Re: [PATCH] tree-optimization: Add register pressure heuristics

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Nov 2, 2023 at 9:50 PM Ajit Agarwal <aagarwa1@linux.ibm.com> wrote:
>
> Hello All:
>
> Currently code sinking heuristics are based on profile data like
> basic block count and sink frequency threshold. We have removed
> such heuristics and added register pressure heuristics based on
> live-in and live-out of early blocks and immediate dominator of
> use blocks of the same loop nesting depth.
>
> Such heuristics reduces register pressure when code sinking is
> done with same loop nesting depth.
>
> High register pressure region is the region where there are live-in of
> early blocks that has been modified by the early block. If there are
> modification of the variables in best block that are live-in in early
> block that are live-out of best block.

?!  Parse error.

> Bootstrapped and regtested on powerpc64-linux-gnu.

What's the effect on code generation?

Note that live is a quadratic problem while sinking was not.  You
are effectively making the pass unfit for -O1.

You are computing "liveness" on GIMPLE where within EBBs there
isn't really any particular order of stmts, so it's kind of a garbage
heuristic.  Likewise you are not computing the effect that moving
a stmt has on liveness as far as I can see but you are just identifying
some odd metrics (I don't really understand them) to rank blocks,
not even taking the register file size into account.

You are replacing the hot/cold heuristic.

IMHO the sinking pass is the totally wrong place to do anything
about register pressure.  You are trying to solve a scheduling
problem by just looking at a single stmt.

Richard.

