[FFmpeg-devel] [PATCH 1/2] libavutil: Add av_visibility_hidden for setting hidden symbol visibility

[Timo Rothenpieler]

On 11.07.2022 16:26, Andreas Rheinhardt wrote:
> Henrik Gramner:
>> On Mon, Jul 11, 2022 at 11:19 AM Martin Storsjö <martin at martin.st> wrote:
>>> +#if (AV_GCC_VERSION_AT_LEAST(4,0) || defined(__clang__)) && (defined(__ELF__) || defined(__MACH__))
>>> +#    define av_visibility_hidden __attribute__((visibility("hidden")))
>>> +#else
>>> +#    define av_visibility_hidden
>>> +#endif
>>
>> The usual approach is to compile with -fvisibility=hidden and
>> explicitly flag exported API symbols.
>>
>> Is there a reason for doing this the other way around?
> 
> -fvisibility=hidden only affects the visibility of symbols defined in
> the currently compiled translation unit. It does not allow the compiler
> to make assumptions about external declarations that are used in this
> translation unit (in other words, it has to presume the worst: That it
> comes from a different DSO). E.g. this is ff_rdft_end on 32bit x86 if
> ff_fft_end is declared with an explicit hidden attribute:
> 
> 000000bb <ff_rdft_end>:
> 
>    bb:	83 44 24 04 18       	addl   $0x18,0x4(%esp)
> 
>    c0:	e9 fc ff ff ff       	jmp    c1 <ff_rdft_end+0x6>
> 
> 			c1: R_386_PC32	ff_fft_end
> 
> 
> And this is the same function if one uses -fvisibility=hidden instead of
> the attribute:
> 
> 000000bb <ff_rdft_end>:
> 
>    bb:	53                   	push   %ebx
> 
>    bc:	e8 fc ff ff ff       	call   bd <ff_rdft_end+0x2>
> 
> 			bd: R_386_PC32	__x86.get_pc_thunk.bx
> 
>    c1:	81 c3 02 00 00 00    	add    $0x2,%ebx
> 
> 			c3: R_386_GOTPC	_GLOBAL_OFFSET_TABLE_
> 
>    c7:	83 ec 14             	sub    $0x14,%esp
> 
>    ca:	8b 44 24 1c          	mov    0x1c(%esp),%eax
> 
>    ce:	83 c0 18             	add    $0x18,%eax
> 
>    d1:	50                   	push   %eax
> 
>    d2:	e8 fc ff ff ff       	call   d3 <ff_rdft_end+0x18>
> 
> 			d3: R_386_PLT32	ff_fft_end
> 
>    d7:	83 c4 18             	add    $0x18,%esp
> 
>    da:	5b                   	pop    %ebx
> 
>    db:	c3                   	ret
> 
> 
> The code is the same as if one had not used -fvisibility=hidden at all.
> 
> Of course, adding the attribute to every function/object is way too much
> effort; that's why the pragma exists.

Is this still true if you also add -fno-semantic-interposition?