tree-optimization/99912 - delete trivially dead stmts during DSE

DSE performs a backwards walk over stmts removing stores but it
leaves removing resulting dead SSA defs to later passes.  This
eats into its own alias walking budget if the removed stores kept
loads live.  The following patch adds removal of trivially dead
SSA defs which helps in this situation and reduces the amount of
garbage followup passes need to deal with.

Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.

2021-04-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/99912
	* tree-ssa-dse.c (dse_dom_walker::m_need_cfg_cleanup): New.
	(dse_dom_walker::todo): Likewise.
	(dse_dom_walker::dse_optimize_stmt): Move VDEF check to the
	caller.
	(dse_dom_walker::before_dom_children): Remove trivially
	dead SSA defs and schedule CFG cleanup if we removed all
	PHIs in a block.
	(pass_dse::execute): Get TODO as computed by the DOM walker
	and return it.  Wipe dominator info earlier.
---
 gcc/tree-ssa-dse.c | 65 +++++++++++++++++++++++++++++++++++++---------
 1 file changed, 53 insertions(+), 12 deletions(-)

On Tue, 27 Apr 2021 at 19:19, Richard Biener <rguenther@suse.de> wrote:
>
> DSE performs a backwards walk over stmts removing stores but it
> leaves removing resulting dead SSA defs to later passes.  This
> eats into its own alias walking budget if the removed stores kept
> loads live.  The following patch adds removal of trivially dead
> SSA defs which helps in this situation and reduces the amount of
> garbage followup passes need to deal with.
>
> Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
>
> 2021-04-27  Richard Biener  <rguenther@suse.de>
>
>         PR tree-optimization/99912
>         * tree-ssa-dse.c (dse_dom_walker::m_need_cfg_cleanup): New.
>         (dse_dom_walker::todo): Likewise.
>         (dse_dom_walker::dse_optimize_stmt): Move VDEF check to the
>         caller.
>         (dse_dom_walker::before_dom_children): Remove trivially
>         dead SSA defs and schedule CFG cleanup if we removed all
>         PHIs in a block.
>         (pass_dse::execute): Get TODO as computed by the DOM walker
>         and return it.  Wipe dominator info earlier.
> ---
>  gcc/tree-ssa-dse.c | 65 +++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 53 insertions(+), 12 deletions(-)
>
> diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
> index 4967a5a9927..f5f39cbe903 100644
> --- a/gcc/tree-ssa-dse.c
> +++ b/gcc/tree-ssa-dse.c
> @@ -963,16 +963,25 @@ public:
>    dse_dom_walker (cdi_direction direction)
>      : dom_walker (direction),
>      m_live_bytes (param_dse_max_object_size),
> -    m_byte_tracking_enabled (false) {}
> +    m_byte_tracking_enabled (false),
> +    m_need_cfg_cleanup (false) {}
>
>    virtual edge before_dom_children (basic_block);
> +  unsigned todo () const;
>
>  private:
>    auto_sbitmap m_live_bytes;
>    bool m_byte_tracking_enabled;
> +  bool m_need_cfg_cleanup;
>    void dse_optimize_stmt (gimple_stmt_iterator *);
>  };
>
> +unsigned
> +dse_dom_walker::todo () const
> +{
> +  return m_need_cfg_cleanup ? TODO_cleanup_cfg : 0;
> +}
> +
>  /* Delete a dead call at GSI, which is mem* call of some kind.  */
>  static void
>  delete_dead_or_redundant_call (gimple_stmt_iterator *gsi, const char *type)
> @@ -1049,11 +1058,6 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
>  {
>    gimple *stmt = gsi_stmt (*gsi);
>
> -  /* If this statement has no virtual defs, then there is nothing
> -     to do.  */
> -  if (!gimple_vdef (stmt))
> -    return;
> -
>    /* Don't return early on *this_2(D) ={v} {CLOBBER}.  */
>    if (gimple_has_volatile_ops (stmt)
>        && (!gimple_clobber_p (stmt)
> @@ -1180,12 +1184,47 @@ dse_dom_walker::before_dom_children (basic_block bb)
>
>    for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
>      {
> -      dse_optimize_stmt (&gsi);
> +      gimple *stmt = gsi_stmt (gsi);
> +
> +      if (gimple_vdef (stmt))
> +       dse_optimize_stmt (&gsi);
> +      else if (def_operand_p def_p = single_ssa_def_operand (stmt, SSA_OP_DEF))
> +       {
> +         /* When we remove dead stores make sure to also delete trivially
> +            dead SSA defs.  */
> +         if (has_zero_uses (DEF_FROM_PTR (def_p))
> +             && !gimple_has_side_effects (stmt))
> +           {
> +             if (dump_file && (dump_flags & TDF_DETAILS))
> +               {
> +                 fprintf (dump_file, "  Deleted trivially dead stmt: ");
> +                 print_gimple_stmt (dump_file, stmt, 0, dump_flags);
> +                 fprintf (dump_file, "\n");
> +               }
> +             if (gsi_remove (&gsi, true) && need_eh_cleanup)
> +               bitmap_set_bit (need_eh_cleanup, bb->index);
> +             release_defs (stmt);
> +           }
> +       }
>        if (gsi_end_p (gsi))
>         gsi = gsi_last_bb (bb);
>        else
>         gsi_prev (&gsi);
>      }
> +  bool removed_phi = false;
> +  for (gphi_iterator si = gsi_start_phis (bb); !gsi_end_p (si);)
> +    {
> +      gphi *phi = si.phi ();
> +      if (has_zero_uses (gimple_phi_result (phi)))
> +       {
> +         remove_phi_node (&si, true);
> +         removed_phi = true;
> +       }
Hi Richard,
Just curious if this is missing dumping info about removed phi node to
dump_file ?

