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llvm-mirror/lib/Target/AArch64/AArch64ConditionalCompares.cpp
Nicolai Hähnle d4eb4d6675 DomTree: Remove getChildren() accessor 
Summary:
Avoid exposing details about how children are stored. This will enable
subsequent type-erasure changes.

New methods are introduced to cover common access patterns.

Change-Id: Idb5f4b1b9c84e4cc71ddb39bb52a388682f5674f

Reviewers: arsenm, RKSimon, mehdi_amini, courbet

Subscribers: qcolombet, sdardis, wdng, hiraditya, jrtc27, zzheng, atanasyan, asbirlea, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83083
2020-07-06 21:58:11 +02:00

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//===-- AArch64ConditionalCompares.cpp --- CCMP formation for AArch64 -----===//
//
// 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 AArch64ConditionalCompares pass which reduces
// branching and code size by using the conditional compare instructions CCMP,
// CCMN, and FCMP.
//
// The CFG transformations for forming conditional compares are very similar to
// if-conversion, and this pass should run immediately before the early
// if-conversion pass.
//
//===----------------------------------------------------------------------===//
#include "AArch64.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineTraceMetrics.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "aarch64-ccmp"
// Absolute maximum number of instructions allowed per speculated block.
// This bypasses all other heuristics, so it should be set fairly high.
static cl::opt<unsigned> BlockInstrLimit(
"aarch64-ccmp-limit", cl::init(30), cl::Hidden,
cl::desc("Maximum number of instructions per speculated block."));
// Stress testing mode - disable heuristics.
static cl::opt<bool> Stress("aarch64-stress-ccmp", cl::Hidden,
cl::desc("Turn all knobs to 11"));
STATISTIC(NumConsidered, "Number of ccmps considered");
STATISTIC(NumPhiRejs, "Number of ccmps rejected (PHI)");
STATISTIC(NumPhysRejs, "Number of ccmps rejected (Physregs)");
STATISTIC(NumPhi2Rejs, "Number of ccmps rejected (PHI2)");
STATISTIC(NumHeadBranchRejs, "Number of ccmps rejected (Head branch)");
STATISTIC(NumCmpBranchRejs, "Number of ccmps rejected (CmpBB branch)");
STATISTIC(NumCmpTermRejs, "Number of ccmps rejected (CmpBB is cbz...)");
STATISTIC(NumImmRangeRejs, "Number of ccmps rejected (Imm out of range)");
STATISTIC(NumLiveDstRejs, "Number of ccmps rejected (Cmp dest live)");
STATISTIC(NumMultNZCVUses, "Number of ccmps rejected (NZCV used)");
STATISTIC(NumUnknNZCVDefs, "Number of ccmps rejected (NZCV def unknown)");
STATISTIC(NumSpeculateRejs, "Number of ccmps rejected (Can't speculate)");
STATISTIC(NumConverted, "Number of ccmp instructions created");
STATISTIC(NumCompBranches, "Number of cbz/cbnz branches converted");
//===----------------------------------------------------------------------===//
// SSACCmpConv
//===----------------------------------------------------------------------===//
//
// The SSACCmpConv class performs ccmp-conversion on SSA form machine code
// after determining if it is possible. The class contains no heuristics;
// external code should be used to determine when ccmp-conversion is a good
// idea.
//
// CCmp-formation works on a CFG representing chained conditions, typically
// from C's short-circuit || and && operators:
//
// From: Head To: Head
// / | CmpBB
// / | / |
// | CmpBB / |
// | / | Tail |
// | / | | |
// Tail | | |
// | | | |
// ... ... ... ...
//
// The Head block is terminated by a br.cond instruction, and the CmpBB block
// contains compare + br.cond. Tail must be a successor of both.
//
// The cmp-conversion turns the compare instruction in CmpBB into a conditional
// compare, and merges CmpBB into Head, speculatively executing its
// instructions. The AArch64 conditional compare instructions have an immediate
// operand that specifies the NZCV flag values when the condition is false and
// the compare isn't executed. This makes it possible to chain compares with
// different condition codes.
//
// Example:
//
// if (a == 5 || b == 17)
// foo();
//
// Head:
// cmp w0, #5
// b.eq Tail
// CmpBB:
// cmp w1, #17
// b.eq Tail
// ...
// Tail:
// bl _foo
//
// Becomes:
//
// Head:
// cmp w0, #5
// ccmp w1, #17, 4, ne ; 4 = nZcv
// b.eq Tail
// ...
// Tail:
// bl _foo
//
// The ccmp condition code is the one that would cause the Head terminator to
// branch to CmpBB.
