Re: [FFmpeg-devel] [PATCH 2/2] libavcodec: Set hidden visibility on global symbols accessed from AArch64 assembly

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Martin Storsjö:
> The AArch64 assembly accesses those symbols directly, without
> indirection via e.g. the GOT on ELF. In order for this not to
> require text relocations, those symbols need to be resolved fully
> at link time, i.e. those symbols can't be interposable.
> 
> Normally, so far, this is achieved when linking shared libraries
> in two ways; we have a version script (libavcodec/libavcodec.v) which
> marks all symbols that don't start with av* as local. Additionally,
> we try to add -Wl,-Bsymbolic to the linker options if supported,
> making sure that such symbol references are resolved fully at link
> time, instead of making them interposable.
> 
> When the libavcodec static library is linked into another shared
> library, there's no guarantee that it uses similar options (even though
> that would be favourable), which would end up requiring text relocations
> in the AArch64 assembly.
> 
> Explicitly mark the symbols that are accessed from AArch64 assembly
> as hidden, so that they are resolved fully at link time even without
> the version script and -Wl,-Bsymbolic.
> 
> Signed-off-by: Martin Storsjö <martin@martin.st>
> ---
>  libavcodec/aacsbrdata.h | 2 +-
>  libavcodec/fft.h        | 2 +-
>  libavcodec/vp9dsp.c     | 2 +-
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/libavcodec/aacsbrdata.h b/libavcodec/aacsbrdata.h
> index 7a11594c9b..aa382c4b52 100644
> --- a/libavcodec/aacsbrdata.h
> +++ b/libavcodec/aacsbrdata.h
> @@ -268,7 +268,7 @@ static const int8_t sbr_offset[6][16] = {
>  };
>  
>  /* First eight entries repeated at end to simplify SIMD implementations. */
> -const DECLARE_ALIGNED(16, INTFLOAT, AAC_RENAME(ff_sbr_noise_table))[][2] = {
> +const av_visibility_hidden DECLARE_ALIGNED(16, INTFLOAT, AAC_RENAME(ff_sbr_noise_table))[][2] = {
>  {Q31(-0.99948153278296f), Q31(-0.59483417516607f)}, {Q31( 0.97113454393991f), Q31(-0.67528515225647f)},
>  {Q31( 0.14130051758487f), Q31(-0.95090983575689f)}, {Q31(-0.47005496701697f), Q31(-0.37340549728647f)},
>  {Q31( 0.80705063769351f), Q31( 0.29653668284408f)}, {Q31(-0.38981478896926f), Q31( 0.89572605717087f)},
> diff --git a/libavcodec/fft.h b/libavcodec/fft.h
> index 706c9d07f5..c2241fbc7c 100644
> --- a/libavcodec/fft.h
> +++ b/libavcodec/fft.h
> @@ -114,7 +114,7 @@ void ff_init_ff_cos_tabs(int index);
>  #endif
>  
>  #define COSTABLE(size) \
> -    COSTABLE_CONST DECLARE_ALIGNED(32, FFTSample, FFT_NAME(ff_cos_##size))[size/2]
> +    COSTABLE_CONST av_visibility_hidden DECLARE_ALIGNED(32, FFTSample, FFT_NAME(ff_cos_##size))[size/2]
>  
>  extern COSTABLE(16);
>  extern COSTABLE(32);
> diff --git a/libavcodec/vp9dsp.c b/libavcodec/vp9dsp.c
> index d8ddf74d4f..1be942d78b 100644
> --- a/libavcodec/vp9dsp.c
> +++ b/libavcodec/vp9dsp.c
> @@ -29,7 +29,7 @@
>  
>  #include "vp9dsp.h"
>  
> -const DECLARE_ALIGNED(16, int16_t, ff_vp9_subpel_filters)[3][16][8] = {
> +const av_visibility_hidden DECLARE_ALIGNED(16, int16_t, ff_vp9_subpel_filters)[3][16][8] = {

Shouldn't you set this in vp9dsp.h, so that all users of it can benefit
from knowing that this symbol is always in the same DSO?

>      [FILTER_8TAP_REGULAR] = {
>          {  0,  0,   0, 128,   0,   0,  0,  0 },
>          {  0,  1,  -5, 126,   8,  -3,  1,  0 },

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