>From c62cffde2be0debcbb35590a1ffc993161fcdb28 Mon Sep 17 00:00:00 2001
From: "Richard W.M. Jones" <rjones@redhat.com>
Date: Thu, 7 Jul 2022 20:21:39 +0100
Subject: [PATCH] common/include: Rename BUILD_BUG_ON_ZERO to something more
 meaningful

Updates: commit 0fa23df5cd5dc97a55857416ea81d5de6d867c18
Thanks: Laszlo Ersek, Eric Blake
---
 common/include/array-size.h      |  2 +-
 common/include/compiler-macros.h | 22 ++++++++++++----------
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/common/include/array-size.h b/common/include/array-size.h
index b6d33dde..3212d9dc 100644
--- a/common/include/array-size.h
+++ b/common/include/array-size.h
@@ -36,6 +36,6 @@
 #include "compiler-macros.h"
 
 #define ARRAY_SIZE(a) \
-  ((sizeof (a) / sizeof ((a)[0])) + BUILD_BUG_ON_ZERO (!TYPE_IS_ARRAY(a)))
+  ((sizeof (a) / sizeof ((a)[0])) + BUILD_BUG_UNLESS_TRUE (TYPE_IS_ARRAY(a)))
 
 #endif /* NBDKIT_ARRAY_SIZE_H */
diff --git a/common/include/compiler-macros.h b/common/include/compiler-macros.h
index 504e0085..1a6cba2b 100644
--- a/common/include/compiler-macros.h
+++ b/common/include/compiler-macros.h
@@ -35,24 +35,26 @@
 
 #ifndef __cplusplus
 
-/* This expression fails at compile time if 'expr' is true.  It does
- * this by constructing a struct which has an impossible
- * (negative-sized) array.
+/* BUILD_BUG_UNLESS_TRUE(1) => 0
+ * BUILD_BUG_UNLESS_TRUE(0) => build time failure
  *
- * If 'expr' is false then we subtract the sizes of the two identical
- * structures, returning zero.
+ * It works by constructing a struct which has an impossible
+ * (negative-sized) bit-field in the build failure case.
+ *
+ * The Linux kernel calls this BUILD_BUG_ON_ZERO which is a
+ * confusing name, but has the same semantics as above.
  */
-#define BUILD_BUG_ON_ZERO_SIZEOF(expr) \
-  (sizeof (struct { int _array_size_failed[(expr) ? -1 : 1]; }))
-#define BUILD_BUG_ON_ZERO(expr) \
-  (BUILD_BUG_ON_ZERO_SIZEOF(expr) - BUILD_BUG_ON_ZERO_SIZEOF(expr))
+#define BUILD_BUG_STRUCT_SIZE(expr) \
+  (sizeof (struct { int: (expr) ? 1 : -1; }))
+#define BUILD_BUG_UNLESS_TRUE(expr) \
+  (BUILD_BUG_STRUCT_SIZE(expr) - BUILD_BUG_STRUCT_SIZE(expr))
 
 #define TYPE_IS_ARRAY(a) \
   (!__builtin_types_compatible_p (typeof (a), typeof (&(a)[0])))
 
 #else /* __cplusplus */
 
-#define BUILD_BUG_ON_ZERO(expr) 0
+#define BUILD_BUG_UNLESS_TRUE(expr) 0
 #define TYPE_IS_ARRAY(a) 1
 
 #endif /* __cplusplus */
-- 
2.37.0.rc2