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llvm-project/llvm/lib/Target/SystemZ/SystemZElimCompare.cpp

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//===-- SystemZElimCompare.cpp - Eliminate comparison instructions --------===//
//
// 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 pass:
// (1) tries to remove compares if CC already contains the required information
// (2) fuses compares and branches into COMPARE AND BRANCH instructions
//
//===----------------------------------------------------------------------===//
#include "SystemZ.h"
#include "SystemZInstrInfo.h"
#include "SystemZTargetMachine.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/MC/MCInstrDesc.h"
#include <cassert>
#include <cstdint>
using namespace llvm;
#define DEBUG_TYPE "systemz-elim-compare"
STATISTIC(BranchOnCounts, "Number of branch-on-count instructions");
STATISTIC(LoadAndTraps, "Number of load-and-trap instructions");
STATISTIC(EliminatedComparisons, "Number of eliminated comparisons");
STATISTIC(FusedComparisons, "Number of fused compare-and-branch instructions");
namespace {
// Represents the references to a particular register in one or more
// instructions.
struct Reference {
Reference() = default;
Reference &operator|=(const Reference &Other) {
Def |= Other.Def;
Use |= Other.Use;
return *this;
}
explicit operator bool() const { return Def || Use; }
// True if the register is defined or used in some form, either directly or
// via a sub- or super-register.
bool Def = false;
bool Use = false;
};
class SystemZElimCompare : public MachineFunctionPass {
public:
static char ID;
SystemZElimCompare(const SystemZTargetMachine &tm)
: MachineFunctionPass(ID) {}
StringRef getPassName() const override {
return "SystemZ Comparison Elimination";
}
bool processBlock(MachineBasicBlock &MBB);
bool runOnMachineFunction(MachineFunction &F) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
MachineFunctionProperties::Property::NoVRegs);
}
private:
Reference getRegReferences(MachineInstr &MI, unsigned Reg);
bool convertToBRCT(MachineInstr &MI, MachineInstr &Compare,
SmallVectorImpl<MachineInstr *> &CCUsers);
bool convertToLoadAndTrap(MachineInstr &MI, MachineInstr &Compare,
SmallVectorImpl<MachineInstr *> &CCUsers);
bool convertToLoadAndTest(MachineInstr &MI, MachineInstr &Compare,
SmallVectorImpl<MachineInstr *> &CCUsers);
bool adjustCCMasksForInstr(MachineInstr &MI, MachineInstr &Compare,
SmallVectorImpl<MachineInstr *> &CCUsers,
unsigned ConvOpc = 0);
bool optimizeCompareZero(MachineInstr &Compare,
SmallVectorImpl<MachineInstr *> &CCUsers);
bool fuseCompareOperations(MachineInstr &Compare,
SmallVectorImpl<MachineInstr *> &CCUsers);
const SystemZInstrInfo *TII = nullptr;
const TargetRegisterInfo *TRI = nullptr;
};
char SystemZElimCompare::ID = 0;
} // end anonymous namespace
// Returns true if MI is an instruction whose output equals the value in Reg.
static bool preservesValueOf(MachineInstr &MI, unsigned Reg) {
switch (MI.getOpcode()) {
case SystemZ::LR:
case SystemZ::LGR:
case SystemZ::LGFR:
case SystemZ::LTR:
case SystemZ::LTGR:
case SystemZ::LTGFR:
case SystemZ::LER:
case SystemZ::LDR:
case SystemZ::LXR:
case SystemZ::LTEBR:
case SystemZ::LTDBR:
case SystemZ::LTXBR:
if (MI.getOperand(1).getReg() == Reg)
return true;
}
return false;
}
// Return true if any CC result of MI would (perhaps after conversion)
// reflect the value of Reg.
static bool resultTests(MachineInstr &MI, unsigned Reg) {
if (MI.getNumOperands() > 0 && MI.getOperand(0).isReg() &&
MI.getOperand(0).isDef() && MI.getOperand(0).getReg() == Reg)
return true;
return (preservesValueOf(MI, Reg));
}
// Describe the references to Reg or any of its aliases in MI.
