Re: [FFmpeg-devel] [PATCH 10/10] lavc/hevc_ps: compactify ShortTermRPS

[James Almer <jamrial@gmail.com>]

On 4/10/2024 10:31 AM, Anton Khirnov wrote:
> Do not use larger fields than needed, use size-1 bitfields for flags.
> 
> Reduces sizeof(HEVCSPS) by 1280 bytes.
> ---
>   libavcodec/hevc_ps.c     |  6 +++---
>   libavcodec/hevc_ps.h     | 19 +++++++++++--------
>   libavcodec/vulkan_hevc.c |  2 +-
>   3 files changed, 15 insertions(+), 12 deletions(-)
> 
> diff --git a/libavcodec/hevc_ps.c b/libavcodec/hevc_ps.c
> index 76fe507e7b..7b486ce0af 100644
> --- a/libavcodec/hevc_ps.c
> +++ b/libavcodec/hevc_ps.c
> @@ -143,11 +143,11 @@ int ff_hevc_decode_short_term_rps(GetBitContext *gb, AVCodecContext *avctx,
>           for (i = 0; i <= rps_ridx->num_delta_pocs; i++) {
>               used[k] = get_bits1(gb);
>   
> -            rps->use_delta_flag = 0;
> +            rps->use_delta = 0;
>               if (!used[k])
> -                rps->use_delta_flag = get_bits1(gb);
> +                rps->use_delta = get_bits1(gb);
>   
> -            if (used[k] || rps->use_delta_flag) {
> +            if (used[k] || rps->use_delta) {
>                   if (i < rps_ridx->num_delta_pocs)
>                       delta_poc = delta_rps + rps_ridx->delta_poc[i];
>                   else
> diff --git a/libavcodec/hevc_ps.h b/libavcodec/hevc_ps.h
> index 92b85115f7..a8d07aa1b2 100644
> --- a/libavcodec/hevc_ps.h
> +++ b/libavcodec/hevc_ps.h
> @@ -70,16 +70,19 @@ typedef struct HEVCHdrParams {
>   } HEVCHdrParams;
>   
>   typedef struct ShortTermRPS {
> -    uint8_t rps_predict;
> -    unsigned int delta_idx;
> -    uint8_t use_delta_flag;
> -    uint8_t delta_rps_sign;
> -    unsigned int abs_delta_rps;
> -    unsigned int num_negative_pics;
> -    int num_delta_pocs;
> -    int rps_idx_num_delta_pocs;
>       int32_t delta_poc[32];
>       uint32_t used;
> +
> +    uint8_t delta_idx;
> +    uint8_t num_negative_pics;
> +    uint8_t num_delta_pocs;
> +    uint8_t rps_idx_num_delta_pocs;
> +
> +    uint16_t abs_delta_rps;
> +    unsigned delta_rps_sign:1;

Wont the compiler add two bytes of padding if you put this here?

> +
> +    unsigned rps_predict:1;
> +    unsigned use_delta:1;
>   } ShortTermRPS;
>   
>   typedef struct HEVCWindow {
> diff --git a/libavcodec/vulkan_hevc.c b/libavcodec/vulkan_hevc.c
> index c2b65fc201..b109475194 100644
> --- a/libavcodec/vulkan_hevc.c
> +++ b/libavcodec/vulkan_hevc.c
> @@ -359,7 +359,7 @@ static void set_sps(const HEVCSPS *sps, int sps_idx,
>                   .delta_rps_sign = sps->st_rps[i].delta_rps_sign,
>               },
>               .delta_idx_minus1 = sps->st_rps[i].delta_idx - 1,
> -            .use_delta_flag = sps->st_rps[i].use_delta_flag,
> +            .use_delta_flag = sps->st_rps[i].use_delta,
>               .abs_delta_rps_minus1 = sps->st_rps[i].abs_delta_rps - 1,
>               .used_by_curr_pic_flag    = 0x0,
>               .used_by_curr_pic_s0_flag = 0x0,
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