//
// FIXME: It should also be possible to speculate a block on the critical edge
// between Head and Tail, just like if-converting a diamond.
//
// FIXME: Handle PHIs in Tail by turning them into selects (if-conversion).
namespace {
class SSACCmpConv {
MachineFunction *MF;
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
MachineRegisterInfo *MRI;
const MachineBranchProbabilityInfo *MBPI;
public:
/// The first block containing a conditional branch, dominating everything
/// else.
MachineBasicBlock *Head;
/// The block containing cmp+br.cond with a successor shared with Head.
MachineBasicBlock *CmpBB;
/// The common successor for Head and CmpBB.
MachineBasicBlock *Tail;
/// The compare instruction in CmpBB that can be converted to a ccmp.
MachineInstr *CmpMI;
private:
/// The branch condition in Head as determined by analyzeBranch.
SmallVector<MachineOperand, 4> HeadCond;
/// The condition code that makes Head branch to CmpBB.
AArch64CC::CondCode HeadCmpBBCC;
/// The branch condition in CmpBB.
SmallVector<MachineOperand, 4> CmpBBCond;
/// The condition code that makes CmpBB branch to Tail.
AArch64CC::CondCode CmpBBTailCC;
/// Check if the Tail PHIs are trivially convertible.
bool trivialTailPHIs();
/// Remove CmpBB from the Tail PHIs.
void updateTailPHIs();
/// Check if an operand defining DstReg is dead.
bool isDeadDef(unsigned DstReg);
/// Find the compare instruction in MBB that controls the conditional branch.
/// Return NULL if a convertible instruction can't be found.
MachineInstr *findConvertibleCompare(MachineBasicBlock *MBB);
/// Return true if all non-terminator instructions in MBB can be safely
/// speculated.
bool canSpeculateInstrs(MachineBasicBlock *MBB, const MachineInstr *CmpMI);
public:
/// runOnMachineFunction - Initialize per-function data structures.
void runOnMachineFunction(MachineFunction &MF,
const MachineBranchProbabilityInfo *MBPI) {
this->MF = &MF;
this->MBPI = MBPI;
TII = MF.getSubtarget().getInstrInfo();
TRI = MF.getSubtarget().getRegisterInfo();
MRI = &MF.getRegInfo();
}
/// If the sub-CFG headed by MBB can be cmp-converted, initialize the
/// internal state, and return true.
bool canConvert(MachineBasicBlock *MBB);
/// Cmo-convert the last block passed to canConvertCmp(), assuming
/// it is possible. Add any erased blocks to RemovedBlocks.
void convert(SmallVectorImpl<MachineBasicBlock *> &RemovedBlocks);
/// Return the expected code size delta if the conversion into a
/// conditional compare is performed.
int expectedCodeSizeDelta() const;
};
} // end anonymous namespace
// Check that all PHIs in Tail are selecting the same value from Head and CmpBB.
// This means that no if-conversion is required when merging CmpBB into Head.
bool SSACCmpConv::trivialTailPHIs() {
for (auto &I : *Tail) {
if (!I.isPHI())
break;
unsigned HeadReg = 0, CmpBBReg = 0;
// PHI operands come in (VReg, MBB) pairs.
for (unsigned oi = 1, oe = I.getNumOperands(); oi != oe; oi += 2) {
MachineBasicBlock *MBB = I.getOperand(oi + 1).getMBB();
Register Reg = I.getOperand(oi).getReg();
if (MBB == Head) {
assert((!HeadReg || HeadReg == Reg) && "Inconsistent PHI operands");
HeadReg = Reg;
}
if (MBB == CmpBB) {
assert((!CmpBBReg || CmpBBReg == Reg) && "Inconsistent PHI operands");
CmpBBReg = Reg;
}
}
if (HeadReg != CmpBBReg)
return false;
}
return true;
}
// Assuming that trivialTailPHIs() is true, update the Tail PHIs by simply
// removing the CmpBB operands. The Head operands will be identical.
void SSACCmpConv::updateTailPHIs() {
for (auto &I : *Tail) {
if (!I.isPHI())
break;
// I is a PHI. It can have multiple entries for CmpBB.
for (unsigned oi = I.getNumOperands(); oi > 2; oi -= 2) {
// PHI operands are (Reg, MBB) at (oi-2, oi-1).
if (I.getOperand(oi - 1).getMBB() == CmpBB) {
I.RemoveOperand(oi - 1);
I.RemoveOperand(oi - 2);
}
}
}
}
// This pass runs before the AArch64DeadRegisterDefinitions pass, so compares
// are still writing virtual registers without any uses.
bool SSACCmpConv::isDeadDef(unsigned DstReg) {
// Writes to the zero register are dead.
if (DstReg == AArch64::WZR || DstReg == AArch64::XZR)
return true;
if (!Register::isVirtualRegister(DstReg))
return false;
// A virtual register def without any uses will be marked dead later, and
// eventually replaced by the zero register.
return MRI->use_nodbg_empty(DstReg);
}
// Parse a condition code returned by analyzeBranch, and compute the CondCode
// corresponding to TBB.
