Re: [PATCH v6 06/10] util/bufferiszero: Improve scalar variant

[Philippe Mathieu-Daudé]

On 25/4/24 00:57, Richard Henderson wrote:
> Split less-than and greater-than 256 cases.
> Use unaligned accesses for head and tail.
> Avoid using out-of-bounds pointers in loop boundary conditions.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>   util/bufferiszero.c | 85 +++++++++++++++++++++++++++------------------
>   1 file changed, 51 insertions(+), 34 deletions(-)
>
> diff --git a/util/bufferiszero.c b/util/bufferiszero.c
> index 02df82b4ff..c9a7ded016 100644
> --- a/util/bufferiszero.c
> +++ b/util/bufferiszero.c
> @@ -28,40 +28,57 @@
>   
>   static bool (*buffer_is_zero_accel)(const void *, size_t);
>   
> -static bool buffer_is_zero_integer(const void *buf, size_t len)
> +static bool buffer_is_zero_int_lt256(const void *buf, size_t len)
>   {
> -    if (unlikely(len < 8)) {
> -        /* For a very small buffer, simply accumulate all the bytes.  */
> -        const unsigned char *p = buf;
> -        const unsigned char *e = buf + len;
> -        unsigned char t = 0;
> +    uint64_t t;
> +    const uint64_t *p, *e;
>   
> -        do {
> -            t |= *p++;
> -        } while (p < e);
> -
> -        return t == 0;
> -    } else {
> -        /* Otherwise, use the unaligned memory access functions to
> -           handle the beginning and end of the buffer, with a couple
> -           of loops handling the middle aligned section.  */
> -        uint64_t t = ldq_he_p(buf);
> -        const uint64_t *p = (uint64_t *)(((uintptr_t)buf + 8) & -8);
> -        const uint64_t *e = (uint64_t *)(((uintptr_t)buf + len) & -8);
> -
> -        for (; p + 8 <= e; p += 8) {
> -            if (t) {
> -                return false;
> -            }
> -            t = p[0] | p[1] | p[2] | p[3] | p[4] | p[5] | p[6] | p[7];
> -        }
> -        while (p < e) {
> -            t |= *p++;
> -        }
> -        t |= ldq_he_p(buf + len - 8);
> -
> -        return t == 0;
> +    /*
> +     * Use unaligned memory access functions to handle
> +     * the beginning and end of the buffer.
> +     */
> +    if (unlikely(len <= 8)) {
> +        return (ldl_he_p(buf) | ldl_he_p(buf + len - 4)) == 0;
>       }
> +
> +    t = ldq_he_p(buf) | ldq_he_p(buf + len - 8);

Here we read #0 and #31, ...

> +    p = QEMU_ALIGN_PTR_DOWN(buf + 8, 8);
> +    e = QEMU_ALIGN_PTR_DOWN(buf + len - 1, 8);
> +
> +    /* Read 0 to 31 aligned words from the middle. */

... so here is #1 to #30?

> +    while (p < e) {
> +        t |= *p++;
> +    }
> +    return t == 0;
> +}

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>