Re: [PATCH] mempool: optimize get objects with constant n

[Thomas Monjalon <thomas@monjalon.net>]

07/06/2023 10:03, Morten Brørup:
> > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > Sent: Wednesday, 7 June 2023 09.52
> > 
> > 18/04/2023 14:55, Bruce Richardson:
> > > On Tue, Apr 18, 2023 at 01:29:49PM +0200, Morten Brørup wrote:
> > > > From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> > > > > On Tue, Apr 11, 2023 at 08:48:45AM +0200, Morten Brørup wrote:
> > > > > > +	if (__extension__(__builtin_constant_p(n)) && n <= cache->len) {
> > > > > > +		/*
> > > > > > +		 * The request size is known at build time, and
> > > > > > +		 * the entire request can be satisfied from the cache,
> > > > > > +		 * so let the compiler unroll the fixed length copy loop.
> > > > > > +		 */
> > > > > > +		cache->len -= n;
> > > > > > +		for (index = 0; index < n; index++)
> > > > > > +			*obj_table++ = *--cache_objs;
> > > > > > +
> > > > >
> > > > > This loop looks a little awkward to me. Would it be clearer (and perhaps
> > > > > easier for compilers to unroll efficiently if it was rewritten as:
> > > > >
> > > > > 	cache->len -= n;
> > > > > 	cache_objs = &cache->objs[cache->len];
> > > > > 	for (index = 0; index < n; index++)
> > > > > 		obj_table[index] = cache_objs[index];
> > > >
> > > > The mempool cache is a stack, so the copy loop needs get the objects in
> > decrementing order. I.e. the source index decrements and the destination index
> > increments.
> > > >
> > >
> > > BTW: Please add this as a comment in the code too, above the loop to avoid
> > > future developers (or even future me), asking this question again!
> > 
> > Looks like this request was missed.
> 
> I intentionally omitted it, because I disagree with the suggestion.
> 
> Otherwise, reminders that the mempool cache is a stack should be plastered all over the source code, not just here. For reference, this copy loop (without such a reminder) also exists elsewhere in the same file.
> 
> Apologies for not responding to Bruce's request earlier.

What about doing a general comment at the top of the function,
with the assignment of the pointer at the end of the array:

    /* The cache is a stack, so copy will be in reverse order. */
    cache_objs = &cache->objs[cache->len];

I could do it on apply if there is an agreement.