On 11.07.2022 16:26, Andreas Rheinhardt wrote:
Henrik Gramner:
On Mon, Jul 11, 2022 at 11:19 AM Martin Storsjö <mar...@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?
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