Reference SystemZElimCompare::getRegReferences(MachineInstr &MI, unsigned Reg) {
Reference Ref;
if (MI.isDebugInstr())
return Ref;
for (unsigned I = 0, E = MI.getNumOperands(); I != E; ++I) {
const MachineOperand &MO = MI.getOperand(I);
if (MO.isReg()) {
if (Register MOReg = MO.getReg()) {
if (TRI->regsOverlap(MOReg, Reg)) {
if (MO.isUse())
Ref.Use = true;
else if (MO.isDef())
Ref.Def = true;
}
}
}
}
return Ref;
}
// Return true if this is a load and test which can be optimized the
// same way as compare instruction.
static bool isLoadAndTestAsCmp(MachineInstr &MI) {
// If we during isel used a load-and-test as a compare with 0, the
// def operand is dead.
return (MI.getOpcode() == SystemZ::LTEBR ||
MI.getOpcode() == SystemZ::LTDBR ||
MI.getOpcode() == SystemZ::LTXBR) &&
MI.getOperand(0).isDead();
}
// Return the source register of Compare, which is the unknown value
// being tested.
static unsigned getCompareSourceReg(MachineInstr &Compare) {
unsigned reg = 0;
if (Compare.isCompare())
reg = Compare.getOperand(0).getReg();
else if (isLoadAndTestAsCmp(Compare))
reg = Compare.getOperand(1).getReg();
assert(reg);
return reg;
}
// Compare compares the result of MI against zero. If MI is an addition
// of -1 and if CCUsers is a single branch on nonzero, eliminate the addition
// and convert the branch to a BRCT(G) or BRCTH. Return true on success.
bool SystemZElimCompare::convertToBRCT(
MachineInstr &MI, MachineInstr &Compare,
SmallVectorImpl<MachineInstr *> &CCUsers) {
// Check whether we have an addition of -1.
unsigned Opcode = MI.getOpcode();
unsigned BRCT;
if (Opcode == SystemZ::AHI)
BRCT = SystemZ::BRCT;
else if (Opcode == SystemZ::AGHI)
BRCT = SystemZ::BRCTG;
else if (Opcode == SystemZ::AIH)
BRCT = SystemZ::BRCTH;
else
return false;
if (MI.getOperand(2).getImm() != -1)
return false;
// Check whether we have a single JLH.
if (CCUsers.size() != 1)
return false;
MachineInstr *Branch = CCUsers[0];
if (Branch->getOpcode() != SystemZ::BRC ||
Branch->getOperand(0).getImm() != SystemZ::CCMASK_ICMP ||
Branch->getOperand(1).getImm() != SystemZ::CCMASK_CMP_NE)
return false;
// We already know that there are no references to the register between
// MI and Compare. Make sure that there are also no references between
// Compare and Branch.
unsigned SrcReg = getCompareSourceReg(Compare);
MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
for (++MBBI; MBBI != MBBE; ++MBBI)
if (getRegReferences(*MBBI, SrcReg))
return false;
// The transformation is OK. Rebuild Branch as a BRCT(G) or BRCTH.
MachineOperand Target(Branch->getOperand(2));
while (Branch->getNumOperands())
Branch->RemoveOperand(0);
Branch->setDesc(TII->get(BRCT));
MachineInstrBuilder MIB(*Branch->getParent()->getParent(), Branch);
MIB.add(MI.getOperand(0)).add(MI.getOperand(1)).add(Target);
// Add a CC def to BRCT(G), since we may have to split them again if the
// branch displacement overflows. BRCTH has a 32-bit displacement, so
// this is not necessary there.
