Re: [PATCH 1/2] correct BB frequencies after loop changed

[Jiufu Guo <guojiufu@linux.ibm.com>]

Jiufu Guo <guojiufu@linux.ibm.com> writes:

> Jeff Law <law@redhat.com> writes:
>
>> On 11/18/20 12:28 AM, Richard Biener wrote:
>>> On Tue, 17 Nov 2020, Jeff Law wrote:
>>>
>>>> Minor questions for Jan and Richi embedded below...
>>>>
>>>> On 10/9/20 4:12 AM, guojiufu via Gcc-patches wrote:
>>>>> When investigating the issue from https://gcc.gnu.org/pipermail/gcc-patches/2020-July/549786.html
>>>>> I find the BB COUNTs of loop seems are not accurate in some case.
>>>>> For example:
>>>>>
>>>>> In below figure:
>>>>>
>>>>>
>>>>>                COUNT:268435456<bb 2>  pre-header
>>>>>                         |
>>>>>                         |  .--------------------.
>>>>>                         |  |                    |
>>>>>                         V  v                    |
>>>>>                COUNT:805306369<bb 3>            |
>>>>>                        / \                      |
>>>>>                    33%/   \                     |
>>>>>                      /     \                    |
>>>>>                     v       v                   |
>>>>> COUNT:268435456<bb 10>  COUNT:536870911<bb 15>  | 
>>>>>     exit-edge                 |   latch         |
>>>>>                               ._________________.
>>>>>
>>>>> Those COUNTs have below equations:
>>>>> COUNT of exit-edge:268435456 = COUNT of pre-header:268435456
>>>>> COUNT of exit-edge:268435456 = COUNT of header:805306369 * 33
>>>>> COUNT of header:805306369 = COUNT of pre-header:268435456 + COUNT of latch:536870911
>>>>>
>>>>>
>>>>> While after pcom:
>>>>>
>>>>>                COUNT:268435456<bb 2>  pre-header
>>>>>                         |
>>>>>                         |  .--------------------.
>>>>>                         |  |                    |
>>>>>                         V  v                    |
>>>>>                COUNT:268435456<bb 3>            |
>>>>>                        / \                      |
>>>>>                    50%/   \                     |
>>>>>                      /     \                    |
>>>>>                     v       v                   |
>>>>> COUNT:134217728<bb 10>  COUNT:134217728<bb 15>  | 
>>>>>     exit-edge                 |   latch         |
>>>>>                               ._________________.
>>>>>
>>>>> COUNT<bb 3> != COUNT<bb 2> + COUNT<bb 15>
>>>>> COUNT<bb 10> != COUNT<bb2>
>>>>>
>>>>> In some cases, the probility of exit-edge is easy to estimate, then
>>>>> those COUNTs of other BBs in loop can be re-caculated.
>>>>>
>>>>> Bootstrap and regtest pass on ppc64le. Is this ok for trunk?
>>>>>
>>>>> Jiufu
>>>>>
>>>>> gcc/ChangeLog:
>>>>> 2020-10-09  Jiufu Guo   <guojiufu@linux.ibm.com>
>>>>>
>>>>> 	* cfgloopmanip.h (recompute_loop_frequencies): New function.
>>>>> 	* cfgloopmanip.c (recompute_loop_frequencies): New implementation.
>>>>> 	* tree-ssa-loop-manip.c (tree_transform_and_unroll_loop): Call
>>>>> 	recompute_loop_frequencies.
>>>>>
>>>>> ---
>>>>>  gcc/cfgloopmanip.c        | 53 +++++++++++++++++++++++++++++++++++++++
>>>>>  gcc/cfgloopmanip.h        |  2 +-
>>>>>  gcc/tree-ssa-loop-manip.c | 28 +++------------------
>>>>>  3 files changed, 57 insertions(+), 26 deletions(-)
>>>>>
>>>>> diff --git a/gcc/cfgloopmanip.c b/gcc/cfgloopmanip.c
>>>>> index 73134a20e33..b0ca82a67fd 100644
>>>>> --- a/gcc/cfgloopmanip.c
>>>>> +++ b/gcc/cfgloopmanip.c
>>>>> @@ -31,6 +31,7 @@ along with GCC; see the file COPYING3.  If not see
>>>>>  #include "gimplify-me.h"
