Re: [PATCH v1] tree-ssa-sink: Improve code sinking pass.

[Ajit Agarwal <aagarwa1@linux.ibm.com>]

Hello Richard:

On 30/05/23 12:34 pm, Richard Biener wrote:
> On Tue, May 30, 2023 at 7:06 AM Ajit Agarwal <aagarwa1@linux.ibm.com> wrote:
>>
>> Hello Richard:
>>
>> On 22/05/23 6:26 pm, Richard Biener wrote:
>>> On Thu, May 18, 2023 at 9:14 AM Ajit Agarwal <aagarwa1@linux.ibm.com> wrote:
>>>>
>>>> Hello All:
>>>>
>>>> This patch improves code sinking pass to sink statements before call to reduce
>>>> register pressure.
>>>> Review comments are incorporated.
>>>>
>>>> Bootstrapped and regtested on powerpc64-linux-gnu.
>>>>
>>>> Thanks & Regards
>>>> Ajit
>>>>
>>>>
>>>> tree-ssa-sink: Improve code sinking pass.
>>>>
>>>> Code Sinking sinks the blocks after call. This increases
>>>> register pressure for callee-saved registers. Improves
>>>> code sinking before call in the use blocks or immediate
>>>> dominator of use blocks.
>>>>
>>>> 2023-05-18  Ajit Kumar Agarwal  <aagarwa1@linux.ibm.com>
>>>>
>>>> gcc/ChangeLog:
>>>>
>>>>         * tree-ssa-sink.cc (statement_sink_location): Modifed to
>>>>         move statements before calls.
>>>>         (block_call_p): New function.
>>>>         (def_use_same_block): New function.
>>>>         (select_best_block): Add heuristics to select the best
>>>>         blocks in the immediate post dominator.
>>>>
>>>> gcc/testsuite/ChangeLog:
>>>>
>>>>         * 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 |  16 ++
>>>>  gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c |  20 +++
>>>>  gcc/tree-ssa-sink.cc                        | 159 ++++++++++++++++++--
>>>>  3 files changed, 185 insertions(+), 10 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..716bc1f9257
>>>> --- /dev/null
>>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-20.c
>>>> @@ -0,0 +1,16 @@
>>>> +/* { dg-do compile } */
>>>> +/* { dg-options "-O2 -fdump-tree-sink -fdump-tree-optimized -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-times "Sunk statements: 5" 1 "sink" } } */
>>>
>>> this doesn't verify the place we sink to?
>>>
>>
>> I am not sure how to verify the place we sink to with dg-final.
> 
> I think dejagnu supports matching multi-line regexps so I suggest
> to scan for the sunk expr RHS to be followed by the call?
> 

You meant to use dg-begin-multiline-output and dg-end-multiline-output.

