[PATCH v3 1/4] Fix loop split incorrect count and probability

Xionghu Luo luoxhu@linux.ibm.com
Wed Nov 24 05:11:43 GMT 2021


Gentle ping, thanks.

[PATCH v3] Fix loop split incorrect count and probability

https://gcc.gnu.org/pipermail/gcc-patches/2021-November/583626.html


On 2021/11/8 14:09, Xionghu Luo via Gcc-patches wrote:
> 
> 
> On 2021/10/27 15:44, Jan Hubicka wrote:
>>> On Wed, 27 Oct 2021, Jan Hubicka wrote:
>>>
>>>>>
>>>>> gcc/ChangeLog:
>>>>>
>>>>> 	* tree-ssa-loop-split.c (split_loop): Fix incorrect probability.
>>>>> 	(do_split_loop_on_cond): Likewise.
>>>>> ---
>>>>>  gcc/tree-ssa-loop-split.c | 25 ++++++++++++++++---------
>>>>>  1 file changed, 16 insertions(+), 9 deletions(-)
>>>>>
>>>>> diff --git a/gcc/tree-ssa-loop-split.c b/gcc/tree-ssa-loop-split.c
>>>>> index 3f6ad046623..d30782888f3 100644
>>>>> --- a/gcc/tree-ssa-loop-split.c
>>>>> +++ b/gcc/tree-ssa-loop-split.c
>>>>> @@ -575,7 +575,11 @@ split_loop (class loop *loop1)
>>>>>  					    stmts2);
>>>>>  	tree cond = build2 (guard_code, boolean_type_node, guard_init, border);
>>>>>  	if (!initial_true)
>>>>> -	  cond = fold_build1 (TRUTH_NOT_EXPR, boolean_type_node, cond); 
>>>>> +	  cond = fold_build1 (TRUTH_NOT_EXPR, boolean_type_node, cond);
>>>>> +
>>>>> +	edge true_edge = EDGE_SUCC (bbs[i], 0)->flags & EDGE_TRUE_VALUE
>>>>> +			   ? EDGE_SUCC (bbs[i], 0)
>>>>> +			   : EDGE_SUCC (bbs[i], 1);
>>>>>  
>>>>>  	/* Now version the loop, placing loop2 after loop1 connecting
>>>>>  	   them, and fix up SSA form for that.  */
>>>>> @@ -583,10 +587,10 @@ split_loop (class loop *loop1)
>>>>>  	basic_block cond_bb;
>>>>>  
>>>>>  	class loop *loop2 = loop_version (loop1, cond, &cond_bb,
>>>>> -					   profile_probability::always (),
>>>>> -					   profile_probability::always (),
>>>>> -					   profile_probability::always (),
>>>>> -					   profile_probability::always (),
>>>>> +					   true_edge->probability,
>>>>> +					   true_edge->probability.invert (),
>>>>> +					   true_edge->probability,
>>>>> +					   true_edge->probability.invert (),
>>>>>  					   true);
>>>>
>>>> As discussed yesterday, for loop of form
>>>>
>>>> for (...)
>>>>   if (cond)
>>>>     cond = something();
>>>>   else
>>>>     something2
>>>>
>>>> Split as
>>>
>>> Note that you are missing to conditionalize loop1 execution
>>> on 'cond' (not sure if that makes a difference).
>> You are right - forgot to mention that.
>>
>> Entry conditional makes no difference on scaling stmts inside loop but
>> affects its header and expected trip count. We however need to set up
>> probability of this conditional (and preheader count if it exists)
>> There is no general way to read the probability of this initial
>> conditional from cfg profile.  So I guess we are stuck with guessing
>> some arbitrary value. I guess common case is that cond is true first
>> iteration tough and often we can easily see that fromo PHI node
>> initializing the test variable.
>>
>> Other thing that changes is expected number of iterations of the split
>> loops, so we may want to update the exit conditinal probability
>> accordingly...
