llvm-6502/lib/Target/SystemZ/SystemZElimCompare.cpp
Richard Sandiford 29e873ddb6 [SystemZ] Optimize floating-point comparisons with zero
This follows the same lines as the integer code.  In the end it seemed
easier to have a second 4-bit mask in TSFlags to specify the compare-like
CC values.  That eats one more TSFlags bit than adding a CCHasUnordered
would have done, but it feels more concise.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187883 91177308-0d34-0410-b5e6-96231b3b80d8
2013-08-07 11:10:06 +00:00

472 lines
16 KiB
C++

//===-- SystemZElimCompare.cpp - Eliminate comparison instructions --------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass:
// (1) tries to remove compares if CC already contains the required information
// (2) fuses compares and branches into COMPARE AND BRANCH instructions
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "systemz-elim-compare"
#include "SystemZTargetMachine.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetRegisterInfo.h"
using namespace llvm;
STATISTIC(BranchOnCounts, "Number of branch-on-count 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()
: Def(false), Use(false), IndirectDef(false), IndirectUse(false) {}
Reference &operator|=(const Reference &Other) {
Def |= Other.Def;
IndirectDef |= Other.IndirectDef;
Use |= Other.Use;
IndirectUse |= Other.IndirectUse;
return *this;
}
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;
bool Use;
// True if the register is defined or used indirectly, by a sub- or
// super-register.
bool IndirectDef;
bool IndirectUse;
};
class SystemZElimCompare : public MachineFunctionPass {
public:
static char ID;
SystemZElimCompare(const SystemZTargetMachine &tm)
: MachineFunctionPass(ID), TII(0), TRI(0) {}
virtual const char *getPassName() const {
return "SystemZ Comparison Elimination";
}
bool processBlock(MachineBasicBlock *MBB);
bool runOnMachineFunction(MachineFunction &F);
private:
Reference getRegReferences(MachineInstr *MI, unsigned Reg);
bool convertToBRCT(MachineInstr *MI, MachineInstr *Compare,
SmallVectorImpl<MachineInstr *> &CCUsers);
bool convertToLoadAndTest(MachineInstr *MI);
bool adjustCCMasksForInstr(MachineInstr *MI, MachineInstr *Compare,
SmallVectorImpl<MachineInstr *> &CCUsers);
bool optimizeCompareZero(MachineInstr *Compare,
SmallVectorImpl<MachineInstr *> &CCUsers);
bool fuseCompareAndBranch(MachineInstr *Compare,
SmallVectorImpl<MachineInstr *> &CCUsers);
const SystemZInstrInfo *TII;
const TargetRegisterInfo *TRI;
};
char SystemZElimCompare::ID = 0;
} // end of anonymous namespace
FunctionPass *llvm::createSystemZElimComparePass(SystemZTargetMachine &TM) {
return new SystemZElimCompare(TM);
}
// Return true if CC is live out of MBB.
static bool isCCLiveOut(MachineBasicBlock *MBB) {
for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
SE = MBB->succ_end(); SI != SE; ++SI)
if ((*SI)->isLiveIn(SystemZ::CC))
return true;
return false;
}
// Return true if any CC result of MI would reflect the value of subreg
// SubReg of Reg.
static bool resultTests(MachineInstr *MI, unsigned Reg, unsigned SubReg) {
if (MI->getNumOperands() > 0 &&
MI->getOperand(0).isReg() &&
MI->getOperand(0).isDef() &&
MI->getOperand(0).getReg() == Reg &&
MI->getOperand(0).getSubReg() == SubReg)
return true;
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 &&
MI->getOperand(1).getSubReg() == SubReg)
return true;
}
return false;
}
// Describe the references to Reg in MI, including sub- and super-registers.
Reference SystemZElimCompare::getRegReferences(MachineInstr *MI, unsigned Reg) {
Reference Ref;
for (unsigned I = 0, E = MI->getNumOperands(); I != E; ++I) {
const MachineOperand &MO = MI->getOperand(I);
if (MO.isReg()) {
if (unsigned MOReg = MO.getReg()) {
if (MOReg == Reg || TRI->regsOverlap(MOReg, Reg)) {
if (MO.isUse()) {
Ref.Use = true;
Ref.IndirectUse |= (MOReg != Reg);
}
if (MO.isDef()) {
Ref.Def = true;
Ref.IndirectDef |= (MOReg != Reg);
}
}
}
}
}
return Ref;
}
// 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). 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
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 = Compare->getOperand(0).getReg();
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).
