> I agree - the compiler removes unnessecary code here, but the reason is
> the incomplete asm() instruction. The asm() instruction has 3 parts: the
> instruction itself (here "swpb %0"), the destination of the operand
> (here "+g" (v)) and the source(s). The sources have been omitted here.
>
> asm("<instruction>" : <destination> : <sources>);
>
> Therefore the compiler can't see a source and removes the statements.
Sorry, no. This is incorrect and misleading.
The "+" in the "+g" declaration says that the variable v is a source
as well as a destination.
Further, an asm that does not delcare a source is perfectly legal, and
often used for things like reading special registers on processors that
have them. E.g.
asm("rdtsc" : "=a" (lo), "=d" (hi));
on x86.
(See the discussion of "voltaile" with "asm" in the GCC manual for some
details I'm not going into here.)