Thanks & Regards
Ajit
>>>> 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..ff41e2ea8ae
>>>> --- /dev/null
>>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-sink-21.c
>>>> @@ -0,0 +1,20 @@
>>>> +/* { dg-do compile } */
>>>> +/* { dg-options "-O2 -fdump-tree-sink-stats -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-times "Sunk statements: 5" 1 "sink" } } */
>>>
>>> likewise.  So both tests already pass before the patch?
>>>
>>>> diff --git a/gcc/tree-ssa-sink.cc b/gcc/tree-ssa-sink.cc
>>>> index 87b1d40c174..76556e7795b 100644
>>>> --- a/gcc/tree-ssa-sink.cc
>>>> +++ b/gcc/tree-ssa-sink.cc
>>>> @@ -171,6 +171,72 @@ nearest_common_dominator_of_uses (def_operand_p def_p, bool *debug_stmts)
>>>>    return commondom;
>>>>  }
>>>>
>>>> +/* Return TRUE if immediate uses of the defs in
>>>> +   USE occur in the same block as USE, FALSE otherwise.  */
>>>> +
>>>> +bool
>>>> +def_use_same_block (gimple *stmt)
>>>> +{
>>>> +  use_operand_p use_p;
>>>> +  def_operand_p def_p;
>>>> +  imm_use_iterator imm_iter;
>>>> +  ssa_op_iter iter;
>>>> +
>>>> +  FOR_EACH_SSA_DEF_OPERAND (def_p, stmt, iter, SSA_OP_DEF)
>>>> +    {
>>>> +      FOR_EACH_IMM_USE_FAST (use_p, imm_iter, DEF_FROM_PTR (def_p))
>>>> +       {
>>>> +         if (is_gimple_debug (USE_STMT (use_p)))
>>>> +           continue;
>>>> +
>>>> +         if (use_p
>>>
>>> use_p is never null
>>>
>>>> +             && (gimple_bb (USE_STMT (use_p)) == gimple_bb (stmt)))
>>>> +           return true;
>>>
>>> the function behavior is obviously odd ...
>>>
>>>> +       }
>>>> +     }
>>>> +  return false;
>>>> +}
>>>> +
>>>> +/* Return TRUE if the block has only calls, FALSE otherwise. */
>>>> +
>>>> +bool
>>>> +block_call_p (basic_block bb)
>>>> +{
>>>> +  int i = 0;
>>>> +  bool is_call = false;
>>>> +  gimple_stmt_iterator gsi = gsi_last_bb (bb);
>>>> +  gimple *last_stmt = gsi_stmt (gsi);
>>>> +
>>>> +  if (last_stmt && gimple_code (last_stmt) == GIMPLE_COND)
>>>> +    {
>>>> +      if (!gsi_end_p (gsi))
>>>> +       gsi_prev (&gsi);
>>>> +
>>>> +       for (; !gsi_end_p (gsi);)
>>>> +        {
>>>> +          gimple *stmt = gsi_stmt (gsi);
>>>> +
>>>> +          /* We have already seen a call.  */
>>>> +          if (is_call)
>>>> +            return false;
>>>
>>> Likewise.  Do you want to check whether a block has
>>> a single stmt and that is a call and that is followed by
>>> a condition?  It looks like a very convoluted way to write this.
>>>
>>>> +
>>>> +          if (is_gimple_call (stmt))
>>>> +            is_call = true;
>>>> +          else
>>>> +            return false;
>>>> +
>>>> +          if (!gsi_end_p (gsi))
>>>> +            gsi_prev (&gsi);
>>>> +
>>>> +           ++i;
>>>> +       }
>>>> +     }
>>>> +  if (is_call && i == 1)
>>>> +    return true;
>>>> +
>>>> +  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.
>>>> @@ -190,7 +256,8 @@ 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)
>>>
>>> please update the function comment
>>>
>>>>  {
>>>>    basic_block best_bb = late_bb;
>>>>    basic_block temp_bb = late_bb;
>>>> @@ -230,14 +297,47 @@ select_best_block (basic_block early_bb,
>>>>        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)
>>>>        /* If result of comparsion is unknown, prefer EARLY_BB.
>>>>          Thus use !(...>=..) rather than (...<...)  */
>>>>        && !(best_bb->count * 100 >= early_bb->count * threshold))
>>>> -    return best_bb;
>>>> +    {
>>>> +      basic_block new_best_bb = get_immediate_dominator (CDI_DOMINATORS, best_bb);
>>>> +      /* Return best_bb if def and use are in same block otherwise new_best_bb.
>>>> +
>>>> +        Things to consider:
>>>> +
>>>> +          new_best_bb is not equal to best_bb and early_bb.
>>>> +
>>>> +          stmt is not call.
>>>> +
>>>> +          new_best_bb doesnt have any phis.
>>>> +
>>>> +          use basic block is not equal to early_bb.
>>>> +
>>>> +          use basic block post dominates to new_best_bb.
>>>> +
>>>> +          new_best_bb dominates early_bb. */
>>>> +      if (new_best_bb && use
>>>> +         && (new_best_bb != best_bb)
