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Link error when compiling gcc atomic operation in 32-bit mode

Tags:

gcc

atomic

linker

I have the following program:

~/test> cat test.cc
int main()
{
  int i = 3;
  int j = __sync_add_and_fetch(&i, 1);
  return 0;
}

I'm compiling this program using GCC 4.2.2 on Linux running on a multi-cpu 64-bit Intel machine:

~/test> uname --all
Linux doom 2.6.9-67.ELsmp #1 SMP Wed Nov 7 13:56:44 EST 2007 x86_64 x86_64 x86_64 GNU/Linux

When I compile the program in 64-bit mode, it compiles and links fine:

~/test> /share/tools/gcc-4.2.2/bin/g++ test.cc
~/test>

When I compile it in 32-bit mode, I get the following error:

~/test> /share/tools/gcc-4.2.2/bin/g++ -m32 test.cc
/tmp/ccEVHGkB.o(.text+0x27): In function `main':
: undefined reference to `__sync_add_and_fetch_4'
collect2: ld returned 1 exit status
~/test>

Although I will never actually run on a 32-bit processor, I do need a 32-bit executable so I can link with some 32-bit libraries.

My 2 questions are:

  1. Why do I get a link error when I compile in 32-bit mode?

  2. Is there some way to get the program to compile and link, while still being able to link with a 32-bit library?

like image 533
Bruno Rijsman Avatar asked Sep 25 '08 00:09

Bruno Rijsman


2 Answers

The answer from Dan Udey was close, close enough in fact to allow me to find the real solution.

According to the man page "-mcpu" is a deprecated synonym for "-mtune" and just means "optimize for a particular CPU (but still run on older CPUs, albeit less optimal)". I tried this, and it did not solve the issue.

However, "-march=" means "generate code for a particular CPU (and don't run on older CPUs)". When I tried this it solved the problem: specifying a CPU of i486 or better got rid of the link error.

~/test> /share/tools/gcc-4.2.2/bin/g++ -m32  test.cc
/tmp/ccYnYLj6.o(.text+0x27): In function `main':
: undefined reference to `__sync_add_and_fetch_4'
collect2: ld returned 1 exit status

~/test> /share/tools/gcc-4.2.2/bin/g++ -m32 -march=i386 test.cc
/tmp/ccOr3ww8.o(.text+0x22): In function `main':
: undefined reference to `__sync_add_and_fetch_4'
collect2: ld returned 1 exit status

~/test> /share/tools/gcc-4.2.2/bin/g++ -m32 -march=i486 test.cc

~/test> /share/tools/gcc-4.2.2/bin/g++ -m32 -march=pentium test.cc
like image 107
Bruno Rijsman Avatar answered Oct 27 '22 21:10

Bruno Rijsman


From the GCC page on Atomic Builtins:

Not all operations are supported by all target processors. If a particular operation cannot be implemented on the target processor, a warning will be generated and a call an external function will be generated. The external function will carry the same name as the builtin, with an additional suffix `_n' where n is the size of the data type.

Judging from your compiler output, which refers to __sync_add_and_fetch_4, this is what's happening. For some reason, GCC is not generating the external function properly.

This is likely why you're only getting an error in 32-bit mode - when compiling for 64-bit mode, it compiles for your processor more closely. When compiling for 32-bit, it may well be using a generic arch (i386, for example) which does not natively support those features. Try specifying a specific architecture for your chip family (Xeon, Core 2, etc.) via -mcpu and see if that works.

If not, you'll have to figure out why GCC isn't including the appropriate function that it should be generating.

like image 41
Dan Udey Avatar answered Oct 27 '22 22:10

Dan Udey