Re: [PATCH 2/3] Make early return predictor more precise.

[Christophe Lyon <christophe dot lyon at linaro dot org>]

Hi,


On 21 June 2017 at 14:50, Martin Liška <mliska@suse.cz> wrote:
> On 06/21/2017 10:26 AM, Jan Hubicka wrote:
>>>
>>> Ok, so I fixed that in the described way. There's one remaining fallout of:
>>> gcc/testsuite/gcc.dg/tree-ssa/ipa-split-5.c
>>>
>>> Where a fnsplit is properly done, but then it's again inlined:
>>>
>>> Considering split_me.part.0/5 with 23 size
>>>  to be inlined into test/2 in unknown:0
>>>  Estimated badness is -0.000001, frequency 0.33.
>>>  Inlined split_me.part.0 into test which now has time 50.300000 and size 44, net change of +17.
>>>
>>> Considering split_me.part.0/5 with 23 size
>>>  to be inlined into test/2 in unknown:0
>>>  Estimated badness is -0.000001, frequency 0.33.
>>>  Inlined split_me.part.0 into test which now has time 70.760000 and size 61, net change of +17.
>>>
>>> Considering split_me.part.0/5 with 23 size
>>>  to be inlined into test/2 in unknown:0
>>>  Estimated badness is -0.000001, frequency 0.33.
>>>  Inlined split_me.part.0 into test which now has time 91.220000 and size 78, net change of +17.
>>>
>>> Considering split_me.part.0/5 with 23 size
>>>  to be inlined into test/2 in unknown:0
>>>  Estimated badness is -0.000001, frequency 0.33.
>>>  Inlined split_me.part.0 into test which now has time 111.680000 and size 95, net change of +17.
>>> Unit growth for small function inlining: 61->129 (111%)
>>>
>>> ...
>>>
>>> Any hint how to block the IPA inlining?
>>
>> I guess you only want to make cold part of split_me bigger or
>> just use --param to reduce growth for auto inlining.
>>
>> How the patch reduces split_me_part considerably?
>> Honza
>
> Well, I probably overlooked test results, test works fine.
>
> I'm going to install the patch.
>

Since this commit (r249450), I have noticed a regression:
FAIL:    gcc.dg/tree-ssa/ipa-split-5.c scan-tree-dump optimized "part"
on aarch64/arm