// Return
static bool parseCond(ArrayRef<MachineOperand> Cond, AArch64CC::CondCode &CC) {
// A normal br.cond simply has the condition code.
if (Cond[0].getImm() != -1) {
assert(Cond.size() == 1 && "Unknown Cond array format");
CC = (AArch64CC::CondCode)(int)Cond[0].getImm();
return true;
}
// For tbz and cbz instruction, the opcode is next.
switch (Cond[1].getImm()) {
default:
// This includes tbz / tbnz branches which can't be converted to
// ccmp + br.cond.
return false;
case AArch64::CBZW:
case AArch64::CBZX:
assert(Cond.size() == 3 && "Unknown Cond array format");
CC = AArch64CC::EQ;
return true;
case AArch64::CBNZW:
case AArch64::CBNZX:
assert(Cond.size() == 3 && "Unknown Cond array format");
CC = AArch64CC::NE;
return true;
}
}
MachineInstr *SSACCmpConv::findConvertibleCompare(MachineBasicBlock *MBB) {
MachineBasicBlock::iterator I = MBB->getFirstTerminator();
if (I == MBB->end())
return nullptr;
// The terminator must be controlled by the flags.
if (!I->readsRegister(AArch64::NZCV)) {
switch (I->getOpcode()) {
case AArch64::CBZW:
case AArch64::CBZX:
case AArch64::CBNZW:
case AArch64::CBNZX:
// These can be converted into a ccmp against #0.
return &*I;
}
++NumCmpTermRejs;
LLVM_DEBUG(dbgs() << "Flags not used by terminator: " << *I);
return nullptr;
}
// Now find the instruction controlling the terminator.
for (MachineBasicBlock::iterator B = MBB->begin(); I != B;) {
I = prev_nodbg(I, MBB->begin());
assert(!I->isTerminator() && "Spurious terminator");
switch (I->getOpcode()) {
// cmp is an alias for subs with a dead destination register.
case AArch64::SUBSWri:
case AArch64::SUBSXri:
// cmn is an alias for adds with a dead destination register.
case AArch64::ADDSWri:
case AArch64::ADDSXri:
// Check that the immediate operand is within range, ccmp wants a uimm5.
// Rd = SUBSri Rn, imm, shift
if (I->getOperand(3).getImm() || !isUInt<5>(I->getOperand(2).getImm())) {
LLVM_DEBUG(dbgs() << "Immediate out of range for ccmp: " << *I);
++NumImmRangeRejs;
return nullptr;
}
LLVM_FALLTHROUGH;
case AArch64::SUBSWrr:
case AArch64::SUBSXrr:
case AArch64::ADDSWrr:
case AArch64::ADDSXrr:
if (isDeadDef(I->getOperand(0).getReg()))
return &*I;
LLVM_DEBUG(dbgs() << "Can't convert compare with live destination: "
<< *I);
++NumLiveDstRejs;
return nullptr;
case AArch64::FCMPSrr:
case AArch64::FCMPDrr:
case AArch64::FCMPESrr:
case AArch64::FCMPEDrr:
return &*I;
}
// Check for flag reads and clobbers.
PhysRegInfo PRI = AnalyzePhysRegInBundle(*I, AArch64::NZCV, TRI);
if (PRI.Read) {
// The ccmp doesn't produce exactly the same flags as the original
// compare, so reject the transform if there are uses of the flags
// besides the terminators.
LLVM_DEBUG(dbgs() << "Can't create ccmp with multiple uses: " << *I);
++NumMultNZCVUses;
return nullptr;
}
if (PRI.Defined || PRI.Clobbered) {
LLVM_DEBUG(dbgs() << "Not convertible compare: " << *I);
++NumUnknNZCVDefs;
return nullptr;
}
}
LLVM_DEBUG(dbgs() << "Flags not defined in " << printMBBReference(*MBB)
<< '\n');
return nullptr;
}
/// Determine if all the instructions in MBB can safely
/// be speculated. The terminators are not considered.
///
/// Only CmpMI is allowed to clobber the flags.
