[RFC] Don't move cold code out of loop by checking bb count

Richard Biener richard.guenther@gmail.com
Fri Aug 6 12:15:20 GMT 2021


On Mon, Aug 2, 2021 at 7:05 AM Xiong Hu Luo <luoxhu@linux.ibm.com> wrote:
>
> There was a patch trying to avoid move cold block out of loop:
>
> https://gcc.gnu.org/pipermail/gcc/2014-November/215551.html
>
> Richard suggested to "never hoist anything from a bb with lower execution
> frequency to a bb with higher one in LIM invariantness_dom_walker
> before_dom_children".
>
> This patch does this profile count check in both gimple LIM
> move_computations_worker and RTL loop-invariant.c find_invariants_bb,
> if the loop bb is colder than loop preheader, don't hoist it out of
> loop.
>
> Also, the profile count in loop split pass should be corrected to avoid
> lim2 and lim4 mismatch behavior, currently, the new loop preheader generated
> by loop_version is set to "[count: 0]:", then lim4 after lsplt pass will
> move statement out of loop unexpectely when lim2 didn't move it.  This
> change could fix regression on 544.nab_r from -1.55% to +0.46%.
>
> SPEC2017 performance evaluation shows 1% performance improvement for
> intrate GEOMEAN and no obvious regression for others.  Especially,
> 500.perlbench_r +7.52% (Perf shows function S_regtry of perlbench is
> largely improved.), and 548.exchange2_r+1.98%, 526.blender_r +1.00%
> on P8LE.
>
> Regression and bootstrap tested pass on P8LE, any comments?  Thanks.

While I'm not familiar with the RTL invariant motion pass the patch there
looks reasonable.  Note that we should assess the profile quality
somehow - I'm not sure how to do that, CCed Honza for that.

For the GIMPLE part the patch looks quite complicated - but note it
probably has to be since LIM performs kind of a "CSE" on loads
(and stores for store-motion), so when there are multiple stmts
affected by a hoisting decision the biggest block count has to be
accounted.  Likewise when there are dependent stmts involved
that might include conditional stmts (a "PHI"), but the overall
cost should be looked at.

Now - GIMPLE LIM "costing" is somewhat backward right now
and it isn't set up to consider those multiple involved stmts.  Plus
the store-motion part does not have any cost part (but it depends
on previously decided invariant motions).

I think the way you implemented the check will cause no hoisting
to be performed instead of, say, hoisting to a different loop level
only.  Possibly shown when you consider a loop nest like

  for (;;)
    if (unlikely_cond)
      for (;;)
         invariant;

we want to hoist 'invariant' but only from the inner loop even if it
is invariant also in the outer loop.  But for example if there is
a store motion opportunity like

  for (;;)
     {
        if (unlikely_cond)
          for (;;)
            a = ...;
        a = ...;
     }

we'd still want to perform the store motion on the outer loop.

Note that store-motion already performs part of the transform
before dependent code is moved in move_computations (that
you patched).

IIRC your main concern were the COND_EXPRs we insert
for hoisted conditional stmts?

Thanks,
Richard.

