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

From: "Martin Storsjö" <martin@martin.st>
To: ffmpeg-devel@ffmpeg.org
Cc: "Martin Storsjö" <martin@martin.st>
Subject: [FFmpeg-devel] [PATCH 2/2] libavcodec: Set hidden visibility on global symbols accessed from AArch64 assembly
Date: Mon, 11 Jul 2022 12:18:50 +0300
Message-ID: <20220711091850.4158449-2-martin@martin.st> (raw)
In-Reply-To: <20220711091850.4158449-1-martin@martin.st>

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] = {
     [FILTER_8TAP_REGULAR] = {
         {  0,  0,   0, 128,   0,   0,  0,  0 },
         {  0,  1,  -5, 126,   8,  -3,  1,  0 },
-- 
2.25.1

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