///
bool SSACCmpConv::canSpeculateInstrs(MachineBasicBlock *MBB,
const MachineInstr *CmpMI) {
// Reject any live-in physregs. It's probably NZCV/EFLAGS, and very hard to
// get right.
if (!MBB->livein_empty()) {
LLVM_DEBUG(dbgs() << printMBBReference(*MBB) << " has live-ins.\n");
return false;
}
unsigned InstrCount = 0;
// Check all instructions, except the terminators. It is assumed that
// terminators never have side effects or define any used register values.
for (auto &I : make_range(MBB->begin(), MBB->getFirstTerminator())) {
if (I.isDebugInstr())
continue;
if (++InstrCount > BlockInstrLimit && !Stress) {
LLVM_DEBUG(dbgs() << printMBBReference(*MBB) << " has more than "
<< BlockInstrLimit << " instructions.\n");
return false;
}
// There shouldn't normally be any phis in a single-predecessor block.
if (I.isPHI()) {
LLVM_DEBUG(dbgs() << "Can't hoist: " << I);
return false;
}
// Don't speculate loads. Note that it may be possible and desirable to
// speculate GOT or constant pool loads that are guaranteed not to trap,
// but we don't support that for now.
if (I.mayLoad()) {
LLVM_DEBUG(dbgs() << "Won't speculate load: " << I);
return false;
}
// We never speculate stores, so an AA pointer isn't necessary.
bool DontMoveAcrossStore = true;
if (!I.isSafeToMove(nullptr, DontMoveAcrossStore)) {
LLVM_DEBUG(dbgs() << "Can't speculate: " << I);
return false;
}
// Only CmpMI is allowed to clobber the flags.
if (&I != CmpMI && I.modifiesRegister(AArch64::NZCV, TRI)) {
LLVM_DEBUG(dbgs() << "Clobbers flags: " << I);
return false;
}
}
return true;
}
/// Analyze the sub-cfg rooted in MBB, and return true if it is a potential
/// candidate for cmp-conversion. Fill out the internal state.
///
bool SSACCmpConv::canConvert(MachineBasicBlock *MBB) {
Head = MBB;
Tail = CmpBB = nullptr;
if (Head->succ_size() != 2)
return false;
MachineBasicBlock *Succ0 = Head->succ_begin()[0];
MachineBasicBlock *Succ1 = Head->succ_begin()[1];
// CmpBB can only have a single predecessor. Tail is allowed many.
if (Succ0->pred_size() != 1)
std::swap(Succ0, Succ1);
// Succ0 is our candidate for CmpBB.
if (Succ0->pred_size() != 1 || Succ0->succ_size() != 2)
return false;
CmpBB = Succ0;
Tail = Succ1;
if (!CmpBB->isSuccessor(Tail))
return false;
// The CFG topology checks out.
LLVM_DEBUG(dbgs() << "\nTriangle: " << printMBBReference(*Head) << " -> "
<< printMBBReference(*CmpBB) << " -> "
<< printMBBReference(*Tail) << '\n');
++NumConsidered;
// Tail is allowed to have many predecessors, but we can't handle PHIs yet.
//
// FIXME: Real PHIs could be if-converted as long as the CmpBB values are
// defined before The CmpBB cmp clobbers the flags. Alternatively, it should
// always be safe to sink the ccmp down to immediately before the CmpBB
// terminators.
if (!trivialTailPHIs()) {
LLVM_DEBUG(dbgs() << "Can't handle phis in Tail.\n");
++NumPhiRejs;
return false;
}
if (!Tail->livein_empty()) {
LLVM_DEBUG(dbgs() << "Can't handle live-in physregs in Tail.\n");
++NumPhysRejs;
return false;
}
// CmpBB should never have PHIs since Head is its only predecessor.
// FIXME: Clean them up if it happens.
if (!CmpBB->empty() && CmpBB->front().isPHI()) {
LLVM_DEBUG(dbgs() << "Can't handle phis in CmpBB.\n");
++NumPhi2Rejs;
return false;
}
if (!CmpBB->livein_empty()) {
LLVM_DEBUG(dbgs() << "Can't handle live-in physregs in CmpBB.\n");
++NumPhysRejs;
return false;
}
// The branch we're looking to eliminate must be analyzable.
