compiler_types.h: fix "unused variable" in __compiletime_assert()

This refines commit c03567a8e8d5 ("include/linux/compiler.h: don't perform
compiletime_assert with -O0") and restores #ifdef __OPTIMIZE__ symmetry by
evaluating the 'condition' variable in both compile-time variants of
__compiletimeassert().

As __OPTIMIZE__ is always true by default, this commit does not change
anything by default.  But it fixes warnings with _non-default_ CFLAGS like
for instance this:

 make  CFLAGS_tcp.o='-Og -U__OPTIMIZE__'

                 from net/ipv4/tcp.c:273:

 include/net/sch_generic.h: In function `qdisc_cb_private_validate':
 include/net/sch_generic.h:511:30:
            error: unused variable `qcb' [-Werror=unused-variable]

  {
     struct qdisc_skb_cb *qcb;

     BUILD_BUG_ON(sizeof(skb->cb) < sizeof(*qcb));
     ...
  }

[akpm@linux-foundation.org: regularize comment layout, reflow comment]
Link: https://lkml.kernel.org/r/20250424194048.652571-1-marc.herbert@linux.intel.com
Signed-off-by: Marc Herbert <Marc.Herbert@linux.intel.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Changbin Du <changbin.du@intel.com>
Cc: Jan Hendrik Farr <kernel@jfarr.cc>
Cc: Macro Elver <elver@google.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Tony Ambardar <tony.ambardar@gmail.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 501cffddc2f4..1f2eee8331d3 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -525,6 +525,12 @@ struct ftrace_likely_data {
 	 sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))
 
 #ifdef __OPTIMIZE__
+/*
+ * #ifdef __OPTIMIZE__ is only a good approximation; for instance "make
+ * CFLAGS_foo.o=-Og" defines __OPTIMIZE__, does not elide the conditional code
+ * and can break compilation with wrong error message(s). Combine with
+ * -U__OPTIMIZE__ when needed.
+ */
 # define __compiletime_assert(condition, msg, prefix, suffix)		\
 	do {								\
 		/*							\
@@ -538,7 +544,7 @@ struct ftrace_likely_data {
 			prefix ## suffix();				\
 	} while (0)
 #else
-# define __compiletime_assert(condition, msg, prefix, suffix) do { } while (0)
+# define __compiletime_assert(condition, msg, prefix, suffix) ((void)(condition))
 #endif
 
 #define _compiletime_assert(condition, msg, prefix, suffix) \