[PATCH] c: Extend the -Wpadded message with actual padding size

[Vit Kabele <vit.kabele@sysgo.com>]

When the compiler warns about padding struct to alignment boundary, it
now also informs the user about the size of the alignment that needs to
be added to get rid of the warning.

This removes the need of using pahole or similar tools, or manually
determining the padding size.

Tested on x86_64-pc-linux-gnu.

gcc/ChangeLog:

	* stor-layout.cc (finalize_record_size): Improve warning message

Signed-off-by: Vit Kabele <vit.kabele@sysgo.com>
---
 gcc/stor-layout.cc | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
index 765f22f68b9..57ddb001780 100644
--- a/gcc/stor-layout.cc
+++ b/gcc/stor-layout.cc
@@ -1781,7 +1781,14 @@ finalize_record_size (record_layout_info rli)
       && simple_cst_equal (unpadded_size, TYPE_SIZE (rli->t)) == 0
       && input_location != BUILTINS_LOCATION
       && !TYPE_ARTIFICIAL (rli->t))
-    warning (OPT_Wpadded, "padding struct size to alignment boundary");
+  {
+      tree padding_size
+		= size_binop (MINUS_EXPR,
+			TYPE_SIZE_UNIT (rli->t), unpadded_size_unit);
+      warning (OPT_Wpadded,
+		   "padding struct size to alignment boundary with %E bytes",
+		   padding_size);
+  }
 
   if (warn_packed && TREE_CODE (rli->t) == RECORD_TYPE
       && TYPE_PACKED (rli->t) && ! rli->packed_maybe_necessary
-- 
2.30.2