if (BRCT != SystemZ::BRCTH)
MIB.addReg(SystemZ::CC, RegState::ImplicitDefine | RegState::Dead);
MI.eraseFromParent();
return true;
}
// Compare compares the result of MI against zero. If MI is a suitable load
// instruction and if CCUsers is a single conditional trap on zero, eliminate
// the load and convert the branch to a load-and-trap. Return true on success.
bool SystemZElimCompare::convertToLoadAndTrap(
MachineInstr &MI, MachineInstr &Compare,
SmallVectorImpl<MachineInstr *> &CCUsers) {
unsigned LATOpcode = TII->getLoadAndTrap(MI.getOpcode());
if (!LATOpcode)
return false;
// Check whether we have a single CondTrap that traps on zero.
if (CCUsers.size() != 1)
return false;
MachineInstr *Branch = CCUsers[0];
if (Branch->getOpcode() != SystemZ::CondTrap ||
Branch->getOperand(0).getImm() != SystemZ::CCMASK_ICMP ||
Branch->getOperand(1).getImm() != SystemZ::CCMASK_CMP_EQ)
return false;
// We already know that there are no references to the register between
// MI and Compare. Make sure that there are also no references between
// Compare and Branch.
unsigned SrcReg = getCompareSourceReg(Compare);
MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
for (++MBBI; MBBI != MBBE; ++MBBI)
if (getRegReferences(*MBBI, SrcReg))
return false;
// The transformation is OK. Rebuild Branch as a load-and-trap.
while (Branch->getNumOperands())
Branch->RemoveOperand(0);
Branch->setDesc(TII->get(LATOpcode));
MachineInstrBuilder(*Branch->getParent()->getParent(), Branch)
.add(MI.getOperand(0))
.add(MI.getOperand(1))
.add(MI.getOperand(2))
.add(MI.getOperand(3));
MI.eraseFromParent();
return true;
}
// If MI is a load instruction, try to convert it into a LOAD AND TEST.
// Return true on success.
bool SystemZElimCompare::convertToLoadAndTest(
MachineInstr &MI, MachineInstr &Compare,
SmallVectorImpl<MachineInstr *> &CCUsers) {
// Try to adjust CC masks for the LOAD AND TEST opcode that could replace MI.
unsigned Opcode = TII->getLoadAndTest(MI.getOpcode());
if (!Opcode || !adjustCCMasksForInstr(MI, Compare, CCUsers, Opcode))
return false;
// Rebuild to get the CC operand in the right place.
auto MIB = BuildMI(*MI.getParent(), MI, MI.getDebugLoc(), TII->get(Opcode));
for (const auto &MO : MI.operands())
MIB.add(MO);
MIB.setMemRefs(MI.memoperands());
MI.eraseFromParent();
return true;
}
// The CC users in CCUsers are testing the result of a comparison of some
// value X against zero and we know that any CC value produced by MI would
// also reflect the value of X. ConvOpc may be used to pass the transfomed
// opcode MI will have if this succeeds. Try to adjust CCUsers so that they
// test the result of MI directly, returning true on success. Leave
// everything unchanged on failure.
bool SystemZElimCompare::adjustCCMasksForInstr(
MachineInstr &MI, MachineInstr &Compare,
SmallVectorImpl<MachineInstr *> &CCUsers,
unsigned ConvOpc) {
int Opcode = (ConvOpc ? ConvOpc : MI.getOpcode());
const MCInstrDesc &Desc = TII->get(Opcode);
unsigned MIFlags = Desc.TSFlags;
// See which compare-style condition codes are available.
unsigned ReusableCCMask = SystemZII::getCompareZeroCCMask(MIFlags);
// For unsigned comparisons with zero, only equality makes sense.
unsigned CompareFlags = Compare.getDesc().TSFlags;
if (CompareFlags & SystemZII::IsLogical)
ReusableCCMask &= SystemZ::CCMASK_CMP_EQ;
if (ReusableCCMask == 0)
return false;
unsigned CCValues = SystemZII::getCCValues(MIFlags);
assert((ReusableCCMask & ~CCValues) == 0 && "Invalid CCValues");
bool MIEquivalentToCmp =
(ReusableCCMask == CCValues &&
CCValues == SystemZII::getCCValues(CompareFlags));
if (!MIEquivalentToCmp) {
// Now check whether these flags are enough for all users.
