[PATCH v7] malloc: Optimize small memory clearing for calloc

[H.J. Lu]

On Wed, Dec 4, 2024 at 9:45 AM Guo, Wangyang <wangyang.guo@intel.com> wrote:
>
> On 12/4/2024 8:16 AM, H.J. Lu wrote:
>
> On Wed, Dec 4, 2024 at 7:32 AM Paul Eggert <eggert@cs.ucla.edu> wrote:
>
> On 2024-12-02 20:42, H.J. Lu wrote:
>
> +  /* Use repeated stores with 1 branch, instead of up to 3.  */
> +  *(d + 0) = 0;
> +  *(d + 1) = 0;
> +  *(d + 2) = 0;
> +  *(d + nclears - 2) = 0;
> +  *(d + nclears - 2 + 1) = 0;
> +  if (nclears > 5)
> +    {
> +      *(d + 3) = 0;
> +      *(d + 3 + 1) = 0;
> +      *(d + nclears - 4) = 0;
> +      *(d + nclears - 4 + 1) = 0;
> +    }
>
> The following alternative code has no branches, when I compile with GCC
> 14 -O2 on x86-64. Would that be a win?
>
>    /* Use repeated stores with no branches.  */
>    int big = 3 * ((nclears + 1) >> 3);
>    d[0] = 0;
>    d[1] = 0;
>    d[2] = 0;
>    d[big] = 0;
>    d[big + 1] = 0;
>    d[nclears - big - 1] = 0;
>    d[nclears - big] = 0;
>    d[nclears - 2] = 0;
>    d[nclears - 2 + 1] = 0;
>
> Wangyang,  can you try this against the current master branch?
>
> Thanks.
>
> I try to apply the alternative code with attached patch, but meet coredump error when running bench-calloc-thread:
>
> Fatal glibc error: malloc.c:2601 (sysmalloc): assertion failed: (old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)

Try this.

--
H.J.
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