Re: [Qemu-devel] [PATCHv5 03/10] cutils: add a function to find non-zero content in a buffer

[Juan Quintela <quintela@redhat.com>]

Peter Lieven <pl@kamp.de> wrote:
> this adds buffer_find_nonzero_offset() which is a SSE2/Altivec
> optimized function that searches for non-zero content in a
> buffer.
>
> the function starts full unrolling only after the first few chunks have
> been checked one by one. analyzing real memory page data has revealed
> that non-zero pages are non-zero within the first 256-512 bits in
> most cases. as this function is also heavily used to check for zero memory
> pages this tweak has been made to avoid the high setup costs of the fully
> unrolled check for non-zero pages.
>
> due to the optimizations used in the function there are restrictions
> on buffer address and search length. the function
> can_use_buffer_find_nonzero_content() can be used to check if
> the function can be used safely.
>
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  include/qemu-common.h |   13 ++++++++++++
>  util/cutils.c         |   55 +++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 68 insertions(+)
>
> diff --git a/include/qemu-common.h b/include/qemu-common.h
> index 9022646..7c7c244 100644
> --- a/include/qemu-common.h
> +++ b/include/qemu-common.h
> @@ -472,4 +472,17 @@ void hexdump(const char *buf, FILE *fp, const char *prefix, size_t size);
>  #define ALL_EQ(v1, v2) ((v1) == (v2))
>  #endif
>
> +#define BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR 8
> +static inline bool
> +can_use_buffer_find_nonzero_offset(const void *buf, size_t len)
> +{
> +    if (len % (BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
> +               * sizeof(VECTYPE)) == 0
> +        && ((uintptr_t) buf) % sizeof(VECTYPE) == 0) {
> +        return true;
> +    }
> +    return false;
> +}

This can be spelled as:

return (len % (BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
               * sizeof(VECTYPE)) == 0
       && ((uintptr_t) buf) % sizeof(VECTYPE) == 0);;

But I don't care too much.