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

[Richard Biener <richard.guenther@gmail.com>]

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)

Richard.

> Thanks,
> Andrew
>
> >
> > > +
> > > +
> > >  /* Replace PHI node element whose edge is E in block BB with variable NEW.
> > >     Remove the edge from COND_BLOCK which does not lead to BB (COND_BLOCK
> > >     is known to have two edges, one of which must reach BB).  */
> > > @@ -94,6 +113,7 @@ replace_phi_edge_with_variable (basic_block cond_block,
> > >    basic_block bb = gimple_bb (phi);
> > >    gimple_stmt_iterator gsi;
> > >    tree phi_result = PHI_RESULT (phi);
> > > +  bool deleteboth = false;
> > >
> > >    /* Duplicate range info if they are the only things setting the target PHI.
> > >       This is needed as later on, the new_tree will be replacing
> > > @@ -137,7 +157,14 @@ replace_phi_edge_with_variable (basic_block cond_block,
> > >        keep_edge = EDGE_SUCC (cond_block, 1);
> > >      }
> > >    else if ((keep_edge = find_edge (cond_block, e->src)))
> > > -    ;
> > > +    {
> > > +      basic_block bb1 = EDGE_SUCC (cond_block, 0)->dest;
> > > +      basic_block bb2 = EDGE_SUCC (cond_block, 1)->dest;
> > > +      if (single_pred_p (bb1) && single_pred_p (bb2)
> > > +         && single_succ_p (bb1) && single_succ_p (bb2)
> > > +         && empty_block_p (bb1) && empty_block_p (bb2))
> > > +       deleteboth = true;
> > > +    }
> > >    else
> > >      gcc_unreachable ();
> > >
> > > @@ -148,6 +175,28 @@ replace_phi_edge_with_variable (basic_block cond_block,
> > >        e->probability = profile_probability::always ();
> > >        delete_basic_block (edge_to_remove->dest);
> > >
> > > +      /* Eliminate the COND_EXPR at the end of COND_BLOCK.  */
> > > +      gsi = gsi_last_bb (cond_block);
> > > +      gsi_remove (&gsi, true);
> > > +    }
> > > +  else if (deleteboth)
> > > +    {
> > > +      basic_block bb1 = EDGE_SUCC (cond_block, 0)->dest;
> > > +      basic_block bb2 = EDGE_SUCC (cond_block, 1)->dest;
> > > +
> > > +      edge newedge = redirect_edge_and_branch (keep_edge, bb);
> > > +      /* no new edge should be the same. */
> > > +      gcc_assert (newedge == keep_edge);
> > > +      keep_edge->flags |= EDGE_FALLTHRU;
> > > +      keep_edge->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
> > > +      keep_edge->probability = profile_probability::always ();
> > > +      /* Copy the edge's phi entry from the old one */
> > > +      copy_phi_args(bb, e, keep_edge);
> > > +
> > > +      /* Delete the old 2 empty basic blocks */
> > > +      delete_basic_block (bb1);
> > > +      delete_basic_block (bb2);
> > > +
> > >        /* Eliminate the COND_EXPR at the end of COND_BLOCK.  */
> > >        gsi = gsi_last_bb (cond_block);
> > >        gsi_remove (&gsi, true);
> > > --
> > > 2.31.1
> > >