[PATCH v3 01/12] stddef: Avoid warning with clang with offsetof()

[Simon Glass]

Some bright sparks have decided that a cast on a constant cannot be a
constant, so offsetof() produces this warning on clang-10:

include/intel_gnvs.h:113:1: error: static_assert expression is not an
	integral constant expression
check_member(acpi_global_nvs, unused2, GNVS_CHROMEOS_ACPI_OFFSET);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/kernel.h:284:2: note: expanded from macro 'check_member'
        offsetof(struct structure, member) == (offset), \
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/stddef.h:20:32: note: expanded from macro 'offsetof'
                                ^
include/intel_gnvs.h:113:1: note: cast that performs the conversions of
	a reinterpret_cast is ot allowed in a constant expression
include/linux/stddef.h:20:33: note: expanded from macro 'offsetof'

Fix it by using the compiler built-in version, if available. This syncs
the function to the same implementation as Linux v5.16 in this header
file.

Signed-off-by: Simon Glass <sjg at chromium.org>
---

Changes in v3:
- Update commit message to mention this syncs with Linux

 include/linux/stddef.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/stddef.h b/include/linux/stddef.h
index c540f6100d4..a7f546fdfe5 100644
--- a/include/linux/stddef.h
+++ b/include/linux/stddef.h
@@ -1,6 +1,8 @@
 #ifndef _LINUX_STDDEF_H
 #define _LINUX_STDDEF_H
 
+#include <linux/compiler_types.h>
+
 #undef NULL
 #if defined(__cplusplus)
 #define NULL 0
@@ -14,7 +16,11 @@
 
 #ifndef __CHECKER__
 #undef offsetof
-#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
+#ifdef __compiler_offsetof
+#define offsetof(TYPE, MEMBER)	__compiler_offsetof(TYPE, MEMBER)
+#else
+#define offsetof(TYPE, MEMBER)	((size_t)&((TYPE *)0)->MEMBER)
+#endif
 #endif
 
 #endif
-- 
2.34.1.575.g55b058a8bb-goog