Problem with Link Time Optimization causing undefined symbols with ASM constants

Question

I am compiling mplayer with llvm-gcc-4.2.1.

With '-O1' (which disables link time optimization), the program successfully compiles and links. With '-O2' or '-O1 -flto', ld complains of undefined symbols:

Undefined symbols for architecture x86_64:
  "_MM_FIX_0_707106781", referenced from:
      _filter in vf_fspp.o
  "_MM_FIX_0_541196100", referenced from:
      _filter in vf_fspp.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

fyi, my version of ld:

@(#)PROGRAM:ld  PROJECT:ld64-123.2
llvm version 2.9svn, from Apple Clang 2.0 (build 137)

I'll focus just on MM_FIX_0_707106781, as the other constants all follow the same procedure.

MM_FIX_0_707106781 is initialized with the macro:

DECLARE_ASM_CONST(8, uint64_t, MM_FIX_0_707106781)=FIX64(0.707106781, 14);

which evaluates to:

static const uint64_t __attribute__((used, aligned (8))) MM_FIX_0_707106781=0x2d412d412d412d41;

These constants are used in asm code:

#define MANGLE(a) "_" #a "(%%rip)"

__asm__ volatile(
...
  "pmulhw "MANGLE(MM_FIX_0_707106781)", %%mm7 \n\t"
...
);

I had a similar (the same?) problem with asm functions that I was able to resolve by adding:
".globl "LABLE_MANGLE(functionnamehere)"\n\t"
before each label, but that knowledge has not helped me with these ASM constants.

That is as much information as I can provide, I'm afraid. Once again, with -O1 the code compiles, links, and runs. With -O2 the linker fails to find these asm constants.

Can anyone offer a solution to this problem? Thanks.

user656562 user656562 · Accepted Answer · 2011-03-17T22:47:12

Thanks to all who took the time to consider my question, however I just realized that I screwed up my compile tools, and am now able to compile normally.

The issue was that the mplayer make scripts invoke 'cc' to compile, with the expectation that cc == gcc. This was not the case on my system; cc was symlinked to some different version of gcc. As soon as I symlinked cc to gcc, I got the project to compile with -O4 (as set in the default mplayer configure script).

In conclusion: improperly configured compiler tools were causing conflicts at link time. Resolved by using the same compiler at all stages of build.

Edit: Actually llvm-gcc still fails with -O4, but the other compilers (gcc-4.5.2 and gcc42, which is Apple's gcc version) succeed. Both other compilers do not accept the -flto flag, so link-time-optimization is still failing. I am at least happy that I can compile with -O2, -O3, etc, which is the main reason I was motivated to pose this question.

Naturally I would like to be able to use the llvm-gcc compiler if I wish (at a level above -O1), however you should consider this question semi-resolved, as the other two compilers are working properly with this code.

Problem with Link Time Optimization causing undefined symbols with ASM constants

4 Answers