Re: [PATCH] PHIOPT: Improve replace_phi_edge_with_variable for diamond shapped bb

[Jeff Law <jeffreyalaw@gmail.com>]

On 5/3/23 00:27, Andrew Pinski via Gcc-patches wrote:
> On Tue, May 2, 2023 at 11:14 PM Richard Biener
> <richard.guenther@gmail.com> wrote:
>>
>> On Wed, May 3, 2023 at 12:04 AM Andrew Pinski <pinskia@gmail.com> wrote:
>>>
>>> On Tue, May 2, 2023 at 5:26 AM Richard Biener via Gcc-patches
>>> <gcc-patches@gcc.gnu.org> wrote:
>>>>
>>>> On Sun, Apr 30, 2023 at 11:14 PM Andrew Pinski via Gcc-patches
>>>> <gcc-patches@gcc.gnu.org> wrote:
>>>>>
>>>>> While looking at differences between what minmax_replacement
>>>>> and match_simplify_replacement does. I noticed that they sometimes
>>>>> chose different edges to remove. I decided we should be able to do
>>>>> better and be able to remove both empty basic blocks in the
>>>>> case of match_simplify_replacement as that moves the statements.
>>>>>
>>>>> This also updates the testcases as now match_simplify_replacement
>>>>> will remove the unused MIN/MAX_EXPR and they were checking for
>>>>> those.
>>>>>
>>>>> OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.
>>>>>
>>>>> gcc/ChangeLog:
>>>>>
>>>>>          * tree-ssa-phiopt.cc (copy_phi_args): New function.
>>>>>          (replace_phi_edge_with_variable): Handle diamond form bb
>>>>>          with forwarder only empty blocks better.
>>>>>
>>>>> gcc/testsuite/ChangeLog:
>>>>>
>>>>>          * gcc.dg/tree-ssa/minmax-15.c: Update test.
>>>>>          * gcc.dg/tree-ssa/minmax-16.c: Update test.
>>>>>          * gcc.dg/tree-ssa/minmax-3.c: Update test.
>>>>>          * gcc.dg/tree-ssa/minmax-4.c: Update test.
>>>>>          * gcc.dg/tree-ssa/minmax-5.c: Update test.
>>>>>          * gcc.dg/tree-ssa/minmax-8.c: Update test.
>>>>> ---
>>>>>   gcc/testsuite/gcc.dg/tree-ssa/minmax-15.c |  3 +-
>>>>>   gcc/testsuite/gcc.dg/tree-ssa/minmax-16.c |  9 ++--
>>>>>   gcc/testsuite/gcc.dg/tree-ssa/minmax-3.c  |  2 +-
>>>>>   gcc/testsuite/gcc.dg/tree-ssa/minmax-4.c  |  2 +-
>>>>>   gcc/testsuite/gcc.dg/tree-ssa/minmax-5.c  |  2 +-
>>>>>   gcc/testsuite/gcc.dg/tree-ssa/minmax-8.c  |  2 +-
>>>>>   gcc/tree-ssa-phiopt.cc                    | 51 ++++++++++++++++++++++-
>>>>>   7 files changed, 59 insertions(+), 12 deletions(-)
>>>>>
>>>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/minmax-15.c b/gcc/testsuite/gcc.dg/tree-ssa/minmax-15.c
>>>>> index 8a39871c938..6731f91e6c3 100644
>>>>> --- a/gcc/testsuite/gcc.dg/tree-ssa/minmax-15.c
>>>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/minmax-15.c
>>>>> @@ -30,5 +30,6 @@ main (void)
>>>>>     return 0;
>>>>>   }
>>>>>
>>>>> -/* { dg-final { scan-tree-dump-times "MIN_EXPR" 3 "phiopt1" } } */
>>>>> +/* There should only be two MIN_EXPR left, the 3rd one was removed. */
>>>>> +/* { dg-final { scan-tree-dump-times "MIN_EXPR" 2 "phiopt1" } } */
>>>>>   /* { dg-final { scan-tree-dump-times "MAX_EXPR" 0 "phiopt1" } } */
>>>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/minmax-16.c b/gcc/testsuite/gcc.dg/tree-ssa/minmax-16.c
>>>>> index 623b12b3f74..094364e6424 100644
>>>>> --- a/gcc/testsuite/gcc.dg/tree-ssa/minmax-16.c
>>>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/minmax-16.c
>>>>> @@ -25,11 +25,8 @@ main (void)
>>>>>     return 0;
>>>>>   }
>>>>>
>>>>> -/* After phiopt1, there really should be only 3 MIN_EXPR in the IR (including debug statements).
>>>>> -   But the way phiopt does not cleanup the CFG all the time, the PHI might still reference the
>>>>> -   alternative bb's moved statement.
>>>>> -   Note in the end, we do dce the statement and other debug statements to end up with only 2 MIN_EXPR.
>>>>> -   So check that too. */
>>>>> -/* { dg-final { scan-tree-dump-times "MIN_EXPR" 4 "phiopt1" } } */
>>>>> +/* After phiopt1, will be only 2 MIN_EXPR in the IR (including debug statements). */
>>>>> +/* xk will only have the final result so the extra debug info does not change anything. */
>>>>> +/* { dg-final { scan-tree-dump-times "MIN_EXPR" 2 "phiopt1" } } */
>>>>>   /* { dg-final { scan-tree-dump-times "MIN_EXPR" 2 "optimized" } } */
>>>>>   /* { dg-final { scan-tree-dump-times "MAX_EXPR" 0 "phiopt1" } } */
>>>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/minmax-3.c b/gcc/testsuite/gcc.dg/tree-ssa/minmax-3.c
>>>>> index 2af10776346..521afe3e4d9 100644
>>>>> --- a/gcc/testsuite/gcc.dg/tree-ssa/minmax-3.c
>>>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/minmax-3.c
>>>>> @@ -25,5 +25,5 @@ main (void)
