https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110888
--- Comment #5 from Rimvydas (RJ) <rimvydas.jas at gmail dot com> ---
It is more like this problem:
$ cat foo.c
void foo_(double *x, double *y, double *z)
{
int i;
__builtin_memset(z, 0, 8); /* z[0] = 0.0; */
for (i=0; i<1 ; i++)
z[0] += x[0] * y[0];
}
$ gcc -O2 -Wall -Wextra -c foo.c -S -fdump-tree-optimized
<bb 2> [local count: 536870913]:
__builtin_memset (z_9(D), 0, 8);
_17 = *x_11(D);
_18 = *y_12(D);
_19 = _17 * _18;
_20 = _19 + 0.0;
*z_9(D) = _20;
return;
It would be beneficial for all frontends if the use of __builtin_memset() to
zero out accumulators would be be considered as !HONOR_SIGNED_ZEROS at least
during PRE pass in the middle-end. If that would complicate things, then
easier solution is to add special case in gfortran frontend-passes that simply
transforms expression to drop accumulator: z[0] = x[0] * y[0];
* side note, in C the redundant __builtin_memset() does not get optimized out,
unlike in gfortran "zero" expr version. Might be middle-end optimization
passes ordering issue. At least PRE pass does take accumulator zeroing into
account.
