I don't know the ins and outs of asm and grabbing rsp on Mac, but the OS X port
group probably know something about it. -- John
On Oct 15, 2011, at 8:49 AM, Michael Barker wrote:
> Good luck!
Thanks. I found where the problem is, it's in the
os::current_stack_pointer method in os_bsd_x86.cpp and it depends on
level of compilation. If I compile the code below without
optimisation e.g.:
#include <stdio.h>
class os {
public:
void* current_stack_pointer();
};
void* os::current_stack_pointer() {
register void *esp __asm__ ("rsp");
return esp;
}
int main() {
os o = os();
printf("%p\n", o.current_stack_pointer());
}
# g++ test.cc -o test
It will generate the following assembly:
__ZN2os21current_stack_pointerEv:
0000000000000000 pushq %rbp
0000000000000001 movq %rsp,%rbp
0000000000000004 movq %rdi,0xf8(%rbp)
0000000000000008 movq 0xe0(%rbp),%rax
000000000000000c movq %rax,%rsp
000000000000000f movq %rsp,%rax
0000000000000012 movq %rax,0xe8(%rbp)
0000000000000016 movq 0xe8(%rbp),%rax
000000000000001a movq %rax,0xf0(%rbp)
000000000000001e movq 0xf0(%rbp),%rax
0000000000000022 popq %rbp
And will fail with an illegal instruction. If optimisation is added
(-O1 is sufficient) it works fine:
# g++ -O1 test.cc -o test
And the generated assembly looks far more sane:
0000000000000000 pushq %rbp
0000000000000001 movq %rsp,%rbp
0000000000000004 movq %rsp,%rax
0000000000000007 popq %rbp
0000000000000008 ret
So I've added -01 to the debug flags in
hotspot/make/bsd/makefiles/gcc.make and it now seems to run okay. I'm
not sure that it's the best fix. Is there are better way to get hold
of the stack pointer? I.e. one that doesn't get stomped over by a
lack of optimisation :-). Not sure if this specific to Mac OS 7 or
gcc 4.2.
Mike.
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