> Thanks & Regards
> Ajit
>
> tree-optimization: Add register pressure heuristics
>
> Currently code sinking heuristics are based on profile data like
> basic block count and sink frequency threshold. We have removed
> such heuristics to add register pressure heuristics based on
> live-in and live-out of early blocks and immediate dominator of
> use blocks.
>
> High register pressure region is the region where there are live-in of
> early blocks that has been modified by the early block. If there are
> modification of the variables in best block that are live-in in early
> block that are live-out of best block.
>
> 2023-11-03  Ajit Kumar Agarwal  <aagarwa1@linux.ibm.com>
>
> gcc/ChangeLog:
>
>         * tree-ssa-sink.cc (statement_sink_location): Add tree_live_info_p
>         as paramters.
>         (sink_code_in_bb): Ditto.
>         (select_best_block): Add register pressure heuristics to select
>         the best blocks in the immediate dominator for same loop nest depth.
>         (execute): Add live range analysis.
>         (additional_var_map): New function.
>         * tree-ssa-live.cc (set_var_live_on_entry): Add virtual operand
>         tests on ssa_names.
>         (verify_live_on_entry): Ditto.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/tree-ssa/ssa-sink-21.c: New test.
>         * gcc.dg/tree-ssa/ssa-sink-22.c: New test.
> ---
>  gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c | 15 ++++
>  gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-22.c | 19 +++++
>  gcc/tree-ssa-live.cc                        | 11 ++-
>  gcc/tree-ssa-sink.cc                        | 93 ++++++++++++++-------
>  4 files changed, 104 insertions(+), 34 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-22.c
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c
> new file mode 100644
> index 00000000000..d3b79ca5803
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c
> @@ -0,0 +1,15 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-sink-stats" } */
> +void bar();
> +int j;
> +void foo(int a, int b, int c, int d, int e, int f)
> +{
> +  int l;
> +  l = a + b + c + d +e + f;
> +  if (a != 5)
> +    {
> +      bar();
> +      j = l;
> +    }
> +}
> +/* { dg-final { scan-tree-dump {l_12\s+=\s+_4\s+\+\s+f_11\(D\);\n\s+bar\s+\(\)} sink1 } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-22.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-22.c
> new file mode 100644
> index 00000000000..84e7938c54f
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-22.c
> @@ -0,0 +1,19 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-sink-stats" } */
> +void bar();
> +int j, x;
> +void foo(int a, int b, int c, int d, int e, int f)
> +{
> +  int l;
> +  l = a + b + c + d +e + f;
> +  if (a != 5)
> +    {
> +      bar();
> +      if (b != 3)
> +        x = 3;
> +      else
> +        x = 5;
> +      j = l;
> +    }
> +}
> +/* { dg-final { scan-tree-dump {l_13\s+=\s+_4\s+\+\s+f_12\(D\);\n\s+bar\s+\(\)} sink1 } } */
> diff --git a/gcc/tree-ssa-live.cc b/gcc/tree-ssa-live.cc
> index f06daf23035..998fe588278 100644
> --- a/gcc/tree-ssa-live.cc
> +++ b/gcc/tree-ssa-live.cc
> @@ -1141,7 +1141,8 @@ set_var_live_on_entry (tree ssa_name, tree_live_info_p live)
>      def_bb = ENTRY_BLOCK_PTR_FOR_FN (cfun);
>
>    /* An undefined local variable does not need to be very alive.  */
> -  if (ssa_undefined_value_p (ssa_name, false))
> +  if (virtual_operand_p (ssa_name)
> +      || ssa_undefined_value_p (ssa_name, false))
>      return;
>
>    /* Visit each use of SSA_NAME and if it isn't in the same block as the def,
> @@ -1540,7 +1541,6 @@ debug (tree_live_info_d *ptr)
>
>
>  /* Verify that the info in LIVE matches the current cfg.  */
> -
>  static void
>  verify_live_on_entry (tree_live_info_p live)
>  {
> @@ -1569,11 +1569,13 @@ verify_live_on_entry (tree_live_info_p live)
>           tree d = NULL_TREE;
>           bitmap loe;
>           var = partition_to_var (map, i);
> +         if (var == NULL_TREE)
> +           continue;
>           stmt = SSA_NAME_DEF_STMT (var);
>           tmp = gimple_bb (stmt);
> +
>           if (SSA_NAME_VAR (var))
>             d = ssa_default_def (cfun, SSA_NAME_VAR (var));
> -
>           loe = live_on_entry (live, e->dest);
