Re: [MirageOS-devel] unsigned integers

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On 7 Sep 2016, at 15:00, Hannes Mehnert <hannes@xxxxxxxxxxx> wrote:
> 
> On 07/09/2016 14:50, Anil Madhavapeddy wrote:
>> The optimisation of the use of builtin functions would probably be drowned 
>> out by the extra 2 value allocations happening because a tuple is being 
>> returned.
>> 
>> Would it be crazy to pass in a bool ref that could be reused to check for 
>> overflow?
> 
> Are we afraid of allocation?  Surely, a global bool ref could be used,
> and it even might be safe considering single threading.  But then we'd
> need to allocate the tuple on the Caml side (is that cheaper?) or
> provide an awkward API (add : t -> t -> t, carry : bool) reminding me of
> unix errno (which I have no plans to replicate).
> 
> better allocate than having a messed up API,

I'd prefer to allocate and have a good API, but then I would also prefer to not 
use compiler builtins that may introduce subtle optimisation bugs in the future 
-- or at least until we know that the optimisations are useful.

E.g. the double fetch vulnerabilities that emerged from compiler builtins 
https://xenbits.xen.org/xsa/advisory-155.html (not directly applicable here, 
but my point is that I'd prefer a slightly slower but more consistent first cut 
at this library)

-anil
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