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


On 06/19/2017 01:11 PM, Jan Hubicka wrote:
>> Ok, you're right that we can preserve the predictor. However, let's consider following test-case:
>>
>> static
>> int baz(int a)
>> {
>>   if (a == 1)
>>     return 1;
>>
>>   return 0;
>> }
>>
>>   
>> static
>> int bar(int a)
>> {
>>   if (a == 1)
>>     return baz(a);
>>
>>   return 0;
>> }
>>   
>> static
>> int foo(int a)
>> {
>>   if (a == 1)
>>     return bar(a);
>>
>>   return 12;
>> }
>>
>> int main(int argc, char **argv)
>> {
>>   return foo(argc);
>> }
>>
>> There after einline we have:
>>
>> main (int argc, char * * argv)
>> {
>>   int D.1832;
>>   int _3;
>>   int _4;
>>
>>   <bb 2> [100.00%]:
>>   if (argc_2(D) == 1)
>>     goto <bb 3>; [37.13%]
>>   else
>>     goto <bb 4>; [62.87%]
>>
>>   <bb 3> [37.13%]:
>>   // predicted unlikely by early return (on trees) predictor.
>>   // predicted unlikely by early return (on trees) predictor.
>>   // predicted unlikely by early return (on trees) predictor.
>>
>>   <bb 4> [100.00%]:
>>   # _3 = PHI <12(2), 1(3)>
>>   _5 = _3;
>>   _4 = _5;
>>   return _4;
>>
>> }
>>
>> I'm thinking what's the best place to merge all the predictor
>> statements?
> 
> I wonder if we need to - predictors are relatively short lived.
> In fact they do not need to hit IPA passes but they do as at a time
> I was implementing them I was worrying about introducing yet another
> global IPA pass to remove them (we can't do during early inlining
> because we want to reuse them after inlining).

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?

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


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