vector body, see getCount(), which is
// register $r3 in this example:
//
// $lr = big-itercount-expression
// ..
// t2DoLoopStart renamable $lr
// vector.body:
// ..
// $vpr = MVE_VCTP32 renamable $r3
// renamable $lr = t2LoopDec killed renamable $lr, 1
// t2LoopEnd renamable $lr, %vector.body
// tB %end
//
// What we would like achieve here is to replace the do-loop start pseudo
// instruction t2DoLoopStart with:
//
// $lr = MVE_DLSTP_32 killed renamable $r3
//
// Thus, $r3 which defines the number of elements, is written to $lr,
// and then we want to delete the whole chain that used to define $lr,
// see the comment below how this chain could look like.
//
void ARMLowOverheadLoops::IterationCountDCE(LowOverheadLoop &LoLoop) {
if (!LoLoop.IsTailPredicationLegal())
return;
if (auto *Def = RDA->getReachingMIDef(LoLoop.Start,
LoLoop.Start->getOperand(0).getReg())) {
SmallPtrSet<MachineInstr*, 4> Remove;
SmallPtrSet<MachineInstr*, 4> Ignore = { LoLoop.Start, LoLoop.Dec,
LoLoop.End, LoLoop.InsertPt };
SmallVector<MachineInstr*, 4> Chain = { Def };
while (!Chain.empty()) {
MachineInstr *MI = Chain.back();
Chain.pop_back();
// If an instruction is conditionally executed, we assume here that this
// an IT-block with just this single instruction in it, otherwise we
// continue and can't perform dead-code elimination on it. This will
// capture most cases, because the loop iteration count expression
// that performs a round-up to next multiple of the vector length will
// look like this:
//
// %mull = ..
// %0 = add i32 %mul, 3
// %1 = icmp slt i32 %mul, 4
// %smin = select i1 %1, i32 %mul, i32 4
// %2 = sub i32 %0, %smin
// %3 = lshr i32 %2, 2
// %4 = add nuw nsw i32 %3, 1
//
// There can be a select instruction, checking if we need to execute only
// 1 vector iteration (in this examples that means 4 elements). Thus,
// we conditionally execute one instructions to materialise the iteration
// count.
MachineInstr *IT = nullptr;
if (TII->getPredicate(*MI) != ARMCC::AL) {
auto PrevMI = std::prev(MI->getIterator());
auto NextMI = std::next(MI->getIterator());
if (PrevMI->getOpcode() == ARM::t2IT &&
TII->getPredicate(*NextMI) == ARMCC::AL)
IT = &*PrevMI;
else
// We can't analyse IT-blocks with multiple statements. Be
// conservative here: clear the list, and don't remove any statements
// at all.
return;
}
if (RDA->isSafeToRemove(MI, Remove, Ignore)) {
for (auto &MO : MI->operands()) {
if (!MO.isReg() || !MO.isUse() || MO.getReg() == 0)
continue;
if (auto *Op = RDA->getReachingMIDef(MI, MO.getReg()))
Chain.push_back(Op);
}
Ignore.insert(MI);
if (IT)
Remove.insert(IT);
}
}
LoLoop.ToRemove.insert(Remove.begin(), Remove.end());
}
}
MachineInstr* ARMLowOverheadLoops::ExpandLoopStart(LowOverheadLoop &LoLoop) {
LLVM_DEBUG(dbgs() << "ARM Loops: Expanding LoopStart.\n");
// When using tail-predication, try to delete the dead code that was used to
// calculate the number of loop iterations.
IterationCountDCE(LoLoop);
MachineInstr *InsertPt = LoLoop.InsertPt;
MachineInstr *Start = LoLoop.Start;
MachineBasicBlock *MBB = InsertPt->getParent();
bool IsDo = Start->getOpcode() == ARM::t2DoLoopStart;
unsigned Opc = LoLoop.getStartOpcode();
MachineOperand &Count = LoLoop.getCount();
MachineInstrBuilder MIB =
BuildMI(*MBB, InsertPt, InsertPt->getDebugLoc(), TII->get(Opc));
MIB.addDef(ARM::LR);
MIB.add(Count);
if (!IsDo)
MIB.add(Start->getOperand(1));
// If we're inserting at a mov lr, then remove it as it's redundant.
