Re: PING^1 [PATCH v7] tree-ssa-sink: Improve code sinking pass

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

On Tue, 18 Jul 2023 at 13:26, Ajit Agarwal via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
>
> Ping!
>
> please review.
>
> Thanks & Regards
> Ajit
>
>
> This patch improves code sinking pass to sink statements before call to reduce
> register pressure.
> Review comments are incorporated.
>
> For example :
>
> 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;
>     }
> }
>
> Code Sinking does the following:
>
> void bar();
> int j;
> void foo(int a, int b, int c, int d, int e, int f)
> {
>   int l;
>
>   if (a != 5)
>     {
>       l = a + b + c + d +e + f;
>       bar();
>       j = l;
>     }
> }
>
> Bootstrapped regtested on powerpc64-linux-gnu.
>
> Thanks & Regards
> Ajit
>
>
> tree-ssa-sink: Improve code sinking pass
>
> Currently, code sinking will sink code after function calls.  This increases
> register pressure for callee-saved registers.  The following patch improves
> code sinking by placing the sunk code before calls in the use block or in
> the immediate dominator of the use blocks.
>
> 2023-06-01  Ajit Kumar Agarwal  <aagarwa1@linux.ibm.com>
>
> gcc/ChangeLog:
>
>         PR tree-optimization/81953
>         * tree-ssa-sink.cc (statement_sink_location): Move statements before
>         calls.
>         (def_use_same_block): New function.
>         (select_best_block): Add heuristics to select the best blocks in the
>         immediate post dominator.
>
> gcc/testsuite/ChangeLog:
>
>         PR tree-optimization/81953
>         * gcc.dg/tree-ssa/ssa-sink-20.c: New testcase.
>         * gcc.dg/tree-ssa/ssa-sink-21.c: New testcase.
> ---
>  gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.c | 15 ++++
>  gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c | 19 +++++
>  gcc/tree-ssa-sink.cc                        | 79 ++++++++++++++-------
>  3 files changed, 87 insertions(+), 26 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.c
> new file mode 100644
> index 00000000000..d3b79ca5803
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.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-21.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c
> new file mode 100644
> index 00000000000..84e7938c54f
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.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-sink.cc b/gcc/tree-ssa-sink.cc
> index b1ba7a2ad6c..113c89d0967 100644
> --- a/gcc/tree-ssa-sink.cc
> +++ b/gcc/tree-ssa-sink.cc
> @@ -171,9 +171,28 @@ nearest_common_dominator_of_uses (def_operand_p def_p, bool *debug_stmts)
>    return commondom;
>  }
>
> +/* Return TRUE if immediate defs of STMT and STMT are in same
> + * block, FALSE otherwise.  */
> +
> +static bool
> +def_use_same_block (gimple *stmt)
> +{
> +  def_operand_p def;
> +  ssa_op_iter iter;
> +
> +  FOR_EACH_SSA_DEF_OPERAND (def, stmt, iter, SSA_OP_DEF)
> +    {
> +      gimple *def_stmt = SSA_NAME_DEF_STMT (DEF_FROM_PTR (def));
> +      if ((gimple_bb (def_stmt) == gimple_bb (stmt)))
> +       return true;
Hi Ajit,
Just wondering, won't this always return true since you're iterating over defs,
and def_stmt == stmt ? Sorry, if I misunderstood.

Thanks,
Prathamesh
> +     }
> +  return false;
> +}
> +
>  /* Given EARLY_BB and LATE_BB, two blocks in a path through the dominator
>     tree, return the best basic block between them (inclusive) to place
> -   statements.
> +   statements. The best basic block should be an immediate dominator of
> +   best basic block if the use stmt is after the call.
>
>     We want the most control dependent block in the shallowest loop nest.
>
> @@ -190,11 +209,22 @@ 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,
> +                  gimple *use)
>  {
>    basic_block best_bb = late_bb;
>    basic_block temp_bb = late_bb;
>    int threshold;
> +  /* 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;
> +    }
>
>    while (temp_bb != early_bb)
>      {
> @@ -203,34 +233,33 @@ select_best_block (basic_block early_bb,
>        if (bb_loop_depth (temp_bb) < bb_loop_depth (best_bb))
>         best_bb = 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.  */
> +      if (bb_has_abnormal_pred (temp_bb))
> +       return early_bb;
> +
> +      /* if we have temp_bb post dominated by use block and def
> +        and use are not in same block then immediate dominator
> +        would be our best block.  */
> +      if (use
> +         && bb_loop_depth(temp_bb) == bb_loop_depth (early_bb)
> +         && !(temp_bb->count * 100 >= early_bb->count * threshold)
> +         && dominated_by_p (CDI_POST_DOMINATORS, temp_bb, gimple_bb (use))
> +         && !def_use_same_block (use))
> +       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.  */
> -  if (bb_has_abnormal_pred (best_bb))
> -    return early_bb;
> -
>    /* If we found a shallower loop nest, then we always consider that
>       a win.  This will always give us the most control dependent block
>       within that loop nest.  */
>    if (bb_loop_depth (best_bb) < bb_loop_depth (early_bb))
>      return best_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;
> -    }
> -
>    /* If BEST_BB is at the same nesting level, then require it to have
>       significantly lower execution frequency to avoid gratuitous movement.  */
>    if (bb_loop_depth (best_bb) == bb_loop_depth (early_bb)
> @@ -439,7 +468,7 @@ 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, NULL);
>
>        if (commondom == frombb)
>         return false;
> @@ -456,19 +485,17 @@ 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, use);
>
>           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;
>         }
> @@ -480,7 +507,7 @@ 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, NULL);
>    if (!sinkbb || sinkbb == frombb)
>      return false;
>
> --
> 2.39.3
>