On Friday, 16 July 2021 04:41:17 CEST Jason Merrill via Gcc-patches wrote: > > Currently the patch does not adjust the values based on -march, as in JF's > > proposal. I'll need more guidance from the ARM/AArch64 maintainers about > > how to go about that. --param l1-cache-line-size is set based on -mtune, > > but I don't think we want -mtune to change these ABI-affecting values. > > Are > > there -march values for which a smaller range than 64-256 makes sense?
As a user who cares about ABI but also cares about maximizing performance of builds for a specific HPC setup I'd expect the hardware interference size values to be allowed to break ABIs. The point of these values is to give me better performance portability (but not necessarily binary portability) than my usual "pick 64 as a good average". Wrt, -march / -mtune setting hardware interference size: IMO -mtune=X should be interpreted as "my binary is supposed to be optimized for X, I accept inefficiencies on everything that's not X". On Friday, 16 July 2021 04:48:52 CEST Noah Goldstein wrote: > On intel x86 systems with a private L2 cache the spatial prefetcher > can cause destructive interference along 128 byte aligned boundaries. > https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-3 > 2-architectures-optimization-manual.pdf#page=60 I don't understand how this feature would lead to false sharing. But maybe I misunderstand the spatial prefetcher. The first access to one of the two cache lines pairs would bring both cache lines to LLC (and possibly L2). If a core with a different L2 reads the other cache line the cache line would be duplicated; if it writes to it, it would be exclusive to the other core's L2. The cache line pairs do not affect each other anymore. Maybe there's a minor inefficiency on initial transfer from memory, but isn't that all? That said. Intel documents the spatial prefetcher exclusively for Sandy Bridge. So if you still believe 128 is necessary, set the destructive hardware interference size to 64 for all of x86 except -mtune=sandybridge. -- ────────────────────────────────────────────────────────────────────────── Dr. Matthias Kretz https://mattkretz.github.io GSI Helmholtz Centre for Heavy Ion Research https://gsi.de std::experimental::simd https://github.com/VcDevel/std-simd ──────────────────────────────────────────────────────────────────────────