Christophe


> Martin
>
>>>
>>> Sending new version of patch.
>>> Martin
>>>
>>>>
>>>> I would just move pass_strip_predict_hints pre-IPA and not worry about
>>>> them chaining.
>>>>
>>>> There is problem that after inlining the prediction may expand its scope
>>>> and predict branch that it outside of the original function body,
>>>> but I do not see very easy solution for that besides not re-doing
>>>> prediction (we could also copy probabilities from the inlined function
>>>> when they exists and honnor them in the outer function. I am not sure
>>>> that is going to improve prediction quality though - extra context
>>>> is probably useful)
>>>>
>>>> Thanks,
>>>> Honza
>>>>>
>>>>> Thanks,
>>>>> Martin
>>>>>
>>>>>>
>>>>>> Where did you found this case?
>>>>>> Honza
>>>>>>>
>>>>>>>        /* Create a new deep copy of the statement.  */
>>>>>>>        copy = gimple_copy (stmt);
>>>>>>> --
>>>>>>> 2.13.0
>>>>>>>
>>>
>>
>>> >From 84625a782add6ae2ed29630815b61b34a052770a Mon Sep 17 00:00:00 2001
>>> From: marxin <mliska@suse.cz>
>>> Date: Tue, 6 Jun 2017 10:55:18 +0200
>>> Subject: [PATCH 1/2] Make early return predictor more precise.
>>>
>>> gcc/ChangeLog:
>>>
>>> 2017-05-26  Martin Liska  <mliska@suse.cz>
>>>
>>>      PR tree-optimization/79489
>>>      * gimplify.c (maybe_add_early_return_predict_stmt): New
>>>      function.
>>>      (gimplify_return_expr): Call the function.
>>>      * predict.c (tree_estimate_probability_bb): Remove handling
>>>      of early return.
>>>      * predict.def: Update comment about early return predictor.
>>>      * gimple-predict.h (is_gimple_predict): New function.
>>>      * predict.def: Change default value of early return to 66.
>>>      * tree-tailcall.c (find_tail_calls): Skip GIMPLE_PREDICT
>>>      statements.
>>>      * passes.def: Put pass_strip_predict_hints to the beginning of
>>>      IPA passes.
>>> ---
>>>  gcc/gimple-low.c     |  2 ++
>>>  gcc/gimple-predict.h |  8 ++++++++
>>>  gcc/gimplify.c       | 16 ++++++++++++++++
>>>  gcc/passes.def       |  1 +
>>>  gcc/predict.c        | 41 -----------------------------------------
>>>  gcc/predict.def      | 15 +++------------
>>>  gcc/tree-tailcall.c  |  2 ++
>>>  7 files changed, 32 insertions(+), 53 deletions(-)
>>>
>>> diff --git a/gcc/gimple-low.c b/gcc/gimple-low.c
>>> index 619b9d7bfb1..4ea6c3532f3 100644
>>> --- a/gcc/gimple-low.c
>>> +++ b/gcc/gimple-low.c
>>> @@ -30,6 +30,8 @@ along with GCC; see the file COPYING3.  If not see
>>>  #include "calls.h"
>>>  #include "gimple-iterator.h"
>>>  #include "gimple-low.h"
>>> +#include "predict.h"
>>> +#include "gimple-predict.h"
>>>
>>>  /* The differences between High GIMPLE and Low GIMPLE are the
>>>     following:
>>> diff --git a/gcc/gimple-predict.h b/gcc/gimple-predict.h
>>> index ba58e12e9e9..0e6c2e1ea01 100644
>>> --- a/gcc/gimple-predict.h
>>> +++ b/gcc/gimple-predict.h
>>> @@ -80,4 +80,12 @@ gimple_build_predict (enum br_predictor predictor, enum prediction outcome)
>>>    return p;
>>>  }
>>>
>>> +/* Return true if GS is a GIMPLE_PREDICT statement.  */
>>> +
>>> +static inline bool
>>> +is_gimple_predict (const gimple *gs)
>>> +{
>>> +  return gimple_code (gs) == GIMPLE_PREDICT;
>>> +}
>>> +
>>>  #endif  /* GCC_GIMPLE_PREDICT_H */
>>> diff --git a/gcc/gimplify.c b/gcc/gimplify.c
>>> index 9af95a28704..1c6e1591953 100644
>>> --- a/gcc/gimplify.c
>>> +++ b/gcc/gimplify.c
>>> @@ -1428,6 +1428,20 @@ gimplify_bind_expr (tree *expr_p, gimple_seq *pre_p)
>>>    return GS_ALL_DONE;
>>>  }
>>>
>>> +/* Maybe add early return predict statement to PRE_P sequence.  */
>>> +
>>> +static void
>>> +maybe_add_early_return_predict_stmt (gimple_seq *pre_p)
>>> +{
>>> +  /* If we are not in a conditional context, add PREDICT statement.  */
>>> +  if (gimple_conditional_context ())
>>> +    {
>>> +      gimple *predict = gimple_build_predict (PRED_TREE_EARLY_RETURN,
>>> +                                          NOT_TAKEN);
>>> +      gimplify_seq_add_stmt (pre_p, predict);
>>> +    }
>>> +}
>>> +
>>>  /* Gimplify a RETURN_EXPR.  If the expression to be returned is not a
>>>     GIMPLE value, it is assigned to a new temporary and the statement is
>>>     re-written to return the temporary.
>>> @@ -1458,6 +1472,7 @@ gimplify_return_expr (tree stmt, gimple_seq *pre_p)
>>>        || TREE_CODE (ret_expr) == RESULT_DECL
>>>        || ret_expr == error_mark_node)
>>>      {
>>> +      maybe_add_early_return_predict_stmt (pre_p);
>>>        greturn *ret = gimple_build_return (ret_expr);
>>>        gimple_set_no_warning (ret, TREE_NO_WARNING (stmt));
>>>        gimplify_seq_add_stmt (pre_p, ret);
>>> @@ -1525,6 +1540,7 @@ gimplify_return_expr (tree stmt, gimple_seq *pre_p)
>>>
>>>    gimplify_and_add (TREE_OPERAND (stmt, 0), pre_p);
>>>
>>> +  maybe_add_early_return_predict_stmt (pre_p);
