[Bug tree-optimization/85186] New: ifcvt can peel iterations affecting code form, and causing vectorization not to happen

[hp at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85186

            Bug ID: 85186
           Summary: ifcvt can peel iterations affecting code form, and
                    causing vectorization not to happen
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hp at gcc dot gnu.org
  Target Milestone: ---

Created attachment 43833
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43833&action=edit
original non-vectorizing case: 1

In a build for x86_64-linux (likely also other targets; this originates
elsewhere) at r258635, drop in attached four test-cases into the dg/vect
subdir.

Observe, when running vect.exp:
Running x/gcc/testsuite/gcc.dg/vect/vect.exp ...
FAIL: gcc.dg/vect/prXXXXX-1.c scan-tree-dump-times vect "vectorized 1 loops" 1
FAIL: gcc.dg/vect/prXXXXX-1.c -flto -ffat-lto-objects  scan-tree-dump-times
vect "vectorized 1 loops" 1

I.e. prXXXXX-1.c does not vectorize, while -2..4 does.

Partial analysis: On inspection using gdb and dumps, this appears to be because
the initialization appears to affect ifcvt (just as a catalyst) causing it to
sort-of peel the first iteration of the to-be-vectorized loop, or something to
that effect, anyway changing the form of the code such that it is not be
recognizable for the loop vectorizer pass that comes next.

1: original (derived from some other gcc test long ago)
2: candidate loop with non-conditional contents: vectorizes.
3: arrays in candidate loop are moved global (i.e. aren't initialized right
before the loop): vectorizes.
4: asm("" : : : "memory") inserted after initialization: vectorizes.

All-in-all this PR isn't important (with the test-cases being artificial), but
when revisiting if-conversion and/or investigates ifcvt vs. loop-vectorization
issues, this PR may provide simplified test-cases of an underlying bug.