>>
> Sorry for the late reply.  The below updated patch mainly solves the issues
> you pointed out:
>   - profile count proportion for both original loop and copied loop
> without dropping down the true branch's count;
>   - probability update in the two loops and between the two loops;
>   - number of iterations update/check for split_loop.
> 
> 
> [PATCH v3] Fix loop split incorrect count and probability
> 
> In tree-ssa-loop-split.c, split_loop and split_loop_on_cond does two
> kind of split. split_loop only works for single loop and insert edge at
> exit when split, while split_loop_on_cond is not limited to single loop
> and insert edge at latch when split.  Both split behavior should consider
> loop count and probability update.  For split_loop, loop split condition
> is moved in front of loop1 and loop2; But split_loop_on_cond moves the
> condition between loop1 and loop2, this patch does:
> 1) profile count proportion for both original loop and copied loop
> without dropping down the true branch's count;
> 2) probability update in and between the two loops;
> 3) number of iterations update for split_loop.
> 
> Regression tested pass, OK for master?
> 
> Changes diff for split_loop and split_loop_on_cond cases:
> 
> 1) diff base/loop-split.c.151t.lsplit patched/loop-split.c.152t.lsplit
> ...
>    <bb 2> [local count: 118111600]:
>    if (beg_5(D) < end_8(D))
>      goto <bb 14>; [89.00%]
>    else
>      goto <bb 6>; [11.00%]
> 
>    <bb 14> [local count: 105119324]:
>    if (beg2_6(D) < c_9(D))
> -    goto <bb 15>; [100.00%]
> +    goto <bb 15>; [33.00%]
>    else
> -    goto <bb 16>; [100.00%]
> +    goto <bb 16>; [67.00%]
> 
> -  <bb 15> [local count: 105119324]:
> +  <bb 15> [local count: 34689377]:
>    _25 = beg_5(D) + 1;
>    _26 = end_8(D) - beg_5(D);
>    _27 = beg2_6(D) + _26;
>    _28 = MIN_EXPR <c_9(D), _27>;
> 
> -  <bb 3> [local count: 955630225]:
> +  <bb 3> [local count: 315357973]:
>    # i_16 = PHI <i_11(8), beg_5(D)(15)>
>    # j_17 = PHI <j_12(8), beg2_6(D)(15)>
>    printf ("a: %d %d\n", i_16, j_17);
>    i_11 = i_16 + 1;
>    j_12 = j_17 + 1;
>    if (j_12 < _28)
> -    goto <bb 8>; [89.00%]
> +    goto <bb 8>; [29.37%]
>    else
> -    goto <bb 17>; [11.00%]
> +    goto <bb 17>; [70.63%]
> 
> -  <bb 8> [local count: 850510901]:
> +  <bb 8> [local count: 280668596]:
>    goto <bb 3>; [100.00%]
> 
> -  <bb 16> [local count: 105119324]:
> +  <bb 16> [local count: 70429947]:
>    # i_22 = PHI <beg_5(D)(14), i_29(17)>
>    # j_23 = PHI <beg2_6(D)(14), j_30(17)>
> 
>    <bb 10> [local count: 955630225]:
>    # i_2 = PHI <i_22(16), i_20(13)>
>    # j_1 = PHI <j_23(16), j_21(13)>
>    i_20 = i_2 + 1;
>    j_21 = j_1 + 1;
>    if (end_8(D) > i_20)
> -    goto <bb 13>; [89.00%]
> +    goto <bb 13>; [59.63%]
>    else
> -    goto <bb 9>; [11.00%]
> +    goto <bb 9>; [40.37%]
> 
> -  <bb 13> [local count: 850510901]:
> +  <bb 13> [local count: 569842305]:
>    goto <bb 10>; [100.00%]
> 
>    <bb 17> [local count: 105119324]:
>    # i_29 = PHI <i_11(3)>
>    # j_30 = PHI <j_12(3)>
>    if (end_8(D) > i_29)
>      goto <bb 16>; [80.00%]
>    else
>      goto <bb 9>; [20.00%]
> 
>    <bb 9> [local count: 105119324]:
> 
>    <bb 6> [local count: 118111600]:
>    return 0;
> 
>  }
> 
> 2) diff base/loop-cond-split-1.c.151t.lsplit  patched/loop-cond-split-1.c.151t.lsplit:
> ...