HeadCond.clear();
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
if (TII->analyzeBranch(*Head, TBB, FBB, HeadCond)) {
LLVM_DEBUG(dbgs() << "Head branch not analyzable.\n");
++NumHeadBranchRejs;
return false;
}
// This is weird, probably some sort of degenerate CFG, or an edge to a
// landing pad.
if (!TBB || HeadCond.empty()) {
LLVM_DEBUG(
dbgs() << "analyzeBranch didn't find conditional branch in Head.\n");
++NumHeadBranchRejs;
return false;
}
if (!parseCond(HeadCond, HeadCmpBBCC)) {
LLVM_DEBUG(dbgs() << "Unsupported branch type on Head\n");
++NumHeadBranchRejs;
return false;
}
// Make sure the branch direction is right.
if (TBB != CmpBB) {
assert(TBB == Tail && "Unexpected TBB");
HeadCmpBBCC = AArch64CC::getInvertedCondCode(HeadCmpBBCC);
}
CmpBBCond.clear();
TBB = FBB = nullptr;
if (TII->analyzeBranch(*CmpBB, TBB, FBB, CmpBBCond)) {
LLVM_DEBUG(dbgs() << "CmpBB branch not analyzable.\n");
++NumCmpBranchRejs;
return false;
}
if (!TBB || CmpBBCond.empty()) {
LLVM_DEBUG(
dbgs() << "analyzeBranch didn't find conditional branch in CmpBB.\n");
++NumCmpBranchRejs;
return false;
}
if (!parseCond(CmpBBCond, CmpBBTailCC)) {
LLVM_DEBUG(dbgs() << "Unsupported branch type on CmpBB\n");
++NumCmpBranchRejs;
return false;
}
if (TBB != Tail)
CmpBBTailCC = AArch64CC::getInvertedCondCode(CmpBBTailCC);
LLVM_DEBUG(dbgs() << "Head->CmpBB on "
<< AArch64CC::getCondCodeName(HeadCmpBBCC)
<< ", CmpBB->Tail on "
<< AArch64CC::getCondCodeName(CmpBBTailCC) << '\n');
CmpMI = findConvertibleCompare(CmpBB);
if (!CmpMI)
return false;
if (!canSpeculateInstrs(CmpBB, CmpMI)) {
++NumSpeculateRejs;
return false;
}
return true;
}
void SSACCmpConv::convert(SmallVectorImpl<MachineBasicBlock *> &RemovedBlocks) {
LLVM_DEBUG(dbgs() << "Merging " << printMBBReference(*CmpBB) << " into "
<< printMBBReference(*Head) << ":\n"
<< *CmpBB);
// All CmpBB instructions are moved into Head, and CmpBB is deleted.
// Update the CFG first.
updateTailPHIs();
// Save successor probabilties before removing CmpBB and Tail from their
// parents.
BranchProbability Head2CmpBB = MBPI->getEdgeProbability(Head, CmpBB);
BranchProbability CmpBB2Tail = MBPI->getEdgeProbability(CmpBB, Tail);
Head->removeSuccessor(CmpBB);
CmpBB->removeSuccessor(Tail);
// If Head and CmpBB had successor probabilties, udpate the probabilities to
// reflect the ccmp-conversion.
if (Head->hasSuccessorProbabilities() && CmpBB->hasSuccessorProbabilities()) {
// Head is allowed two successors. We've removed CmpBB, so the remaining
// successor is Tail. We need to increase the successor probability for
// Tail to account for the CmpBB path we removed.
//
// Pr(Tail|Head) += Pr(CmpBB|Head) * Pr(Tail|CmpBB).
assert(*Head->succ_begin() == Tail && "Head successor is not Tail");
BranchProbability Head2Tail = MBPI->getEdgeProbability(Head, Tail);
Head->setSuccProbability(Head->succ_begin(),
Head2Tail + Head2CmpBB * CmpBB2Tail);
// We will transfer successors of CmpBB to Head in a moment without
// normalizing the successor probabilities. Set the successor probabilites
// before doing so.
//
// Pr(I|Head) = Pr(CmpBB|Head) * Pr(I|CmpBB).
for (auto I = CmpBB->succ_begin(), E = CmpBB->succ_end(); I != E; ++I) {
BranchProbability CmpBB2I = MBPI->getEdgeProbability(CmpBB, *I);
CmpBB->setSuccProbability(I, Head2CmpBB * CmpBB2I);
}
}
Head->transferSuccessorsAndUpdatePHIs(CmpBB);
DebugLoc TermDL = Head->getFirstTerminator()->getDebugLoc();
TII->removeBranch(*Head);
// If the Head terminator was one of the cbz / tbz branches with built-in
// compare, we need to insert an explicit compare instruction in its place.
if (HeadCond[0].getImm() == -1) {
++NumCompBranches;
unsigned Opc = 0;
switch (HeadCond[1].getImm()) {
case AArch64::CBZW:
case AArch64::CBNZW:
Opc = AArch64::SUBSWri;
break;
case AArch64::CBZX:
case AArch64::CBNZX:
Opc = AArch64::SUBSXri;
break;
default:
llvm_unreachable("Cannot convert Head branch");
}
const MCInstrDesc &MCID = TII->get(Opc);
// Create a dummy virtual register for the SUBS def.
