Re: [Qemu-devel] [PATCHv4 2/9] cutils: add a function to find non-zero content in a buffer

[Orit Wasserman]

On 03/22/2013 02:46 PM, Peter Lieven wrote:
> this adds buffer_find_nonzero_offset() which is a SSE2/Altivec
> optimized function that searches for non-zero content in a
> buffer.
> 
> 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 <address@hidden>
> ---
>  include/qemu-common.h |   13 +++++++++++++
>  util/cutils.c         |   45 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 58 insertions(+)
> 
> diff --git a/include/qemu-common.h b/include/qemu-common.h
> index e76ade3..078e535 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;
> +}
> +size_t buffer_find_nonzero_offset(const void *buf, size_t len);
> +
>  #endif
> diff --git a/util/cutils.c b/util/cutils.c
> index 1439da4..41c627e 100644
> --- a/util/cutils.c
> +++ b/util/cutils.c
> @@ -143,6 +143,51 @@ int qemu_fdatasync(int fd)
>  }
>  
>  /*
> + * Searches for an area with non-zero content in a buffer
> + *
> + * Attention! The len must be a multiple of
> + * BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR * sizeof(VECTYPE)
> + * and addr must be a multiple of sizeof(VECTYPE) due to
> + * restriction of optimizations in this function.
> + *
> + * can_use_buffer_find_nonzero_offset() can be used to check
> + * these requirements.
> + *
> + * The return value is the offset of the non-zero area rounded
> + * down to BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR * sizeof(VECTYPE).
> + * If the buffer is all zero the return value is equal to len.
> + */
> +
> +size_t buffer_find_nonzero_offset(const void *buf, size_t len)
> +{
> +    VECTYPE *p = (VECTYPE *)buf;
> +    VECTYPE zero = ZERO_SPLAT;
> +    size_t i;
> +
> +    assert(len % (BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
> +        * sizeof(VECTYPE)) == 0);
> +    assert(((uintptr_t) buf) % sizeof(VECTYPE) == 0);
> +
> +    if (*((const long *) buf)) {
> +        return 0;
> +    }
> +
> +    for (i = 0; i < len / sizeof(VECTYPE);
Why not put len/sizeof(VECTYPE) in a variable?
Orit
> +            i += BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR) {
> +        VECTYPE tmp0 = p[i + 0] | p[i + 1];
> +        VECTYPE tmp1 = p[i + 2] | p[i + 3];
> +        VECTYPE tmp2 = p[i + 4] | p[i + 5];
> +        VECTYPE tmp3 = p[i + 6] | p[i + 7];
> +        VECTYPE tmp01 = tmp0 | tmp1;
> +        VECTYPE tmp23 = tmp2 | tmp3;
> +        if (!ALL_EQ(tmp01 | tmp23, zero)) {
> +            break;
> +        }
> +    }
> +    return i * sizeof(VECTYPE);
> +}
> +
> +/*
>   * Checks if a buffer is all zeroes
>   *
>   * Attention! The len must be a multiple of 4 * sizeof(long) due to
>