>           if (loe && bitmap_bit_p (loe, i))
>             {
> @@ -1614,7 +1616,8 @@ verify_live_on_entry (tree_live_info_p live)
>               {
>                 /* An undefined local variable does not need to be very
>                    alive.  */
> -               if (ssa_undefined_value_p (var, false))
> +               if (virtual_operand_p (var)
> +                   || ssa_undefined_value_p (var, false))
>                   continue;
>
>                 /* The only way this var shouldn't be marked live on entry is
> diff --git a/gcc/tree-ssa-sink.cc b/gcc/tree-ssa-sink.cc
> index a360c5cdd6e..d0c9ef1ab86 100644
> --- a/gcc/tree-ssa-sink.cc
> +++ b/gcc/tree-ssa-sink.cc
> @@ -176,6 +176,9 @@ nearest_common_dominator_of_uses (def_operand_p def_p, bool *debug_stmts)
>     tree, return the best basic block between them (inclusive) to place
>     statements.
>
> +   The best basic block should be an immediate dominator of
> +   best basic block if we've moved to same loop nest.
> +
>     We want the most control dependent block in the shallowest loop nest.
>
>     If the resulting block is in a shallower loop nest, then use it.  Else
> @@ -191,24 +194,23 @@ nearest_common_dominator_of_uses (def_operand_p def_p, bool *debug_stmts)
>  static basic_block
>  select_best_block (basic_block early_bb,
>                    basic_block late_bb,
> -                  gimple *stmt)
> +                  gimple *stmt,
> +                  tree_live_info_p &live)
>  {
>    basic_block best_bb = late_bb;
>    basic_block temp_bb = late_bb;
> -  int threshold;
>
>    while (temp_bb != early_bb)
>      {
>        /* If we've moved into a lower loop nest, then that becomes
>          our best block.  */
> -      if (bb_loop_depth (temp_bb) < bb_loop_depth (best_bb))
> +      if (bb_loop_depth (temp_bb) <= bb_loop_depth (best_bb))
>         best_bb = temp_bb;
>
>        /* Walk up the dominator tree, hopefully we'll find a shallower
>          loop nest.  */
>        temp_bb = get_immediate_dominator (CDI_DOMINATORS, temp_bb);
>      }
> -
>    /* Placing a statement before a setjmp-like function would be invalid
>       (it cannot be reevaluated when execution follows an abnormal edge).
>       If we selected a block with abnormal predecessors, just punt.  */
> @@ -233,24 +235,36 @@ select_best_block (basic_block early_bb,
>        && !dominated_by_p (CDI_DOMINATORS, best_bb->loop_father->latch, best_bb))
>      return early_bb;
>
> -  /* Get the sinking threshold.  If the statement to be moved has memory
> -     operands, then increase the threshold by 7% as those are even more
> -     profitable to avoid, clamping at 100%.  */
> -  threshold = param_sink_frequency_threshold;
> -  if (gimple_vuse (stmt) || gimple_vdef (stmt))
> -    {
> -      threshold += 7;
> -      if (threshold > 100)
> -       threshold = 100;
> -    }
> +  int best_bb_liveout_cnt
> +    = bitmap_count_bits (&live->liveout[best_bb->index]);
> +  int early_bb_liveout_cnt
> +     = bitmap_count_bits (&live->liveout[early_bb->index]);
> +  int early_livein_cnt
> +     = bitmap_count_bits (&live->livein[early_bb->index]);
> +
> +  /* High register pressure region is the region where there are live-in of
> +     early blocks that has been modified by the early block. If there are
> +     modification of the variables in best block that are live-in in early
> +     block that are live-out of best block.  */
> +  bool live_in_rgn = (early_livein_cnt != 0
> +                     && early_bb_liveout_cnt <= early_livein_cnt);
> +
> +  bool high_reg_pressure_rgn
> +    = ((best_bb_liveout_cnt != 0 || live_in_rgn)
> +       && best_bb_liveout_cnt <= early_bb_liveout_cnt);
>
>    /* If BEST_BB is at the same nesting level, then require it to have
> -     significantly lower execution frequency to avoid gratuitous movement.  */
> +     significantly high register pressure region to avoid gratuitous
> +      movement.  */
>    if (bb_loop_depth (best_bb) == bb_loop_depth (early_bb)
> -      /* If result of comparsion is unknown, prefer EARLY_BB.
> -        Thus use !(...>=..) rather than (...<...)  */
> -      && !(best_bb->count * 100 >= early_bb->count * threshold))
> -    return best_bb;
> +     && high_reg_pressure_rgn)
> +    {
