[LLVMdev] About user of bitcast/GEP instruction

[guoqing zhang]

Hi,

If the special handling in the meg2reg pass is to look for lifetime
intrinsics, shouldn't it cast to <IntrisicInst> and then use
getInstrinsicID to check for lifetime_start and lifetime_end ?

The thing that I don't understand is the following piece of code, which
finds all the users and cast it to <Instruction> then eraseFromParent().
How can this guarantee that it only erase lifetime instrinsics ?

-guoqing

      // The only users of this bitcast/GEP instruction are lifetime
intrinsics.
      // Follow the use/def chain to erase them now instead of leaving it
for
      // dead code elimination later.
      for (auto UUI = I->user_begin(), UUE = I->user_end(); UUI != UUE;) {
        Instruction *Inst = cast<Instruction>(*UUI);
        ++UUI;
        Inst->eraseFromParent();
      }

On Fri, Jan 30, 2015 at 1:55 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "guoqing zhang" <gqzhang81 at gmail.com>
> > To: llvmdev at cs.uiuc.edu
> > Sent: Friday, January 30, 2015 4:29:16 AM
> > Subject: [LLVMdev] About user of bitcast/GEP instruction
> >
> > Hi,
> >
> >
> > In PromoteMemoryToRegister.cpp, it seems to rely on the fact that the
> > only users of bitcast/GEP instruction are lifetime intrinsics
> > (llvm.lifetime.start/end).
>
> You're misreading that code. There is special handling is that pass for
> looking through those instructions to find lifetime intrinsics, but that's
> all.
>
> > I did some searching in llvm/test folder,
> > it seems to be true.
> >
>
> bitcasts and GEPs are general-purpose instructions. GEPs are most often
> used by loads/stores. Look at test/Transforms/LoopReroll/basic.ll for an
> example with lots of them. test/Analysis/BasicAA/gep-alias.ll is another
> place to look.
>
> >
> > However, by reading LLVM IR manual, I don't see any restriction
> > stated on the possible user of bitcast/GEP instruction. So my
> > question is who impose the restriction ? Is it Clang ?
>
> There is definitely no such restriction.
>
>  -Hal
>
> >
> >
> > Regards
> > -guoqing
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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