>    <bb 2> [local count: 118111600]:
>    if (n_7(D) > 0)
>      goto <bb 4>; [89.00%]
>    else
>      goto <bb 3>; [11.00%]
> 
>    <bb 3> [local count: 118111600]:
>    return;
> 
>    <bb 4> [local count: 105119324]:
>    pretmp_3 = ga;
> 
> -  <bb 5> [local count: 955630225]:
> +  <bb 5> [local count: 315357973]:
>    # i_13 = PHI <i_10(20), 0(4)>
>    # prephitmp_12 = PHI <prephitmp_5(20), pretmp_3(4)>
>    if (prephitmp_12 != 0)
>      goto <bb 6>; [33.00%]
>    else
>      goto <bb 7>; [67.00%]
> 
>    <bb 6> [local count: 315357972]:
>    _2 = do_something ();
>    ga = _2;
> 
> -  <bb 7> [local count: 955630225]:
> +  <bb 7> [local count: 315357973]:
>    # prephitmp_5 = PHI <prephitmp_12(5), _2(6)>
>    i_10 = inc (i_13);
>    if (n_7(D) > i_10)
>      goto <bb 21>; [89.00%]
>    else
>      goto <bb 11>; [11.00%]
> 
>    <bb 11> [local count: 105119324]:
>    goto <bb 3>; [100.00%]
> 
> -  <bb 21> [local count: 850510901]:
> +  <bb 21> [local count: 280668596]:
>    if (prephitmp_12 != 0)
> -    goto <bb 20>; [100.00%]
> +    goto <bb 20>; [33.00%]
>    else
> -    goto <bb 19>; [INV]
> +    goto <bb 19>; [67.00%]
> 
> -  <bb 20> [local count: 850510901]:
> +  <bb 20> [local count: 280668596]:
>    goto <bb 5>; [100.00%]
> 
> -  <bb 19> [count: 0]:
> +  <bb 19> [local count: 70429947]:
>    # i_23 = PHI <i_10(21)>
>    # prephitmp_25 = PHI <prephitmp_5(21)>
> 
> -  <bb 12> [local count: 955630225]:
> +  <bb 12> [local count: 640272252]:
>    # i_15 = PHI <i_23(19), i_22(16)>
>    # prephitmp_16 = PHI <prephitmp_25(19), prephitmp_16(16)>
>    i_22 = inc (i_15);
>    if (n_7(D) > i_22)
> -    goto <bb 16>; [89.00%]
> +    goto <bb 16>; [59.63%]
>    else
> -    goto <bb 11>; [11.00%]
> +    goto <bb 11>; [40.37%]
> 
> -  <bb 16> [local count: 850510901]:
> +  <bb 16> [local count: 569842305]:
>    goto <bb 12>; [100.00%]
> 
>  }
> 
> gcc/ChangeLog:
> 
> 	* tree-ssa-loop-split.c (split_loop): Fix incorrect
> 	profile_count and probability.
> 	(do_split_loop_on_cond): Likewise.
