C++ aliasing rules

[Dan Nicolaescu]

Hi!

I would need some expert C++ advice. 

struct first  {  int i; char a;  int f1;  char f2; double d;};
struct second {  char b;  int f2;  int f3;};


void f (struct first * ps1, struct second * ps2)
{
  ps1->f1++;
  ps2->f2++;
  ps1->f1++;
  ps2->f2++;
}


can it be assumed that given that "first" and "second" are
incompatible then ps1.f1 and ps2.f2 don't alias and we can generate
code like the following: (SPARC assembly)

        !#PROLOGUE# 0
        !#PROLOGUE# 1
        ld      [%o1+8], %o0
        ld      [%o2+4], %o3
        add     %o0, 2, %o0
        add     %o3, 2, %o3
        st      %o0, [%o1+8]
        retl
        st      %o3, [%o2+4]

instead of what is generated now: 


        !#PROLOGUE# 0
        !#PROLOGUE# 1
        ld      [%o0+8], %o3
        add     %o3, 1, %o3
        st      %o3, [%o0+8]
        ld      [%o1+4], %o2
        add     %o2, 1, %o2
        st      %o2, [%o1+4]
        ld      [%o0+8], %o3
        add     %o3, 1, %o3
        st      %o3, [%o0+8]
        ld      [%o1+4], %o0
        add     %o0, 1, %o0
        retl
        st      %o0, [%o1+4]


The more general question is if 2 COMPONENT_REFs that refer to classes
that are in conflicting alias sets (ie alias_sets_conflict_p) can
alias in C++. I have a hunch that this might be true, but I want to
make sure...  If the above is not true, is there a restricted case
when it is true?


(for the curious this is related to the patch at
http://gcc.gnu.org/ml/gcc-patches/2002-03/msg01869.html that proved to
not be needed for java, but the framework might be used for C++. 
For some reason it is not totally correct for C, but I have a hunch
that it might be OK for C++.

Thanks.
        --dan