Re: Memory prefetching while sequentially fetching from SortTuple array, tuplestore

On Thu, Jul 16, 2015 at 8:49 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Meh. I don't like the assumption that non-GCC compilers will be smart
> enough to optimize away the useless-to-them if() tests this adds.
> Please refactor that so that there is exactly 0 new code when the
> intrinsic doesn't exist.

I imagined that there was some value in copying the GCC intrinsic's
behavior, and actually evaluating the "addr" expression even in the
event of no platform support. On reflection, I suppose that that isn't
actually a particularly useful property for Postgres. There will only
ever be a handful of callers.

Attached revision does not rely on such optimization occurring on
platforms that lack __builtin_prefetch(). This allowed me to decouple
availability from actual use, in the style of posix_fadvise(), so that
one can manually disable memory prefetching within pg_config_manual.h.

Clang is compatibile with __builtin_prefetch() intrinsic, FWIW. I'm
not sure if it's worth trying to make the wrapper portable across a
variety of supported compilers. If we were to attempt it, we would not
be the first. I note that ICC's memref_control has an identical
interface to __builtin_prefetch().

--
Peter Geoghegan