> gcc/ChangeLog:
>
>         * loop-invariant.c (find_invariants_bb): Check profile count
>         before motion.
>         (find_invariants_body): Add argument.
>         * tree-ssa-loop-im.c (move_computations_worker): Check profile
>         count before motion.
>         (execute_sm): Likewise.
>         (execute_sm_exit): Check pointer validness.
>         * tree-ssa-loop-split.c (split_loop): Correct probability.
>         (do_split_loop_on_cond): Likewise.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/tree-ssa/recip-3.c: Adjust.
> ---
>  gcc/loop-invariant.c                    |  10 +-
>  gcc/testsuite/gcc.dg/tree-ssa/recip-3.c |   2 +-
>  gcc/tree-ssa-loop-im.c                  | 164 +++++++++++++++++++++++-
>  gcc/tree-ssa-loop-split.c               |  14 +-
>  4 files changed, 177 insertions(+), 13 deletions(-)
>
> diff --git a/gcc/loop-invariant.c b/gcc/loop-invariant.c
> index bdc7b59dd5f..7b5d64d11f9 100644
> --- a/gcc/loop-invariant.c
> +++ b/gcc/loop-invariant.c
> @@ -1183,9 +1183,14 @@ find_invariants_insn (rtx_insn *insn, bool always_reached, bool always_executed)
>     call.  */
>
>  static void
> -find_invariants_bb (basic_block bb, bool always_reached, bool always_executed)
> +find_invariants_bb (class loop *loop, basic_block bb, bool always_reached,
> +                   bool always_executed)
>  {
>    rtx_insn *insn;
> +  basic_block preheader = loop_preheader_edge (loop)->src;
> +
> +  if (preheader->count > bb->count)
> +    return;
>
>    FOR_BB_INSNS (bb, insn)
>      {
> @@ -1214,8 +1219,7 @@ find_invariants_body (class loop *loop, basic_block *body,
>    unsigned i;
>
>    for (i = 0; i < loop->num_nodes; i++)
> -    find_invariants_bb (body[i],
> -                       bitmap_bit_p (always_reached, i),
> +    find_invariants_bb (loop, body[i], bitmap_bit_p (always_reached, i),
>                         bitmap_bit_p (always_executed, i));
>  }
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/recip-3.c b/gcc/testsuite/gcc.dg/tree-ssa/recip-3.c
> index 638bf38db8c..641c91e719e 100644
> --- a/gcc/testsuite/gcc.dg/tree-ssa/recip-3.c
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/recip-3.c
> @@ -23,4 +23,4 @@ float h ()
>         F[0] += E / d;
>  }
>
> -/* { dg-final { scan-tree-dump-times " / " 1 "recip" } } */
> +/* { dg-final { scan-tree-dump-times " / " 5 "recip" } } */
> diff --git a/gcc/tree-ssa-loop-im.c b/gcc/tree-ssa-loop-im.c
> index 7de47edbcb3..2bfb5e8ec15 100644
> --- a/gcc/tree-ssa-loop-im.c
> +++ b/gcc/tree-ssa-loop-im.c
> @@ -1147,6 +1147,61 @@ move_computations_worker (basic_block bb)
>           continue;
>         }
>
> +      edge e = loop_preheader_edge (level);
> +      if (e->src->count > bb->count)
> +       {
> +         if (dump_file && (dump_flags & TDF_DETAILS))
> +           {
> +             fprintf (dump_file, "PHI node NOT moved to %d from %d:\n",
> +                      e->src->index, bb->index);
> +             print_gimple_stmt (dump_file, stmt, 0);
> +             fprintf (dump_file, "(cost %u) out of loop %d.\n\n", cost,
> +                      level->num);
> +           }
> +         gsi_next (&bsi);
> +         continue;
> +       }
> +      else
> +       {
> +         unsigned i;
> +         bool skip_phi_move = false;
> +         for (i = 0; i < gimple_phi_num_args (stmt); i++)
> +           {
> +             tree def = PHI_ARG_DEF (stmt, i);
> +
> +             if (TREE_CODE (def) != SSA_NAME)
> +               continue;
> +
> +             gimple *def_stmt = SSA_NAME_DEF_STMT (def);
> +
> +             if (!gimple_bb (def_stmt))
> +               continue;
> +
> +             if (!dominated_by_p (CDI_DOMINATORS, e->src,
> +                                  gimple_bb (def_stmt)))
> +               {
> +                 if (dump_file && (dump_flags & TDF_DETAILS))
> +                   {
> +                     fprintf (dump_file,
> +                              "PHI node NOT moved to %d [local count:%d] from "
> +                              "%d [local count:%d]:\n",
> +                              e->src->index, e->src->count.value (), bb->index,
> +                              bb->count.value ());
> +                     print_gimple_stmt (dump_file, stmt, 0);
> +                     fprintf (dump_file, "(cost %u) out of loop %d.\n\n", cost,
> +                              level->num);
> +                   }
> +                 skip_phi_move = true;