MachineOperand Target(Branch->getOperand(2));
Branch->RemoveOperand(2);
Branch->RemoveOperand(1);
Branch->RemoveOperand(0);
Branch->setDesc(TII->get(BRCT));
MachineInstrBuilder(*Branch->getParent()->getParent(), Branch)
.addOperand(MI->getOperand(0))
.addOperand(MI->getOperand(1))
.addOperand(Target)
.addReg(SystemZ::CC, RegState::ImplicitDefine);
MI->removeFromParent();
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) {
unsigned Opcode = TII->getLoadAndTest(MI->getOpcode());
if (!Opcode)
return false;
MI->setDesc(TII->get(Opcode));
MachineInstrBuilder(*MI->getParent()->getParent(), MI)
.addReg(SystemZ::CC, RegState::ImplicitDefine);
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. 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) {
int Opcode = 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");
// 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.
int CCDef = MI->findRegisterDefOperandIdx(SystemZ::CC, false, true, TRI);
assert(CCDef >= 0 && "Couldn't find CC set");
MI->getOperand(CCDef).setIsDead(false);
// Clear any intervening kills of CC.
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:
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 = Compare->getOperand(0).getReg();
unsigned SrcSubReg = Compare->getOperand(0).getSubReg();
MachineBasicBlock *MBB = Compare->getParent();
MachineBasicBlock::iterator MBBI = Compare, MBBE = MBB->begin();
Reference CCRefs;
Reference SrcRefs;
while (MBBI != MBBE) {
--MBBI;
MachineInstr *MI = MBBI;
if (resultTests(MI, SrcReg, SrcSubReg)) {
// Try to remove both MI and Compare by converting a branch to BRCT(G).
// We don't care in this case whether CC is modified between MI and
// Compare.
if (!CCRefs.Use && !SrcRefs && convertToBRCT(MI, Compare, CCUsers)) {
BranchOnCounts += 1;
return true;
}
// Try to eliminate Compare by reusing a CC result from MI.
if ((!CCRefs && convertToLoadAndTest(MI)) ||
(!CCRefs.Def && adjustCCMasksForInstr(MI, Compare, CCUsers))) {
EliminatedComparisons += 1;
return true;
}
}
SrcRefs |= getRegReferences(MI, SrcReg);
if (SrcRefs.Def)
return false;
CCRefs |= getRegReferences(MI, SystemZ::CC);
if (CCRefs.Use && CCRefs.Def)
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::
fuseCompareAndBranch(MachineInstr *Compare,
SmallVectorImpl<MachineInstr *> &CCUsers) {
// See whether we have a comparison that can be fused.
unsigned FusedOpcode = TII->getCompareAndBranch(Compare->getOpcode(),
Compare);
if (!FusedOpcode)
return false;
// See whether we have a single branch with which to fuse.
if (CCUsers.size() != 1)
return false;
MachineInstr *Branch = CCUsers[0];
if (Branch->getOpcode() != SystemZ::BRC)
return false;
// Make sure that the operands are available at the branch.
unsigned SrcReg = Compare->getOperand(0).getReg();
unsigned SrcReg2 = (Compare->getOperand(1).isReg() ?
Compare->getOperand(1).getReg() : 0);
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 and target.
MachineOperand CCMask(MBBI->getOperand(1));
MachineOperand Target(MBBI->getOperand(2));
assert((CCMask.getImm() & ~SystemZ::CCMASK_ICMP) == 0 &&
"Invalid condition-code mask for integer comparison");
// Clear out all current operands.
int CCUse = MBBI->findRegisterUseOperandIdx(SystemZ::CC, false, TRI);
assert(CCUse >= 0 && "BRC must use CC");
Branch->RemoveOperand(CCUse);
Branch->RemoveOperand(2);
Branch->RemoveOperand(1);
Branch->RemoveOperand(0);
// Rebuild Branch as a fused compare and branch.
Branch->setDesc(TII->get(FusedOpcode));
MachineInstrBuilder(*Branch->getParent()->getParent(), Branch)
.addOperand(Compare->getOperand(0))
.addOperand(Compare->getOperand(1))
.addOperand(CCMask)
.addOperand(Target)
.addReg(SystemZ::CC, RegState::ImplicitDefine);
// 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.
bool CompleteCCUsers = !isCCLiveOut(MBB);
SmallVector<MachineInstr *, 4> CCUsers;
MachineBasicBlock::iterator MBBI = MBB->end();
while (MBBI != MBB->begin()) {
MachineInstr *MI = --MBBI;
if (CompleteCCUsers &&
MI->isCompare() &&
(optimizeCompareZero(MI, CCUsers) ||
fuseCompareAndBranch(MI, CCUsers))) {
++MBBI;
MI->removeFromParent();
Changed = true;
CCUsers.clear();
CompleteCCUsers = true;
continue;
}
Reference CCRefs(getRegReferences(MI, SystemZ::CC));
if (CCRefs.Def) {
CCUsers.clear();
CompleteCCUsers = !CCRefs.IndirectDef;
}
if (CompleteCCUsers && CCRefs.Use)
CCUsers.push_back(MI);
}
return Changed;
}
bool SystemZElimCompare::runOnMachineFunction(MachineFunction &F) {
TII = static_cast<const SystemZInstrInfo *>(F.getTarget().getInstrInfo());
TRI = &TII->getRegisterInfo();
bool Changed = false;
for (MachineFunction::iterator MFI = F.begin(), MFE = F.end();
MFI != MFE; ++MFI)
Changed |= processBlock(MFI);
return Changed;
}