[Bug tree-optimization/80015] auto vectorization leaves scalar epilogue even if it is unreachable due to dominating n % 4 check

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80015

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-03-13
            Version|unknown                     |7.0.1
             Blocks|                            |53947
            Summary|auto vectorization leaves   |auto vectorization leaves
                   |scalar code even if it is   |scalar epilogue even if it
                   |unreachable                 |is unreachable due to
                   |                            |dominating n % 4 check
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
With __builtin_unreachable(); we early compute n as having low-order bits unset
while with the return 0 this is obviously not true.  Given we have no way to
flow-sensitively attach that info to the other path the vectorizer isn't told
about this.  There is also no CCP pass after vectorization which eventually
could recover from this.

The two variants are not semantically equivalent btw.

Confirmed.  I think it should be viewed as a missing feature in niter analysis.


Referenced Bugs:

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