[LLVMdev] Fwd: Clang question

[Ryan Taylor]

Eric,

  Ok, thanks, looks like I'll need to figure something out. I was hoping
scalarrepl would take care of this for me, but it's not lowering the
structure (I haven't look at the opt code to see why, I"m sure there's some
valid reason I'm unaware of atm).



On Mon, Mar 5, 2012 at 12:35 PM, Eric Christopher <echristo at apple.com>wrote:

> You'll need to do the work then. I'd also question why? On most platforms
> a decent memcpy exists.
>
> -eric
>
> On Mar 5, 2012, at 12:28 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:
>
> > I would like it to always be lowered, I don't want it.
> >
> > On Mon, Mar 5, 2012 at 12:27 PM, Eric Christopher <echristo at apple.com>
> wrote:
> > You don't have memcpy or want it to always lower it?
> >
> > -eric
> >
> > On Mar 5, 2012, at 11:56 AM, Ryan Taylor <ryta1203 at gmail.com> wrote:
> >
> > > Christoph,
> > >
> > >  Yes, you are correct on the lifetime calls, they are just markers for
> liveness.
> > >
> > > However, the backend is not optimizing these calls away. I could try
> to deal with them outside of llvm but I was hoping for a cleaner solution
> using llvm?
> > >
> > > On Mon, Mar 5, 2012 at 11:51 AM, Christoph Erhardt <
> christoph at sicherha.de> wrote:
> > > Hi Ryan,
> > >
> > > the compiler is free to insert implicit calls to memcpy(), for instance
> > > for assignments from one struct/class variable to another. The same
> goes
> > > for memset(), which may be inserted implicitly for the initialization
> of
> > > local structs or arrays.
> > >
> > > The good news is that the backend normally optimizes these calls away
> > > where possible, replacing them with simple moves - at least as long as
> > > the number of bytes to copy does not exceed a certain threshold.
> > >
> > > As for the llvm.lifetime intrinsics, take a look at the documentation:
> > > http://llvm.org/docs/LangRef.html#int_memorymarkers
> > > If I'm not mistaken, these calls seem to be used to mark the lifespan
> of
> > > a stack-allocated object.
> > >
> > > Regards,
> > > Christoph
> > >
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