[FFmpeg-devel] [PATCH] Fix missing used attribute for inline assembly variables

[James Almer]

On 10/30/2017 4:17 PM, Thomas Köppe wrote:
> Variables used in inline assembly need to be marked with attribute((used)).
> Static constants already were, via the define of DECLARE_ASM_CONST.
> But DECLARE_ALIGNED does not add this attribute, and some of the variables
> defined with it are const only used in inline assembly, and therefore
> appeared dead.
> 
> This change makes FFMPEG linkable with Clang's ThinLTO.
> 
> ---
>  libavcodec/cabac.c | 2 +-
>  libavutil/mem.h    | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/cabac.c b/libavcodec/cabac.c
> index dd2b057c6d..7321b48901 100644
> --- a/libavcodec/cabac.c
> +++ b/libavcodec/cabac.c
> @@ -32,7 +32,7 @@
>  #include "cabac.h"
>  #include "cabac_functions.h"
> 
> -const uint8_t ff_h264_cabac_tables[512 + 4*2*64 + 4*64 + 63] = {
> +DECLARE_ALIGNED(1, const uint8_t, ff_h264_cabac_tables)[512 + 4*2*64 +
> 4*64 + 63] = {
>      9,8,7,7,6,6,6,6,5,5,5,5,5,5,5,5,
>      4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,
>      3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,
> diff --git a/libavutil/mem.h b/libavutil/mem.h
> index 527cd03191..db06e109be 100644
> --- a/libavutil/mem.h
> +++ b/libavutil/mem.h
> @@ -98,7 +98,7 @@
>          AV_PRAGMA(DATA_ALIGN(v,n))                      \
>          static const t __attribute__((aligned(n))) v
>  #elif defined(__DJGPP__)

Is your intention to change the DJGPP path, or the gnuc/clang path?

> -    #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned
> (FFMIN(n, 16)))) v
> +    #define DECLARE_ALIGNED(n,t,v)      t av_used __attribute__ ((aligned
> (FFMIN(n, 16)))) v
>      #define DECLARE_ASM_CONST(n,t,v)    static const t av_used
> __attribute__ ((aligned (FFMIN(n, 16)))) v
>  #elif defined(__GNUC__) || defined(__clang__)
>      #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned (n))) v
>