Re: [PATCH] Hoist loop invariant statements containing data refs with zero-step during loop-versioning in vectorization.

[Cong Hou <congh at google dot com>]

Hi Richard

You mentioned that Micha has a patch pending that enables of zero-step
stores. What is the status of this patch? I could not find it through
searching "Micha".

Thank you!


Cong


On Wed, Oct 16, 2013 at 2:02 AM, Richard Biener <rguenther@suse.de> wrote:
> On Tue, 15 Oct 2013, Cong Hou wrote:
>
>> Thank you for your reminder, Jeff! I just noticed Richard's comment. I
>> have modified the patch according to that.
>>
>> The new patch is attached.
>
> (posting patches inline is easier for review, now you have to deal
> with no quoting markers ;))
>
> Comments inline.
>
> diff --git a/gcc/ChangeLog b/gcc/ChangeLog
> index 8a38316..2637309 100644
> --- a/gcc/ChangeLog
> +++ b/gcc/ChangeLog
> @@ -1,3 +1,8 @@
> +2013-10-15  Cong Hou  <congh@google.com>
> +
> +       * tree-vect-loop-manip.c (vect_loop_versioning): Hoist loop invariant
> +       statement that contains data refs with zero-step.
> +
>  2013-10-14  David Malcolm  <dmalcolm@redhat.com>
>
>         * dumpfile.h (gcc::dump_manager): New class, to hold state
> diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
> index 075d071..9d0f4a5 100644
> --- a/gcc/testsuite/ChangeLog
> +++ b/gcc/testsuite/ChangeLog
> @@ -1,3 +1,7 @@
> +2013-10-15  Cong Hou  <congh@google.com>
> +
> +       * gcc.dg/vect/pr58508.c: New test.
> +
>  2013-10-14  Tobias Burnus  <burnus@net-b.de>
>
>         PR fortran/58658
> diff --git a/gcc/testsuite/gcc.dg/vect/pr58508.c b/gcc/testsuite/gcc.dg/vect/pr58508.c
> new file mode 100644
> index 0000000..cb22b50
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/vect/pr58508.c
> @@ -0,0 +1,20 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -ftree-vectorize -fdump-tree-vect-details" } */
> +
> +
> +/* The GCC vectorizer generates loop versioning for the following loop
> +   since there may exist aliasing between A and B.  The predicate checks
> +   if A may alias with B across all iterations.  Then for the loop in
> +   the true body, we can assert that *B is a loop invariant so that
> +   we can hoist the load of *B before the loop body.  */
> +
> +void foo (int* a, int* b)
> +{
> +  int i;
> +  for (i = 0; i < 100000; ++i)
> +    a[i] = *b + 1;
> +}
> +
> +
> +/* { dg-final { scan-tree-dump-times "hoist" 2 "vect" } } */
> +/* { dg-final { cleanup-tree-dump "vect" } } */
> diff --git a/gcc/tree-vect-loop-manip.c b/gcc/tree-vect-loop-manip.c
> index 574446a..f4fdec2 100644
> --- a/gcc/tree-vect-loop-manip.c
> +++ b/gcc/tree-vect-loop-manip.c
> @@ -2477,6 +2477,92 @@ vect_loop_versioning (loop_vec_info loop_vinfo,
>        adjust_phi_and_debug_stmts (orig_phi, e, PHI_RESULT (new_phi));
>      }
>
>
> Note that applying this kind of transform at this point invalidates
> some of the earlier analysis the vectorizer performed (namely the
> def-kind which now effectively gets vect_external_def from
> vect_internal_def).  In this case it doesn't seem to cause any
> issues (we re-compute the def-kind everytime we need it (how wasteful)).
>
> +  /* Extract load and store statements on pointers with zero-stride
> +     accesses.  */
> +  if (LOOP_REQUIRES_VERSIONING_FOR_ALIAS (loop_vinfo))
> +    {
> +      /* In the loop body, we iterate each statement to check if it is a load
> +        or store.  Then we check the DR_STEP of the data reference.  If
> +        DR_STEP is zero, then we will hoist the load statement to the loop
> +        preheader, and move the store statement to the loop exit.  */
>
> We don't move the store yet.  Micha has a patch pending that enables
> vectorization of zero-step stores.
>
> +      for (gimple_stmt_iterator si = gsi_start_bb (loop->header);
> +          !gsi_end_p (si);)
>
> While technically ok now (vectorized loops contain a single basic block)
> please use LOOP_VINFO_BBS () to get at the vector of basic-blcoks
> and iterate over them like other code does.
>
> +       {
> +         gimple stmt = gsi_stmt (si);
> +         stmt_vec_info stmt_info = vinfo_for_stmt (stmt);
> +         struct data_reference *dr = STMT_VINFO_DATA_REF (stmt_info);
> +
> +         if (dr && integer_zerop (DR_STEP (dr)))
> +           {
> +             if (DR_IS_READ (dr))
> +               {
> +                 if (dump_enabled_p ())
> +                   {
> +                     dump_printf_loc
> +                         (MSG_NOTE, vect_location,
> +                          "hoist the statement to outside of the loop ");
>
> "hoisting out of the vectorized loop: "
>
> +                     dump_gimple_stmt (MSG_NOTE, TDF_SLIM, stmt, 0);
