On Wed, Jul 30, 2025 at 09:37:10AM +0200, Jakub Jelinek wrote:
> On Fri, Jul 25, 2025 at 10:53:39AM +0800, Yang Yujie wrote:
> > In BEXTC, whether a _BitInt object is properly extended is examined
> > by a value comparison against a copied object in a wider _BitInt
> > type that utilizes all of the partial limb.
> >
> > Since the (implicit) conversion to the wider type may be optimized
> > away and cause the result of the comparison to always be true,
> > we need to cast the copied object down to the original type to
> > force a extension, so that it can serve as our reference.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * gcc.dg/bitintext.h (BEXTC): Convert the copied object back
> > to the original type before comparison.
>
> This makes no sense at all, you're not testing anything then.
> typeof (x) is the narrower type (smaller N). The lower bits will be always
> equal. The point was to test the padding bits.
> So, if the earlier patch optimizes away the extension, then you should
> just use volatile.
> So say
> _BitInt(PROMOTED_SIZE (x) * __CHAR_BIT__) __x;
> volatile __typeof_unqual__ (x) __y = (x);
> do_copy (&__x, &__y, sizeof (__x));
> if (__x != __y)
> __builtin_abort ();
> etc.
Though that risks that the assignment __y = (x); will extend the padding
bits.
So maybe better:
_BitInt(PROMOTED_SIZE (x) * __CHAR_BIT__) __x;
volatile __typeof_unqual__ (x) __y = (x);
do_copy ((void *) &__y, &(x), sizeof (__y));
do_copy (&__x, (const void *) &__y, sizeof (__x));
if (__x != __y)
__builtin_abort ();
If even that optimizes the extension away, we might need to use 2 shifts
on the volatile __y, left followed by right, by the difference of N
from typeof (x) and PROMOTED_SIZE (x) * __CHAR_BIT__.
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;
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
