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[PowerPC] eliminate partially redundant compare instruction
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This is a follow-on of D37211.
D37211 eliminates a compare instruction if two conditional branches can be made based on the one compare instruction, e.g.
if (a == 0) { ... }
else if (a < 0) { ... }

This patch extends this optimization to support partially redundant cases, which often happen in while loops.
For example, one compare instruction is moved from the loop body into the preheader by this optimization in the following example.
do {
  if (a == 0) dummy1();
  a = func(a);
} while (a > 0);

Differential Revision: https://reviews.llvm.org/D38236

llvm-svn: 314390
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@inouehrs
inouehrs committed Sep 28, 2017
1 parent 9d3f125 commit 79c0bec
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Showing 2 changed files with 214 additions and 14 deletions.
194 changes: 180 additions & 14 deletions llvm/lib/Target/PowerPC/PPCMIPeephole.cpp
View file
Expand Up @@ -542,7 +542,44 @@ static unsigned getPredicateToIncImm(MachineInstr *BI, MachineInstr *CMPI) {
return 0;
}

// This takes a Phi node and returns a register value for the spefied BB.
static unsigned getIncomingRegForBlock(MachineInstr *Phi,
MachineBasicBlock *MBB) {
for (unsigned I = 2, E = Phi->getNumOperands() + 1; I != E; I += 2) {
MachineOperand &MO = Phi->getOperand(I);
if (MO.getMBB() == MBB)
return Phi->getOperand(I-1).getReg();
}
llvm_unreachable("invalid src basic block for this Phi node\n");
return 0;
}

// This function tracks the source of the register through register copy.
// If BB1 and BB2 are non-NULL, we also track PHI instruction in BB2
// assuming that the control comes from BB1 into BB2.
static unsigned getSrcVReg(unsigned Reg, MachineBasicBlock *BB1,
MachineBasicBlock *BB2, MachineRegisterInfo *MRI) {
unsigned SrcReg = Reg;
while (1) {
unsigned NextReg = SrcReg;
MachineInstr *Inst = MRI->getVRegDef(SrcReg);
if (BB1 && Inst->getOpcode() == PPC::PHI && Inst->getParent() == BB2) {
NextReg = getIncomingRegForBlock(Inst, BB1);
// We track through PHI only once to avoid infinite loop.
BB1 = nullptr;
}
else if (Inst->isFullCopy())
NextReg = Inst->getOperand(1).getReg();
if (NextReg == SrcReg || !TargetRegisterInfo::isVirtualRegister(NextReg))
break;
SrcReg = NextReg;
}
return SrcReg;
}

static bool eligibleForCompareElimination(MachineBasicBlock &MBB,
MachineBasicBlock *&PredMBB,
MachineBasicBlock *&MBBtoMoveCmp,
MachineRegisterInfo *MRI) {

auto isEligibleBB = [&](MachineBasicBlock &BB) {
Expand Down Expand Up @@ -571,12 +608,61 @@ static bool eligibleForCompareElimination(MachineBasicBlock &MBB,
return false;
};

if (MBB.pred_size() != 1)
// If this BB has more than one successor, we can create a new BB and
// move the compare instruction in the new BB.
// So far, we do not move compare instruction to a BB having multiple
// successors to avoid potentially increasing code size.
auto isEligibleForMoveCmp = [](MachineBasicBlock &BB) {
return BB.succ_size() == 1;
};

if (!isEligibleBB(MBB))
return false;