Thanks,
Prathamesh
> +      else
> +       gsi_next (&si);
> +    }
> +  if (removed_phi && gimple_seq_empty_p (phi_nodes (bb)))
> +    m_need_cfg_cleanup = true;
>    return NULL;
>  }
>
> @@ -1234,21 +1273,23 @@ pass_dse::execute (function *fun)
>
>    /* Dead store elimination is fundamentally a walk of the post-dominator
>       tree and a backwards walk of statements within each block.  */
> -  dse_dom_walker (CDI_POST_DOMINATORS).walk (fun->cfg->x_exit_block_ptr);
> +  dse_dom_walker walker (CDI_POST_DOMINATORS);
> +  walker.walk (fun->cfg->x_exit_block_ptr);
> +  free_dominance_info (CDI_POST_DOMINATORS);
> +
> +  unsigned todo = walker.todo ();
>
>    /* Removal of stores may make some EH edges dead.  Purge such edges from
>       the CFG as needed.  */
>    if (!bitmap_empty_p (need_eh_cleanup))
>      {
>        gimple_purge_all_dead_eh_edges (need_eh_cleanup);
> -      cleanup_tree_cfg ();
> +      todo |= TODO_cleanup_cfg;
>      }
>
>    BITMAP_FREE (need_eh_cleanup);
>
> -  /* For now, just wipe the post-dominator information.  */
> -  free_dominance_info (CDI_POST_DOMINATORS);
> -  return 0;
> +  return todo;
>  }
>
>  } // anon namespace
> --
> 2.26.2

On Tue, 27 Apr 2021, Prathamesh Kulkarni wrote:

