[LLVMdev] alloc_size metadata

[Nuno Lopes]

>> 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 }
>
> suppose the size is %n+4.  Then in the function you have to compute
>    %s = add i32 %n, 4
> and then put %s in the metadata
>    !0 = metadata !{ i32 (i32)* @size, null, i32 %s }
> However then the only use of %s is in the metadata, so basically any
> optimization pass will zap it, right?  So won't this only work if the
> size etc calculation doesn't actually require any calculation, eg it
> would work if you only pass function parameters and globals but that's
> about it.  Am I missing something?

Actually that's not what I had in mind.
Metadata will take %n (and not %s) as the parameter for @size function, and 
then @size(%n) itself returns %n+4.  So no computations are supposed to be 
added to the caller of an allocation function. The metadata should only 
reference the parameters given to the allocation function. If the function 
is not inlined, then there is no risk of these parameters disappearing. If 
it is inlined, the probability is probably low, because you will most likely 
need the original parameters to compute the allocated size in the inlined 
function as well.

Nuno