Re: [PATCH] powerpc64le: add optimized strlen for P9

["Lucas A. M. Magalhaes via Libc-alpha" <libc-alpha@sourceware.org>]

Quoting Paul E. Murphy via Libc-alpha (2020-05-21 16:10:48)
> This is a followup to rawmemchr/strlen from Anton.  I missed
> his original strlen patch, and likewise I wasn't happy with
> the 3-4% performance drop for larger strings which occurs
> around 2.5kB as the P8 vector loop is a bit faster.  As noted,
> this is up to 50% faster for small strings, and about 1% faster
> for larger strings (I hazard to guess this some uarch difference
> between lxv and lvx).
> 
> I guess this is a semi-V2 of the patch.  Likewise, I need to
> double check binutils 2.26 supports the P9 insn used here.
> 
> ---8<---
> 
> This started as a trivial change to Anton's rawmemchr.  I got
> carried away.  This is a hybrid between P8's asympotically
> faster 64B checks with extremely efficient small string checks
> e.g <64B (and sometimes a little bit more depending on alignment).
> 
> The second trick is to align to 64B by running a 48B checking loop
> 16B at a time until we naturally align to 64B (i.e checking 48/96/144
> bytes/iteration based on the alignment after the first 5 comparisons).
> This allieviates the need to check page boundaries.
> 
> Finally, explicly use the P7 strlen with the runtime loader when building
> P9.  We need to be cautious about vector/vsx extensions here on P9 only
> builds.
> ---
>  .../powerpc/powerpc64/le/power9/rtld-strlen.S |   1 +
>  sysdeps/powerpc/powerpc64/le/power9/strlen.S  | 215 ++++++++++++++++++
>  sysdeps/powerpc/powerpc64/multiarch/Makefile  |   2 +-
>  .../powerpc64/multiarch/ifunc-impl-list.c     |   4 +
>  .../powerpc64/multiarch/strlen-power9.S       |   2 +
>  sysdeps/powerpc/powerpc64/multiarch/strlen.c  |   5 +
>  6 files changed, 228 insertions(+), 1 deletion(-)
>  create mode 100644 sysdeps/powerpc/powerpc64/le/power9/rtld-strlen.S
>  create mode 100644 sysdeps/powerpc/powerpc64/le/power9/strlen.S
>  create mode 100644 sysdeps/powerpc/powerpc64/multiarch/strlen-power9.S
> 
> diff --git a/sysdeps/powerpc/powerpc64/le/power9/rtld-strlen.S b/sysdeps/powerpc/powerpc64/le/power9/rtld-strlen.S
> new file mode 100644
> index 0000000000..e9d83323ac
> --- /dev/null
> +++ b/sysdeps/powerpc/powerpc64/le/power9/rtld-strlen.S
> @@ -0,0 +1 @@
> +#include <sysdeps/powerpc/powerpc64/power7/strlen.S>
> diff --git a/sysdeps/powerpc/powerpc64/le/power9/strlen.S b/sysdeps/powerpc/powerpc64/le/power9/strlen.S
> new file mode 100644
> index 0000000000..084d6e31a8
> --- /dev/null
> +++ b/sysdeps/powerpc/powerpc64/le/power9/strlen.S
> @@ -0,0 +1,215 @@
> +
> +/* Optimized rawmemchr implementation for PowerPC64/POWER9.
s/rawmemchr/strlen

Still trying to understand the rest of the patch though. =)

---
Lucas A. M. Magalhães