[LLVMdev] What is dead def?

[Thomson]

Thanks Mehdi. Below is an example of "dead def", it looks like unused
definition as you mentioned, could you confirm? I also saw dead defs in
some generated code before register allocation, like "%reg1030<def> =
ADD8rr %reg1028<kill>, %reg1029<kill>, %EFLAGS<*imp-def,dead*>", does it
mean similar for EFLAGS here?

// AggressiveAntiDepBreaker.cpp

void AggressiveAntiDepBreaker::PrescanInstruction(MachineInstr *MI,
                                                  unsigned Count,
                                             std::set<unsigned>&
PassthruRegs) {
  std::vector<unsigned> &DefIndices = State->GetDefIndices();
  std::multimap<unsigned, AggressiveAntiDepState::RegisterReference>&
    RegRefs = State->GetRegRefs();

  // Handle *dead defs* by simulating a last-use of the register just
  // after the def. A dead def can occur because the def is truly
  // dead, or because only a subregister is live at the def. If we
  // don't do this the dead def will be incorrectly merged into the
  // previous def.
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    MachineOperand &MO = MI->getOperand(i);
    if (!MO.isReg() || !MO.isDef()) continue;
    unsigned Reg = MO.getReg();
    if (Reg == 0) continue;

    HandleLastUse(Reg, Count + 1, "", "\t*Dead Def*: ", "\n");
  }


On Fri, Jan 2, 2015 at 4:38 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:

>
> > On Jan 1, 2015, at 7:24 AM, Thomson <lilotom at gmail.com> wrote:
> >
> > I saw there is reference to dead def on registers in LLVM source code. I
> am not aware of this concept from the traditional course material. What are
> the properties of dead def?
>
> Could it be a def that is never used (hence dead)?
> If you mention where you saw it, it may be easier to help :)
>
> Best,
>
> Mehdi
>
>
> >
> > Cheers
> > Thomson
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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