[PATCH] Improve DSE to handle redundant zero initializations.

[Richard Biener]

On Tue, Aug 13, 2019 at 10:18 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Thanks for doing this.
>
> Matthew Beliveau <mbelivea@redhat.com> writes:
> > diff --git gcc/tree-ssa-dse.c gcc/tree-ssa-dse.c
> > index 5b7c4fc6d1a..dcaeb8edbfe 100644
> > --- gcc/tree-ssa-dse.c
> > +++ gcc/tree-ssa-dse.c
> > @@ -628,11 +628,8 @@ dse_optimize_redundant_stores (gimple *stmt)
> >        tree fndecl;
> >        if ((is_gimple_assign (use_stmt)
> >          && gimple_vdef (use_stmt)
> > -        && ((gimple_assign_rhs_code (use_stmt) == CONSTRUCTOR
> > -             && CONSTRUCTOR_NELTS (gimple_assign_rhs1 (use_stmt)) == 0
> > -             && !gimple_clobber_p (stmt))
> > -            || (gimple_assign_rhs_code (use_stmt) == INTEGER_CST
> > -                && integer_zerop (gimple_assign_rhs1 (use_stmt)))))
> > +        && initializer_zerop (gimple_op (use_stmt, 1), NULL)
> > +        && !gimple_clobber_p (stmt))
> >         || (gimple_call_builtin_p (use_stmt, BUILT_IN_NORMAL)
> >             && (fndecl = gimple_call_fndecl (use_stmt)) != NULL
> >             && (DECL_FUNCTION_CODE (fndecl) == BUILT_IN_MEMSET
> > @@ -1027,15 +1024,13 @@ dse_dom_walker::dse_optimize_stmt (gimple_stmt_iterator *gsi)
> >      {
> >        bool by_clobber_p = false;
> >
> > -      /* First see if this store is a CONSTRUCTOR and if there
> > -      are subsequent CONSTRUCTOR stores which are totally
> > -      subsumed by this statement.  If so remove the subsequent
> > -      CONSTRUCTOR store.
> > +      /* Check if this store initalizes zero, or some aggregate of zeros,
> > +      and check if there are subsequent stores which are subsumed by this
> > +      statement.  If so, remove the subsequent store.
> >
> >        This will tend to make fewer calls into memset with longer
> >        arguments.  */
> > -      if (gimple_assign_rhs_code (stmt) == CONSTRUCTOR
> > -       && CONSTRUCTOR_NELTS (gimple_assign_rhs1 (stmt)) == 0
> > +      if (initializer_zerop (gimple_op (stmt, 1), NULL)
> >         && !gimple_clobber_p (stmt))
> >       dse_optimize_redundant_stores (stmt);
> >
>
> In addition to Jeff's comment, the original choice of gimple_assign_rhs1
> is the preferred way to write this (applies to both hunks).

And the !gimple_clobber_p test is now redundant.

> Richard