Register DestReg =
MRI->createVirtualRegister(TII->getRegClass(MCID, 0, TRI, *MF));
// Insert a SUBS Rn, #0 instruction instead of the cbz / cbnz.
BuildMI(*Head, Head->end(), TermDL, MCID)
.addReg(DestReg, RegState::Define | RegState::Dead)
.add(HeadCond[2])
.addImm(0)
.addImm(0);
// SUBS uses the GPR*sp register classes.
MRI->constrainRegClass(HeadCond[2].getReg(),
TII->getRegClass(MCID, 1, TRI, *MF));
}
Head->splice(Head->end(), CmpBB, CmpBB->begin(), CmpBB->end());
// Now replace CmpMI with a ccmp instruction that also considers the incoming
// flags.
unsigned Opc = 0;
unsigned FirstOp = 1; // First CmpMI operand to copy.
bool isZBranch = false; // CmpMI is a cbz/cbnz instruction.
switch (CmpMI->getOpcode()) {
default:
llvm_unreachable("Unknown compare opcode");
case AArch64::SUBSWri: Opc = AArch64::CCMPWi; break;
case AArch64::SUBSWrr: Opc = AArch64::CCMPWr; break;
case AArch64::SUBSXri: Opc = AArch64::CCMPXi; break;
case AArch64::SUBSXrr: Opc = AArch64::CCMPXr; break;
case AArch64::ADDSWri: Opc = AArch64::CCMNWi; break;
case AArch64::ADDSWrr: Opc = AArch64::CCMNWr; break;
case AArch64::ADDSXri: Opc = AArch64::CCMNXi; break;
case AArch64::ADDSXrr: Opc = AArch64::CCMNXr; break;
case AArch64::FCMPSrr: Opc = AArch64::FCCMPSrr; FirstOp = 0; break;
case AArch64::FCMPDrr: Opc = AArch64::FCCMPDrr; FirstOp = 0; break;
case AArch64::FCMPESrr: Opc = AArch64::FCCMPESrr; FirstOp = 0; break;
case AArch64::FCMPEDrr: Opc = AArch64::FCCMPEDrr; FirstOp = 0; break;
case AArch64::CBZW:
case AArch64::CBNZW:
Opc = AArch64::CCMPWi;
FirstOp = 0;
isZBranch = true;
break;
case AArch64::CBZX:
case AArch64::CBNZX:
Opc = AArch64::CCMPXi;
FirstOp = 0;
isZBranch = true;
break;
}
// The ccmp instruction should set the flags according to the comparison when
// Head would have branched to CmpBB.
// The NZCV immediate operand should provide flags for the case where Head
// would have branched to Tail. These flags should cause the new Head
// terminator to branch to tail.
unsigned NZCV = AArch64CC::getNZCVToSatisfyCondCode(CmpBBTailCC);
const MCInstrDesc &MCID = TII->get(Opc);
MRI->constrainRegClass(CmpMI->getOperand(FirstOp).getReg(),
TII->getRegClass(MCID, 0, TRI, *MF));
if (CmpMI->getOperand(FirstOp + 1).isReg())
MRI->constrainRegClass(CmpMI->getOperand(FirstOp + 1).getReg(),
TII->getRegClass(MCID, 1, TRI, *MF));
MachineInstrBuilder MIB = BuildMI(*Head, CmpMI, CmpMI->getDebugLoc(), MCID)
.add(CmpMI->getOperand(FirstOp)); // Register Rn
if (isZBranch)
MIB.addImm(0); // cbz/cbnz Rn -> ccmp Rn, #0
else
MIB.add(CmpMI->getOperand(FirstOp + 1)); // Register Rm / Immediate
MIB.addImm(NZCV).addImm(HeadCmpBBCC);
// If CmpMI was a terminator, we need a new conditional branch to replace it.
// This now becomes a Head terminator.
if (isZBranch) {
bool isNZ = CmpMI->getOpcode() == AArch64::CBNZW ||
CmpMI->getOpcode() == AArch64::CBNZX;
BuildMI(*Head, CmpMI, CmpMI->getDebugLoc(), TII->get(AArch64::Bcc))
.addImm(isNZ ? AArch64CC::NE : AArch64CC::EQ)
.add(CmpMI->getOperand(1)); // Branch target.
}
CmpMI->eraseFromParent();
Head->updateTerminator(CmpBB->getNextNode());
RemovedBlocks.push_back(CmpBB);
CmpBB->eraseFromParent();
LLVM_DEBUG(dbgs() << "Result:\n" << *Head);
++NumConverted;
}
int SSACCmpConv::expectedCodeSizeDelta() const {
int delta = 0;
// If the Head terminator was one of the cbz / tbz branches with built-in
// compare, we need to insert an explicit compare instruction in its place
// plus a branch instruction.
if (HeadCond[0].getImm() == -1) {
switch (HeadCond[1].getImm()) {
case AArch64::CBZW:
case AArch64::CBNZW:
case AArch64::CBZX:
case AArch64::CBNZX:
// Therefore delta += 1
delta = 1;
break;
default:
llvm_unreachable("Cannot convert Head branch");
}
}
// If the Cmp terminator was one of the cbz / tbz branches with
// built-in compare, it will be turned into a compare instruction
// into Head, but we do not save any instruction.
// Otherwise, we save the branch instruction.
switch (CmpMI->getOpcode()) {
default:
--delta;
break;
case AArch64::CBZW:
case AArch64::CBNZW:
case AArch64::CBZX:
case AArch64::CBNZX:
break;
}
return delta;
}
//===----------------------------------------------------------------------===//
// AArch64ConditionalCompares Pass
//===----------------------------------------------------------------------===//
namespace {
class AArch64ConditionalCompares : public MachineFunctionPass {
const MachineBranchProbabilityInfo *MBPI;
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
MCSchedModel SchedModel;
// Does the proceeded function has Oz attribute.
bool MinSize;
MachineRegisterInfo *MRI;
MachineDominatorTree *DomTree;
MachineLoopInfo *Loops;
MachineTraceMetrics *Traces;
MachineTraceMetrics::Ensemble *MinInstr;
SSACCmpConv CmpConv;
public:
static char ID;
AArch64ConditionalCompares() : MachineFunctionPass(ID) {
initializeAArch64ConditionalComparesPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnMachineFunction(MachineFunction &MF) override;
StringRef getPassName() const override {
return "AArch64 Conditional Compares";
}
private:
bool tryConvert(MachineBasicBlock *);
void updateDomTree(ArrayRef<MachineBasicBlock *> Removed);
void updateLoops(ArrayRef<MachineBasicBlock *> Removed);
void invalidateTraces();
bool shouldConvert();
};
} // end anonymous namespace
char AArch64ConditionalCompares::ID = 0;
INITIALIZE_PASS_BEGIN(AArch64ConditionalCompares, "aarch64-ccmp",
"AArch64 CCMP Pass", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_DEPENDENCY(MachineTraceMetrics)
INITIALIZE_PASS_END(AArch64ConditionalCompares, "aarch64-ccmp",
"AArch64 CCMP Pass", false, false)
FunctionPass *llvm::createAArch64ConditionalCompares() {
return new AArch64ConditionalCompares();
}
void AArch64ConditionalCompares::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<MachineBranchProbabilityInfo>();
AU.addRequired<MachineDominatorTree>();
AU.addPreserved<MachineDominatorTree>();
AU.addRequired<MachineLoopInfo>();
AU.addPreserved<MachineLoopInfo>();
AU.addRequired<MachineTraceMetrics>();
AU.addPreserved<MachineTraceMetrics>();
MachineFunctionPass::getAnalysisUsage(AU);
}
/// Update the dominator tree after if-conversion erased some blocks.
void AArch64ConditionalCompares::updateDomTree(
ArrayRef<MachineBasicBlock *> Removed) {
// convert() removes CmpBB which was previously dominated by Head.
// CmpBB children should be transferred to Head.
MachineDomTreeNode *HeadNode = DomTree->getNode(CmpConv.Head);
for (MachineBasicBlock *RemovedMBB : Removed) {
MachineDomTreeNode *Node = DomTree->getNode(RemovedMBB);
assert(Node != HeadNode && "Cannot erase the head node");
assert(Node->getIDom() == HeadNode && "CmpBB should be dominated by Head");
while (Node->getNumChildren())
DomTree->changeImmediateDominator(Node->back(), HeadNode);
DomTree->eraseNode(RemovedMBB);
}
}
/// Update LoopInfo after if-conversion.
void
AArch64ConditionalCompares::updateLoops(ArrayRef<MachineBasicBlock *> Removed) {
if (!Loops)
return;
for (MachineBasicBlock *RemovedMBB : Removed)
Loops->removeBlock(RemovedMBB);
}
/// Invalidate MachineTraceMetrics before if-conversion.
void AArch64ConditionalCompares::invalidateTraces() {
Traces->invalidate(CmpConv.Head);
Traces->invalidate(CmpConv.CmpBB);
}
/// Apply cost model and heuristics to the if-conversion in IfConv.
