linux-kernel - [PATCH v3] include/uapi/linux/swab.h: move explicit cast outside ternary

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Message-Id: <20220608223539.470472-1-justinstitt@google.com>
Date:   Wed,  8 Jun 2022 15:35:39 -0700
From:   Justin Stitt <jstitt007@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Al Viro <viro@...iv.linux.org.uk>,
        Justin Stitt <jstitt007@...il.com>
Subject: [PATCH v3] include/uapi/linux/swab.h: move explicit cast outside ternary

From: Justin Stitt <jstitt007@...il.com>

A cast inside __builtin_constant_p doesn't do anything since it should
evaluate as constant at compile time irrespective of this cast. Instead,
I moved this cast outside the ternary to ensure the return type is as
expected.

Additionally, if __HAVE_BUILTIN_BSWAP16__ was not defined then __swab16
is actually returning an `int` not a `u16` due to integer promotion.

As Al Viro notes:
You *can't* get smaller-than-int out of ? :, same as you can't get it
out of addition, etc.

This also fixes some clang -Wformat warnings involving default
argument promotion.

Link: https://github.com/ClangBuiltLinux/linux/issues/378
Suggested-by: Al Viro <viro@...iv.linux.org.uk>
Suggested-by: Nathan Chancellor <nathan@...nel.org>
Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Justin Stitt <jstitt007@...il.com>
---
 diff from v2 -> v3:
  * re-insert respective (u16, u32, u64) cast to __builtin_bswap as per
     Nick's suggestion
  * added note from Al Viro

 include/uapi/linux/swab.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/uapi/linux/swab.h b/include/uapi/linux/swab.h
index 7272f85d6d6a..0723a9cce747 100644
--- a/include/uapi/linux/swab.h
+++ b/include/uapi/linux/swab.h
@@ -102,7 +102,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
 #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x))
 #else
 #define __swab16(x)				\
-	(__builtin_constant_p((__u16)(x)) ?	\
+	(__u16)(__builtin_constant_p(x) ?	\
 	___constant_swab16(x) :			\
 	__fswab16(x))
 #endif
@@ -115,7 +115,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
 #define __swab32(x) (__u32)__builtin_bswap32((__u32)(x))
 #else
 #define __swab32(x)				\
-	(__builtin_constant_p((__u32)(x)) ?	\
+	(__u32)(__builtin_constant_p(x) ?	\
 	___constant_swab32(x) :			\
 	__fswab32(x))
 #endif
@@ -128,7 +128,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
 #define __swab64(x) (__u64)__builtin_bswap64((__u64)(x))
 #else
 #define __swab64(x)				\
-	(__builtin_constant_p((__u64)(x)) ?	\
+	(__u64)(__builtin_constant_p(x) ?	\
 	___constant_swab64(x) :			\
 	__fswab64(x))
 #endif
-- 
2.36.1.476.g0c4daa206d-goog