For reasons to long and boring to mention here, I need to be able to call the
128-bit atomic compare-and-exchange operator on an amd64 platform.
Having no previous experience with in-line assembler etc, I cribbed something
together based on a few examples on the net and on the os sourcecode
repository. Initially I just wanted to create my own version of the
atomic_cas_64 call (on the basis that this would be a simpler case to debug,
and then I could easily 'upgrade' to the 128-bit version).
What I have at the moment is this assembler:
[b]testasm.il:[/b]
.inline matt_cas_64,0
movq %rsi, %rax
lock
cmpxchgq %rdx, (%rdi)
ret
.end
and this c code:
[b]test.c:[/b]
#include <stdio.h>
#include <sys/types.h>
uint64_t matt_cas_64(uint64_t volatile*, uint64_t, uint64_t);
int main(int argc, char* argv[])
{
volatile uint64_t foo = 12;
uint64_t bar = 21;
if (12 == matt_cas_64(&foo, 12, bar))
{
printf("Success: %ld\n", foo);
}
else
{
printf("Failure\n");
}
return 0;
}
Compile and link (using Studio11) thusly:
[b]cc -xarch=amd64 testasm.il test.c[/b]
to get a binary which dies unpleasantly when run:
> dbx a.out
Reading a.out
Reading ld.so.1
Reading libc.so.1
(dbx) run
Running: a.out
(process id 16963)
signal SEGV (no mapping at the fault address) in (unknown) at 0xc
0x000000000000000c: <bad address 0xc>
(dbx) where
[1] 0xc(0xfffffd7fffdff870, 0xc, 0x15, 0x0, 0xc, 0x0), at 0xc
So, something is clearly very wrong with my assembler or how I'm trying to use
it. Can anyone point me in the right direction?
Thanks,
Matt.
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