Re: [Qemu-devel] [PATCH v3 0/9] Improve buffer_is_zero

[Dr. David Alan Gilbert]

* Richard Henderson (address@hidden) wrote:

Have you considered contributing something similar to this to glibc?
I filed https://sourceware.org/bugzilla/show_bug.cgi?id=19920  a while back
suggesting it would be useful to have it in libc to be used
by things other than just qemu.

Dave

> Changes from v2 to v3:
> 
>   * Unit testing.  This includes having x86 attempt all versions of
>     the accelerator that will run on the hardware.  Thus an avx2 host
>     will run the basic test 5 times (1.5sec on my laptop).
> 
>   * Drop the ppc and aarch64 specializations.  I have improved the
>     basic integer version to the point that those vectorized versions
>     are not a win.
> 
>     In the case of my aarch64 mustang, the integer version is 4 times
>     faster than the neon version that I delete.  With effort I was
>     able to rewrite the neon version to come to within a factor of 1.1,
>     but it remained slower than the integer.  To be fair, gcc6 makes
>     very good use of ldp, so the integer path is *also* loading 16 bytes
>     per insn.
> 
>     I can forward my standalone aarch64 benchmark if anyone is interested.
> 
>     Note however that at least the avx2 acceleration is still very much
>     a win, being about 3 times faster on my laptop.  Of course, it's
>     handling 4 times as much data per loop as the integer version, so
>     one can still see the overhead caused by using vector insns.
> 
>     For grins I wrote an avx512 version, if someone has a skylake upon
>     which to test and benchmark.  That requires additional configure
>     checks, so I didn't bother to include it here.
> 
> 
> r~
> 
> 
> Richard Henderson (9):
>   cutils: Move buffer_is_zero and subroutines to a new file
>   cutils: Remove SPLAT macro
>   cutils: Export only buffer_is_zero
>   cutils: Rearrange buffer_is_zero acceleration
>   cutils: Add test for buffer_is_zero
>   cutils: Add generic prefetch
>   cutils: Rewrite x86 buffer zero checking
>   cutils: Remove aarch64 buffer zero checking
>   cutils: Remove ppc buffer zero checking
> 
>  configure                 |  21 +--
>  include/qemu/cutils.h     |   3 +-
>  migration/ram.c           |   2 +-
>  migration/rdma.c          |   5 +-
>  tests/Makefile.include    |   3 +
>  tests/test-bufferiszero.c |  78 +++++++++++
>  util/Makefile.objs        |   1 +
>  util/bufferiszero.c       | 332 
> ++++++++++++++++++++++++++++++++++++++++++++++
>  util/cutils.c             | 244 ----------------------------------
>  9 files changed, 423 insertions(+), 266 deletions(-)
>  create mode 100644 tests/test-bufferiszero.c
>  create mode 100644 util/bufferiszero.c
> 
> -- 
> 2.7.4
> 
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK