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

["Martin Liška" <mliska@suse.cz>]

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.

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
>>
>