MachineBasicBlock *PredMBB = *MBB.pred_begin();
if (isEligibleBB(MBB) && isEligibleBB(*PredMBB))
unsigned NumPredBBs = MBB.pred_size();
if (NumPredBBs == 1) {
MachineBasicBlock *TmpMBB = *MBB.pred_begin();
if (isEligibleBB(*TmpMBB)) {
PredMBB = TmpMBB;
MBBtoMoveCmp = nullptr;
return true;
}
}
else if (NumPredBBs == 2) {
// We check for partially redundant case.
// So far, we support cases with only two predecessors
// to avoid increasing the number of instructions.
MachineBasicBlock::pred_iterator PI = MBB.pred_begin();
MachineBasicBlock *Pred1MBB = *PI;
MachineBasicBlock *Pred2MBB = *(PI+1);

if (isEligibleBB(*Pred1MBB) && isEligibleForMoveCmp(*Pred2MBB)) {
// We assume Pred1MBB is the BB containing the compare to be merged and
// Pred2MBB is the BB to which we will append a compare instruction.
// Hence we can proceed as is.
}
else if (isEligibleBB(*Pred2MBB) && isEligibleForMoveCmp(*Pred1MBB)) {
// We need to swap Pred1MBB and Pred2MBB to canonicalize.
std::swap(Pred1MBB, Pred2MBB);
}
else return false;

// Here, Pred2MBB is the BB to which we need to append a compare inst.
// We cannot move the compare instruction if operands are not available
// in Pred2MBB (i.e. defined in MBB by an instruction other than PHI).
MachineInstr *BI = &*MBB.getFirstInstrTerminator();
MachineInstr *CMPI = MRI->getVRegDef(BI->getOperand(1).getReg());
for (int I = 1; I <= 2; I++)
if (CMPI->getOperand(I).isReg()) {
MachineInstr *Inst = MRI->getVRegDef(CMPI->getOperand(I).getReg());
if (Inst->getParent() == &MBB && Inst->getOpcode() != PPC::PHI)
return false;
}

PredMBB = Pred1MBB;
MBBtoMoveCmp = Pred2MBB;
return true;
}

return false;
}
Expand Down Expand Up @@ -607,19 +693,44 @@ bool PPCMIPeephole::eliminateRedundantCompare(void) {
bool Simplified = false;

for (MachineBasicBlock &MBB2 : *MF) {
// We only consider two basic blocks MBB1 and MBB2 if
MachineBasicBlock *MBB1 = nullptr, *MBBtoMoveCmp = nullptr;

// For fully redundant case, we select two basic blocks MBB1 and MBB2
// as an optimization target if
// - both MBBs end with a conditional branch,
// - MBB1 is the only predecessor of MBB2, and
// - compare does not take a physical register as a operand in both MBBs.
if (!eligibleForCompareElimination(MBB2, MRI))
// In this case, eligibleForCompareElimination sets MBBtoMoveCmp nullptr.
//
// As partially redundant case, we additionally handle if MBB2 has one
// additional predecessor, which has only one successor (MBB2).
// In this case, we move the compre instruction originally in MBB2 into
// MBBtoMoveCmp. This partially redundant case is typically appear by
// compiling a while loop; here, MBBtoMoveCmp is the loop preheader.
//
// Overview of CFG of related basic blocks
// Fully redundant case Partially redundant case
// -------- ---------------- --------
// | MBB1 | (w/ 2 succ) | MBBtoMoveCmp | | MBB1 | (w/ 2 succ)
// -------- ---------------- --------
// | \ (w/ 1 succ) \ | \
// | \ \ | \
// | \ |
// -------- --------
// | MBB2 | (w/ 1 pred | MBB2 | (w/ 2 pred
// -------- and 2 succ) -------- and 2 succ)
// | \ | \
// | \ | \
//
if (!eligibleForCompareElimination(MBB2, MBB1, MBBtoMoveCmp, MRI))
continue;

MachineBasicBlock *MBB1 = *MBB2.pred_begin();
MachineInstr *BI1 = &*MBB1->getFirstInstrTerminator();
MachineInstr *CMPI1 = MRI->getVRegDef(BI1->getOperand(1).getReg());

MachineInstr *BI2 = &*MBB2.getFirstInstrTerminator();
MachineInstr *CMPI2 = MRI->getVRegDef(BI2->getOperand(1).getReg());
bool IsPartiallyRedundant = (MBBtoMoveCmp != nullptr);

