Re: [MirageOS-devel] unsigned integers

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On 7 Sep 2016, at 14:45, Hannes Mehnert <hannes@xxxxxxxxxxx> wrote:
> 
> hello,
> 
> for several projects (i.e. sequence numbers) I had the need to have
> fixed width unsigned integers.  Finally I managed to implement them (8,
> 16, 32, 64 bit), and would like to hear back from you what you think.
> The crucial bit for me is that arithmetic expression expose the carry
> bit explicitly (by being t -> t -> t * bool).
> 
> I don't have the plan to complete it with all possible arithmetics
> operations (div, pow, ..), since I don't have use cases for these.
> 
> The type t is on purpose a public alias to (int, int, int32, int64),
> thus you can use bit operations from these modules directly (and, xor,
> or).  This also means that you have interoperability with e.g. cstruct.
> 
> The documentation is up at
> https://hannesm.github.io/usane/doc/Usane.html , code at
> https://github.com/hannesm/usane
> 
> The implementation uses __builtin_add/sub/mul_overflow if available
> (since clang 3.4 / gcc 5.1).

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?

Alternatively, the existing allocation may be -flambda safe and optimisable 
away for common usage patterns, but I'm not sure this is the case without 
actually inspecting the output of some tests...

Anil
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