> ---
>  gcc/tree-ssa-loop-split.c | 110 +++++++++++++++++++++++++++++++++++---
>  1 file changed, 102 insertions(+), 8 deletions(-)
> 
> diff --git a/gcc/tree-ssa-loop-split.c b/gcc/tree-ssa-loop-split.c
> index 3f6ad046623..102766241fb 100644
> --- a/gcc/tree-ssa-loop-split.c
> +++ b/gcc/tree-ssa-loop-split.c
> @@ -575,7 +575,10 @@ split_loop (class loop *loop1)
>  					    stmts2);
>  	tree cond = build2 (guard_code, boolean_type_node, guard_init, border);
>  	if (!initial_true)
> -	  cond = fold_build1 (TRUTH_NOT_EXPR, boolean_type_node, cond); 
> +	  cond = fold_build1 (TRUTH_NOT_EXPR, boolean_type_node, cond);
> +
> +	edge true_edge, false_edge;
> +	extract_true_false_edges_from_block (bbs[i], &true_edge, &false_edge);
> 
>  	/* Now version the loop, placing loop2 after loop1 connecting
>  	   them, and fix up SSA form for that.  */
> @@ -583,11 +586,11 @@ split_loop (class loop *loop1)
>  	basic_block cond_bb;
> 
>  	class loop *loop2 = loop_version (loop1, cond, &cond_bb,
> -					   profile_probability::always (),
> -					   profile_probability::always (),
> -					   profile_probability::always (),
> -					   profile_probability::always (),
> -					   true);
> +					  true_edge->probability,
> +					  true_edge->probability.invert (),
> +					  profile_probability::always (),
> +					  profile_probability::always (),
> +					  true);
>  	gcc_assert (loop2);
> 
>  	edge new_e = connect_loops (loop1, loop2);
> @@ -607,6 +610,53 @@ split_loop (class loop *loop1)
>  	tree guard_next = PHI_ARG_DEF_FROM_EDGE (phi, loop_latch_edge (loop1));
>  	patch_loop_exit (loop1, guard_stmt, guard_next, newend, initial_true);
> 
> +	/* Check first loop's number of iterations.  */
> +	update_ssa (TODO_update_ssa);
> +	gcc_assert (number_of_iterations_exit (loop1, single_exit (loop1),
> +					       &niter, false, true));
> +
> +	/* Proportion first loop's bb counts except those dominated by true
> +	   branch to avoid drop 1s down.  */
> +	basic_block *bbs1, *bbs2;
> +	bbs1 = get_loop_body (loop1);
> +	unsigned j;
> +	for (j = 0; j < loop1->num_nodes; j++)
> +	  if (bbs1[j] == loop1->latch
> +	      || !dominated_by_p (CDI_DOMINATORS, bbs1[j], true_edge->dest))
> +	    bbs1[j]->count
> +	      = bbs1[j]->count.apply_probability (true_edge->probability);
> +	free (bbs1);
> +
> +	/* Fix first loop's exit probability after scaling.  */
> +	edge exit_to_latch1 = single_pred_edge (loop1->latch);
> +	exit_to_latch1->probability = exit_to_latch1->probability.apply_scale (
> +	  true_edge->probability.to_reg_br_prob_base (), REG_BR_PROB_BASE);
> +	single_exit (loop1)->probability
> +	  = exit_to_latch1->probability.invert ();
> +
> +	/* Check second loop's number of iterations.  */
> +	class tree_niter_desc niter2;
> +	gcc_assert (number_of_iterations_exit (loop2, single_exit (loop2),
> +					       &niter2, false, true));
> +
> +	/* Proportion second loop's bb counts except those dominated by false
> +	   branch to avoid drop 1s down.  */
> +	basic_block bbi_copy = get_bb_copy (false_edge->dest);
> +	bbs2 = get_loop_body (loop2);
> +	for (j = 0; j < loop2->num_nodes; j++)
> +	  if (bbs2[j] == loop2->latch
> +	      || !dominated_by_p (CDI_DOMINATORS, bbs2[j], bbi_copy))
> +	    bbs2[j]->count = bbs2[j]->count.apply_probability (
> +	      true_edge->probability.invert ());
