Date: Fri, 9 May 2025 22:50:35 GMT From: Lexi Winter <ivy@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: 7e0c5e0128c4 - main - sys/cdefs.h: add __nodiscard annotation Message-ID: <202505092250.549MoZsx028151@gitrepo.freebsd.org>
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The branch main has been updated by ivy: URL: https://cgit.FreeBSD.org/src/commit/?id=7e0c5e0128c43bbae78190911aead8e1d9475ba5 commit 7e0c5e0128c43bbae78190911aead8e1d9475ba5 Author: Lexi Winter <ivy@FreeBSD.org> AuthorDate: 2025-05-09 21:28:14 +0000 Commit: Lexi Winter <ivy@FreeBSD.org> CommitDate: 2025-05-09 22:50:23 +0000 sys/cdefs.h: add __nodiscard annotation __nodiscard adds the [[nodiscard]] attribute to a function, type or constructor in C or C++, causing a value so marked to issue a compiler warning if it is discarded (i.e., not used or assigned) other than by casting it to void. this replaces the existing __result_use_or_ignore_check, which has a similar purpose but different semantics. since __nodiscard provides more functionality (at least in GCC) and __result_use_or_ignore_check only had a single user, remove __result_use_or_ignore_check. [[nodiscard]] has been supported in C++ since C++17, but only in C since C23; however, both LLVM and GCC implement it even in older language versions, so it should always be available with a relatively modern compiler. for Clang, [[nodiscard]] in C is only available since LLVM 17, but we can fall back to __attribute__((__warn_unused_result__)) which has the same semantics and provides support back to (at least) LLVM 11. GCC supports [[nodiscard]] in both C and C++ since at least GCC 11. for GCC, we can't provide a fallback as the semantics of its warn_unused_result are different, but since __result_use_or_ignore_check isn't defined for GCC anyway, we don't lose anything here. MFC after: 2 weeks Reviewed by: des, emaste Approved by: des (mentor) Differential Revision: https://reviews.freebsd.org/D50217 --- share/man/man9/cdefs.9 | 19 +++++++++++++++---- sys/sys/cdefs.h | 50 ++++++++++++++++++++++++++++++++++++++------------ sys/sys/systm.h | 14 +++++++------- 3 files changed, 60 insertions(+), 23 deletions(-) diff --git a/share/man/man9/cdefs.9 b/share/man/man9/cdefs.9 index 4efce132d393..397ddb0891bb 100644 --- a/share/man/man9/cdefs.9 +++ b/share/man/man9/cdefs.9 @@ -3,7 +3,7 @@ .\" .\" SPDX-License-Identifier: BSD-2-Clause .\" -.Dd January 6, 2025 +.Dd May 9, 2025 .Dt CDEFS 9 .Os .Sh NAME @@ -118,9 +118,20 @@ family of functions. .It Sy __fastcall Ta Use the .Dq fastcall ABI to call and name mangle this function. -.It Sy __result_use_check Ta Warn if function caller does not use it's return value -.It Sy __result_use_or_ignore_check Ta Warn if function caller does not use it's return value. -Allows the value to be explicitly ignored with a (void) cast. +.It Sy __result_use_check Ta Warn if function caller does not use its return value +.It Sy __nodiscard Ta Equivalent to the standard +.Dq [[nodiscard]] +attribute. +If applied to a function, warn if function caller does not use its +return value. +The warning may be silenced using a cast to +.Vt void , +or in C++, using an assignment to +.Va std::ignore . +If applied to a struct, C++ class or enum, this applies to all functions +returning values of that type. +If applied to a C++ constructor, this applies to creating instances of +the class using that constructor. .It Sy __returns_twice Ta Returns multiple times, like .Xr fork 2 .It Sy __unreachable Ta This code is not reachable at runtime diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h index c46be9f35842..e79cf2972f3f 100644 --- a/sys/sys/cdefs.h +++ b/sys/sys/cdefs.h @@ -253,18 +253,6 @@ #define __noinline __attribute__ ((__noinline__)) #define __fastcall __attribute__((__fastcall__)) #define __result_use_check __attribute__((__warn_unused_result__)) -#ifdef __clang__ -/* - * clang and gcc have different semantics for __warn_unused_result__: the latter - * does not permit the use of a void cast to suppress the warning. Use - * __result_use_or_ignore_check in places where a void cast is acceptable. - * This can be implemented by [[nodiscard]] from C23. - */ -#define __result_use_or_ignore_check __result_use_check -#else -#define __result_use_or_ignore_check -#endif /* !