>>>>>  #include "tree-ssa-loop-manip.h"
>>>>>  #include "dumpfile.h"
>>>>> +#include "cfgrtl.h"
>>>>>  
>>>>>  static void copy_loops_to (class loop **, int,
>>>>>  			   class loop *);
>>>>> @@ -1773,3 +1774,55 @@ loop_version (class loop *loop,
>>>>>  
>>>>>    return nloop;
>>>>>  }
>>>>> +
>>>>> +/* Recalculate the COUNTs of BBs in LOOP, if the probability of exit edge
>>>>> +   is NEW_PROB.  */
>>>>> +
>>>>> +bool
>>>>> +recompute_loop_frequencies (class loop *loop, profile_probability new_prob)
>>>>> +{
>>>>> +  edge exit = single_exit (loop);
>>>>> +  if (!exit)
>>>>> +    return false;
>>>>> +
>>>>> +  edge e;
>>>>> +  edge_iterator ei;
>>>>> +  edge non_exit;
>>>>> +  basic_block * bbs;
>>>>> +  profile_count exit_count = loop_preheader_edge (loop)->count ();
>>>>> +  profile_probability exit_p = exit_count.probability_in (loop->header->count);
>>>>> +  profile_count base_count = loop->header->count;
>>>>> +  profile_count after_num = base_count.apply_probability (exit_p);
>>>>> +  profile_count after_den = base_count.apply_probability (new_prob);
>>>>> +
>>>>> +  /* Update BB counts in loop body.
>>>>> +     COUNT<exit> = COUNT<preheader>
>>>>> +     COUNT<exit> = COUNT<header> * exit_edge_probility
>>>>> +     The COUNT<new_header> = COUNT<old_header> * old_exit_p / new_prob.  */
>>>>> +  bbs = get_loop_body (loop);
>>>>> +  scale_bbs_frequencies_profile_count (bbs, loop->num_nodes, after_num,
>>>>> +						     after_den);
>>>>> +  free (bbs);
>>>>> +
>>>>> +  /* Update probability and count of the BB besides exit edge (maybe latch).  */
>>>>> +  FOR_EACH_EDGE (e, ei, exit->src->succs)
>>>>> +    if (e != exit)
>>>>> +      break;
>>>>> +  non_exit = e;
>>>> Are we sure that exit->src has just two successors (will that case be
>>>> canonicalized before we get here?).? If it has > 2 successors, then I'm
>>>> pretty sure the frequencies get mucked up.? Richi could probably answer
>>>> whether or not the block with the loop exit edge can have > 2 successors.
>>> There's nothing preventing more than two edges in the SRC generally
>>> (the exit could be an edge off a switch).
>> That's precisely the case I was concerned about.
>>
>>>   But of course this function
>>> is very likely called on loops that are countable which means niter
>>> analysis has to handle it which in turn means all exits are controlled
>>> by simple conditions on IVs.
>> Thanks.  It sounds like it's unlikely we'll have > 2 out edges.
>>>
>>> I guess adding a gcc_assert (EDGE_COUNT (exit->src->succs) == 2) can't 
>>> hurt (with a comment reflecting the above).
>> Sounds good to me.   Just catching this case if it happens is good
>> enough for me -- if it trips we can come back and adjust the code to
>> distribute across the edges.
> With this gcc_assert, run bootstrap and regression test, no failure
> occur.
> For this patch, in the original code, there is code:
> -  new_nonexit = single_pred_edge (loop->latch);
> -  prob = new_nonexit->probability;
> -  new_nonexit->probability = new_exit->probability.invert ();
> Which is also assume 2 successors.  So, it may relative safe.
>
> Thanks,
> Jiufu Guo.
>
>>
>> So if Jan could chime in on the downstream edge updates question then I
>> think we'd be ready to move forward.
>>
>> jeff

Hi,

I saw Jeff say ok for patch [PATCH 2/2]
https://gcc.gnu.org/pipermail/gcc-patches/2020-December/560833.html.

I'm wondering if we can approval this patch [PATCH 1/2]
https://gcc.gnu.org/pipermail/gcc-patches/2020-October/555871.html.
and then I may commit these patches to trunk. :)

Thanks,
Jiufu Guo.