//
//===----------------------------------------------------------------------===//
//
-// This pass moves instructions into successor blocks, when possible, so that
+// This pass moves instructions into successor blocks when possible, so that
// they aren't executed on paths where their results aren't needed.
//
// This pass is not intended to be a replacement or a complete alternative
public:
static char ID; // Pass identification
MachineSinking() : MachineFunctionPass(&ID) {}
-
+
virtual bool runOnMachineFunction(MachineFunction &MF);
-
+
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB) const;
};
} // end anonymous namespace
-
+
char MachineSinking::ID = 0;
static RegisterPass<MachineSinking>
X("machine-sink", "Machine code sinking");
/// AllUsesDominatedByBlock - Return true if all uses of the specified register
/// occur in blocks dominated by the specified block.
-bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
+bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
MachineBasicBlock *MBB) const {
assert(TargetRegisterInfo::isVirtualRegister(Reg) &&
"Only makes sense for vregs");
// This may leave a referencing dbg_value in the original block, before
// the definition of the vreg. Dwarf generator handles this although the
// user might not get the right info at runtime.
- for (MachineRegisterInfo::use_nodbg_iterator I =
- RegInfo->use_nodbg_begin(Reg),
- E = RegInfo->use_nodbg_end(); I != E; ++I) {
+ for (MachineRegisterInfo::use_nodbg_iterator
+ I = RegInfo->use_nodbg_begin(Reg), E = RegInfo->use_nodbg_end();
+ I != E; ++I) {
// Determine the block of the use.
MachineInstr *UseInst = &*I;
MachineBasicBlock *UseBlock = UseInst->getParent();
+
if (UseInst->isPHI()) {
// PHI nodes use the operand in the predecessor block, not the block with
// the PHI.
UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB();
}
+
// Check that it dominates.
if (!DT->dominates(MBB, UseBlock))
return false;
}
+
return true;
}
bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
DEBUG(dbgs() << "******** Machine Sinking ********\n");
-
+
const TargetMachine &TM = MF.getTarget();
TII = TM.getInstrInfo();
TRI = TM.getRegisterInfo();
AllocatableSet = TRI->getAllocatableSet(MF);
bool EverMadeChange = false;
-
+
while (1) {
bool MadeChange = false;
// Process all basic blocks.
- for (MachineFunction::iterator I = MF.begin(), E = MF.end();
+ for (MachineFunction::iterator I = MF.begin(), E = MF.end();
I != E; ++I)
MadeChange |= ProcessBlock(*I);
-
+
// If this iteration over the code changed anything, keep iterating.
if (!MadeChange) break;
EverMadeChange = true;
- }
+ }
return EverMadeChange;
}
if (MBB.succ_size() <= 1 || MBB.empty()) return false;
// Don't bother sinking code out of unreachable blocks. In addition to being
- // unprofitable, it can also lead to infinite looping, because in an unreachable
- // loop there may be nowhere to stop.
+ // unprofitable, it can also lead to infinite looping, because in an
+ // unreachable loop there may be nowhere to stop.
if (!DT->isReachableFromEntry(&MBB)) return false;
bool MadeChange = false;
bool ProcessedBegin, SawStore = false;
do {
MachineInstr *MI = I; // The instruction to sink.
-
+
// Predecrement I (if it's not begin) so that it isn't invalidated by
// sinking.
ProcessedBegin = I == MBB.begin();
if (SinkInstruction(MI, SawStore))
++NumSunk, MadeChange = true;
-
+
// If we just processed the first instruction in the block, we're done.
} while (!ProcessedBegin);
-
+
return MadeChange;
}
// Check if it's safe to move the instruction.
if (!MI->isSafeToMove(TII, AA, SawStore))
return false;
-
+
// FIXME: This should include support for sinking instructions within the
// block they are currently in to shorten the live ranges. We often get
// instructions sunk into the top of a large block, but it would be better to
// be careful not to *increase* register pressure though, e.g. sinking
// "x = y + z" down if it kills y and z would increase the live ranges of y
// and z and only shrink the live range of x.
-
+
// Loop over all the operands of the specified instruction. If there is
// anything we can't handle, bail out.