> +     /* Avoid sinking to immediate dominator if the statement to be moved
> +       has memory operand and same loop nest.  */
> +      if (best_bb != late_bb && gimple_vuse (stmt))
> +       return late_bb;
> +      return best_bb;
> +    }
>
>    /* No better block found, so return EARLY_BB, which happens to be the
>       statement's original block.  */
> @@ -265,7 +279,8 @@ select_best_block (basic_block early_bb,
>  static bool
>  statement_sink_location (gimple *stmt, basic_block frombb,
>                          gimple_stmt_iterator *togsi, bool *zero_uses_p,
> -                        virtual_operand_live &vop_live)
> +                        virtual_operand_live &vop_live,
> +                        tree_live_info_p &live)
>  {
>    gimple *use;
>    use_operand_p one_use = NULL_USE_OPERAND_P;
> @@ -413,7 +428,7 @@ statement_sink_location (gimple *stmt, basic_block frombb,
>        if (!dominated_by_p (CDI_DOMINATORS, commondom, frombb))
>         return false;
>
> -      commondom = select_best_block (frombb, commondom, stmt);
> +      commondom = select_best_block (frombb, commondom, stmt, live);
>
>        if (commondom == frombb)
>         return false;
> @@ -430,19 +445,17 @@ statement_sink_location (gimple *stmt, basic_block frombb,
>             continue;
>           break;
>         }
> +
>        use = USE_STMT (one_use);
>
>        if (gimple_code (use) != GIMPLE_PHI)
>         {
> -         sinkbb = select_best_block (frombb, gimple_bb (use), stmt);
> +         sinkbb = select_best_block (frombb, gimple_bb (use), stmt, live);
>
>           if (sinkbb == frombb)
>             return false;
>
> -         if (sinkbb == gimple_bb (use))
> -           *togsi = gsi_for_stmt (use);
> -         else
> -           *togsi = gsi_after_labels (sinkbb);
> +         *togsi = gsi_after_labels (sinkbb);
>
>           return true;
>         }
> @@ -454,7 +467,7 @@ statement_sink_location (gimple *stmt, basic_block frombb,
>    if (!sinkbb)
>      return false;
>
> -  sinkbb = select_best_block (frombb, sinkbb, stmt);
> +  sinkbb = select_best_block (frombb, sinkbb, stmt, live);
>    if (!sinkbb || sinkbb == frombb)
>      return false;
>
> @@ -643,7 +656,8 @@ sink_common_stores_to_bb (basic_block bb)
>  /* Perform code sinking on BB */
>
>  static unsigned
> -sink_code_in_bb (basic_block bb, virtual_operand_live &vop_live)
> +sink_code_in_bb (basic_block bb, virtual_operand_live &vop_live,
> +               tree_live_info_p &live)
>  {
>    gimple_stmt_iterator gsi;
>    edge_iterator ei;
> @@ -670,7 +684,8 @@ sink_code_in_bb (basic_block bb, virtual_operand_live &vop_live)
>        gimple_stmt_iterator togsi;
>        bool zero_uses_p;
>
> -      if (!statement_sink_location (stmt, bb, &togsi, &zero_uses_p, vop_live))
> +      if (!statement_sink_location (stmt, bb, &togsi, &zero_uses_p,
> +                                   vop_live, live))
>         {
>           gimple_stmt_iterator saved = gsi;
>           if (!gsi_end_p (gsi))
> @@ -814,6 +829,15 @@ private:
>    bool unsplit_edges;
>  }; // class pass_sink_code
>
> +void additional_var_map (var_map &map)
> +{
> +  map->bmp_bbs = BITMAP_ALLOC (NULL);
> +  map->outofssa_p = false;
> +  basic_block bb;
> +  FOR_EACH_BB_FN (bb, cfun)
> +    bitmap_set_bit (map->bmp_bbs, bb->index);
> +}
> +
>  unsigned int
>  pass_sink_code::execute (function *fun)
>  {
> @@ -827,12 +851,21 @@ pass_sink_code::execute (function *fun)
>    calculate_dominance_info (CDI_DOMINATORS);
>
>    virtual_operand_live vop_live;
> +  var_map map = init_var_map (num_ssa_names);
> +  additional_var_map (map);
> +  tree_live_info_p live = calculate_live_ranges (map, true);
>
>    int *rpo = XNEWVEC (int, n_basic_blocks_for_fn (cfun));
>    int n = inverted_rev_post_order_compute (fun, rpo);
> +
>    for (int i = 0; i < n; ++i)
> -    todo |= sink_code_in_bb (BASIC_BLOCK_FOR_FN (fun, rpo[i]), vop_live);
> +    todo |= sink_code_in_bb (BASIC_BLOCK_FOR_FN (fun, rpo[i]), vop_live, live);
>    free (rpo);
> +  if (live)
> +    delete_tree_live_info (live);
> +
> +  if (map)
> +    delete_var_map (map);
>
>    statistics_counter_event (fun, "Sunk statements", sink_stats.sunk);
>    statistics_counter_event (fun, "Commoned stores", sink_stats.commoned);
> --
> 2.39.3
>
>