[Bug tree-optimization/99415] s115 benchmark of TSVC is vectorized by icc and not by gcc

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99415

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |tree-optimization
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
             Blocks|                            |53947
   Last reconfirmed|                            |2021-03-08
           Keywords|                            |missed-optimization

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The benchmark is written badly to confuse our loop header copying it seems. 
Writing

        for (int j = 0; j < LEN_2D-1; j++) {
            for (int i = j+1; i < LEN_2D; i++) {
                a[i] -= aa[j][i] * a[j];
            }
        }

fixes the vectorizing.

Possibly a mistake users do, so probably worth investigating further.  Not
sure how to most easily address this - we'd like to peel the last iteration
of the outer loop, noting it does nothing.  Maybe loop-splitting can figure
this out?  Alternatively loop header copying should just do its job...

Hmm, actually loop-header copying does do its job but then there's jump
threading messing this up again (the loop header check is redundant for
all but the last iteration of the outer loop).  So -fno-tree-dominator-opts
fixes this as well.  And for some reason ch_vect thinks the loops are
all do-while loops.


Referenced Bugs:

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