> On Tue, 27 Apr 2021 at 19:19, Richard Biener <rguenther@suse.de> wrote:
> >
> > DSE performs a backwards walk over stmts removing stores but it
> > leaves removing resulting dead SSA defs to later passes.  This
> > eats into its own alias walking budget if the removed stores kept
> > loads live.  The following patch adds removal of trivially dead
> > SSA defs which helps in this situation and reduces the amount of
> > garbage followup passes need to deal with.
> >
> > Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
> >
> > 2021-04-27  Richard Biener  <rguenther@suse.de>
> >
> >         PR tree-optimization/99912
> >         * tree-ssa-dse.c (dse_dom_walker::m_need_cfg_cleanup): New.
> >         (dse_dom_walker::todo): Likewise.
> >         (dse_dom_walker::dse_optimize_stmt): Move VDEF check to the
> >         caller.
> >         (dse_dom_walker::before_dom_children): Remove trivially
> >         dead SSA defs and schedule CFG cleanup if we removed all
> >         PHIs in a block.
> >         (pass_dse::execute): Get TODO as computed by the DOM walker
> >         and return it.  Wipe dominator info earlier.
> > ---
> >  gcc/tree-ssa-dse.c | 65 +++++++++++++++++++++++++++++++++++++---------
> >  1 file changed, 53 insertions(+), 12 deletions(-)
> >
> > diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
> > index 4967a5a9927..f5f39cbe903 100644
> > --- a/gcc/tree-ssa-dse.c
> > +++ b/gcc/tree-ssa-dse.c
> > @@ -963,16 +963,25 @@ public:
> >    dse_dom_walker (cdi_direction direction)
> >      : dom_walker (direction),
> >      m_live_bytes (param_dse_max_object_size),
> > -    m_byte_tracking_enabled (false) {}
> > +    m_byte_tracking_enabled (false),
> > +    m_need_cfg_cleanup (false) {}
> >
> >    virtual edge before_dom_children (basic_block);
> > +  unsigned todo () const;
> >
> >  private:
> >    auto_sbitmap m_live_bytes;
> >    bool m_byte_tracking_enabled;
> > +  bool m_need_cfg_cleanup;
> >    void dse_optimize_stmt (gimple_stmt_iterator *);
> >  };
> >
> > +unsigned
> > +dse_dom_walker::todo () const
> > +{
> > +  return m_need_cfg_cleanup ? TODO_cleanup_cfg : 0;
> > +}
> > +
> >  /* Delete a dead call at GSI, which is mem* call of some kind.  */
> >  static void
> >  delete_dead_or_redundant_call (gimple_stmt_iterator *gsi, const char *type)
> > @@ -1049,11 +1058,6 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
> >  {
> >    gimple *stmt = gsi_stmt (*gsi);
> >
> > -  /* If this statement has no virtual defs, then there is nothing
> > -     to do.  */
> > -  if (!gimple_vdef (stmt))
> > -    return;
> > -
> >    /* Don't return early on *this_2(D) ={v} {CLOBBER}.  */
> >    if (gimple_has_volatile_ops (stmt)
> >        && (!gimple_clobber_p (stmt)
> > @@ -1180,12 +1184,47 @@ dse_dom_walker::before_dom_children (basic_block bb)
> >
> >    for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
> >      {
> > -      dse_optimize_stmt (&gsi);
> > +      gimple *stmt = gsi_stmt (gsi);
> > +
> > +      if (gimple_vdef (stmt))
> > +       dse_optimize_stmt (&gsi);
> > +      else if (def_operand_p def_p = single_ssa_def_operand (stmt, SSA_OP_DEF))
> > +       {
> > +         /* When we remove dead stores make sure to also delete trivially
> > +            dead SSA defs.  */
> > +         if (has_zero_uses (DEF_FROM_PTR (def_p))
> > +             && !gimple_has_side_effects (stmt))
> > +           {
> > +             if (dump_file && (dump_flags & TDF_DETAILS))
> > +               {
> > +                 fprintf (dump_file, "  Deleted trivially dead stmt: ");
> > +                 print_gimple_stmt (dump_file, stmt, 0, dump_flags);
> > +                 fprintf (dump_file, "\n");
> > +               }
> > +             if (gsi_remove (&gsi, true) && need_eh_cleanup)
> > +               bitmap_set_bit (need_eh_cleanup, bb->index);
> > +             release_defs (stmt);
> > +           }
> > +       }
> >        if (gsi_end_p (gsi))
> >         gsi = gsi_last_bb (bb);
> >        else
> >         gsi_prev (&gsi);
> >      }
> > +  bool removed_phi = false;
> > +  for (gphi_iterator si = gsi_start_phis (bb); !gsi_end_p (si);)
> > +    {
> > +      gphi *phi = si.phi ();
> > +      if (has_zero_uses (gimple_phi_result (phi)))
> > +       {
> > +         remove_phi_node (&si, true);
> > +         removed_phi = true;
> > +       }
> Hi Richard,
> Just curious if this is missing dumping info about removed phi node to
> dump_file ?

Yes, I'll add this.  I also have to investigate some testsuite fallout,
so there'll be v2.

Richard.

> Thanks,
> Prathamesh
> > +      else
> > +       gsi_next (&si);
> > +    }
> > +  if (removed_phi && gimple_seq_empty_p (phi_nodes (bb)))
> > +    m_need_cfg_cleanup = true;
> >    return NULL;
> >  }
> >
> > @@ -1234,21 +1273,23 @@ pass_dse::execute (function *fun)
> >
> >    /* Dead store elimination is fundamentally a walk of the post-dominator
> >       tree and a backwards walk of statements within each block.  */
> > -  dse_dom_walker (CDI_POST_DOMINATORS).walk (fun->cfg->x_exit_block_ptr);
> > +  dse_dom_walker walker (CDI_POST_DOMINATORS);
> > +  walker.walk (fun->cfg->x_exit_block_ptr);
> > +  free_dominance_info (CDI_POST_DOMINATORS);
> > +
> > +  unsigned todo = walker.todo ();
> >
> >    /* Removal of stores may make some EH edges dead.  Purge such edges from
> >       the CFG as needed.  */
> >    if (!bitmap_empty_p (need_eh_cleanup))
> >      {
> >        gimple_purge_all_dead_eh_edges (need_eh_cleanup);
> > -      cleanup_tree_cfg ();
> > +      todo |= TODO_cleanup_cfg;
> >      }
> >
> >    BITMAP_FREE (need_eh_cleanup);
> >
> > -  /* For now, just wipe the post-dominator information.  */
> > -  free_dominance_info (CDI_POST_DOMINATORS);
> > -  return 0;
> > +  return todo;
> >  }
> >
> >  } // anon namespace
> > --
> > 2.26.2
>