MachineBasicBlock *ParentBlock = MI->getParent();
-
+
// SuccToSinkTo - This is the successor to sink this instruction to, once we
// decide.
MachineBasicBlock *SuccToSinkTo = 0;
-
+
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg()) continue; // Ignore non-register operands.
-
+
unsigned Reg = MO.getReg();
if (Reg == 0) continue;
-
+
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
if (MO.isUse()) {
// If the physreg has no defs anywhere, it's just an ambient register
// it could get allocated to something with a def during allocation.
if (!RegInfo->def_empty(Reg))
return false;
+
if (AllocatableSet.test(Reg))
return false;
+
// Check for a def among the register's aliases too.
for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
unsigned AliasReg = *Alias;
if (!RegInfo->def_empty(AliasReg))
return false;
+
if (AllocatableSet.test(AliasReg))
return false;
}
// If it's not safe to move defs of the register class, then abort.
if (!TII->isSafeToMoveRegClassDefs(RegInfo->getRegClass(Reg)))
return false;
-
+
// FIXME: This picks a successor to sink into based on having one
// successor that dominates all the uses. However, there are cases where
// sinking can happen but where the sink point isn't a successor. For
// example:
+ //
// x = computation
// if () {} else {}
// use x
- // the instruction could be sunk over the whole diamond for the
+ //
+ // the instruction could be sunk over the whole diamond for the
// if/then/else (or loop, etc), allowing it to be sunk into other blocks
// after that.
-
+
// Virtual register defs can only be sunk if all their uses are in blocks
// dominated by one of the successors.
if (SuccToSinkTo) {
// If a previous operand picked a block to sink to, then this operand
// must be sinkable to the same block.
- if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo))
+ if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo))
return false;
+
continue;
}
-
+
// Otherwise, we should look at all the successors and decide which one
// we should sink to.
for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
break;
}
}
-
+
// If we couldn't find a block to sink to, ignore this instruction.
if (SuccToSinkTo == 0)
return false;
}
}
-
+
// If there are no outputs, it must have side-effects.
if (SuccToSinkTo == 0)
return false;
// landing pad is implicitly defined.
if (SuccToSinkTo->isLandingPad())
return false;
-
+
// It is not possible to sink an instruction into its own block. This can
// happen with loops.
if (MI->getParent() == SuccToSinkTo)
return false;
-
- DEBUG(dbgs() << "Sink instr " << *MI);
- DEBUG(dbgs() << "to block " << *SuccToSinkTo);
-
+
+ // If the instruction to move defines a dead physical register which is live
+ // when leaving the basic block, don't move it because it could turn into a
+ // "zombie" define of that preg. E.g., EFLAGS. (<rdar://problem/8030636>)
+ for (unsigned I = 0, E = MI->getNumOperands(); I != E; ++I) {
+ const MachineOperand &MO = MI->getOperand(I);
+ if (!MO.isReg()) continue;
+ unsigned Reg = MO.getReg();
+ if (Reg == 0 || !TargetRegisterInfo::isPhysicalRegister(Reg)) continue;
+ if (SuccToSinkTo->isLiveIn(Reg))
+ return false;
+ }
+
+ DEBUG(dbgs() << "Sink instr " << *MI << "\tinto block " << *SuccToSinkTo);
+
// If the block has multiple predecessors, this would introduce computation on
// a path that it doesn't already exist. We could split the critical edge,
// but for now we just punt.
// Otherwise we are OK with sinking along a critical edge.
DEBUG(dbgs() << "Sinking along critical edge.\n");
}
-
- // Determine where to insert into. Skip phi nodes.
+
+ // Determine where to insert into. Skip phi nodes.
MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
while (InsertPos != SuccToSinkTo->end() && InsertPos->isPHI())
++InsertPos;
-
+
// Move the instruction.
SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
++MachineBasicBlock::iterator(MI));
- // Conservatively, clear any kill flags, since it's possible that
- // they are no longer correct.
+ // Conservatively, clear any kill flags, since it's possible that they are no
+ // longer correct.
MI->clearKillInfo();
return true;
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