> +                 break;
> +               }
> +           }
> +         if (skip_phi_move)
> +           {
> +             gsi_next (&bsi);
> +             continue;
> +           }
> +       }
> +
>        if (dump_file && (dump_flags & TDF_DETAILS))
>         {
>           fprintf (dump_file, "Moving PHI node\n");
> @@ -1184,14 +1239,13 @@ move_computations_worker (basic_block bb)
>           tree lhs = gimple_assign_lhs (new_stmt);
>           SSA_NAME_RANGE_INFO (lhs) = NULL;
>         }
> -      gsi_insert_on_edge (loop_preheader_edge (level), new_stmt);
> +      gsi_insert_on_edge (e, new_stmt);
>        remove_phi_node (&bsi, false);
>      }
>
>    for (gimple_stmt_iterator bsi = gsi_start_bb (bb); !gsi_end_p (bsi); )
>      {
>        edge e;
> -
>        gimple *stmt = gsi_stmt (bsi);
>
>        lim_data = get_lim_data (stmt);
> @@ -1214,7 +1268,90 @@ move_computations_worker (basic_block bb)
>        /* We do not really want to move conditionals out of the loop; we just
>          placed it here to force its operands to be moved if necessary.  */
>        if (gimple_code (stmt) == GIMPLE_COND)
> -       continue;
> +       {
> +         gsi_next (&bsi);
> +         continue;
> +       }
> +
> +      e = loop_preheader_edge (level);
> +      if (e->src->count > bb->count)
> +       {
> +         if (dump_file && (dump_flags & TDF_DETAILS))
> +           {
> +             fprintf (dump_file,
> +                      "stmt: Statement NOT moved to %d [local count:%d] from "
> +                      "%d [local count:%d]:\n",
> +                      e->src->index, e->src->count.value (), bb->index,
> +                      bb->count.value ());
> +             print_gimple_stmt (dump_file, stmt, 0);
> +             fprintf (dump_file, "(cost %u) out of loop %d.\n\n", cost,
> +                      level->num);
> +           }
> +         gsi_next (&bsi);
> +         continue;
> +       }
> +      else
> +       {
> +         if (is_gimple_assign (stmt))
> +           {
> +             tree rhs1 = gimple_assign_rhs1 (stmt);
> +             tree rhs2 = gimple_assign_rhs2 (stmt);
> +             if (TREE_CODE (rhs1) == MEM_REF)
> +               {
> +                 rhs2 = TREE_OPERAND (rhs1, 1);
> +                 rhs1 = TREE_OPERAND (rhs1, 0);
> +               }
> +             gimple *stmt1 = NULL, *stmt2 = NULL;
> +             basic_block def_bb;
> +             if (rhs1 && TREE_CODE (rhs1) == SSA_NAME)
> +               {
> +                 stmt1 = SSA_NAME_DEF_STMT (rhs1);
> +                 def_bb = gimple_bb (stmt1);
> +                 if (stmt1
> +                     && def_bb
> +                     && (def_bb == bb
> +                         || !dominated_by_p (CDI_DOMINATORS, e->src, def_bb)))
> +                   {
> +                     if (dump_file && (dump_flags & TDF_DETAILS))
> +                       {
> +                         fprintf (dump_file,
> +                                  "stmt1: Statement NOT moved to %d [local "
> +                                  "count:%d] from %d [local count:%d]:\n",
> +                                  e->src->index, e->src->count.value (),
> +                                  bb->index, bb->count.value ());
> +                         print_gimple_stmt (dump_file, stmt, 0);
> +                         fprintf (dump_file, "(cost %u) out of loop %d.\n\n",
> +                                  cost, level->num);
> +                       }
> +                     gsi_next (&bsi);
> +                     continue;
> +                   }
> +               }
> +             if (rhs2 && TREE_CODE (rhs2) == SSA_NAME)
> +               {
> +                 stmt2 = SSA_NAME_DEF_STMT (rhs2);
> +                 def_bb = gimple_bb (stmt2);
> +                 if (stmt2 && def_bb
> +                     && (def_bb == bb
> +                         || !dominated_by_p (CDI_DOMINATORS, e->src, def_bb)))
> +                   {
> +                     if (dump_file && (dump_flags & TDF_DETAILS))
> +                       {
> +                         fprintf (dump_file,
> +                                  "stmt2: Statement NOT moved to %d [local "
> +                                  "count:%d] from %d [local count:%d]:\n",
> +                                  e->src->index, e->src->count.value (),
> +                                  bb->index, bb->count.value ());
