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

[Andrew Pinski <pinskia@gmail.com>]

On Tue, May 2, 2023 at 11:36 PM Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
>
> 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?

It looks to be a semi-generalization of the function I created. That
is, it supports phi nodes for the edges in different bbs while the one
I created assumes both edges now point to the same bb.
Let me look into moving that into a common location and using that in
the two other places (and also in phiopt).

Thanks,
Andrew

>
> jeff