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From: "linkw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/109069] [12/13 Regression] Vector truncation test
 program produces incorrect result since r12-6537-g080a06fcb076b3
Date: Fri, 10 Mar 2023 09:58:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109069

Kewen Lin <linkw at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Vector truncation test      |[12/13 Regression] Vector
                   |program produces incorrect  |truncation test program
                   |result on big-endian        |produces incorrect result
                   |powerpc64-linux-gnu with    |since
                   |-mcpu=3Dpower10 -O2           |r12-6537-g080a06fcb076b3
      Known to work|                            |11.3.0

--- Comment #7 from Kewen Lin <linkw at gcc dot gnu.org> ---
One test case can be:

#include <altivec.h>

__attribute__ ((noipa))
vector signed int
test ()
{
  vector signed int v =3D {-16, -16, -16, -16};
  vector signed int res =3D vec_sld (v, v, 3);
  return res;
}

int
main ()
{
  vector signed int res =3D test ();
  if (res[0] !=3D 0xf0ffffff)
    __builtin_abort ();
  return 0;
}

It fails on Power7,8,9,10. It starts to fail from r12-6537-g080a06fcb076b3,
when the two input operands are the same, it's only safe to optimize it int=
o a
vector move no matter what's the shift count is that the given vector is a
const vec_duplicate and the duplicated element is affordable in a byte. The
culprit commit is incorrect to use easy_vector_constant as predicate, which
just guards those vector constants without memory loading.

The reason why the reporter only saw this on Power10 is that the given cons=
tant
0xFA578D00u can only satisfy "easy_vector_constant" till power10. The
constructed test case above adopts one constant which can be built with
vspltisw, then we can easily see the failure.=