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

[Richard Sandiford <richard.sandiford@arm.com>]

Richard Biener <richard.guenther@gmail.com> writes:
> 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?

Overall I see a ~2.5% slowdown and a 4.7% increase in load size.
That comes almost entirely from the (RTL) DSE side; this patch
and gimple DSE part don't seem to make much difference.

If I keep the gimple passes as-is and just disable RTL DSE, the slowdown
is still ~2.5% and there's a 4.4% increase in load size.

These are all measuring cc1plus (built from post-patch sources)
and using -O2 -g tree-into-ssa.ii for the speed checks.

> Can you restrict the keep-all-vdefs to user variables (and measure the
> difference this makes)?

In order to avoid wrong debug for pointer dereferences, I think it would
have to be keep-all-vdefs for writes to either user variables or unknown
locations.  But as above, I can't measure a significant difference with
the patch.

> Again I wonder if this makes C++ with -Og impractical runtime-wise.

Got a particular test in mind?

Richard