https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114403
--- Comment #23 from Richard Biener <rguenth at gcc dot gnu.org> ---
Maybe easier to understand testcase:
long x[9];
long a[20];
struct { long x; long b[40]; } b;
int __attribute__((noipa))
foo (int n)
{
int i = 0;
int k = 0;
do
{
if (x[k++]) // early exit, loop upper bound is 8 because of this
break;
a[i] = b.b[2*i]; // the misaligned 2*i access causes peeling for gaps
}
while (++i < n);
return i;
}
int main()
{
x[8] = 1;
if (foo (20) != 8)
__builtin_abort ();
return 0;
}
with -O3 -msse4.1 -fno-vect-cost-model we return 20 instead of 8. Adding
-fdisable-tree-cunroll avoids the issue. The upper bound we set on the
vector loop causes us to force taking the IV exit which continues
with i == (niter - 1) / VF * VF, but 'niter' is 20 here.
