Question

We recently upgraded the code to gcc4.3.3 from gcc4.2.4.

void testAndSet(  volatile int*  s, int* val )
{
   __asm__ __volatile__ ( "btsl   $0, %0\n   "
           "jc   bitSet\n   "
           "movl   $0, %1\n   "
           "jmp   returnVector\n"
           "bitSet:\n   "
           "movl   $1, %1\n"
           "returnVector: " : "=m"(*s), "=m"(*val) );
}

Our code now fails with the following errors,

lock.cxx: Assembler messages:
lock.cxx:59: Error: symbol `bitSet' is already defined
lock.cxx:61: Error: symbol `returnVector' is already defined
lock.cxx:59: Error: symbol `bitSet' is already defined
lock.cxx:61: Error: symbol `returnVector' is already defined

Those symbols weren’t found anywhere else. (Renaming them causes the same error with the new name).

What’s up with this? why do I get the errors twice?

Answer 1

Probably the optimizer has changed and is now inlining your testAndSet() function into 2 places. Because you are using global names for your labels, this does not work. You should use local names instead. E.g:

   __asm__ __volatile__ ( "btsl   $0, %0\n   "
           "jc   0f\n   "
           "movl   $0, %1\n   "
           "jmp   1f\n"
           "0:\n   "
           "movl   $1, %1\n"
           "1: " : "=m"(*s), "=m"(*val) );

Local labels are just numbers; to disambiguate cases where there are many labels called “0” you need to use “jmp 0f” for forward jumps and “jmp 0b” for backward jumps.