[LLVMdev] Clang question

[Villmow, Micah]

Ryan,
In a backend, set this in the TargetLowering:

  maxStoresPerMemcpy  = 4096;
  maxStoresPerMemmove = 4096;
  maxStoresPerMemset  = 4096;

We don't have memcpy in our backend so we have to expand it to a sequence of stores.

Micah

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Eric Christopher
> Sent: Monday, March 05, 2012 12:35 PM
> To: Ryan Taylor
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Clang question
> 
> 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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