>>>    ret = gimple_build_return (result);
>>>    gimple_set_no_warning (ret, TREE_NO_WARNING (stmt));
>>>    gimplify_seq_add_stmt (pre_p, ret);
>>> diff --git a/gcc/passes.def b/gcc/passes.def
>>> index c14f6b9f63c..316e19d12e3 100644
>>> --- a/gcc/passes.def
>>> +++ b/gcc/passes.def
>>> @@ -107,6 +107,7 @@ along with GCC; see the file COPYING3.  If not see
>>>           early optimizations again.  It is thus good idea to do this
>>>            late.  */
>>>        NEXT_PASS (pass_split_functions);
>>> +      NEXT_PASS (pass_strip_predict_hints);
>>>        POP_INSERT_PASSES ()
>>>        NEXT_PASS (pass_release_ssa_names);
>>>        NEXT_PASS (pass_rebuild_cgraph_edges);
>>> diff --git a/gcc/predict.c b/gcc/predict.c
>>> index 60d1a094d10..790be9fbf16 100644
>>> --- a/gcc/predict.c
>>> +++ b/gcc/predict.c
>>> @@ -2739,7 +2739,6 @@ tree_estimate_probability_bb (basic_block bb, bool local_only)
>>>  {
>>>    edge e;
>>>    edge_iterator ei;
>>> -  gimple *last;
>>>
>>>    FOR_EACH_EDGE (e, ei, bb->succs)
>>>      {
>>> @@ -2766,46 +2765,6 @@ tree_estimate_probability_bb (basic_block bb, bool local_only)
>>>          }
>>>      }
>>>
>>> -      /* Predict early returns to be probable, as we've already taken
>>> -     care for error returns and other cases are often used for
>>> -     fast paths through function.
>>> -
>>> -     Since we've already removed the return statements, we are
>>> -     looking for CFG like:
>>> -
>>> -     if (conditional)
>>> -     {
>>> -     ..
>>> -     goto return_block
>>> -     }
>>> -     some other blocks
>>> -     return_block:
>>> -     return_stmt.  */
>>> -      if (e->dest != bb->next_bb
>>> -      && e->dest != EXIT_BLOCK_PTR_FOR_FN (cfun)
>>> -      && single_succ_p (e->dest)
>>> -      && single_succ_edge (e->dest)->dest == EXIT_BLOCK_PTR_FOR_FN (cfun)
>>> -      && (last = last_stmt (e->dest)) != NULL
>>> -      && gimple_code (last) == GIMPLE_RETURN)
>>> -    {
>>> -      edge e1;
>>> -      edge_iterator ei1;
>>> -
>>> -      if (single_succ_p (bb))
>>> -        {
>>> -          FOR_EACH_EDGE (e1, ei1, bb->preds)
>>> -            if (!predicted_by_p (e1->src, PRED_NULL_RETURN)
>>> -                && !predicted_by_p (e1->src, PRED_CONST_RETURN)
>>> -                && !predicted_by_p (e1->src, PRED_NEGATIVE_RETURN))
>>> -              predict_edge_def (e1, PRED_TREE_EARLY_RETURN, NOT_TAKEN);
>>> -        }
>>> -      else
>>> -        if (!predicted_by_p (e->src, PRED_NULL_RETURN)
>>> -            && !predicted_by_p (e->src, PRED_CONST_RETURN)
>>> -            && !predicted_by_p (e->src, PRED_NEGATIVE_RETURN))
>>> -          predict_edge_def (e, PRED_TREE_EARLY_RETURN, NOT_TAKEN);
>>> -    }
>>> -
>>>        /* Look for block we are guarding (ie we dominate it,
>>>       but it doesn't postdominate us).  */
>>>        if (e->dest != EXIT_BLOCK_PTR_FOR_FN (cfun) && e->dest != bb
>>> diff --git a/gcc/predict.def b/gcc/predict.def
>>> index fcda6c48f11..f7b2bf7738c 100644
>>> --- a/gcc/predict.def
>>> +++ b/gcc/predict.def
>>> @@ -128,18 +128,9 @@ DEF_PREDICTOR (PRED_POLYMORPHIC_CALL, "polymorphic call", HITRATE (59), 0)
>>>     indefinitely.  */
>>>  DEF_PREDICTOR (PRED_RECURSIVE_CALL, "recursive call", HITRATE (75), 0)
>>>
>>> -/* Branch causing function to terminate is probably not taken.
>>> -   FIXME: early return currently predicts code:
>>> -   int foo (int a)
>>> -   {
>>> -      if (a)
>>> -    bar();
>>> -      else
>>> -    bar2();
>>> -   }
>>> -   even though there is no return statement involved.  We probably want to track
>>> -   this from FE or retire the predictor.  */
>>> -DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (54), 0)
>>> +/* Branch causing function to terminate is probably not taken.  */
>>> +DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (66),
>>> +           0)
>>>
>>>  /* Branch containing goto is probably not taken.
>>>     FIXME: Currently not used.  */
>>> diff --git a/gcc/tree-tailcall.c b/gcc/tree-tailcall.c
>>> index b7053387e91..6aa9a56462e 100644
>>> --- a/gcc/tree-tailcall.c
>>> +++ b/gcc/tree-tailcall.c
>>> @@ -421,6 +421,7 @@ find_tail_calls (basic_block bb, struct tailcall **ret)
>>>        if (gimple_code (stmt) == GIMPLE_LABEL
>>>        || gimple_code (stmt) == GIMPLE_RETURN
>>>        || gimple_code (stmt) == GIMPLE_NOP
>>> +      || gimple_code (stmt) == GIMPLE_PREDICT
>>>        || gimple_clobber_p (stmt)
>>>        || is_gimple_debug (stmt))
>>>      continue;
>>> @@ -555,6 +556,7 @@ find_tail_calls (basic_block bb, struct tailcall **ret)
>>>
>>>        if (gimple_code (stmt) == GIMPLE_LABEL
>>>        || gimple_code (stmt) == GIMPLE_NOP
>>> +      || gimple_code (stmt) == GIMPLE_PREDICT
>>>        || gimple_clobber_p (stmt)
>>>        || is_gimple_debug (stmt))
>>>      continue;
>>> --
>>> 2.13.1
>>>
>>
>