SmallVector<MachineOperand *, 4> AlterMasks;
for (unsigned int I = 0, E = CCUsers.size(); I != E; ++I) {
MachineInstr *MI = CCUsers[I];
// Fail if this isn't a use of CC that we understand.
unsigned Flags = MI->getDesc().TSFlags;
unsigned FirstOpNum;
if (Flags & SystemZII::CCMaskFirst)
FirstOpNum = 0;
else if (Flags & SystemZII::CCMaskLast)
FirstOpNum = MI->getNumExplicitOperands() - 2;
else
return false;
// Check whether the instruction predicate treats all CC values
// outside of ReusableCCMask in the same way. In that case it
// doesn't matter what those CC values mean.
unsigned CCValid = MI->getOperand(FirstOpNum).getImm();
unsigned CCMask = MI->getOperand(FirstOpNum + 1).getImm();
unsigned OutValid = ~ReusableCCMask & CCValid;
unsigned OutMask = ~ReusableCCMask & CCMask;
if (OutMask != 0 && OutMask != OutValid)
return false;
AlterMasks.push_back(&MI->getOperand(FirstOpNum));
AlterMasks.push_back(&MI->getOperand(FirstOpNum + 1));
}
// All users are OK. Adjust the masks for MI.
for (unsigned I = 0, E = AlterMasks.size(); I != E; I += 2) {
AlterMasks[I]->setImm(CCValues);
unsigned CCMask = AlterMasks[I + 1]->getImm();
if (CCMask & ~ReusableCCMask)
AlterMasks[I + 1]->setImm((CCMask & ReusableCCMask) |
(CCValues & ~ReusableCCMask));
}
}
// CC is now live after MI.
if (!ConvOpc)
MI.clearRegisterDeads(SystemZ::CC);
// Check if MI lies before Compare.
bool BeforeCmp = false;
MachineBasicBlock::iterator MBBI = MI, MBBE = MI.getParent()->end();
for (++MBBI; MBBI != MBBE; ++MBBI)
if (MBBI == Compare) {
BeforeCmp = true;
break;
}
// Clear any intervening kills of CC.
if (BeforeCmp) {
MachineBasicBlock::iterator MBBI = MI, MBBE = Compare;
for (++MBBI; MBBI != MBBE; ++MBBI)
MBBI->clearRegisterKills(SystemZ::CC, TRI);
}
return true;
}
// Return true if Compare is a comparison against zero.
static bool isCompareZero(MachineInstr &Compare) {
switch (Compare.getOpcode()) {
case SystemZ::LTEBRCompare:
case SystemZ::LTDBRCompare:
case SystemZ::LTXBRCompare:
return true;
default:
if (isLoadAndTestAsCmp(Compare))
return true;
return Compare.getNumExplicitOperands() == 2 &&
Compare.getOperand(1).isImm() && Compare.getOperand(1).getImm() == 0;
}
}
// Try to optimize cases where comparison instruction Compare is testing
// a value against zero. Return true on success and if Compare should be
// deleted as dead. CCUsers is the list of instructions that use the CC
// value produced by Compare.
bool SystemZElimCompare::optimizeCompareZero(
MachineInstr &Compare, SmallVectorImpl<MachineInstr *> &CCUsers) {
if (!isCompareZero(Compare))
return false;
// Search back for CC results that are based on the first operand.
unsigned SrcReg = getCompareSourceReg(Compare);
MachineBasicBlock &MBB = *Compare.getParent();
Reference CCRefs;
Reference SrcRefs;
for (MachineBasicBlock::reverse_iterator MBBI =
std::next(MachineBasicBlock::reverse_iterator(&Compare)),
MBBE = MBB.rend(); MBBI != MBBE;) {
MachineInstr &MI = *MBBI++;
if (resultTests(MI, SrcReg)) {
// Try to remove both MI and Compare by converting a branch to BRCT(G).