/// Return true if the conversion is a good idea.
///
bool AArch64ConditionalCompares::shouldConvert() {
// Stress testing mode disables all cost considerations.
if (Stress)
return true;
if (!MinInstr)
MinInstr = Traces->getEnsemble(MachineTraceMetrics::TS_MinInstrCount);
// Head dominates CmpBB, so it is always included in its trace.
MachineTraceMetrics::Trace Trace = MinInstr->getTrace(CmpConv.CmpBB);
// If code size is the main concern
if (MinSize) {
int CodeSizeDelta = CmpConv.expectedCodeSizeDelta();
LLVM_DEBUG(dbgs() << "Code size delta: " << CodeSizeDelta << '\n');
// If we are minimizing the code size, do the conversion whatever
// the cost is.
if (CodeSizeDelta < 0)
return true;
if (CodeSizeDelta > 0) {
LLVM_DEBUG(dbgs() << "Code size is increasing, give up on this one.\n");
return false;
}
// CodeSizeDelta == 0, continue with the regular heuristics
}
// Heuristic: The compare conversion delays the execution of the branch
// instruction because we must wait for the inputs to the second compare as
// well. The branch has no dependent instructions, but delaying it increases
// the cost of a misprediction.
//
// Set a limit on the delay we will accept.
unsigned DelayLimit = SchedModel.MispredictPenalty * 3 / 4;
// Instruction depths can be computed for all trace instructions above CmpBB.
unsigned HeadDepth =
Trace.getInstrCycles(*CmpConv.Head->getFirstTerminator()).Depth;
unsigned CmpBBDepth =
Trace.getInstrCycles(*CmpConv.CmpBB->getFirstTerminator()).Depth;
LLVM_DEBUG(dbgs() << "Head depth: " << HeadDepth
<< "\nCmpBB depth: " << CmpBBDepth << '\n');
if (CmpBBDepth > HeadDepth + DelayLimit) {
LLVM_DEBUG(dbgs() << "Branch delay would be larger than " << DelayLimit
<< " cycles.\n");
return false;
}
// Check the resource depth at the bottom of CmpBB - these instructions will
// be speculated.
unsigned ResDepth = Trace.getResourceDepth(true);
LLVM_DEBUG(dbgs() << "Resources: " << ResDepth << '\n');
// Heuristic: The speculatively executed instructions must all be able to
// merge into the Head block. The Head critical path should dominate the
// resource cost of the speculated instructions.
if (ResDepth > HeadDepth) {
LLVM_DEBUG(dbgs() << "Too many instructions to speculate.\n");
return false;
}
return true;
}
bool AArch64ConditionalCompares::tryConvert(MachineBasicBlock *MBB) {
bool Changed = false;
while (CmpConv.canConvert(MBB) && shouldConvert()) {
invalidateTraces();
SmallVector<MachineBasicBlock *, 4> RemovedBlocks;
CmpConv.convert(RemovedBlocks);
Changed = true;
updateDomTree(RemovedBlocks);
updateLoops(RemovedBlocks);
}
return Changed;
}
bool AArch64ConditionalCompares::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG(dbgs() << "********** AArch64 Conditional Compares **********\n"
<< "********** Function: " << MF.getName() << '\n');
if (skipFunction(MF.getFunction()))
return false;
TII = MF.getSubtarget().getInstrInfo();
TRI = MF.getSubtarget().getRegisterInfo();
SchedModel = MF.getSubtarget().getSchedModel();
MRI = &MF.getRegInfo();
DomTree = &getAnalysis<MachineDominatorTree>();
Loops = getAnalysisIfAvailable<MachineLoopInfo>();
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
Traces = &getAnalysis<MachineTraceMetrics>();
MinInstr = nullptr;
MinSize = MF.getFunction().hasMinSize();
bool Changed = false;
CmpConv.runOnMachineFunction(MF, MBPI);
// Visit blocks in dominator tree pre-order. The pre-order enables multiple
// cmp-conversions from the same head block.
// Note that updateDomTree() modifies the children of the DomTree node
// currently being visited. The df_iterator supports that; it doesn't look at
// child_begin() / child_end() until after a node has been visited.
for (auto *I : depth_first(DomTree))
if (tryConvert(I->getBlock()))
Changed = true;
return Changed;
}
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