// We cannot optimize an unsupported compare opcode or
// a mix of 32-bit and 64-bit comaprisons
Expand All @@ -631,6 +742,7 @@ bool PPCMIPeephole::eliminateRedundantCompare(void) {
unsigned NewOpCode = 0;
unsigned NewPredicate1 = 0, NewPredicate2 = 0;
int16_t Imm1 = 0, NewImm1 = 0, Imm2 = 0, NewImm2 = 0;
bool SwapOperands = false;

if (CMPI1->getOpcode() != CMPI2->getOpcode()) {
// Typically, unsigned comparison is used for equality check, but
Expand All @@ -649,10 +761,15 @@ bool PPCMIPeephole::eliminateRedundantCompare(void) {
if (CMPI1->getOperand(2).isReg() && CMPI2->getOperand(2).isReg()) {
// In case of comparisons between two registers, these two registers
// must be same to merge two comparisons.
unsigned Cmp1Operand1 = CMPI1->getOperand(1).getReg();
unsigned Cmp1Operand2 = CMPI1->getOperand(2).getReg();
unsigned Cmp2Operand1 = CMPI2->getOperand(1).getReg();
unsigned Cmp2Operand2 = CMPI2->getOperand(2).getReg();
unsigned Cmp1Operand1 = getSrcVReg(CMPI1->getOperand(1).getReg(),
nullptr, nullptr, MRI);
unsigned Cmp1Operand2 = getSrcVReg(CMPI1->getOperand(2).getReg(),
nullptr, nullptr, MRI);
unsigned Cmp2Operand1 = getSrcVReg(CMPI2->getOperand(1).getReg(),
MBB1, &MBB2, MRI);
unsigned Cmp2Operand2 = getSrcVReg(CMPI2->getOperand(2).getReg(),
MBB1, &MBB2, MRI);

if (Cmp1Operand1 == Cmp2Operand1 && Cmp1Operand2 == Cmp2Operand2) {
// Same pair of registers in the same order; ready to merge as is.
}
Expand All @@ -661,13 +778,20 @@ bool PPCMIPeephole::eliminateRedundantCompare(void) {
// We reverse the predicate to merge compare instructions.
PPC::Predicate Pred = (PPC::Predicate)BI2->getOperand(0).getImm();
NewPredicate2 = (unsigned)PPC::getSwappedPredicate(Pred);
// In case of partial redundancy, we need to swap operands
// in another compare instruction.
SwapOperands = true;
}
else continue;
}
else if (CMPI1->getOperand(2).isImm() && CMPI2->getOperand(2).isImm()){
// In case of comparisons between a register and an immediate,
// the operand register must be same for two compare instructions.
if (CMPI1->getOperand(1).getReg() != CMPI2->getOperand(1).getReg())
unsigned Cmp1Operand1 = getSrcVReg(CMPI1->getOperand(1).getReg(),
nullptr, nullptr, MRI);
unsigned Cmp2Operand1 = getSrcVReg(CMPI2->getOperand(1).getReg(),
MBB1, &MBB2, MRI);
if (Cmp1Operand1 != Cmp2Operand1)
continue;

NewImm1 = Imm1 = (int16_t)CMPI1->getOperand(2).getImm();
Expand Down Expand Up @@ -747,16 +871,58 @@ bool PPCMIPeephole::eliminateRedundantCompare(void) {
CMPI1->getOperand(2).setImm(NewImm1);
}

