On Wed, Jul 30, 2025 at 10:17:42AM GMT, Jakub Jelinek wrote:
> On Wed, Jul 30, 2025 at 10:16:06AM +0200, Jakub Jelinek wrote:
> > Or maybe better
> > _BitInt(PROMOTED_SIZE (x) * __CHAR_BIT__) __x;
> > int __y = S (x);
> > do_copy (&__x, &(x), sizeof (__x));
> > if (__y < PROMOTED_SIZE (x) * __CHAR_BIT__)
> > {
> > int __neg = __x < 0;
> > __x >>= PROMOTED_SIZE (x) * __CHAR_BIT__ - __y;
>
> Obviously __x >>= __y;
> We want to shift away all the normal bits and keep just the padding bits
> which aren't padding in the promoted type.
>
> > if (__x != (__neg ? (typeof (__x)) -1 : (__typeof (__x)) 0))
> > __builtin_abort ();
> > }
> > and just no __neg and if (__x) in the unsigned case?
> > The intent is not to invoke any UB, just have the memcpy hidden
> > from the optimizers.
>
> Jakub
I'm not sure if this is correct, but isn't accessing __x in its own type
after do_copy already an undefined behavior?
N3301 J.2 (34)
> An object has its stored value accessed other than by an lvalue of
> an allowable type (6.5.1).
where "allowable type" refers to some variant of a compatible type,
and "_BitInt(PROMOTED_SIZE (x) * __CHAR_BIT__)" is not an allowable
type to access the object of (x).
