[FFmpeg-devel] [PATCH] avutil/buffer: add av_buffer_fast_alloc()

[wm4]

On Tue, 13 Mar 2018 20:48:56 -0300
James Almer <jamrial at gmail.com> wrote:

> Same concept as av_fast_malloc(). If the buffer passed to it is writable
> and big enough it will be reused, otherwise a new one will be allocated
> instead.
> 
> Signed-off-by: James Almer <jamrial at gmail.com>
> ---
> TODO: Changelog and APIChanges entries, version bump.
> 
>  libavutil/buffer.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  libavutil/buffer.h | 42 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 105 insertions(+)
> 
> diff --git a/libavutil/buffer.c b/libavutil/buffer.c
> index 8d1aa5fa84..16ce1b82e2 100644
> --- a/libavutil/buffer.c
> +++ b/libavutil/buffer.c
> @@ -215,6 +215,69 @@ int av_buffer_realloc(AVBufferRef **pbuf, int size)
>      return 0;
>  }
>  
> +static av_always_inline int buffer_fast_alloc(AVBufferRef **pbuf, int size, int zero_alloc)
> +{
> +    AVBufferRef *buf = *pbuf;
> +    AVBuffer *b;
> +    uint8_t *data;
> +
> +    if (!buf || !av_buffer_is_writable(buf) || buf->data != buf->buffer->data) {
> +        /* Buffer can't be reused, and neither can the entire AVBufferRef.
> +         * Unref the latter and alloc a new one. */
> +        av_buffer_unref(pbuf);
> +
> +        buf = av_buffer_alloc(size);
> +        if (!buf)
> +            return AVERROR(ENOMEM);
> +
> +        if (zero_alloc)
> +            memset(buf->data, 0, size);
> +
> +        *pbuf = buf;
> +        return 0;
> +    }
> +    b = buf->buffer;
> +
> +    if (size <= b->size) {
> +        /* Buffer can be reused. Update the size of AVBufferRef but leave the
> +         * AVBuffer untouched. */
> +        buf->size = FFMAX(0, size);
> +        return 0;
> +    }
> +
> +    /* Buffer can't be reused, but there's no need to alloc new AVBuffer and
> +     * AVBufferRef structs. Free the existing buffer, allocate a new one, and
> +     * reset AVBuffer and AVBufferRef to default values. */
> +    b->free(b->opaque, b->data);
> +    b->free   = av_buffer_default_free;
> +    b->opaque = NULL;
> +    b->flags  = 0;
> +
> +    data = av_malloc(size);
> +    if (!data) {
> +        av_buffer_unref(pbuf);
> +        return AVERROR(ENOMEM);
> +    }
> +
> +    if (zero_alloc)
> +        memset(data, 0, size);
> +
> +    b->data = buf->data = data;
> +    b->size = buf->size = size;
> +
> +    return 0;
> +}
> +
> +int av_buffer_fast_alloc(AVBufferRef **pbuf, int size)
> +{
> +    return buffer_fast_alloc(pbuf, size, 0);
> +}
> +
> +int av_buffer_fast_allocz(AVBufferRef **pbuf, int size)
> +{
> +    return buffer_fast_alloc(pbuf, size, 1);
> +}
> +
>  AVBufferPool *av_buffer_pool_init2(int size, void *opaque,
>                                     AVBufferRef* (*alloc)(void *opaque, int size),
>                                     void (*pool_free)(void *opaque))
> diff --git a/libavutil/buffer.h b/libavutil/buffer.h
> index 73b6bd0b14..1166017d22 100644
> --- a/libavutil/buffer.h
> +++ b/libavutil/buffer.h
> @@ -197,6 +197,48 @@ int av_buffer_make_writable(AVBufferRef **buf);
>   */
>  int av_buffer_realloc(AVBufferRef **buf, int size);
>  
> +/**
> + * Allocate a buffer, reusing the given one if writable and large enough.
> + *
> + * @code{.c}
> + * AVBufferRef *buf = ...;
> + * int ret = av_buffer_fast_alloc(&buf, size);
> + * if (ret < 0) {
> + *     // Allocation failed; buf already freed
> + *     return ret;
> + * }
> + * @endcode
> + *
> + * @param buf  A buffer reference. *buf may be NULL. On success, a new buffer
> + *             reference will be written in its place. On failure, it will be
> + *             unreferenced and set to NULL.
> + * @param size Required buffer size.
> + *
> + * @return 0 on success, a negative AVERROR on failure.
> + *
> + * @see av_buffer_realloc()
> + * @see av_buffer_fast_allocz()
> + */
> +int av_buffer_fast_alloc(AVBufferRef **buf, int size);
> +
> +/**
> + * Allocate and clear a buffer, reusing the given one if writable and large
> + * enough.
> + *
> + * Like av_buffer_fast_alloc(), but all newly allocated space is initially
> + * cleared. Reused buffer is not cleared.
> + *
> + * @param buf  A buffer reference. *buf may be NULL. On success, a new buffer
> + *             reference will be written in its place. On failure, it will be
> + *             unreferenced and set to NULL.
> + * @param size Required buffer size.
> + *
> + * @return 0 on success, a negative AVERROR on failure.
> + *
> + * @see av_buffer_fast_alloc()
> + */
> +int av_buffer_fast_allocz(AVBufferRef **buf, int size);
> +
>  /**
>   * @}
>   */

Wouldn't it be better to avoid writing a lot of new allocation code
(with possibly subtle problems), and just use av_buffer_realloc() to
handle reallocation? (Possibly it could be moved to an internal
function to avoid the memcpy() required by realloc.)