Re: [PATCH 3/5] xen/sort: Switch to an extern inline implementation

[Andrew Cooper <amc96@xxxxxxxx>]

On 16/11/2021 00:36, Stefano Stabellini wrote:
> On Thu, 11 Nov 2021, Julien Grall wrote:
>> On 11/11/2021 17:57, Andrew Cooper wrote:
>>> There are exactly 3 callers of sort() in the hypervisor.
>>>
>>> Both arm callers pass in NULL for the swap function.  While this might seem
>>> like an attractive option at first, it causes generic_swap() to be used
>>> which
>>> forced a byte-wise copy.  Provide real swap functions which the compiler can
>>> optimise sensibly.
>> I understand the theory, but none of the two calls are in hot paths or deal
>> with large set on Arm. So I am rather hesitant to introduce specialised swap
>> in each case as it doesn't seem we will gain much from this change.
> While I like Andrew's optimization, like Julien, I would also rather not
> have to introduce specialized swap functions any time a sort() is
> called. Why not keeping around generic_swap as extern gnu_inline in
> sort.h as well so that the ARM callers can simply:
>
>     sort(bootinfo.mem.bank, bootinfo.mem.nr_banks, sizeof(struct membank),
>          cmp_memory_node, generic_swap);
>
> ?
>
> That looks like a good option, although it would still result in a
> small increase in bloat.

That is basically what Jan suggested.

I'm still unconvinced that you actually want to be doing byte-wise
swapping, even if it isn't a hotpath.  A custom swap function will
compile to less code than using generic_swap() like this, and run faster.

~Andrew