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

Question

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:
Why do I get a link error when I compile in 32-bit mode?
Is there some way to get the program to compile and link, while still being able to link with a 32-bit library?

Answer 1

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

Answer 2

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.