if (InsertPt != Start)
LoLoop.ToRemove.insert(InsertPt);
LoLoop.ToRemove.insert(Start);
LLVM_DEBUG(dbgs() << "ARM Loops: Inserted start: " << *MIB);
return &*MIB;
}
void ARMLowOverheadLoops::ConvertVPTBlocks(LowOverheadLoop &LoLoop) {
auto RemovePredicate = [](MachineInstr *MI) {
LLVM_DEBUG(dbgs() << "ARM Loops: Removing predicate from: " << *MI);
if (int PIdx = llvm::findFirstVPTPredOperandIdx(*MI)) {
assert(MI->getOperand(PIdx).getImm() == ARMVCC::Then &&
"Expected Then predicate!");
MI->getOperand(PIdx).setImm(ARMVCC::None);
MI->getOperand(PIdx+1).setReg(0);
} else
llvm_unreachable("trying to unpredicate a non-predicated instruction");
};
// There are a few scenarios which we have to fix up:
// 1) A VPT block with is only predicated by the vctp and has no internal vpr
// defs.
// 2) A VPT block which is only predicated by the vctp but has an internal
// vpr def.
// 3) A VPT block which is predicated upon the vctp as well as another vpr
// def.
// 4) A VPT block which is not predicated upon a vctp, but contains it and
// all instructions within the block are predicated upon in.
for (auto &Block : LoLoop.getVPTBlocks()) {
SmallVectorImpl<PredicatedMI> &Insts = Block.getInsts();
if (Block.HasNonUniformPredicate()) {
PredicatedMI *Divergent = Block.getDivergent();
if (isVCTP(Divergent->MI)) {
// The vctp will be removed, so the size of the vpt block needs to be
// modified.
uint64_t Size = getARMVPTBlockMask(Block.size() - 1);
Block.getVPST()->getOperand(0).setImm(Size);
LLVM_DEBUG(dbgs() << "ARM Loops: Modified VPT block mask.\n");
} else if (Block.IsOnlyPredicatedOn(LoLoop.VCTP)) {
// The VPT block has a non-uniform predicate but it's entry is guarded
// only by a vctp, which means we:
// - Need to remove the original vpst.
// - Then need to unpredicate any following instructions, until
// we come across the divergent vpr def.
// - Insert a new vpst to predicate the instruction(s) that following
// the divergent vpr def.
// TODO: We could be producing more VPT blocks than necessary and could
// fold the newly created one into a proceeding one.
for (auto I = ++MachineBasicBlock::iterator(Block.getVPST()),
E = ++MachineBasicBlock::iterator(Divergent->MI); I != E; ++I)
RemovePredicate(&*I);
unsigned Size = 0;
auto E = MachineBasicBlock::reverse_iterator(Divergent->MI);
auto I = MachineBasicBlock::reverse_iterator(Insts.back().MI);
MachineInstr *InsertAt = nullptr;
while (I != E) {
InsertAt = &*I;
++Size;
++I;
}
MachineInstrBuilder MIB = BuildMI(*InsertAt->getParent(), InsertAt,
InsertAt->getDebugLoc(),
TII->get(ARM::MVE_VPST));
MIB.addImm(getARMVPTBlockMask(Size));
LLVM_DEBUG(dbgs() << "ARM Loops: Removing VPST: " << *Block.getVPST());
LLVM_DEBUG(dbgs() << "ARM Loops: Created VPST: " << *MIB);
LoLoop.ToRemove.insert(Block.getVPST());
}
} else if (Block.IsOnlyPredicatedOn(LoLoop.VCTP)) {
// A vpt block which is only predicated upon vctp and has no internal vpr
// defs:
// - Remove vpst.
// - Unpredicate the remaining instructions.
LLVM_DEBUG(dbgs() << "ARM Loops: Removing VPST: " << *Block.getVPST());
LoLoop.ToRemove.insert(Block.getVPST());
for (auto &PredMI : Insts)
RemovePredicate(PredMI.MI);
}
}
LLVM_DEBUG(dbgs() << "ARM Loops: Removing VCTP: " << *LoLoop.VCTP);
LoLoop.ToRemove.insert(LoLoop.VCTP);
}
void ARMLowOverheadLoops::Expand(LowOverheadLoop &LoLoop) {
// Combine the LoopDec and LoopEnd instructions into LE(TP).
auto ExpandLoopEnd = [this](LowOverheadLoop &LoLoop) {
MachineInstr *End = LoLoop.End;
MachineBasicBlock *MBB = End->getParent();
unsigned Opc = LoLoop.IsTailPredicationLegal() ?
