[FFmpeg-devel] [PATCH] remove __attribute__((unused))

[Måns Rullgård]

Reimar Doeffinger <Reimar.Doeffinger at stud.uni-karlsruhe.de> writes:

> Hello,
> attached patch would just remove them, since I don't really know what
> their point is (probably just removing warnings?).
> Probably a better fix would be to define attribute_unused just as it was
> done for attribute_used.

There is already such a macro.

> Index: libavcodec/h264.c
> ===================================================================
> --- libavcodec/h264.c	(revision 9014)
> +++ libavcodec/h264.c	(working copy)
> @@ -1800,7 +1800,7 @@
>      const int lt= src[-1-1*stride];
>      LOAD_TOP_EDGE
>      LOAD_LEFT_EDGE
> -    const __attribute__((unused)) int unu= l3;
> +    const int unu= l3;

This and similar cases is to kill a warning about l3 (or whatever)
being unused.  The variable l3 is declared by the macro
LOAD_LEFT_EDGE, but isn't used everywhere this macro is invoked.  I
think the proper solution is to put attribute_unused on all the
variables declared by those macros.

> @@ -5655,7 +5655,7 @@
>      return ctx + 4 * cat;
>  }
>
> -static const __attribute((used)) uint8_t last_coeff_flag_offset_8x8[63] = {
> +static const uint8_t last_coeff_flag_offset_8x8[63] = {
>      0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
>      2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
>      3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4,

This needs attribute_used since it might be used only from inline asm,
in which case gcc doesn't notice it and optimises it out with failed
link as a result.

> Index: libavcodec/dvdata.h
> ===================================================================
> --- libavcodec/dvdata.h	(revision 9014)
> +++ libavcodec/dvdata.h	(working copy)
> @@ -2534,7 +2534,7 @@
>    {  31,  67, 103,  21,  57,  93,  11,  47,  83},
>  };
>
> -static const __attribute__((unused)) int dv_audio_frequency[3] = {
> +static const int dv_audio_frequency[3] = {
>      48000, 44100, 32000,
>  };

Very nasty definition of data in a header file.  This array is only
referenced from some files that #include dvdata.h, with the rest
causing an unused static warning.  Proper fix is to put the actual
data in a C file and declare it extern in the header.  Roman?

> Index: libavcodec/4xm.c
> ===================================================================
> --- libavcodec/4xm.c	(revision 9014)
> +++ libavcodec/4xm.c	(working copy)
> @@ -555,7 +555,7 @@
>      uint16_t *dst= (uint16_t*)f->current_picture.data[0];
>      const int stride= f->current_picture.linesize[0]>>1;
>      const unsigned int bitstream_size= get32(buf);
> -    const int token_count __attribute__((unused)) = get32(buf + bitstream_size + 8);
> +    const int token_count = get32(buf + bitstream_size + 8);
>      unsigned int prestream_size= 4*get32(buf + bitstream_size + 4);
>      uint8_t *prestream= buf + bitstream_size + 12;

This looks like the read value is never used.  It is assigned to a
variable so the other get32() calls can be mixed with declarations.
Without the attribute, it generates an unused variable warning.  Using
attribute_unused instead would be a bit cleaner.

-- 
M?ns Rullg?rd
mans at mansr.com