[LLVMdev] alloc_size metadata

[Hal Finkel]

On Tue, 5 Jun 2012 15:57:25 +0100
"Nuno Lopes" <nunoplopes at sapo.pt> wrote:

> >> Hi,
> >>
> >> So here is a new proposal:
> >>
> >> !0 = metadata !{ alloc_siz_fn, offset_fn, parameters* }
> >>
> >> alloc_size_fn and offset_fn are functions that return either
> >> i32/i64 depending on the platform, and they must have the same
> >> number of arguments (not necessarily the same as the as allocation
> >> function). The parameters are given in the metadata as well.
> >> To accommodate the common case, offer_fn can be null, meaning it is
> >> a zero offset.
> >>
> >> The usage would be something like this:
> >>
> >> %r = call i8* @my_realloc(i32* %ptr, i32 %n), !alloc_size !0
> >> !0 = metadata !{ i32 (i32)* @size, null, i32 %n }
> >>
> >> Even if my_realloc() gets inlined later, the metadata can still be
> >> applied to the returned value (since it is not really specific to a
> >> call site). Of course some parameters of the allocation function
> >> may be deleted if the function gets inlined (i.e., nulled in the
> >> metadata), but I don't think we can workaround that problem. This
> >> is a best-effort approach, anyway.
> >>
> >>
> >> To avoid these functions being removed, I propose a new linkage
> >> type. Something like internal_metadata (or hopefully a better
> >> name). This linkage would mean that a function can only be removed
> >> in codegen, and if it has no users. The difference to internal
> >> linkage, is that internal functions with no users can be deleted
> >> at any time.
> >
> > Is it possible to determine which functions are referenced in
> > metadata even though the metadata is not listed as a user? It seems
> > like we could do this without defining another linkage class.
> 
> Right now, no.
> We would need a map from Function* to metadata, since we don't really
> want to scan the whole metadata every time we want to delete a
> function. So, I don't know.. I'm pretty agnostic to either solution.

Fair enough. I'd prefer a more descriptive name if we use a linkage
class; how about metadata_referenced or referenced_by_metadata or
used_by_metadata, etc. (maybe also with the internal_ prefix as well)?

If there is an efficient way to maintain the map, I think that would be
better.

Thanks again,
Hal

> 
> Nuno 
> 



-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory