[RFC 0/1] replace inline assembly prefetch with cc builtins

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

This is an unorthodox RFC in that I think it probably shouldn't be
accepted.

I thought I'd try replacing the x86 inline assembly for DPDK software
prefetch with GCC __builtin_prefetch().

Being essentially a black box for the compiler, inline assembly
hinders certain optimizations. The hope was beyond DPDK source code
simplification there would also be a performance upside.

I evaluate this change in an eventdev-based application with a
multi-stage pipeline, which heavily relies on software prefetching to
mitigate the cost of core-to-core transitions. After I replaced the
lib/eal/x86/include/rte_prefetch.h inline assembly with calls to GCC
built-ins, the reference application saw a ~50 cc/stage slowdown on
GCC 13.3.0 and GCC 14.2.0.

So unfortunately GCC uses its newfound freedom to make the code
slower. I'm guessing it reorders the prefetches to happen at a
too-late time.

On clang 17.0.6 and clang 18.1.3, the application-level performance
remains the same after the change.

I've tested only a single application, so it would be of great value
of we could get some more experience (preferably from real-world
apps).

rte_prefetch*_write() is using GCC built-ins, so I tried moving that
to inline assembly. Initially a saw a 10 cc/stage gain from this move
(on GCC), but after reorganizing the prefetch logic, that gain was
gone. I have a patch for that change as well, if anyone is
interested. (It's a little more complicated that you might think,
since you need to detect if prefetchw is available in the target ISA.)

Mattias Rönnblom (1):
  eal/x86: replace inline assembly prefetch with cc builtins

 lib/eal/x86/include/rte_prefetch.h | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

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2.43.0