[Mesa-dev] [PATCH] util/ralloc: Remove double zero'ing of rzalloc buffers

[Nicolai Hähnle]

On 22.01.2016 02:53, Jordan Justen wrote:
> Juha-Pekka found this back in May 2015:
> <1430915727-28677-1-git-send-email-juhapekka.heikkila at gmail.com>
>
>  From the discussion, obviously it would be preferable to make
> ralloc_size no longer return zeroed memory, but Juha-Pekka found that
> it would break Mesa.
>
> For now, let's point out the flaw, and stop doing the double zeroing
> of rzalloc buffers.
>
> Signed-off-by: Jordan Justen <jordan.l.justen at intel.com>
> Cc: Juha-Pekka Heikkila <juhapekka.heikkila at gmail.com>
> Cc: Kenneth Graunke <kenneth at whitecape.org>
> ---
>
>   For a release build, I saw the code size shrink by 64 bytes.
>
>   src/util/ralloc.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/src/util/ralloc.c b/src/util/ralloc.c
> index 6d4032b..24c1eee 100644
> --- a/src/util/ralloc.c
> +++ b/src/util/ralloc.c
> @@ -49,6 +49,14 @@ _CRTIMP int _vscprintf(const char *format, va_list argptr);
>   #endif
>   #endif
>
> +/* ralloc_size has always used calloc to allocate memory. This has allowed
> + * code using ralloc_size to depend on always receiving a cleared buffer.
> + *
> + * FIXME: Clean up the code base to allow this to be set to false, and then
> + * remove it altogether.
> + */
> +static const bool always_allocate_zeroed_memory = true;
> +
>   #define CANARY 0x5A1106
>
>   struct ralloc_header
> @@ -110,7 +118,10 @@ ralloc_context(const void *ctx)
>   void *
>   ralloc_size(const void *ctx, size_t size)
>   {
> -   void *block = calloc(1, size + sizeof(ralloc_header));
> +   void *block =
> +      always_allocate_zeroed_memory ?
> +      calloc(1, size + sizeof(ralloc_header)) :
> +      malloc(size + sizeof(ralloc_header));

There's an integer overflow here which would be good to fix. Since it 
was there already in the older version, the patch is

Reviewed-by: Nicolai Hähnle <nicolai.haehnle at amd.com>

as is.

>      ralloc_header *info;
>      ralloc_header *parent;
>
> @@ -132,7 +143,7 @@ void *
>   rzalloc_size(const void *ctx, size_t size)
>   {
>      void *ptr = ralloc_size(ctx, size);
> -   if (likely(ptr != NULL))
> +   if (!always_allocate_zeroed_memory && likely(ptr != NULL))
>         memset(ptr, 0, size);
>      return ptr;
>   }
>