Implement `std::launder`

[mclow.lists]

Summary

std::launder was introduced into c++17 as a compiler optimization barrier. It's something that the compiler 'knows about', and affects codegen.

See https://wg21.link/p0137r1 for more.

[• Quuxplusone]

test/std/language.support/support.dynamic/ptr.launder/launder.fail.cpp

26

Would one also expect a decent static analyzer to warn that you are laundering a null pointer? (and should we care?)

[efriedma]

include/new

174

How is the compiler supposed to know that "std::__1::launder()" has special semantics?

[efriedma]

include/new

174

Oh, wait, is this actually not in the __1 namespace? Sort of hard to tell because the patch wasn't posted with enough context.

It isn't exactly great to special-case functions named "std::launder"... but wouldn't be the first name in the std namespace which has special compiler semantics.

[tcanens]

At least for GCC, it should use __builtin_launder.

Also needs to implement "The program is ill-formed if T is a function type or cv void."

[hfinkel]

In D40144#927828, @tcanens wrote:

At least for GCC, it should use __builtin_launder.

I presume we'll need to add something similar for Clang as well.

Also needs to implement "The program is ill-formed if T is a function type or cv void."

[rsmith]

In D40144#927866, @hfinkel wrote:

In D40144#927828, @tcanens wrote:

At least for GCC, it should use __builtin_launder.

I presume we'll need to add something similar for Clang as well.

Yes, we agreed to use the same builtin name for this in Clang and GCC quite a while back, but haven't actually got around to implementing it in Clang yet. You should use __has_builtin to detect the existence of this builtin in Clang. In GCC, I believe this is available iff __GNUC__ is 7 or greater.

[mclow.lists]

include/new

174

I'm about to move it into the _1 namespace. Since it's calling a compiler intrinsic, it doesn't need to be in a special place.

[mclow.lists]

Move the launder function into the main libc++ namespace.
Call __builtin_launder when available.
Check to see when it's available (for gcc and clang)
Add failing tests.

[EricWF]

I think we may want a __launder function that we can use internally in all dialects.

include/__config

458 ↗

(On Diff #123422)

These macros should take the negative _LIBCPP_HAS_NO_BUILTIN_LAUNDER form to be consistent.

include/new

260

Typically diagnostics don't start with capitals.

Small nit on the use of a contraction too.

261

Since this is C++17 only, we can use the trait_t and trait_v versions to be less verbose.

[EricWF]

I've made an attempt to add __builtin_launder to clang in D40218.

[mclow.lists]

Made an internal function __launder which is not c++17 specific.
Fixed some wording for the standard asserts.

[tcanens]

include/new

260

Technically, the attempt is to launder pointers to cv void/functions. :)

Also, since __launder is non-C++17-specific, I guess it can't use _t and _v after all... - and I suppose it'll also need to parens-protect the is_same check for the fallback static_assert macro?

[mclow.lists]

include/new

260

Oh, *bother* (and yes, you're correct)

[mclow.lists]

Un-c++17'ed the internal function __launder

[EricWF]

include/new

274

The call should probably be qualified to _VSTD::__launder.

test/std/language.support/support.dynamic/ptr.launder/launder.nodiscard.fail.cpp

16 ↗

(On Diff #123663)

Why is this test unsupported with older compilers? The version of std::launder in this patch should work regardless of compiler support for __builtin_launder.

Although, it's possible the compiler requirements were intended to avoid older clang versions without [[nodiscard]]. However, I don't think clang-3.8 supports C++2a, so the condition seems redundant.

test/std/language.support/support.dynamic/ptr.launder/launder.types.fail.cpp

28 ↗

(On Diff #123663)

Alternatively you could de-duplicate the four identical checks into the single check:

// expected-error-re@new:* 4 {{static_assert failed{{.*}} "can't launder cv-void"}}

[mclow.lists]

test/std/language.support/support.dynamic/ptr.launder/launder.nodiscard.fail.cpp

16 ↗

(On Diff #123663)

We're testing the [[nodiscard]] bit here, not the launder bit. And that didn't come in until clang 3.9

[EricWF]

test/std/language.support/support.dynamic/ptr.launder/launder.nodiscard.fail.cpp

16 ↗

(On Diff #123663)

Perhaps we should fall back to __attribute__((warn_unused_result)) when we don't have [[nodiscard]]? All supported Clang versions should provide that, as well as GCC 4.9 and newer.

[mclow.lists]

_VSTD:: qualify the call to __launder.
De-dup error messages in test.

[mclow.lists]

landed as revision 318864

include/new

 new_handler set_new_handler(new_handler new_p) noexcept;
 new_handler get_new_handler() noexcept;
 
+// 21.6.4, pointer optimization barrier
+template <class T> constexpr T* launder(T* p) noexcept; // C++17
 }  // std
 
 void* operator new(std::size_t size);                                   // replaceable

 #endif
 #endif
 
+#if _LIBCPP_STD_VER > 14
+template <class _Tp>
+_LIBCPP_NODISCARD_AFTER_CXX17 inline _LIBCPP_INLINE_VISIBILITY
+constexpr _Tp* launder(_Tp* __p) noexcept { return __p;}
+#endif
 }  // std
 
 #if defined(_LIBCPP_CXX03_LANG)

test/std/language.support/support.dynamic/ptr.launder/launder.fail.cpp

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <new>
+
+// template <class T> constexpr T* launder(T* p) noexcept;
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+// UNSUPPORTED: clang-3.3, clang-3.4, clang-3.5, clang-3.6, clang-3.7, clang-3.8
+
+#include <new>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main ()
+{
+    int *p = nullptr;
+    std::launder(p);  // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
+}

test/std/language.support/support.dynamic/ptr.launder/launder.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <new>
+
+// template <class T> constexpr T* launder(T* p) noexcept;
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+#include <new>
+#include <cassert>
+
+#include "test_macros.h"
+
+constexpr int gi = 5;
+constexpr float gf = 8.f;
+
+int main() {
+  static_assert(std::launder(&gi) == &gi, "" );
+  static_assert(std::launder(&gf) == &gf, "" );
+
+    const int *i = &gi;
+    const float *f = &gf;
+    static_assert(std::is_same<decltype(i), decltype(std::launder(i))>::value, "");
+    static_assert(std::is_same<decltype(f), decltype(std::launder(f))>::value, "");
+
+  assert(std::launder(i) == i);
+  assert(std::launder(f) == f);
+}

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