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

[Michael Niedermayer]

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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