Compiling for i386 on 64bit OSX is hard

[Hendrik Schreiber]

Summary of the bug:

When building on OS X 10.7.5 (64bit), it is hard to build a small or shared 32bit version.

How to reproduce:

Specifying --arch=i386 has no effect, as configure differentiates between 32 and 64 bit by compiling some code.

Forcing 32bit via --cc="clang -m32" works, but then the flag --enable-small leads to a compilation error:

CC	libavcodec/h264_cabac.o
In file included from libavcodec/h264_cabac.c:43:
libavcodec/x86/h264_i386.h:137:9: error: ran out of registers during register allocation
        "mov %1, %6                             \n\t"

Since the failing code is assembler, specifying --disable-asm avoids the failure.

What if we use gcc instead of clang? Well, that works partially. I.e. it does not work with shared libs enabled:

--enable-small --enable-shared --disable-static --arch=i386 --cc="gcc -m32"

again leads to

CC	libavcodec/h264_cabac.o
cc1: error in backend: Ran out of registers during register allocation!
make: *** [libavcodec/h264_cabac.o] Error 1

$ gcc --version
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.9.00)

$ clang --version
Apple clang version 3.1 (tags/Apple/clang-318.0.58) (based on LLVM 3.1svn)
Target: x86_64-apple-darwin11.4.2
Thread model: posix

[Hendrik Schreiber]

It looks like the issue has been discussed before in ​https://lists.ffmpeg.org/pipermail/ffmpeg-user/2012-June/006886.html - but, given that the discussion is old, the issue has not been fixed, yet.

[Carl Eugen Hoyos]

Do I understand correctly that only building with --enable-small is "hard", --enable-shared (alone) works fine?

[Hendrik Schreiber]

Sorry for being unclear.

The answer is: No. It depends on the used compiler.

./configure --enable-shared --disable-static --arch=i386 --cc="gcc -m32"
also leads to

CC	libavcodec/h264_cabac.o
cc1: error in backend: Ran out of registers during register allocation!
make: *** [libavcodec/h264_cabac.o] Error 1

./configure --enable-shared --disable-static --arch=i386 --cc="clang -m32"
works.

But
./configure --enable-shared --disable-static --arch=i386 --cc="clang -m32" --enable-small
leads to

CC	libavcodec/h264_cabac.o
In file included from libavcodec/h264_cabac.c:43:
libavcodec/x86/h264_i386.h:137:9: error: ran out of registers during register allocation
        "mov %1, %6                             \n\t"
        ^

But in any case, whoever attempts to build ffmpeg for i386 on OSX has to know, that --arch=i386 is ignored. AFAIK, this is not documented anywhere that's easily accessible. And probably it shouldn't be ignored to begin with.

One apparently has to specify --cc somehow to get this to build for i386 at all.
And when you do, some things still don't work. That's what makes it hard, as in: it takes a few hours to achieve a working result.

Thank you for looking into this.

[Michael Niedermayer]

Is this still reproduceable ? (there was a change in the cabac asm code since then)
if it still happens maybe another function has to be added under #if BROKEN_COMPILER

[Carl Eugen Hoyos]

I tested the following configure lines successfully:

$ ./configure --enable-small --cc='cc -m32'
$ ./configure --enable-shared --disable-static --cc='cc -m32'
$ ./configure --enable-shared --disable-static --enable-small --cc='cc -m32'

The used compiler was:

$ cc -v
Apple LLVM version 5.0 (clang-500.2.75) (based on LLVM 3.3svn)
Target: x86_64-apple-darwin12.5.0
Thread model: posix