[Bug c++/98544] New: [11 regression] Wrong code generated by tree vectorizer

[martin@mpa-garching.mpg.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98544

            Bug ID: 98544
           Summary: [11 regression] Wrong code generated by tree
                    vectorizer
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: martin@mpa-garching.mpg.de
  Target Milestone: ---

Created attachment 49893
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49893&action=edit
test case

The attached test case (sorry, I don't have the time to reduce this properly at
the moment ...) tests real-valued FFTs of different lengths and should not
output anything if compiled properly.

However, with current trunk and the following compiler command line, some of
the results are really wrong:

martin@debian:~/codes/ducc/bug$ g++ -std=c++17 -O1 -mavx -ftree-vectorize
bug2.cc
martin@debian:~/codes/ducc/bug$ ./a.out
problem at length 15; discrepancy is 4.40898
problem at length 20; discrepancy is 4.54691
problem at length 21; discrepancy is 3.70442
problem at length 25; discrepancy is 8.40318
problem at length 27; discrepancy is 8.44956
problem at length 28; discrepancy is 3.12203
problem at length 30; discrepancy is 8.81795

The discrepancies should be below 1e-15.

The failure goes away when removing either "-ftree-vectorize" or "-mavx". On
released versions of gcc, it runs fine.