https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87621
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2018-10-16
CC| |rguenth at gcc dot gnu.org
Blocks| |53947
Summary|auto-vectorization fails |outer loop
|for exponentiation code |auto-vectorization fails
| |for exponentiation code
Ever confirmed|0 |1
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue is the unsupported reduction. We can't vectorize a
x = x*x;
reduction. And I don't see how we could.
We could eventually vectorize the outer loop but outer loop vectorization
is "confused" by the if-conversion we need to do to the inner loop.
Fixing that (y *= n%2 ? x : 1) yields outer loop vectorization failure like
t.ii:20:20: note: vect_is_simple_use: operand y_36 = PHI <1(3),
prephitmp_27(10)>, type of def: unknown
t.ii:20:20: missed: Unsupported pattern.
t.ii:17:6: missed: not vectorized: unsupported use in stmt.
t.ii:20:20: missed: unexpected pattern.
t.ii:20:20: missed: couldn't vectorize loop
that is because we "simplified" the multiplication by 1 and thus the
reduction op becomes
y = n%2 ? new_y : y;
and appearantly we do not like this (not sure why the reduction structure
is relevant for outer loop vectorization). We do not actually detect this
as reduction, but we could simply identify inner loop reductions by
looking for the loop-closed PHIs.
So - were you expecting outer loop vectorization to happen?
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations
