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From: "zsojka at seznam dot cz" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/100672] wrong code with vector shift and unary minus
Date: Wed, 19 May 2021 12:00:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100672

--- Comment #10 from Zdenek Sojka <zsojka at seznam dot cz> ---
(In reply to Zdenek Sojka from comment #9)
> (In reply to Richard Biener from comment #8)
> > (In reply to Zdenek Sojka from comment #7)
> > > (In reply to Richard Biener from comment #2)
> > > > Maybe sth is wrong with the testcase?  clang also results in an abo=
rt.  Isn't
> > > > right-shift of negative values undefined?
> > > >=20
> > >=20
> > > Thank you for having a look.
> > >=20
> > > I believe the behavior is fully defined. The vectors are unsigned, an=
d the
> > > conversion to unsigned is done by adding 2**64; this behaves the same:
> > >=20
> > > $ cat testcase.c
> > > typedef unsigned long long __attribute__((__vector_size__ (32))) V;
> > >=20
> > > V
> > > foo (V v)
> > > {
> > >   return -(v >> 1);
> >=20
> > but this is a logical right shift, thus gives 0x7ff...e, ...
> > and negating that doesn't yield 1.
> >=20
>=20
> Shame on me. You are very right.

I did the mistake while reducing the testcase.
The original testcase had positive arguments:

$ cat testcase.c
typedef unsigned long long __attribute__((__vector_size__ (32))) V;

V
foo (V v)
{
  return -(v >> 1);
}

int
main (void)
{
  V v =3D foo ((V) { 2, 4, 6, 8 });
  if (v[0] !=3D 0xffffffffffffffff || v[1] !=3D 0xfffffffffffffffe ||
      v[2] !=3D 0xfffffffffffffffd || v[3] !=3D 0xfffffffffffffffc)
    __builtin_abort ();
  return 0;
}


I wanted to prevent the long 0xfffff... constants, but broke the testcase i=
n he
process. Thank you for correcting me.=