https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65335
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Status|UNCONFIRMED |NEW
Last reconfirmed| |2015-03-09
CC| |law at gcc dot gnu.org,
| |rguenth at gcc dot gnu.org
Ever confirmed|0 |1
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
This is if-conversion at work making the code vectorizable. You can disable it
with -fno-tree-loop-if-convert.
It's not easy to assess profitablility here (well, locally it should use
bb frequencies), as computing a*a*a*a*a always could be a win if vectorizing
the rest of the loop compensates for the cost.
In this case the optimal thing to do is to turn the loop into a loop nest.
Not sure how you'd call this kind of loop transform - sth with unswitching
IV-based conditions (it's not exactly splitting in this case). We want
for (int j = 0; j < 10000/1000; j++){
int i;
for (i = j*1000; i < 999; ++i)
buffer[i] = a;
a = a * a * a * a * a;
buffer[i] = a;
}
so we can vectorize the inner loop and don't need to evaluate the conditional
there.