>>>> +         && (new_best_bb != early_bb)
>>>> +         && !is_gimple_call (stmt)
>>>> +         && gsi_end_p (gsi_start_phis (new_best_bb))
>>>> +         && (gimple_bb (use) != early_bb)
>>>> +         && !is_gimple_call (use)
>>>> +         && dominated_by_p (CDI_POST_DOMINATORS, new_best_bb, gimple_bb(use))
>>>> +         && dominated_by_p (CDI_DOMINATORS, new_best_bb, early_bb)
>>>> +         && block_call_p (new_best_bb))
>>>> +       {
>>>> +         if (def_use_same_block (use))
>>>> +           return best_bb;
>>>
>>> given the odd implementation of the predicates this matches very very
>>> specific cases.
>>>
>>> Consider
>>>
>>>  if (..)
>>>   {
>>>     foo();
>>>     bar();
>>>     ... = l;
>>>   }
>>>
>>> and C++ where foo and bar might throw.  You then likely want to sink
>>> before foo ().
>>>
>>> What's the reason to only consider blocks with exactly 'call; cond;' ?
>>>
>>>> +
>>>> +         return new_best_bb;
>>>> +       }
>>>> +       return best_bb;
>>>> +    }
>>>>
>>>>    /* No better block found, so return EARLY_BB, which happens to be the
>>>>       statement's original block.  */
>>>> @@ -439,7 +539,7 @@ 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 +556,58 @@ 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);
>>>> +          gimple *def_stmt = SSA_NAME_DEF_STMT (DEF_FROM_PTR (def_p));
>>>> +
>>>> +          if ((gimple_bb (def_stmt) == gimple_bb (use))
>>>> +               && (gimple_bb (use) != sinkbb))
>>>> +            sinkbb = gimple_bb (use);
>>>> +
>>>> +           if (sinkbb == gimple_bb (use))
>>>> +             {
>>>> +               gimple_stmt_iterator gsi = gsi_last_bb (sinkbb);
>>>> +               gimple *def_stmt = SSA_NAME_DEF_STMT (DEF_FROM_PTR (def_p));
>>>> +               gimple *last_stmt = gsi_stmt (gsi);
>>>> +
>>>> +               /* Update sinking point as stmt before call if the sinking block
>>>> +                  has only calls. Otherwise update sinking point as the use
>>>> +                  stmt. */
>>>> +               if (gsi_stmt (gsi) == use
>>>> +                   && !is_gimple_call (last_stmt)
>>>> +                   && (gimple_code (last_stmt) != GIMPLE_SWITCH)
>>>> +                   && (gimple_code (last_stmt) != GIMPLE_COND)
>>>> +                   && (gimple_code (last_stmt) != GIMPLE_GOTO)
>>>> +                   && (!gimple_vdef (use) || !def_use_same_block (def_stmt)))
>>>> +                 {
>>>> +                   if (!gsi_end_p (gsi))
>>>> +                     gsi_prev (&gsi);
>>>> +
>>>> +                   gimple *stmt = gsi_stmt (gsi);
>>>> +
>>>> +                   if (!gsi_end_p (gsi))
>>>> +                     gsi_prev (&gsi);
>>>> +
>>>> +                   if (gsi_end_p (gsi) && stmt && is_gimple_call (stmt)
>>>> +                       && gsi_end_p (gsi_start_phis (sinkbb))
>>>> +                       && !is_gimple_call (def_stmt))
>>>> +                     *togsi = gsi_for_stmt (stmt);
>>>> +                   else
>>>> +                     *togsi = gsi_for_stmt (use);
>>>> +                  }
>>>> +               else
>>>> +                 *togsi = gsi_for_stmt(use);
>>>> +              }
>>>> +            else
>>>> +               *togsi = gsi_after_labels (sinkbb);
>>>
>>> This is very convoluted.  I think that in the end you want to compute (once) the
>>> position of the first call in each block.  Since we're waking the CFG backwards
>>> in post-dominator order this information can be gathered during this walk.
>>> This would determine the location to sink to iff the use stmt is dominated by
>>> this location (you can for example use gimple_uid to mark stmts before it).
>>>
>>> The alternative is to simply always sink to the start of blocks even for the
>>> use stmt block in case that has a call before the use (but you still need to
>>> efficiently compute that).
>>>
>>
>> Incorporated the above comments and sent a separate patch.
>>
>> Thanks & Regards
>> Ajit
>>
>>> Richard.
>>>
>>>>
>>>>           return true;
>>>>         }
>>>> @@ -480,7 +619,7 @@ 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;
>>>>
>>>> --
>>>> 2.31.1
>>>>