> +                     dump_printf (MSG_NOTE, "\n");
> +                   }
> +
> +                 gsi_remove (&si, false);
> +                 gsi_insert_on_edge_immediate (loop_preheader_edge (loop), stmt);
>
> Note that this will result in a bogus VUSE on the stmt at this point which
> will be only fixed because of implementation details of loop versioning.
> Either get the correct VUSE from the loop header virtual PHI node
> preheader edge (if there is none then the current VUSE is the correct one
> to use) or clear it.
>
> +               }
> +             /* TODO: We also consider vectorizing loops containing zero-step
> +                data refs as writes.  For example:
> +
> +                int a[N], *s;
> +                for (i = 0; i < N; i++)
> +                  *s += a[i];
> +
> +                In this case the write to *s can be also moved after the
> +                loop.  */
>
> Note that you then invalidate even more vectorizer analysis - you
> basically introduce a scalar reduction (you have to add a PHI node).
> Which means that the transform has to happen elsewhere.
>
> As Jakub now tries with if-conversion this would also be a candidate
> for applying the loop versioning before even starting the rest of the
> vectorizer analysis code.
>
> That said, I'd remove the TODO at this point because it's clearly not
> possible to implement just here ;)
>
> +             continue;
> +           }
> +         else if (!dr)
> +         {
> +           bool hoist = true;
> +           for (size_t i = 0; i < gimple_num_ops (stmt); i++)
>
> You are checking all kinds of statements, including assignments
> of which you are also checking the LHS ... restricting to
> assignments you can start walking at i = 1.
>
> +             {
> +               tree op = gimple_op (stmt, i);
> +               if (TREE_CODE (op) == INTEGER_CST
> +                   || TREE_CODE (op) == REAL_CST)
>
> There are other constants as well - just check
>
>                 if (is_gimple_min_invariant (op))
>
> +                 continue;
> +               if (TREE_CODE (op) == SSA_NAME)
> +                 {
> +                   gimple def = SSA_NAME_DEF_STMT (op);
> +                   if (def == stmt
>
> with starting at op 1 you can drop this.
>
> +                       || gimple_nop_p (def)
> +                       || !flow_bb_inside_loop_p (loop, gimple_bb (def)))
> +                     continue;
> +                 }
>
> Note that you fail to hoist if-converted code this way because
> op1 of
>
>    name_1 = a_2 < b_4 ? x_5 : y_6;
>
> is 'a_2 < b_4', a tree expression and not an SSA name (ugh).  Maybe
> we don't care (it's just a missed optimization), but if you are
> restricting yourself to hoisting assignments without a data-ref
> then you can walk over SSA uses on the stmt (instead of over gimple
> ops) with
>
>                 FOR_EACH_SSA_TREE_OPERAND (op, stmt, iter, SSA_USE)
>
> which would automagically take care of that case.
>
> Can you add a testcase which involves invariant if-conversion?
> Can you add a testcase with just an invariant store to make sure
> we don't wreck it?  Can you add a testcase with an invariant store
> and load (the reduction case), too?
>
> +               hoist = false;
> +               break;
> +             }
>
> For example you'll hoist all labels this way (no ops), as well as the
> loop exit GIMPLE_COND in case it's operands were loop invariant,
> breaking the CFG ... so please add && is_gimple_assign () to the
> if (!dr) check ;)
>
> +           if (hoist)
> +             {
> +               gsi_remove (&si, false);
> +               gsi_insert_on_edge_immediate (loop_preheader_edge (loop), stmt);
> +
> +               if (dump_enabled_p ())
> +                 {
> +                   dump_printf_loc
> +                       (MSG_NOTE, vect_location,
> +                        "hoist the statement to outside of the loop ");
> +                   dump_gimple_stmt (MSG_NOTE, TDF_SLIM, stmt, 0);
> +                   dump_printf (MSG_NOTE, "\n");
> +                 }
> +               continue;
> +             }
> +         }
> +         gsi_next (&si);
> +       }
> +    }
> +
>    /* End loop-exit-fixes after versioning.  */
>
>    if (cond_expr_stmt_list)
>
>>
>> thanks,
>> Cong
>>
>>
>> On Tue, Oct 15, 2013 at 12:33 PM, Jeff Law <law@redhat.com> wrote:
>> > On 10/14/13 17:31, Cong Hou wrote:
>> >>
>> >> Any comment on this patch?
>> >
>> > Richi replied in the BZ you opened.
>> >
>> > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58508
>> >
>> > Essentially he said emit the load on the edge rather than in the block
>> > itself.
>> > jeff
>> >