http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57742
--- Comment #7 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #5)
> We have walk_aliased_vdefs for this. Basically the first callback
> you receive has to be the malloc, otherwise there is an aliasing
> stmt inbetween. Initialize the ao_ref with ao_ref_init_from_ptr_and_size.
Hmm, there is a problem with that: I don't get a callback for malloc.
stmt_may_clobber_ref_p_1 only looks at the lhs of a call statement if it isn't
an SSA_NAME, so it considers that p=malloc(n) does not clobber MEM_REF[p]. This
kind of makes sense, it creates this memory, which is different from
clobbering. I can look at the def_stmt of the first argument of memset to find
the malloc, at least, but that doesn't help me with the memory checks.
Also, for this testcase:
void* f(int n,double*d){
int* p=__builtin_malloc(n);
++*d;
__builtin_memset(p,0,n);
return p;
}
I actually get a callback for the store in *d, which gcc believes might alias
:-(
For this example:
void g(int*);
void* f(int n){
int* p=__builtin_malloc(n);
for(int i=0;i<10000;++i){
__builtin_memset(p,0,n);
g(p);
p[5]=10;
}
return p;
}
if I modify the aliasing machinery to make it believe that p=malloc does alias,
malloc is the first callback. I haven't added the dominance checks, but I
assume they will tell me that malloc dominates memset and memset postdominates
malloc, although I still shouldn't do the transformation.
Pretty depressed at this point...
