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

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

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 can
also introduce wrong debug info for earlier references (second test
in Og-dce-3.c) or make earlier references unavailable (first test
in 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).

2019-07-29  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.

From-SVN: r273872