On Mon, 14 Sep 2020 at 20:12, Horia Geantă <horia.geanta@xxxxxxx> wrote: > > On 9/14/2020 7:28 PM, Ard Biesheuvel wrote: > > On Mon, 14 Sep 2020 at 19:24, Horia Geantă <horia.geanta@xxxxxxx> wrote: > >> > >> On 9/9/2020 1:10 AM, Herbert Xu wrote: > >>> On Tue, Sep 08, 2020 at 01:35:04PM +0300, Horia Geantă wrote: > >>>> > >>>>> Just go with the get_unaligned unconditionally. > >>>> > >>>> Won't this lead to sub-optimal code for ARMv7 > >>>> in case the IV is aligned? > >>> > >>> If this should be optimised in ARMv7 then that should be done > >>> in get_unaligned itself and not open-coded. > >>> > >> I am not sure what's wrong with avoiding using the unaligned accessors > >> in case data is aligned. > >> > >> Documentation/core-api/unaligned-memory-access.rst clearly states: > >> These macros work for memory accesses of any length (not just 32 bits as > >> in the examples above). Be aware that when compared to standard access of > >> aligned memory, using these macros to access unaligned memory can be costly in > >> terms of performance. > >> > >> So IMO it makes sense to use get_unaligned() only when needed. > >> There are several cases of users doing this, e.g. siphash. > >> > > > > For ARMv7 code, using the unaligned accessors unconditionally is fine, > > and it will not affect performance. > > > > In general, when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is defined, > > you can use the unaligned accessors. If it is not, it helps to have > > different code paths. > > > arch/arm/include/asm/unaligned.h doesn't make use of > linux/unaligned/access_ok.h, even if CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS > is set. > > I understand the comment in the file, however using get_unaligned() > unconditionally takes away the opportunity to generate optimized code > (using ldrd/ldm) when data is aligned. > But the minimal optimization that is possible here (one ldrd/ldm instruction vs two ldr instructions) is defeated by the fact that you are using a conditional branch to select between the two. And this is not even a hot path to begin with, > > This is a bit murky, and through the years, the interpretation of > > unaligned-memory-access.rst has shifted a bit, but in this case, it > > makes no sense to make the distinction. > > > > Thanks, > Horia
