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(-) -- 2.43.0
