[Ffmpeg-devel] [Patch] fix swscale shared lib on x86-64

[sean]

building swscale on x86-64 fails:

/usr/bin/ld: yuv2rgb.o: relocation R_X86_64_PC32 against 
`mmx_00ffw' can not be used when making a shared object; 
recompile with -fPIC
/usr/bin/ld: final link failed: Bad value

With a lot of help from the binutils list, here a patch that 
  allows libswscale to build. It adds 
__attribute__((visibility("hidden"))).


Alan Modra < amodra at bigpond.net.au > described what the 
problem is:

 >>>mmx_00ffw is just a constant in yuv2rgb.c:
 >>>> >>
 >>>> >>uint64_t attribute_used __attribute__((aligned(8)))
 >>>> >>mmx_00ffw = 0x00ff00ff00ff00ffULL;


 >The trouble is that this variable is used in asm 
statements >using rip addressing, and the variable is 
global.  ELF >shared libraries have particular rules 
regarding global >symbol resolution;  A global symbol in a 
shared library may >be overridden by the same symbol in the 
main app code (or >another shared lib).  rip addressing 
won't work in this
 >situation for two reasons: a) the rip offset is in 
 >read-only code, so relocating the offset means the library 
 >can't share that page of text, and more seriously, b) the 
 >rip offset is only 32-bit so might not reach to the app 
 >symbol or another library sym (64-bit address space).

Works on x86-64. Dunno if it breaks anything on i386.

sean
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