subreg question

[Dale Johannesen]

On Thursday, December 13, 2001, at 12:28 PM, Daniel Jacobowitz wrote:

> On Thu, Dec 13, 2001 at 12:21:33PM -0800, Dale Johannesen wrote:
>>
>> On Thursday, December 13, 2001, at 10:16 AM, Aldy Hernandez wrote:
>>
>>>>>>>> "Dale" == Dale Johannesen <dalej@apple.com> writes:
>>>
>>>> What cases of subreg are supposed to be handled exactly?
>>>> double x(float y) {
>>>>     double z;
>>>>     *((float *)&z) = y;     <-- boo!
>>>>     return z;  }
>>>
>>> i believe this is invalid code, hence implementation dependent.  You
>>> can't access an lvalue as a different type as it was declared.
>>
>> I don't see why not.  (C89) 3.3.4 says the cast might produce an invalid
>> pointer if it's not aligned properly, which is not the case here.
>> Other than that I don't find any relevant restriction in the standard.
>> What did you have in mind?
>> (FWIW, gcc doesn't warn about this even with -pedantic.)
>
> It's not an invalid pointer.  It doesn't alias with Z, though.

Why not?

> That
> assignment does not define Z, but only clobbers some memory.

Seems to me it defines however much of Z is covered by a float-sized
object (4 out of 8 bytes, on most machines).  The other half of Z,
and its value, are indeed undefined.  If we had another assignment
to  *((float *)((char*)&z+4)), then all of Z would be defined (somehow).

> You don't
> get a warning because no one ever wrote the warning code for alias
> violations; but this returns an undefined value, not y.

Certainly.  I'm not suggesting this particular example does something
useful, just questioning the generated rtl.  Anyway, even an invalid
program doesn't justify generating invalid rtl.