On 7/8/19 5:34 AM, Richard Biener wrote:
> On Sun, Jul 7, 2019 at 9:07 PM Jeff Law <l...@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.sandif...@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.I've never really seen DSE
> be terribly important for any codebase. It's
unfortunate, but that's my experience.
Jeff