> +                         print_gimple_stmt (dump_file, stmt, 0);
> +                         fprintf (dump_file, "(cost %u) out of loop %d.\n\n",
> +                                  cost, level->num);
> +                       }
> +                     gsi_next (&bsi);
> +                     continue;
> +                   }
> +               }
> +           }
> +       }
>
>        if (dump_file && (dump_flags & TDF_DETAILS))
>         {
> @@ -1224,7 +1361,6 @@ move_computations_worker (basic_block bb)
>                    cost, level->num);
>         }
>
> -      e = loop_preheader_edge (level);
>        gcc_assert (!gimple_vdef (stmt));
>        if (gimple_vuse (stmt))
>         {
> @@ -2094,6 +2230,19 @@ execute_sm (class loop *loop, im_mem_ref *ref,
>    bool multi_threaded_model_p = false;
>    gimple_stmt_iterator gsi;
>    sm_aux *aux = new sm_aux;
> +  basic_block bb = gimple_bb (first_mem_ref_loc (loop, ref)->stmt);
> +
> +  edge e = loop_preheader_edge (loop);
> +  if (e->src->count > bb->count)
> +    {
> +      if (dump_file && (dump_flags & TDF_DETAILS))
> +       {
> +         fprintf (dump_file, "Don't execute store motion of ");
> +         print_generic_expr (dump_file, ref->mem.ref);
> +         fprintf (dump_file, " from loop %d\n", loop->num);
> +       }
> +      return;
> +    }
>
>    if (dump_file && (dump_flags & TDF_DETAILS))
>      {
> @@ -2202,7 +2351,12 @@ execute_sm_exit (class loop *loop, edge ex, vec<seq_entry> &seq,
>         }
>        else
>         {
> -         sm_aux *aux = *aux_map.get (ref);
> +         sm_aux **paux = aux_map.get (ref);
> +         sm_aux *aux;
> +         if (paux)
> +           aux = *paux;
> +         else
> +           continue;
>           if (!aux->store_flag || kind == sm_ord)
>             {
>               gassign *store;
> diff --git a/gcc/tree-ssa-loop-split.c b/gcc/tree-ssa-loop-split.c
> index 3a09bbc39e5..4cae82936b9 100644
> --- a/gcc/tree-ssa-loop-split.c
> +++ b/gcc/tree-ssa-loop-split.c
> @@ -577,14 +577,17 @@ split_loop (class loop *loop1)
>         if (!initial_true)
>           cond = fold_build1 (TRUTH_NOT_EXPR, boolean_type_node, cond);
>
> +       edge true_edge = EDGE_SUCC (bbs[i], 0)->flags & EDGE_TRUE_VALUE
> +                          ? EDGE_SUCC (bbs[i], 0)
> +                          : EDGE_SUCC (bbs[i], 1);
>         /* Now version the loop, placing loop2 after loop1 connecting
>            them, and fix up SSA form for that.  */
>         initialize_original_copy_tables ();
>         basic_block cond_bb;
>
>         class loop *loop2 = loop_version (loop1, cond, &cond_bb,
> -                                          profile_probability::always (),
> -                                          profile_probability::always (),
> +                                          true_edge->probability,
> +                                          true_edge->probability.invert (),
>                                            profile_probability::always (),
>                                            profile_probability::always (),
>                                            true);
> @@ -1486,8 +1489,8 @@ do_split_loop_on_cond (struct loop *loop1, edge invar_branch)
>    initialize_original_copy_tables ();
>
>    struct loop *loop2 = loop_version (loop1, boolean_true_node, NULL,
> -                                    profile_probability::always (),
> -                                    profile_probability::never (),
> +                                    invar_branch->probability,
> +                                    invar_branch->probability.invert (),
>                                      profile_probability::always (),
>                                      profile_probability::always (),
>                                      true);
> @@ -1530,6 +1533,9 @@ do_split_loop_on_cond (struct loop *loop1, edge invar_branch)
>    to_loop1->flags |= true_invar ? EDGE_FALSE_VALUE : EDGE_TRUE_VALUE;
>    to_loop2->flags |= true_invar ? EDGE_TRUE_VALUE : EDGE_FALSE_VALUE;
>
> +  to_loop1->probability = invar_branch->probability.invert ();
> +  to_loop2->probability = invar_branch->probability;
> +
>    /* Due to introduction of a control flow edge from loop1 latch to loop2
>       pre-header, we should update PHIs in loop2 to reflect this connection
>       between loop1 and loop2.  */
> --
> 2.27.0.90.geebb51ba8c
>


More information about the Gcc-patches mailing list