[LLVMdev] alloc_size metadata

[Hal Finkel]

On Mon, 04 Jun 2012 18:37:31 +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.

> 
> So, what do you think about this new proposal?  I guess it addresses  
> all issues raised so far.

I think this is good.

 -Hal

> 
> Thanks,
> Nuno
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-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory