[Sablevm-user] Re: 64 bit alignment

[Etienne M. G. <eg...@j-...>]

xli wrote:
> 
> Thank you for your reply.
> 
> > On a 64 bit platform, sizeof(void *)  will be 64 bits.  All we have to
> > do, now, is make sure the remaining of the stack is aligned (which I
> > have to double check).
> 
> I think you are right for a 64 bit platform. However, I tried your idea
> on a SPARC architecture which is a 32 bit platform but requires 64 bit
> allignment for doubles (longs).  For example, the following code:
> 
> #define allignment() {if ((int)(p)&0x00000007)  p++;}
> 
> main(){
> int *p;
> double d1, d2, d3;
> 
>         d1 = 123.5;
>         d2 = 45.7892;
> 
>         p = (int*) malloc(20*sizeof(void*));
> 
>         p++;
> 
>         // allignment();
> 
>         *((double*) p) = d1;
> 
>         p += 2;
> 
>         *((double*) p) = d2;
> 
>         d3 = *(double*) p + *(double*) (p-2);
> 
>         printf(" %f + %f = %f \n", d1, d2, d3);
> }
> 
> will cause a bus error if the allignment macro is ommitted.
> 

I didn't know about that.  In this case, there's an ugly solution (maybe
not that ugly): 

union _svmt_stack_frame_entry_union
{
  jint jint;
  jfloat jfloat;
  void *addr;
#ifdef DOUBLE_ALIGNMENT
  jdouble dummy_jdouble;
#endif
};



> I also tried the above code on my PC, it works because ix86 architecture
> does not require such kind of allignment (I guess).
> 
> >This is an important point to check.  I will need a lot of feedback from
> >people like you to make sure SableVM is easily portable to various
> >systems with a minimal effort.
> 
> This is also an important point for me, as I am now working on a
> Prolog virtual machine which should be portable to various platforms.
> 
> Cheers.
> 
> Li

-- 
----------------------------------------------------------------------
Etienne M. Gagnon, M.Sc.                     e-mail: eg...@j-...
Author of SableCC:                 http://www.sable.mcgill.ca/sablecc/
and SableVM:                                   http://www.sablevm.org/
----------------------------------------------------------------------