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| //===------------- PPCEarlyReturn.cpp - Form Early Returns ----------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// A pass that form early (predicated) returns. If-conversion handles some of
// this, but this pass picks up some remaining cases.
//
//===----------------------------------------------------------------------===//
#include "MCTargetDesc/PPCPredicates.h"
#include "PPC.h"
#include "PPCInstrBuilder.h"
#include "PPCInstrInfo.h"
#include "PPCMachineFunctionInfo.h"
#include "PPCTargetMachine.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineMemOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/TargetRegistry.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "ppc-early-ret"
STATISTIC(NumBCLR, "Number of early conditional returns");
STATISTIC(NumBLR, "Number of early returns");
namespace {
// PPCEarlyReturn pass - For simple functions without epilogue code, move
// returns up, and create conditional returns, to avoid unnecessary
// branch-to-blr sequences.
struct PPCEarlyReturn : public MachineFunctionPass {
static char ID;
PPCEarlyReturn() : MachineFunctionPass(ID) {
initializePPCEarlyReturnPass(*PassRegistry::getPassRegistry());
}
const TargetInstrInfo *TII;
protected:
bool processBlock(MachineBasicBlock &ReturnMBB) {
bool Changed = false;
MachineBasicBlock::iterator I = ReturnMBB.begin();
I = ReturnMBB.SkipPHIsLabelsAndDebug(I);
// The block must be essentially empty except for the blr.
if (I == ReturnMBB.end() ||
(I->getOpcode() != PPC::BLR && I->getOpcode() != PPC::BLR8) ||
I != ReturnMBB.getLastNonDebugInstr())
return Changed;
SmallVector<MachineBasicBlock*, 8> PredToRemove;
for (MachineBasicBlock::pred_iterator PI = ReturnMBB.pred_begin(),
PIE = ReturnMBB.pred_end(); PI != PIE; ++PI) {
bool OtherReference = false, BlockChanged = false;
if ((*PI)->empty())
continue;
for (MachineBasicBlock::iterator J = (*PI)->getLastNonDebugInstr();;) {
if (J == (*PI)->end())
break;
if (J->getOpcode() == PPC::B) {
if (J->getOperand(0).getMBB() == &ReturnMBB) {
// This is an unconditional branch to the return. Replace the
// branch with a blr.
BuildMI(**PI, J, J->getDebugLoc(), TII->get(I->getOpcode()))
.copyImplicitOps(*I);
MachineBasicBlock::iterator K = J--;
K->eraseFromParent();
BlockChanged = true;
++NumBLR;
continue;
}
} else if (J->getOpcode() == PPC::BCC) {
if (J->getOperand(2).getMBB() == &ReturnMBB) {
// This is a conditional branch to the return. Replace the branch
// with a bclr.
BuildMI(**PI, J, J->getDebugLoc(), TII->get(PPC::BCCLR))
.addImm(J->getOperand(0).getImm())
.addReg(J->getOperand(1).getReg())
.copyImplicitOps(*I);
MachineBasicBlock::iterator K = J--;
K->eraseFromParent();
BlockChanged = true;
++NumBCLR;
continue;
}
} else if (J->getOpcode() == PPC::BC || J->getOpcode() == PPC::BCn) {
if (J->getOperand(1).getMBB() == &ReturnMBB) {
// This is a conditional branch to the return. Replace the branch
// with a bclr.
BuildMI(
**PI, J, J->getDebugLoc(),
TII->get(J->getOpcode() == PPC::BC ? PPC::BCLR : PPC::BCLRn))
.addReg(J->getOperand(0).getReg())
.copyImplicitOps(*I);
MachineBasicBlock::iterator K = J--;
K->eraseFromParent();
BlockChanged = true;
++NumBCLR;
continue;
}
} else if (J->isBranch()) {
if (J->isIndirectBranch()) {
if (ReturnMBB.hasAddressTaken())
OtherReference = true;
} else
for (unsigned i = 0; i < J->getNumOperands(); ++i)
if (J->getOperand(i).isMBB() &&
J->getOperand(i).getMBB() == &ReturnMBB)
OtherReference = true;
} else if (!J->isTerminator() && !J->isDebugInstr())
break;
if (J == (*PI)->begin())
break;
--J;
}
if ((*PI)->canFallThrough() && (*PI)->isLayoutSuccessor(&ReturnMBB))
OtherReference = true;
// Predecessors are stored in a vector and can't be removed here.
if (!OtherReference && BlockChanged) {
PredToRemove.push_back(*PI);
}
if (BlockChanged)
Changed = true;
}
for (unsigned i = 0, ie = PredToRemove.size(); i != ie; ++i)
PredToRemove[i]->removeSuccessor(&ReturnMBB, true);
if (Changed && !ReturnMBB.hasAddressTaken()) {
// We now might be able to merge this blr-only block into its
// by-layout predecessor.
if (ReturnMBB.pred_size() == 1) {
MachineBasicBlock &PrevMBB = **ReturnMBB.pred_begin();
if (PrevMBB.isLayoutSuccessor(&ReturnMBB) && PrevMBB.canFallThrough()) {
// Move the blr into the preceding block.
PrevMBB.splice(PrevMBB.end(), &ReturnMBB, I);
PrevMBB.removeSuccessor(&ReturnMBB, true);
}
}
if (ReturnMBB.pred_empty())
ReturnMBB.eraseFromParent();
}
return Changed;
}
public:
bool runOnMachineFunction(MachineFunction &MF) override {
if (skipFunction(MF.getFunction()))
return false;
TII = MF.getSubtarget().getInstrInfo();
bool Changed = false;
// If the function does not have at least two blocks, then there is
// nothing to do.
if (MF.size() < 2)
return Changed;
// We can't use a range-based for loop due to clobbering the iterator.
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E;) {
MachineBasicBlock &B = *I++;
Changed |= processBlock(B);
}
return Changed;
}
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
MachineFunctionProperties::Property::NoVRegs);
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineFunctionPass::getAnalysisUsage(AU);
}
};
}
INITIALIZE_PASS(PPCEarlyReturn, DEBUG_TYPE,
"PowerPC Early-Return Creation", false, false)
char PPCEarlyReturn::ID = 0;
FunctionPass*
llvm::createPPCEarlyReturnPass() { return new PPCEarlyReturn(); }
|