Since SCTLR isn't allowed on userland, there is no way to detect unalignment access support without trap. Generally, unalignement access causes SIGBUS, so we might get a data from crash reporter. Android armv7-a ABI doesn't define that hardware configuration has to set alignment bit of SCTLR, so we should consider both unfortunately.
ARM document of Cortex-A8 says [*1], alignment identifier is 64 (VLD2.16@64), it requires 2 cycles, but alignment identifier is 128 (VLD2.16@128), it is 1 cycle. And on Cortex-A9, unalignment access requires additional cycles [*2]. When I was investigating string issue, I created a benchmark result for optimization on Cortex-A7. So I will look for this data. -- Makoto [*1] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0344h/ch16s06s07.html [*2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0344h/ch16s06s07.html On Tue, Mar 27, 2018 at 7:44 PM, 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? > > -- > Henri Sivonen > hsivo...@hsivonen.fi > https://hsivonen.fi/ > _______________________________________________ > dev-platform mailing list > dev-platform@lists.mozilla.org > https://lists.mozilla.org/listinfo/dev-platform > _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