// or a load-and-trap instruction. We don't care in this case whether
// CC is modified between MI and Compare.
if (!CCRefs.Use && !SrcRefs) {
if (convertToBRCT(MI, Compare, CCUsers)) {
BranchOnCounts += 1;
return true;
}
if (convertToLoadAndTrap(MI, Compare, CCUsers)) {
LoadAndTraps += 1;
return true;
}
}
// Try to eliminate Compare by reusing a CC result from MI.
if ((!CCRefs && convertToLoadAndTest(MI, Compare, CCUsers)) ||
(!CCRefs.Def && adjustCCMasksForInstr(MI, Compare, CCUsers))) {
EliminatedComparisons += 1;
return true;
}
}
SrcRefs |= getRegReferences(MI, SrcReg);
if (SrcRefs.Def)
break;
CCRefs |= getRegReferences(MI, SystemZ::CC);
if (CCRefs.Use && CCRefs.Def)
break;
}
// Also do a forward search to handle cases where an instruction after the
// compare can be converted, like
// LTEBRCompare %f0s, %f0s; %f2s = LER %f0s => LTEBRCompare %f2s, %f0s
for (MachineBasicBlock::iterator MBBI =
std::next(MachineBasicBlock::iterator(&Compare)), MBBE = MBB.end();
MBBI != MBBE;) {
MachineInstr &MI = *MBBI++;
if (preservesValueOf(MI, SrcReg)) {
// Try to eliminate Compare by reusing a CC result from MI.
if (convertToLoadAndTest(MI, Compare, CCUsers)) {
EliminatedComparisons += 1;
return true;
}
}
if (getRegReferences(MI, SrcReg).Def)
return false;
if (getRegReferences(MI, SystemZ::CC))
return false;
}
return false;
}
// Try to fuse comparison instruction Compare into a later branch.
// Return true on success and if Compare is therefore redundant.
bool SystemZElimCompare::fuseCompareOperations(
MachineInstr &Compare, SmallVectorImpl<MachineInstr *> &CCUsers) {
// See whether we have a single branch with which to fuse.
if (CCUsers.size() != 1)
return false;
MachineInstr *Branch = CCUsers[0];
SystemZII::FusedCompareType Type;
switch (Branch->getOpcode()) {
case SystemZ::BRC:
Type = SystemZII::CompareAndBranch;
break;
case SystemZ::CondReturn:
Type = SystemZII::CompareAndReturn;
break;
case SystemZ::CallBCR:
Type = SystemZII::CompareAndSibcall;
break;
case SystemZ::CondTrap:
Type = SystemZII::CompareAndTrap;
break;
default:
return false;
}
// See whether we have a comparison that can be fused.
unsigned FusedOpcode =
TII->getFusedCompare(Compare.getOpcode(), Type, &Compare);
if (!FusedOpcode)
return false;
// Make sure that the operands are available at the branch.
// SrcReg2 is the register if the source operand is a register,
// 0 if the source operand is immediate, and the base register
// if the source operand is memory (index is not supported).
Register SrcReg = Compare.getOperand(0).getReg();
Register SrcReg2 =
Compare.getOperand(1).isReg() ? Compare.getOperand(1).getReg() : Register();
MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
for (++MBBI; MBBI != MBBE; ++MBBI)
if (MBBI->modifiesRegister(SrcReg, TRI) ||
(SrcReg2 && MBBI->modifiesRegister(SrcReg2, TRI)))
return false;
// Read the branch mask, target (if applicable), regmask (if applicable).
MachineOperand CCMask(MBBI->getOperand(1));
assert((CCMask.getImm() & ~SystemZ::CCMASK_ICMP) == 0 &&
"Invalid condition-code mask for integer comparison");
// This is only valid for CompareAndBranch.
