[Pixman] [PATCH 1/4] pixman-fast-path: Add over_n_8888 fast path (disabled)

[Ben Avison]

On Thu, 27 Aug 2015 10:59:52 +0100, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> It would be *really* nice if we could somehow use a benchmark mode
> where we could run an operation with every possible implementation and
> compare them. I wonder, can we already do that with PIXMAN_DISABLE?

That would certainly help detect some issues, where we get worse
performance at supposedly more advanced implementation levels. Obviously
it wouldn't make any sense to bother with certain combinations, such as
PIXMAN_DISABLE=arm-neon when testing on an x86.

One thing it wouldn't be able to detect, though, would be where the fetch/
combine/writeback iterators are faster than fast paths for the *same*
implementation level - such as with the ARMv6 nearest-scaled patches I
was revisiting recently. In that specific case, it turned out that my
original solution of bespoke C wrappers for the fetchers turned out to be
even faster - but we don't have any way at present of detecting if there
are other cases where we would be better off deleting the fast paths and
letting the iterators do the work instead.

Ben