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GCC 7.2 support (-Wnoexcept-type)

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P0012R1, accepted into C++17, makes a function’s “noexcept” (or
“throw()”) specification part of its signature. GCC 7.2 provides a
warning, -Wnoexcept-type, that is triggered when a function pointer type
with an exception specification is used in pre-C++17 code in such a way
as to pose an ABI incompatibility with C++17 code.

https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/C_002b_002b-Dialect-Options.html#index-Wnoexcept-type

Warnings are of the form:

In file included from ../../util/misc/from_pointer_cast_test.cc:15:0:
../../util/misc/from_pointer_cast.h:64:1: error: mangled name for ‘typename std::enable_if<(std::is_pointer<From>::value && std::is_pointer<_Tp>::value), To>::type crashpad::FromPointerCast(From) [with To = const volatile void*; From = void* (*)(long unsigned int) throw ()]’ will change in C++17 because the exception specification is part of a function type [-Werror=noexcept-type]
 FromPointerCast(From from) {
 ^~~~~~~~~~~~~~~
../../util/misc/from_pointer_cast.h:64:1: error: mangled name for ‘typename std::enable_if<(std::is_pointer<From>::value && std::is_pointer<_Tp>::value), To>::type crashpad::FromPointerCast(From) [with To = volatile void*; From = void* (*)(long unsigned int) throw ()]’ will change in C++17 because the exception specification is part of a function type [-Werror=noexcept-type]

In Crashpad, this warning is triggered by the two FromPointerCast<>()
variants that accept function pointer “From” arguments. This occurs when
using glibc as the standard C library, since glibc declares its
functions as “throw()”. FromPointerCast<>() is used with pointers to
glibc functions such as malloc() and getpid().

The warning is disabled for the FromPointerCast<>() variants that would
trigger it. The warning is not useful or actionable in this internal
Crashpad code where ABI changes due to language version (including
mangling changes) are not a concern.

Clang 4.0 has the similar -Wc++1z-compat-mangling option (also available
as -Wc++17-compat-mangling and the GCC-compatible -Wnoexcept-type in
Clang 5.0) but it is not triggered by this pattern.

Change-Id: Id293db3954be415f67a55476ca72bfb7d399aa3b
Reviewed-on: https://chromium-review.googlesource.com/738292
Reviewed-by: Joshua Peraza <jperaza@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
This commit is contained in:
Mark Mentovai 2017-10-25 13:52:12 -04:00 committed by Commit Bot
parent 6d5bd1d04d
commit fbc365fa9e
1 changed files with 20 additions and 0 deletions
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20
util/misc/from_pointer_cast.h
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@ -21,6 +21,7 @@
#include <type_traits>
#include "base/numerics/safe_conversions.h"
#include "build/build_config.h"
namespace crashpad {
@ -56,6 +57,20 @@ FromPointerCast(From) {
return To();
}
// FromPointerCast<>() with a function pointer “From” type raises
// -Wnoexcept-type in GCC 7.2 if the function pointer type has a throw() or
// noexcept specification. This is the case for all standard C library functions
// provided by glibc. Various tests make use of FromPointerCast<>() with
// pointers to standard C library functions.
//
// Clang has the similar -Wc++1z-compat-mangling, which is not triggered by this
// pattern.
#if defined(COMPILER_GCC) && !defined(__clang__) && \
(__GNUC__ > 7 || (__GNUC__ == 7 && __GNUC_MINOR__ >= 2))
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wnoexcept-type"
#endif
// Cast a pointer to any other pointer type.
template <typename To, typename From>
typename std::enable_if<std::is_pointer<From>::value &&
@ -88,6 +103,11 @@ FromPointerCast(From from) {
return base::checked_cast<To>(intermediate);
}
#if defined(COMPILER_GCC) && !defined(__clang__) && \
(__GNUC__ > 7 || (__GNUC__ == 7 && __GNUC_MINOR__ >= 2))
#pragma GCC diagnostic pop
#endif
#endif // DOXYGEN
} // namespace crashpad