MachineOperand Target(MBBI->getOperand(
Type == SystemZII::CompareAndBranch ? 2 : 0));
const uint32_t *RegMask;
if (Type == SystemZII::CompareAndSibcall)
RegMask = MBBI->getOperand(2).getRegMask();
// Clear out all current operands.
int CCUse = MBBI->findRegisterUseOperandIdx(SystemZ::CC, false, TRI);
assert(CCUse >= 0 && "BRC/BCR must use CC");
Branch->RemoveOperand(CCUse);
// Remove target (branch) or regmask (sibcall).
if (Type == SystemZII::CompareAndBranch ||
Type == SystemZII::CompareAndSibcall)
Branch->RemoveOperand(2);
Branch->RemoveOperand(1);
Branch->RemoveOperand(0);
// Rebuild Branch as a fused compare and branch.
// SrcNOps is the number of MI operands of the compare instruction
// that we need to copy over.
unsigned SrcNOps = 2;
if (FusedOpcode == SystemZ::CLT || FusedOpcode == SystemZ::CLGT)
SrcNOps = 3;
Branch->setDesc(TII->get(FusedOpcode));
MachineInstrBuilder MIB(*Branch->getParent()->getParent(), Branch);
for (unsigned I = 0; I < SrcNOps; I++)
MIB.add(Compare.getOperand(I));
MIB.add(CCMask);
if (Type == SystemZII::CompareAndBranch) {
// Only conditional branches define CC, as they may be converted back
// to a non-fused branch because of a long displacement. Conditional
// returns don't have that problem.
MIB.add(Target).addReg(SystemZ::CC,
RegState::ImplicitDefine | RegState::Dead);
}
if (Type == SystemZII::CompareAndSibcall)
MIB.addRegMask(RegMask);
// Clear any intervening kills of SrcReg and SrcReg2.
MBBI = Compare;
for (++MBBI; MBBI != MBBE; ++MBBI) {
MBBI->clearRegisterKills(SrcReg, TRI);
if (SrcReg2)
MBBI->clearRegisterKills(SrcReg2, TRI);
}
FusedComparisons += 1;
return true;
}
// Process all comparison instructions in MBB. Return true if something
// changed.
bool SystemZElimCompare::processBlock(MachineBasicBlock &MBB) {
bool Changed = false;
// Walk backwards through the block looking for comparisons, recording
// all CC users as we go. The subroutines can delete Compare and
// instructions before it.
LivePhysRegs LiveRegs(*TRI);
LiveRegs.addLiveOuts(MBB);
bool CompleteCCUsers = !LiveRegs.contains(SystemZ::CC);
SmallVector<MachineInstr *, 4> CCUsers;
MachineBasicBlock::iterator MBBI = MBB.end();
while (MBBI != MBB.begin()) {
MachineInstr &MI = *--MBBI;
if (CompleteCCUsers && (MI.isCompare() || isLoadAndTestAsCmp(MI)) &&
(optimizeCompareZero(MI, CCUsers) ||
fuseCompareOperations(MI, CCUsers))) {
++MBBI;
MI.eraseFromParent();
Changed = true;
CCUsers.clear();
continue;
}
if (MI.definesRegister(SystemZ::CC)) {
CCUsers.clear();
CompleteCCUsers = true;
}
if (MI.readsRegister(SystemZ::CC) && CompleteCCUsers)
CCUsers.push_back(&MI);
}
return Changed;
}
bool SystemZElimCompare::runOnMachineFunction(MachineFunction &F) {
if (skipFunction(F.getFunction()))
return false;
TII = static_cast<const SystemZInstrInfo *>(F.getSubtarget().getInstrInfo());
TRI = &TII->getRegisterInfo();
bool Changed = false;
for (auto &MBB : F)
Changed |= processBlock(MBB);
return Changed;
}
FunctionPass *llvm::createSystemZElimComparePass(SystemZTargetMachine &TM) {
return new SystemZElimCompare(TM);
}
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