Re: cast lvalues optimizing and strict-aliasing

[Christian BRUEL <christian dot bruel at st dot com>]

thanks for your answer, this is what I fearing. However, I'm surprised 
when you say that the struct value as is accessed as a short, despite 
the char * cast: (from 9899:1999 6.5.4 "Preceding an expression by a 
parenthesized type name converts the value of the expresion to the named 
type").

Now you seem to imply that the aliasing rule only looks at the final 
points-to object's effective type, not the expression's lvalue type 
implied by the cast or the temporary pointer.

many thanks

-c

Ian Lance Taylor wrote:
> Christian BRUEL <christian.bruel@st.com> writes:
>
> > From my understanding of the type-punning aliasing rule, the
> > following 
> > code is correct, and indeed compiles fine with gcc/trunk :
> >
> > typedef struct {
> >   short a : 16;
> >   short b : 16;
> > } c;
> >
> > foo(c t1)
> > {
> >   char *pt1 = (char*)&t1;
> >
> >   if (*(int*)(pt1))
> >     return 0;
> >
> >   return 1;
> > }
>
> No, this is not OK in C.  The rule is that you may access a stored value
> only via a union, or a pointer to a compatible type, or a pointer to a
> character type.  You You are (presumably) storing the value as short,
> and are accessing it as int.  That is not OK.  The rule is about how you
> access the value; passing the address of the value through a char *
> pointer does not sanitize it.
>
> gcc's aliasing warnings do their best but they are not intended to catch
> every erroneous case.
>
>
> > However if I try to rewrite it using an intermediate cast :
> >
> > foo(c t1)
> > {
> >   if (*((int*)((char*)&t1)))
> >     return 0;
> >
> >   return 1;
> > }
>
> This isn't OK either.
>
> Ian