On Tue, Aug 05, 2025 at 11:58:45AM GMT, Jakub Jelinek wrote:
> On Sat, Aug 02, 2025 at 05:14:21PM +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.
>
> The ChangeLog doesn't describe what the patch does, e.g. the introduction of
> __BEXTC macro.
>
> > @@ -25,22 +25,30 @@ do_copy (void *p, const void *q, __SIZE_TYPE__ r)
> > /* Macro to test whether (on targets where psABI requires it) _BitInt
> > with padding bits have those filled with sign or zero extension. */
> > #if defined(__s390x__) || defined(__arm__) || defined(__loongarch__)
> > +#define __BEXTC(x, __x, __y) \
> > + do { \
> > + typeof (x) __z; \
> > + do_copy (&(__x), &(x), sizeof (__x)); \
> > + __z = (typeof (x)) __x; \
> > + do_copy (&(__y), &__z, sizeof (__y)); \
> > + if ((__x) != (__y)) \
> > + __builtin_abort (); \
> > + } while (0)
>
> I'd suggest a change. I'd just use a name of the macro
> not reserved for implementation, and instead of adding __x and __y
> arguments, let it take
> #define BEXTC1(x, uns) \
> do { \
> uns _BitInt(PROMOTED_SIZE (x) * __CHAR_BIT__) __x; \
> do_copy (&__x, &(x), sizeof (__x)); \
> if ((__x) != (typeof (x)) __x) \
> __builtin_abort (); \
> } while (0)
>
> and then just use BEXTC1 (x, signed); for the signed case
> and BEXTC1 (x, unsigned); for the unsigned one.
> I think the above can't be optimized away, __x is the wider type and
> after do_copy which is noipa the compiler can't know anything about
> the state of the upper bits. So, cast from __x to typeof (x) and
> back to typeof (__x) for the comparison has to sign or zero extend
> depending on whether typeof (x) is signed or unsigned.
>
> Jakub
Ok. This is much simpler, I wasn't really sure what's legitimate.
Thanks for the review.
Yujie
