On Tue, Mar 27, 2018 at 3:44 AM, Henri Sivonen <hsivo...@hsivonen.fi> wrote:
> I'm having trouble finding reliable information about the performance > of unaligned NEON memory access on ARMv7 phones. > > What I can find is: > > * ARMv7 seems to allow unaligned access to be a trap-to-kernel kind > of performance disaster, but it's hard to find information about > whether the phone SoCs we care about are actually disastrous like > that. > > * On aarch64, unaligned access is the same instruction as aligned > access and gets dynamically penalized, but only minimally, if the > access crosses a cache line boundary. *Presumably* ARMv7 code running > on an ARMv8 core gets the same benefit. > > Do we know what performance characteristics we can assume for > unaligned NEON loads/stores on Android phones that have ARMv7 cores > and recent enough Android that Fennec runs in the first place? Is http://fastcompression.blogspot.fr/2015/08/accessing-unaligned-memory.html and/or the comments for MEM_FORCE_MEMORY_ACCESS at https://github.com/facebook/zstd/blob/dev/lib/common/mem.h useful? I could also introduce you to the zstandard developers if you think it would be useful (compression often spends a large portion of its execution time accessing and moving memory and I'm pretty sure they know arcane memory access details like this). Reply privately if you want that introduction. _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
