[FFmpeg-devel] Suggested fix for: Linker error when using libavcodec.a within shared library on ARM plattform

[Andreas Haupt]

>
>
>Michael Niedermayer <michaelni at gmx.at> schrieb am 13:01 Donnerstag, 12.Februar 2015:
> 
>
>On Thu, Feb 12, 2015 at 10:56:58AM +0000, Andreas Haupt wrote:
>> Hi,
>> 
>> I found an issue when linking against the ffmpeg libavcodec.a static libary on armhf which can probably consider to be a bug.
>> 
>> Summary of the bug:
>> When I try to build a shared library in which I include libavcodec.a I receive the following linker error (on ARM plattform - banana pi board).
>> Affected versions of ffmpeg are at least 2.5.3 and git-master, affected plattform is armhf (linux).
>> 
>> Error message from linker:
>> 
>> /home/hpt/ffmpeg/arm_gcc/2.5.3/lib/static/libavcodec.a(videodsp_armv5te.o): In function `ff_prefetch_arm':
>> /root/build_ffmpeg/ffmpeg/libavcodec/arm/videodsp_armv5te.S:29:(.text+0x8): relocation truncated to fit: R_ARM_THM_JUMP19 against symbol `ff_prefetch_arm' defined in .text section in /home/hpt/ffmpeg/arm_gcc/2.5.3/lib/static/libavcodec.a(videodsp_armv5te.o)
>> collect2: ld returned 1 exit status
>> 
>> How to reproduce:
>> Use the attached cpp file to build a shared object library on arm.
>> 
>> When I try to create the shared object library I get the linker error from above. I used the following command to create the shared object library:
>> 
>> % g++ -shared -D__STDC_CONSTANT_MACROS -o test.so -I/home/hpt/ffmpeg/arm_gcc/2.5.3/include/ sample_cpp_file_for_bug_reproduction.cpp /home/hpt/ffmpeg/arm_gcc/2.5.3/lib/static/libavcodec.a
>> 
>> There seems to be some problem in the libavcodec/arm/videodsp_armv5te.S assembler file. In that file the label 'ff_prefetch_arm' (line 25) is exported and further down used by a conditional near jump (line 29). The near jump cannot be done if the label is exported within a shared object library. That's at least the explanation I found for myself.
>> 
>> I used the attached (untested) patch to fix the linker error from above. As far as I can see this patch should fix the problem without any side effects. At least it fixes the  linker error I get.
>
>does this work too: ?
>diff --git a/libavcodec/arm/videodsp_armv5te.S b/libavcodec/arm/videodsp_armv5te.S
>index 55bcce5..aff1161 100644
>--- a/libavcodec/arm/videodsp_armv5te.S
>+++ b/libavcodec/arm/videodsp_armv5te.S
>@@ -23,9 +23,10 @@
>#include "libavutil/arm/asm.S"
>
>function ff_prefetch_arm, export=1
>
>+1:
>         subs            r2,  r2,  #1
>         pld             [r0]
>         add             r0,  r0,  r1
>-        bne             X(ff_prefetch_arm)
>+        bne             1b
>         bx              lr
>endfunc
>
>[...]
>
>-- 
>Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
>Frequently ignored answer#1 FFmpeg bugs should be sent to our bugtracker. User
>questions about the command line tools should be sent to the ffmpeg-user ML.
>And questions about how to use libav* should be sent to the libav-user ML.
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>

>

Yes, the diff you suggested does work. (They don't produce the earlier linker error.)

Andreas