[Bug target/110979] Miss-optimization for O2 fully masked loop on floating point reduction.

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110979

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amonakov at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
   Last reconfirmed|                            |2023-08-11
     Ever confirmed|0                           |1
             Blocks|                            |53947
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think there's a duplicate bug showing in-order reduction vectorization
results in bad code (effectively unrolling VF times).  I don't think this is in
any way connected to using partial vectors though.

Now, I think this is also wrong-code to some extent as without
-fno-signed-zeros adding 0.0 can result in a wrong sign?

Without partial vectors and i != 96 you get similar unrolling, but at least
we won't execute adds for not executed lanes.  With -O3 you'd get an
unrolled main loop and an epilog loop (with the original loop bound).


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations