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
jeff