[Bug tree-optimization/67681] New: Missed vectorization: induction variable used after loop

[Bug tree-optimization/67681] New: Missed vectorization: induction variable used after loop

[alalaw01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67681

            Bug ID: 67681
           Summary: Missed vectorization: induction variable used after
                    loop
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alalaw01 at gcc dot gnu.org
  Target Milestone: ---

The inner loop here:
void addlog2 (int *data)
{
  int i = 1;
  for (int j=0; j<=30; j++) {
    int max = 1 << j;
    if (FOO && i>max) break;

    for (; i <= max; i++)
      data[i] += j;
  }
}

does not vectorize if the if(FOO...) is present:
$ /work/alalaw01/build-aarch64-none-elf/install/bin/aarch64-none-elf-gcc -S -O2
-ftree-vectorize -fdump-tree-vect-details=stdout loop9b.c -DFOO=1 | grep
vectorized
loop9b.c:1:6: note: not vectorized: inner-loop count not invariant.
loop9b.c:8:5: note: === vect_mark_stmts_to_be_vectorized ===
loop9b.c:8:5: note: not vectorized: value used after loop.
loop9b.c:8:5: note: === vect_mark_stmts_to_be_vectorized ===
loop9b.c:8:5: note: not vectorized: value used after loop.
loop9b.c:1:6: note: vectorized 0 loops in function.


$ aarch64-none-elf-gcc -S -O2 -ftree-vectorize -fdump-tree-vect-details=stdout
loop9b.c -DFOO=0 | grep vectorized
loop9b.c:4:3: note: not vectorized: inner-loop count not invariant.
loop9b.c:8:5: note: === vect_mark_stmts_to_be_vectorized ===
loop9b.c:8:5: note: loop vectorized
loop9b.c:1:6: note: vectorized 1 loops in function.

Same with -O3. Of course clever analysis could figure out that i>max is never
true, but even without that, we should be able to get 'i' back afterwards.

[Bug tree-optimization/67681] Missed vectorization: induction variable used after loop

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67681

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The outer loop also has multiple exits.  The testcase is incomplete (FOO is
undefined).

[Bug tree-optimization/67681] Missed vectorization: induction variable used after loop

[alalaw01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67681

--- Comment #2 from alalaw01 at gcc dot gnu.org ---
Being stupid here, but why does the outer loop having multiple exits matter -
it's the inner loop that should be vectorized?

FOO was a macro used to selectively make the test i>max disappear (enabling
vectorization) - the two commandlines had -DFOO=0 (vectorizes) and -DFOO=1
(doesn't).