> +	free (bbs2);
> +
> +	/* Fix second loop's exit probability after scaling.  */
> +	edge exit_to_latch2 = single_pred_edge (loop2->latch);
> +	exit_to_latch2->probability = exit_to_latch2->probability.apply_scale (
> +	  false_edge->probability.to_reg_br_prob_base (), REG_BR_PROB_BASE);
> +	single_exit (loop2)->probability
> +	  = exit_to_latch2->probability.invert ();
> +
>  	/* Finally patch out the two copies of the condition to be always
>  	   true/false (or opposite).  */
>  	gcond *force_true = as_a<gcond *> (last_stmt (bbs[i]));
> @@ -1486,8 +1536,8 @@ do_split_loop_on_cond (struct loop *loop1, edge invar_branch)
>    initialize_original_copy_tables ();
> 
>    struct loop *loop2 = loop_version (loop1, boolean_true_node, NULL,
> -				     profile_probability::always (),
> -				     profile_probability::never (),
> +				     invar_branch->probability.invert (),
> +				     invar_branch->probability,
>  				     profile_probability::always (),
>  				     profile_probability::always (),
>  				     true);
> @@ -1535,6 +1585,50 @@ do_split_loop_on_cond (struct loop *loop1, edge invar_branch)
>       between loop1 and loop2.  */
>    connect_loop_phis (loop1, loop2, to_loop2);
> 
> +  update_ssa (TODO_update_ssa);
> +
> +  edge true_edge, false_edge, skip_edge1, skip_edge2;
> +  extract_true_false_edges_from_block (cond_bb, &true_edge, &false_edge);
> +
> +  /* Proportion first loop's bb counts except those dominated by true
> +     branch to avoid drop 1s down.  */
> +  skip_edge1 = true_invar ? false_edge : true_edge;
> +  skip_edge2 = true_invar ? true_edge : false_edge;
> +  basic_block *bbs1, *bbs2;
> +  bbs1 = get_loop_body (loop1);
> +  unsigned j;
> +  for (j = 0; j < loop1->num_nodes; j++)
> +    if (bbs1[j] == loop1->latch
> +	|| !dominated_by_p (CDI_DOMINATORS, bbs1[j], skip_edge1->dest))
> +      bbs1[j]->count
> +	= bbs1[j]->count.apply_probability (skip_edge1->probability);
> +  free (bbs1);
> +
> +  /* Fix first loop's exit probability after scaling.  */
> +  to_loop1->probability = invar_branch->probability.invert ();
> +  to_loop2->probability = invar_branch->probability;
> +
> +  /* Proportion second loop's bb counts except those dominated by false
> +     branch to avoid drop 1s down.  */
> +  basic_block bbi_copy = get_bb_copy (skip_edge2->dest);
> +  bbs2 = get_loop_body (loop2);
> +  for (j = 0; j < loop2->num_nodes; j++)
> +    if (bbs2[j] == loop2->latch
> +	|| !dominated_by_p (CDI_DOMINATORS, bbs2[j], bbi_copy))
> +      bbs2[j]->count
> +	= bbs2[j]->count.apply_probability (skip_edge2->probability);
> +  free (bbs2);
> +
> +  /* Fix second loop's exit probability after scaling.  */
> +  edge loop2_latch_exit;
> +  edge exit_to_latch2 = single_pred_edge (loop2->latch);
> +  exit_to_latch2->probability = exit_to_latch2->probability.apply_scale (
> +    skip_edge2->probability.to_reg_br_prob_base (), REG_BR_PROB_BASE);
> +  loop2_latch_exit = EDGE_SUCC (exit_to_latch2->src, 0) == exit_to_latch2
> +		       ? EDGE_SUCC (exit_to_latch2->src, 1)
> +		       : EDGE_SUCC (exit_to_latch2->src, 0);
> +  loop2_latch_exit->probability = exit_to_latch2->probability.invert ();
> +
>    free_original_copy_tables ();
> 
>    return true;
> 

-- 
Thanks,
Xionghu


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