https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82946
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #0)
> In the program below, while GCC optimizes the strlen call in f() to a
> constant it doesn't do the same for the equivalent function g().
>
> I suspect this is caused by the same underlying assumptions as pr80944:
> i.e., that the strcpy (a->d, "123") call could change a->d if a->d pointed
> at or into itself. While that might be true in other circumstances, it's
> not possible here. Since the array at a->d is subsequently accessed by the
> call to strlen, the strcpy call cannot change a->d in a valid program
> because "123" (or any other string literal) cannot be a valid representation
> of a pointer. (The only way for a conforming program to obtain a valid
> pointer is by assigning to it the value of another valid pointer. Even if
> the bit pattern of the literal "123" happened to match a valid address in a
> program, copying the literal into a pointer and then using that pointer is
> undefined.)
>
> So a->d can be assumed not to change in either function and the strlen
> optimization below is safe in both.
>
> $ cat c.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout a.c
>
> char* strcpy (char*, const char*);
> __SIZE_TYPE__ strlen (const char*);
>
> struct A { char *d; };
>
> unsigned f (struct A *a)
> {
> char *d = a->d;
> strcpy (d, "123");
> return strlen (d); // folded into 3
> }
>
> unsigned g (struct A *a)
> {
> strcpy (a->d, "123");
> return strlen (a->d); // not folded but can be
> }
>
>
> ;; Function f (f, funcdef_no=0, decl_uid=1898, cgraph_uid=0, symbol_order=0)
>
> f (struct A * a)
> {
> char * d;
>
> <bb 2> [local count: 10000]:
> d_4 = a_3(D)->d;
> __builtin_memcpy (d_4, "123", 4);
> return 3;
>
> }
>
>
>
> ;; Function g (g, funcdef_no=1, decl_uid=1902, cgraph_uid=1, symbol_order=1)
>
> g (struct A * a)
> {
> char * _1;
> char * _2;
> long unsigned int _3;
> unsigned int _7;
>
> <bb 2> [local count: 10000]:
> _1 = a_5(D)->d;
> __builtin_memcpy (_1, "123", 4);
> _2 = a_5(D)->d;
Clearly because GCC has to assume a_5(D)->d points to itself and thus
memcpy clobbering it.
Can't see how you can rule that out for a valid program. Thus - INVALID?
Richard.
> _3 = strlen (_2);
> _7 = (unsigned int) _3;
> return _7;
>
> }
