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

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

I fixed the formatting and added the test.

The test has first element 32bit so that it should work on both 32 and
64bit architectures, even without the aligned attribute.

If there is some better way how to write the test properly formatted
(i.e. not on a single line), please let me know.

-- >8 --
Subject: [PATCH] c: Extend the -Wpadded message with actual padding size

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): Extend warning message.

gcc/testsuite/ChangeLog:

	* c-c++-common/Wpadded.c: New test.

Signed-off-by: Vit Kabele <vit.kabele@sysgo.com>
---
 gcc/stor-layout.cc                   |  7 ++++++-
 gcc/testsuite/c-c++-common/Wpadded.c | 10 ++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/c-c++-common/Wpadded.c

diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
index 765f22f68b9..88923c4136b 100644
--- a/gcc/stor-layout.cc
+++ b/gcc/stor-layout.cc
@@ -1781,7 +1781,12 @@ 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 pad_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", pad_size);
+  }
 
   if (warn_packed && TREE_CODE (rli->t) == RECORD_TYPE
       && TYPE_PACKED (rli->t) && ! rli->packed_maybe_necessary
diff --git a/gcc/testsuite/c-c++-common/Wpadded.c b/gcc/testsuite/c-c++-common/Wpadded.c
new file mode 100644
index 00000000000..e8f1044a36b
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wpadded.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-Wpadded" } */
+
+/*
+ * The struct is on single line, because C++ compiler emits the -Wpadded
+ * warning at the first line of the struct, while the C compiler at the last
+ * line of the struct definition. This way the test passes on both
+ */
+struct S { __UINT32_TYPE__ i; char c; }; /* { dg-warning "padding struct size to alignment boundary with 3 bytes" } */
+
-- 
2.30.2