#272: altivec specific code segfaults on ppc when compiled with --enable-pic
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Reporter: kim | Owner:
Type: defect | Status: open
Priority: normal | Component: avcodec
Version: git-master | Resolution:
Keywords: ppc, PIC | Blocked By:
Blocking: | Reproduced: 1
Analyzed: 0 |
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Comment (by kim):
Replying to [comment:7 reimar]:
> Are you sure this isn't specific to your compiler version?
> LLVM and possible also gcc made an ABI change that optimized the TOC
register r2 away.
> The current code thus can't work anymore.
I think that's what is happening. However I'm using a fairly standard
compiler (which I think is the default in any recent distro: gcc 4.5.x)
Also tested with gcc 4.4.5 with the same results so I guess this change in
gcc is quite old. However I came up with a proof of concept patch to load
the address of local symbols in a pic friendly way. (I use the standard
trick of computing the space between a fixed label and the symbol to be
loaded and then add this to the adresss of the fixed label which is
determined at runtime using a phony branch instruction). I've tested it on
linux ppc 32bit and I'm now able to play the above sample without
problems.
{{{
diff --git a/libavcodec/ppc/asm.S b/libavcodec/ppc/asm.S
index e372d53..30e0953 100644
--- a/libavcodec/ppc/asm.S
+++ b/libavcodec/ppc/asm.S
@@ -67,7 +67,11 @@ X(\name):
.macro movrel rd, sym
#if CONFIG_PIC
- lwz \rd, \sym@got(r2)
+ bcl 20, 31, lab_pic_\@
+lab_pic_\@:
+ mflr \rd
+ addis \rd, \rd, (\sym - lab_pic_\@)@ha
+ addi \rd, \rd, (\sym - lab_pic_\@)@l
#else
lis \rd, \sym@ha
la \rd, \sym@l(\rd)
}}}
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Ticket URL: <https://avcodec.org/trac/ffmpeg/ticket/272#comment:8>
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