linux-kernel - [PATCH next 1/1] Use _Generic() to implement __is

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Date:   Fri, 10 Nov 2023 10:34:31 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "'linux@...musvillemoes.dk'" <linux@...musvillemoes.dk>,
        'Steven Rostedt' <rostedt@...dmis.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "'bvanassche@....org'" <bvanassche@....org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "'Martin.Uecker@....uni-goettingen.de'" 
        <Martin.Uecker@....uni-goettingen.de>
Subject: [PATCH next 1/1] Use _Generic() to implement __is_constexpr()

This is somewhat more obvious than relying on sizeof(void) being 1.
It is also portable to any c11 compiler.

Change the constants to 0 - they have no bearing on the result and
just confuse the reader.

It relies on the type of (expr ? NULL : (type *)any) being 'type *'
but that of (expr ? (void *)x : (type *)any) being 'void *'.

Signed-off-by: David Laight <david.laight@...lab.com>
---

I'm not 100% sure all of 'tools' sets -std=gnu99 or -std=c11 to get
_Generic() supported.

This version will also be usable in 'nolibc' where sizeof(void) is
an error.

 include/linux/compiler.h       | 2 +-
 tools/include/linux/compiler.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index bb1339c7057b..2e11e216fd69 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -233,7 +233,7 @@ static inline void *offset_to_ptr(const int *off)
  * Glory to Martin Uecker <Martin.Uecker@....uni-goettingen.de>
  */
 #define __is_constexpr(x) \
-	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
+	_Generic(0 ? (void *)((long)(x) * 0) : (char *)0, char *: 1, void *: 0)
 
 /*
  * Whether 'type' is a signed type or an unsigned type. Supports scalar types,
diff --git a/tools/include/linux/compiler.h b/tools/include/linux/compiler.h
index 7b65566f3e42..6e7f05196414 100644
--- a/tools/include/linux/compiler.h
+++ b/tools/include/linux/compiler.h
@@ -69,7 +69,7 @@
  * Glory to Martin Uecker <Martin.Uecker@....uni-goettingen.de>
  */
 #define __is_constexpr(x) \
-	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
+	_Generic(0 ? (void *)((long)(x) * 0) : (char *)0, char *: 1, void *: 0)
 
 #ifdef __ANDROID__
 /*
-- 
2.17.1

-
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