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

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