Illegal Instruction in debug_build

[John Rose]

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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