__clang__ */ - #define __returns_twice __attribute__((__returns_twice__)) #define __unreachable() __builtin_unreachable() @@ -295,6 +283,44 @@ #define __noexcept_if(__c) #endif +/* + * nodiscard attribute added in C++17 and C23, but supported by both LLVM and + * GCC in earlier language versions, so we use __has_c{,pp}_attribute to test + * for it. + * + * __nodiscard may be used on a function: + * __nodiscard int f(); + * + * or on a struct, union or enum: + * struct __nodiscard S{}; + * struct S f(); + * + * or in C++, on an object constructor. + */ + +#if defined(__cplusplus) && defined(__has_cpp_attribute) +#if __has_cpp_attribute(nodiscard) +#define __nodiscard [[nodiscard]] +#endif +#elif defined(__STDC_VERSION__) && defined(__has_c_attribute) +#if __has_c_attribute(__nodiscard__) +#define __nodiscard [[__nodiscard__]] +#endif +#endif + +#ifndef __nodiscard +/* + * LLVM 16 and earlier don't support [[nodiscard]] in C, but they do support + * __warn_unused_result__ with the same semantics, so fall back to that. + * We can't do this for GCC because the semantics are different. + */ +#ifdef __clang__ +#define __nodiscard __attribute__((__warn_unused_result__)) +#else +#define __nodiscard +#endif +#endif + /* * We use `__restrict' as a way to define the `restrict' type qualifier * without disturbing older software that is unaware of C99 keywords. diff --git a/sys/sys/systm.h b/sys/sys/systm.h index f542a9a86018..7cc02c77bea4 100644 --- a/sys/sys/systm.h +++ b/sys/sys/systm.h @@ -309,9 +309,9 @@ int __result_use_check copyin(const void * __restrict udaddr, void * _Nonnull __restrict kaddr, size_t len); int __result_use_check copyin_nofault(const void * __restrict udaddr, void * _Nonnull __restrict kaddr, size_t len); -int __result_use_or_ignore_check copyout(const void * _Nonnull __restrict kaddr, +__nodiscard int copyout(const void * _Nonnull __restrict kaddr, void * __restrict udaddr, size_t len); -int __result_use_or_ignore_check copyout_nofault( +__nodiscard int copyout_nofault( const void * _Nonnull __restrict kaddr, void * __restrict udaddr, size_t len); @@ -334,11 +334,11 @@ int64_t fuword64(volatile const void *base); int __result_use_check fueword(volatile const void *base, long *val); int __result_use_check fueword32(volatile const void *base, int32_t *val); int __result_use_check fueword64(volatile const void *base, int64_t *val); -int __result_use_or_ignore_check subyte(volatile void *base, int byte); -int __result_use_or_ignore_check suword(volatile void *base, long word); -int __result_use_or_ignore_check suword16(volatile void *base, int word); -int __result_use_or_ignore_check suword32(volatile void *base, int32_t word); -int __result_use_or_ignore_check suword64(volatile void *base, int64_t word); +__nodiscard int subyte(volatile void *base, int byte); +__nodiscard int suword(volatile void *base, long word); +__nodiscard int suword16(volatile void *base, int word); +__nodiscard int suword32(volatile void *base, int32_t word); +__nodiscard int suword64(volatile void *base, int64_t word); uint32_t casuword32(volatile uint32_t *base, uint32_t oldval, uint32_t newval); u_long casuword(volatile u_long *p, u_long oldval, u_long newval); int casueword32(volatile uint32_t *base, uint32_t oldval, uint32_t *oldvalp,
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