[PATCH 03/34] rs6000: Add the rest of the [altivec] stanza to the builtins file

Bill Schmidt wschmidt@linux.ibm.com
Tue Aug 10 13:02:24 GMT 2021

On 8/10/21 7:48 AM, Segher Boessenkool wrote:
> On Tue, Aug 10, 2021 at 07:17:54AM -0500, Bill Schmidt wrote:
>> On 8/9/21 6:44 PM, Segher Boessenkool wrote:
>>> This is not a documented GCC extension either, and it might even
>>> conflict with the existing void * extension (allowing arithmetic on it,
>>> by defining sizeof(void)).  In either case it is not currently defined.
>> I'm not sure how you get to this, but all we're doing here is standard C.
> Arithmetic on void* is the GCC extension.  sizeof(void) is 1 as GCC
> extension, instead of being undefined.  Pointer arithmetic is only
> defined for arrays of the type being pointed to, and you cannot have an
> array of void.  You can do this as GCC extension though, it behaves as
> if it was a char* instead.
>> x.c:
>> char
>> foo (const void *x)
>> {
>>    const char *y = (const char *) x;
>>    return *y;
>> }
> And this behaves exactly the same if you do s/const void/void/ .  The
> const qualifier is meaningless on things of type void, since you cannot
> have an lvalue of that type anyway.  And all type qualifiers can be cast
> away (or cast into existence).
>> y.c:
>> void
>> foo (const void *x, char c)
>> {
>>    const char *y = (const char *) x;
>>    *y = c;
>> }
>> wschmidt@rain6p1:~/src$ gcc -c x.c
>> wschmidt@rain6p1:~/src$ gcc -c y.c
>> y.c: In function 'foo':
>> y.c:5:6: error: assignment of read-only location '*y'
>>     *y = c;
>>        ^
> Yes, *y is an lvalue.  *x is not: *x is an error.
> It *is* allowed to have a "const void", but it means exactly the same as
> just "void" (you cannot assign to either!)  And, they are compatible
> types, too, (they are the *same* type in fact!), so if you ever would
> treat them differently it would be mightily confusing :-)

The whole point is that this data type is only used for interfaces, as 
shown in the example code.  Nobody wants to define const void as 
anything.  The const serves only as a contract that the pointed-to 
object, no matter what it is cast to, will not be modified.

I think you're over-thinking this. :-)


> Segher

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