https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107160
Kewen Lin <linkw at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu.org,
| |rsandifo at gcc dot gnu.org
--- Comment #7 from Kewen Lin <linkw at gcc dot gnu.org> ---
One reduced test case is:
============================================================
#include <stdio.h>
#include <math.h>
#define N 128
float fl[N];
__attribute__ ((noipa, optimize (0))) void
init ()
{
for (int i = 0; i < N; i++)
fl[i] = i;
}
__attribute__ ((noipa)) float
foo (int n1)
{
float sum0, sum1, sum2, sum3;
sum0 = sum1 = sum2 = sum3 = 0.0f;
int n = (n1 / 4) * 4;
for (int i = 0; i < n; i += 4)
{
sum0 += fabs (fl[i]);
sum1 += fabs (fl[i + 1]);
sum2 += fabs (fl[i + 2]);
sum3 += fabs (fl[i + 3]);
}
return sum0 + sum1 + sum2 + sum3;
}
__attribute__ ((optimize (0))) int
main ()
{
init ();
float res = foo (80);
__builtin_printf ("res:%f\n", res);
return 0;
}
============================================================
incorrect result "res:670.000000" vs expected result "res:3160.000000"
It looks it exposes one bug in vectorization reduction support. The reduction
epilogue handling looks wrong, it generates gimple code like:
# vect_sum3_31.16_101 = PHI <vect_sum3_31.16_97(3)>
# vect_sum3_31.16_102 = PHI <vect_sum3_31.16_98(3)>
# vect_sum3_31.16_103 = PHI <vect_sum3_31.16_99(3)>
# vect_sum3_31.16_104 = PHI <vect_sum3_31.16_100(3)>
_105 = BIT_FIELD_REF <vect_sum3_31.16_101, 32, 0>;
_106 = BIT_FIELD_REF <vect_sum3_31.16_101, 32, 32>;
_107 = BIT_FIELD_REF <vect_sum3_31.16_101, 32, 64>;
_108 = BIT_FIELD_REF <vect_sum3_31.16_101, 32, 96>;
_109 = BIT_FIELD_REF <vect_sum3_31.16_102, 32, 0>;
_110 = BIT_FIELD_REF <vect_sum3_31.16_102, 32, 32>;
_111 = BIT_FIELD_REF <vect_sum3_31.16_102, 32, 64>;
_112 = BIT_FIELD_REF <vect_sum3_31.16_102, 32, 96>;
...
it doesn't consider the reduced results vect_sum3_31.16_10{1,2,3,4} from the
loop can be reduced again in loop exit block as they are in the same slp group.