// We finally eliminate compare instruction in MBB2.
BI2->getOperand(1).setReg(BI1->getOperand(1).getReg());
if (IsPartiallyRedundant) {
// We touch up the compare instruction in MBB2 and move it to
// a previous BB to handle partially redundant case.
if (SwapOperands) {
unsigned Op1 = CMPI2->getOperand(1).getReg();
unsigned Op2 = CMPI2->getOperand(2).getReg();
CMPI2->getOperand(1).setReg(Op2);
CMPI2->getOperand(2).setReg(Op1);
}
if (NewImm2 != Imm2)
CMPI2->getOperand(2).setImm(NewImm2);

for (int I = 1; I <= 2; I++) {
if (CMPI2->getOperand(I).isReg()) {
MachineInstr *Inst = MRI->getVRegDef(CMPI2->getOperand(I).getReg());
if (Inst->getParent() != &MBB2)
continue;

assert(Inst->getOpcode() == PPC::PHI &&
"We cannot support if an operand comes from this BB.");
unsigned SrcReg = getIncomingRegForBlock(Inst, MBBtoMoveCmp);
CMPI2->getOperand(I).setReg(SrcReg);
}
}
auto I = MachineBasicBlock::iterator(MBBtoMoveCmp->getFirstTerminator());
MBBtoMoveCmp->splice(I, &MBB2, MachineBasicBlock::iterator(CMPI2));

DebugLoc DL = CMPI2->getDebugLoc();
unsigned NewVReg = MRI->createVirtualRegister(&PPC::CRRCRegClass);
BuildMI(MBB2, MBB2.begin(), DL,
TII->get(PPC::PHI), NewVReg)
.addReg(BI1->getOperand(1).getReg()).addMBB(MBB1)
.addReg(BI2->getOperand(1).getReg()).addMBB(MBBtoMoveCmp);
BI2->getOperand(1).setReg(NewVReg);
}
else {
// We finally eliminate compare instruction in MBB2.
BI2->getOperand(1).setReg(BI1->getOperand(1).getReg());
CMPI2->eraseFromParent();
}
BI2->getOperand(1).setIsKill(true);
BI1->getOperand(1).setIsKill(false);
CMPI2->eraseFromParent();

DEBUG(dbgs() << "into a compare and two branches:\n");
DEBUG(CMPI1->dump());
DEBUG(BI1->dump());
DEBUG(BI2->dump());
if (IsPartiallyRedundant) {
DEBUG(dbgs() << "The following compare is moved into BB#" <<
MBBtoMoveCmp->getNumber() << " to handle partial redundancy.\n");
DEBUG(CMPI2->dump());
}

Simplified = true;
}
Expand Down
34 changes: 34 additions & 0 deletions llvm/test/CodeGen/PowerPC/cmp_elimination.ll
View file
Expand Up @@ -714,9 +714,43 @@ if.end:
ret void
}

; partially redundant case
define void @func28(i32 signext %a) {
; CHECK-LABEL: @func28
; CHECK: cmplwi [[REG1:[0-9]+]], [[REG2:[0-9]+]]
; CHECK: .[[LABEL1:[A-Z0-9_]+]]:
; CHECK-NOT: cmp
; CHECK: bne 0, .[[LABEL2:[A-Z0-9_]+]]
; CHECK: bl dummy1
; CHECK: .[[LABEL2]]:
; CHECK: cmpwi [[REG1]], [[REG2]]
; CHECK: bgt 0, .[[LABEL1]]
; CHECK: blr
entry:
br label %do.body

do.body:
%a.addr.0 = phi i32 [ %a, %entry ], [ %call, %if.end ]
%cmp = icmp eq i32 %a.addr.0, 0
br i1 %cmp, label %if.then, label %if.end

if.then:
tail call void @dummy1() #2
br label %if.end

if.end:
%call = tail call signext i32 @func(i32 signext %a.addr.0) #2
%cmp1 = icmp sgt i32 %call, 0
br i1 %cmp1, label %do.body, label %do.end

do.end:
ret void
}

declare void @dummy1()
declare void @dummy2()
declare void @dummy3()
declare signext i32 @func(i32 signext)

!1 = !{!"branch_weights", i32 2000, i32 1}
!2 = !{!"branch_weights", i32 1, i32 2000}

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