>>>>>     return 0;
>>>>>   }
>>>>>
>>>>> -/* { dg-final { scan-tree-dump-times "MIN_EXPR" 3 "phiopt1" } } */
>>>>> +/* { dg-final { scan-tree-dump-times "MIN_EXPR" 2 "phiopt1" } } */
>>>>>   /* { dg-final { scan-tree-dump-times "MAX_EXPR" 0 "phiopt1" } } */
>>>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/minmax-4.c b/gcc/testsuite/gcc.dg/tree-ssa/minmax-4.c
>>>>> index 973f39bfed3..49e27185b5e 100644
>>>>> --- a/gcc/testsuite/gcc.dg/tree-ssa/minmax-4.c
>>>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/minmax-4.c
>>>>> @@ -26,4 +26,4 @@ main (void)
>>>>>   }
>>>>>
>>>>>   /* { dg-final { scan-tree-dump-times "MIN_EXPR" 0 "phiopt1" } } */
>>>>> -/* { dg-final { scan-tree-dump-times "MAX_EXPR" 3 "phiopt1" } } */
>>>>> +/* { dg-final { scan-tree-dump-times "MAX_EXPR" 2 "phiopt1" } } */
>>>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/minmax-5.c b/gcc/testsuite/gcc.dg/tree-ssa/minmax-5.c
>>>>> index 34e4e720511..194c881cc98 100644
>>>>> --- a/gcc/testsuite/gcc.dg/tree-ssa/minmax-5.c
>>>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/minmax-5.c
>>>>> @@ -25,5 +25,5 @@ main (void)
>>>>>     return 0;
>>>>>   }
>>>>>
>>>>> -/* { dg-final { scan-tree-dump-times "MIN_EXPR" 2 "phiopt1" } } */
>>>>> +/* { dg-final { scan-tree-dump-times "MIN_EXPR" 1 "phiopt1" } } */
>>>>>   /* { dg-final { scan-tree-dump-times "MAX_EXPR" 1 "phiopt1" } } */
>>>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/minmax-8.c b/gcc/testsuite/gcc.dg/tree-ssa/minmax-8.c
>>>>> index 0160e573fef..d5cb53145ea 100644
>>>>> --- a/gcc/testsuite/gcc.dg/tree-ssa/minmax-8.c
>>>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/minmax-8.c
>>>>> @@ -26,4 +26,4 @@ main (void)
>>>>>   }
>>>>>
>>>>>   /* { dg-final { scan-tree-dump-times "MIN_EXPR" 1 "phiopt1" } } */
>>>>> -/* { dg-final { scan-tree-dump-times "MAX_EXPR" 2 "phiopt1" } } */
>>>>> +/* { dg-final { scan-tree-dump-times "MAX_EXPR" 1 "phiopt1" } } */
>>>>> diff --git a/gcc/tree-ssa-phiopt.cc b/gcc/tree-ssa-phiopt.cc
>>>>> index 65b3deea34a..311423efeb5 100644
>>>>> --- a/gcc/tree-ssa-phiopt.cc
>>>>> +++ b/gcc/tree-ssa-phiopt.cc
>>>>> @@ -82,6 +82,25 @@ single_non_singleton_phi_for_edges (gimple_seq seq, edge e0, edge e1)
>>>>>     return phi;
>>>>>   }
>>>>>
>>>>> +/* For each PHI in BB, copy the argument associated with SRC_E to TGT_E.  */
>>>>> +
>>>>> +static void
>>>>> +copy_phi_args (basic_block bb, edge src_e, edge tgt_e)
>>>>> +{
>>>>> +  gphi_iterator gsi;
>>>>> +  int src_indx = src_e->dest_idx;
>>>>> +
>>>>> +  for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
>>>>> +    {
>>>>> +      gphi *phi = gsi.phi ();
>>>>> +      tree def = gimple_phi_arg_def (phi, src_indx);
>>>>> +      location_t locus = gimple_phi_arg_location (phi, src_indx);
>>>>> +
>>>>> +      add_phi_arg (phi, def, tgt_e, locus);
>>>>> +    }
>>>>> +}
>>>>
>>>> Doesn't flush_pending_stmts (tgt_e) do this?
>>>
>>> No, In fact the above code is very similar to the code from
>>> remove_forwarder_block in tree-cfgcleanup.cc (I copied it and changed
>>> it from copy_phi_args in tree-ssa-threadupdate.cc though as I don't
>>> need a mapping).
>>> Let me factor out the code from remove_forwarder_block  and put it in
>>> some common spot and then use that; it will be the same logic even.
>>
>> Hmm, but it's odd - if you redirect an edge on GIMPLE then there should
>> be helpers available to do all this.  I think you're doing something wrong
>> (without actually looking too close)
> 
> Maybe some (crude) diagrams are needed to explain why we need to copy
> the entries for the phi nodes from one edge to another.
> 
> So the original BB structure is:
> 
>                  BB
>             /e1       \e2
>          BB1       BB2
>            \e3      /e4
>                BB3
> BB3 has a few phi nodes (except for one of the phi nodes, the entries
> for BB1, BB2 are all the same).
> When you redirect e1 (or e2) to BB3, we create new entries in the phi
> nodes for that edge now as it was not there before.
> So the shape is:
>         BB
>           |e1 (or e2)
>         BB3
> 
> but since it is a new entry in the PHI node, it will be a nullptr. So
> we need to copy them from the e3 or e4 entries.
> Does that make sense on why the new function is needed here? This is
> not a normal operation done by any other pass either.
Jump threading does some of this kind of stuff.   Does 
copy_phi_arg_into_existing_phi look like something you could use?

jeff