ARM::MVE_LETP : ARM::t2LEUpdate;
MachineInstrBuilder MIB = BuildMI(*MBB, End, End->getDebugLoc(),
TII->get(Opc));
MIB.addDef(ARM::LR);
MIB.add(End->getOperand(0));
MIB.add(End->getOperand(1));
LLVM_DEBUG(dbgs() << "ARM Loops: Inserted LE: " << *MIB);
LoLoop.Dec->eraseFromParent();
End->eraseFromParent();
return &*MIB;
};
// TODO: We should be able to automatically remove these branches before we
// get here - probably by teaching analyzeBranch about the pseudo
// instructions.
// If there is an unconditional branch, after I, that just branches to the
// next block, remove it.
auto RemoveDeadBranch = [](MachineInstr *I) {
MachineBasicBlock *BB = I->getParent();
MachineInstr *Terminator = &BB->instr_back();
if (Terminator->isUnconditionalBranch() && I != Terminator) {
MachineBasicBlock *Succ = Terminator->getOperand(0).getMBB();
if (BB->isLayoutSuccessor(Succ)) {
LLVM_DEBUG(dbgs() << "ARM Loops: Removing branch: " << *Terminator);
Terminator->eraseFromParent();
}
}
};
if (LoLoop.Revert) {
if (LoLoop.Start->getOpcode() == ARM::t2WhileLoopStart)
RevertWhile(LoLoop.Start);
else
LoLoop.Start->eraseFromParent();
bool FlagsAlreadySet = RevertLoopDec(LoLoop.Dec);
RevertLoopEnd(LoLoop.End, FlagsAlreadySet);
} else {
LoLoop.Start = ExpandLoopStart(LoLoop);
RemoveDeadBranch(LoLoop.Start);
LoLoop.End = ExpandLoopEnd(LoLoop);
RemoveDeadBranch(LoLoop.End);
if (LoLoop.IsTailPredicationLegal())
ConvertVPTBlocks(LoLoop);
for (auto *I : LoLoop.ToRemove) {
LLVM_DEBUG(dbgs() << "ARM Loops: Erasing " << *I);
I->eraseFromParent();
}
}
PostOrderLoopTraversal DFS(LoLoop.ML, *MLI);
DFS.ProcessLoop();
const SmallVectorImpl<MachineBasicBlock*> &PostOrder = DFS.getOrder();
for (auto *MBB : PostOrder) {
recomputeLiveIns(*MBB);
// FIXME: For some reason, the live-in print order is non-deterministic for
// our tests and I can't out why... So just sort them.
MBB->sortUniqueLiveIns();
}
for (auto *MBB : reverse(PostOrder))
recomputeLivenessFlags(*MBB);
}
bool ARMLowOverheadLoops::RevertNonLoops() {
LLVM_DEBUG(dbgs() << "ARM Loops: Reverting any remaining pseudos...\n");
bool Changed = false;
for (auto &MBB : *MF) {
SmallVector<MachineInstr*, 4> Starts;
SmallVector<MachineInstr*, 4> Decs;
SmallVector<MachineInstr*, 4> Ends;
for (auto &I : MBB) {
if (isLoopStart(I))
Starts.push_back(&I);
else if (I.getOpcode() == ARM::t2LoopDec)
Decs.push_back(&I);
else if (I.getOpcode() == ARM::t2LoopEnd)
Ends.push_back(&I);
}
if (Starts.empty() && Decs.empty() && Ends.empty())
continue;
Changed = true;
for (auto *Start : Starts) {
if (Start->getOpcode() == ARM::t2WhileLoopStart)
RevertWhile(Start);
else
Start->eraseFromParent();
}
for (auto *Dec : Decs)
RevertLoopDec(Dec);
for (auto *End : Ends)
RevertLoopEnd(End);
}
return Changed;
}
FunctionPass *llvm::createARMLowOverheadLoopsPass() {
return new ARMLowOverheadLoops();
}
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