https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98854
--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Little bit convoluted testcase:
double a[1024];
int bar();
void foo (int n)
{
double x = 0, y = 0;
int i = 1023;
do
{
x += a[i] + a[i+1];
y += a[i] / a[i+1];
if (bar ())
break;
}
while (--i);
a[0] = x;
a[1] = y;
}
where we end up with the {x, y} vector CTOR inside the loop (and even
spill/reload it because of the call). We have a PHI node-only feed
for the vectorized store:
t.c:16:8: note: Vectorizing SLP tree:
t.c:16:8: note: node 0x3b21ee0 (max_nunits=2, refcnt=1)
t.c:16:8: note: op template: a[0] = x_22;
t.c:16:8: note: stmt 0 a[0] = x_22;
t.c:16:8: note: stmt 1 a[1] = y_21;
t.c:16:8: note: children 0x3b21f68
t.c:16:8: note: node 0x3b21f68 (max_nunits=2, refcnt=1)
t.c:16:8: note: op template: x_22 = PHI <x_26(9), x_25(10)>
t.c:16:8: note: stmt 0 x_22 = PHI <x_26(9), x_25(10)>
t.c:16:8: note: stmt 1 y_21 = PHI <y_24(9), y_23(10)>
t.c:16:8: note: children 0x3b21ff0 0x3b22210
t.c:16:8: note: node 0x3b21ff0 (max_nunits=2, refcnt=1)
t.c:16:8: note: op template: x_26 = PHI <x_14(3)>
t.c:16:8: note: stmt 0 x_26 = PHI <x_14(3)>
t.c:16:8: note: stmt 1 y_24 = PHI <y_15(3)>
t.c:16:8: note: children 0x3b22320
t.c:16:8: note: node (external) 0x3b22320 (max_nunits=1, refcnt=1)
t.c:16:8: note: { x_14, y_15 }
t.c:16:8: note: node 0x3b22210 (max_nunits=2, refcnt=1)
t.c:16:8: note: op template: x_25 = PHI <x_14(4)>
t.c:16:8: note: stmt 0 x_25 = PHI <x_14(4)>
t.c:16:8: note: stmt 1 y_23 = PHI <y_15(4)>
t.c:16:8: note: children 0x3b223a8
t.c:16:8: note: node (external) 0x3b223a8 (max_nunits=1, refcnt=1)
t.c:16:8: note: { x_14, y_15 }
fixing this issue fixes the slowdown. Testing a patch.
