[FFmpeg-devel] [PATCH] avutil/mem: Mark DECLARE_ASM_ALIGNED as visibility("hidden") for __GNUC__

[Fāng-ruì Sòng]

Sorry if this doesn't attach to the correct thread as I didn't
subscribe to this list and don't know the Message-ID of the thread.

> The word "also" indicates here that this should be an independent patch.

I added `#if defined(__GNUC__) && !(defined(_WIN32) ||
defined(__CYGWIN__))`, not `#if (defined(__GNUC__) ||
defined(__clang__)) && !(defined(_WIN32) || defined(__CYGWIN__))`. For
consistency I removed the defined(__clang__) below. If that change
should be an independent one, here is the amended version without the
removal of defined(__clang__)


Inline asm code assumes these DECLARE_ASM_ALIGNED declared global
constants are non-preemptive, e.g.

libavcodec/x86/cabac.h
        "lea    "MANGLE(ff_h264_cabac_tables)", %0      \n\t"

On ELF platforms, if -Wl,-Bsymbolic
-Wl,--version-script,libavcodec/libavcodec.ver are removed from the
linker command line, the symbol will be considered preemptive and fail
to link to a DSO:

    ld.lld: error: relocation R_X86_64_PC32 cannot be used against
symbol ff_h264_cabac_tables; recompile with -fPIC

It is better to express the intention explicitly and mark such global
constants hidden (non-preemptive). It also improves portability as no
linker magic is required.

DECLARE_ASM_CONST uses the "static" specifier to indicate internal
linkage. The visibility annotation is unnecessary.

Signed-off-by: Fangrui Song <maskray at google.com>
---
 libavutil/mem.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/libavutil/mem.h b/libavutil/mem.h
index 5fb1a02dd9..9afeed0b43 100644
--- a/libavutil/mem.h
+++ b/libavutil/mem.h
@@ -100,6 +100,12 @@
  * @param v Name of the variable
  */

+#if defined(__GNUC__) && !(defined(_WIN32) || defined(__CYGWIN__))
+    #define DECLARE_HIDDEN __attribute__ ((visibility ("hidden")))
+#else
+    #define DECLARE_HIDDEN
+#endif
+
 #if defined(__INTEL_COMPILER) && __INTEL_COMPILER < 1110 || defined(__SUNPRO_C)
     #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned (n))) v
     #define DECLARE_ASM_ALIGNED(n,t,v)  t __attribute__ ((aligned (n))) v
@@ -110,7 +116,7 @@
     #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
-    #define DECLARE_ASM_ALIGNED(n,t,v)  t av_used __attribute__
((aligned (n))) v
+    #define DECLARE_ASM_ALIGNED(n,t,v)  t av_used __attribute__
((aligned (n))) DECLARE_HIDDEN v
     #define DECLARE_ASM_CONST(n,t,v)    static const t av_used
__attribute__ ((aligned (n))) v
 #elif defined(_MSC_VER)
     #define DECLARE_ALIGNED(n,t,v)      __declspec(align(n)) t v