This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Prevent tree-ssa-dce.c from deleting stores at -Og


On Sun, Jul 7, 2019 at 9:07 PM Jeff Law <law@redhat.com> wrote:
>
> On 7/7/19 3:45 AM, Richard Sandiford wrote:
> > DCE tries to delete dead stores to local data and also tries to insert
> > debug binds for simple cases:
> >
> >   /* If this is a store into a variable that is being optimized away,
> >      add a debug bind stmt if possible.  */
> >   if (MAY_HAVE_DEBUG_BIND_STMTS
> >       && gimple_assign_single_p (stmt)
> >       && is_gimple_val (gimple_assign_rhs1 (stmt)))
> >     {
> >       tree lhs = gimple_assign_lhs (stmt);
> >       if ((VAR_P (lhs) || TREE_CODE (lhs) == PARM_DECL)
> >         && !DECL_IGNORED_P (lhs)
> >         && is_gimple_reg_type (TREE_TYPE (lhs))
> >         && !is_global_var (lhs)
> >         && !DECL_HAS_VALUE_EXPR_P (lhs))
> >       {
> >         tree rhs = gimple_assign_rhs1 (stmt);
> >         gdebug *note
> >           = gimple_build_debug_bind (lhs, unshare_expr (rhs), stmt);
> >         gsi_insert_after (i, note, GSI_SAME_STMT);
> >       }
> >     }
> >
> > But this doesn't help for things like "print *ptr" when ptr points
> > to the local variable (tests Og-dce-1.c and Og-dce-2.c).  It also tends
> > to make the *live* -- and thus useful -- values optimised out, because
> > we can't yet switch back to tracking the memory location as it evolves
> > over time (test Og-dce-3.c).
> >
> > So for -Og I think it'd be better not to delete any stmts with
> > vdefs for now.  This also means that we can avoid the potentially
> > expensive vop walks (which already have a cut-off, but still).
> >
> > The patch also fixes the Og failures in gcc.dg/guality/pr54970.c
> > (PR 86638).
> >
> > Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?
> >
> > Richard
> >
> >
> > 2019-07-07  Richard Sandiford  <richard.sandiford@arm.com>
> >
> > gcc/
> >       PR debug/86638
> >       * tree-ssa-dce.c (keep_all_vdefs_p): New function.
> >       (mark_stmt_if_obviously_necessary): Mark all stmts with vdefs as
> >       necessary if keep_all_vdefs_p is true.
> >       (mark_aliased_reaching_defs_necessary): Add a gcc_checking_assert
> >       that keep_all_vdefs_p is false.
> >       (mark_all_reaching_defs_necessary): Likewise.
> >       (propagate_necessity): Skip the vuse scan if keep_all_vdefs_p is true.
> >
> > gcc/testsuite/
> >       * c-c++-common/guality/Og-dce-1.c: New test.
> >       * c-c++-common/guality/Og-dce-2.c: Likewise.
> >       * c-c++-common/guality/Og-dce-3.c: Likewise.
> OK

I wonder how code size (and compile-time) is affected by the DSE/DCE patch?
Say just look at -Og built cc1?  Can you restrict the keep-all-vdefs to
user variables (and measure the difference this makes)?  Again I wonder if
this makes C++ with -Og impractical runtime-wise.

Richard.

> jeff


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]