mfbt/OperatorNewExtensions.h
author Georg Fritzsche <georg.fritzsche@googlemail.com>
Mon, 22 May 2017 15:33:29 +0700
changeset 410493 af4327fd49cd93c717328e61caf1af692a0e52a0
parent 352850 522135225936a32c7b084ba7eea082ee025ea7d5
child 508163 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1361661 - Part 1: Generate headers with process data from Processes.yaml. r=dexter Adding the Gecko enums to Processes.yaml allows us to generate mappings from ProcessID to GeckoProcessType. We generate string tables with the Telemetry process names, so we can use these names consistently throughout Telemetry.

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* A version of |operator new| that eschews mandatory null-checks. */

#ifndef mozilla_OperatorNewExtensions_h
#define mozilla_OperatorNewExtensions_h

#include "mozilla/Assertions.h"

// Credit goes to WebKit for this implementation, cf.
// https://bugs.webkit.org/show_bug.cgi?id=74676
namespace mozilla {
enum NotNullTag {
  KnownNotNull,
};
} // namespace mozilla

/*
 * The logic here is a little subtle.  [expr.new] states that if the allocation
 * function being called returns null, then object initialization must not be
 * done, and the entirety of the new expression must return null.  Non-throwing
 * (noexcept) functions are defined to return null to indicate failure.  The
 * standard placement operator new is defined in such a way, and so it requires
 * a null check, even when that null check would be extraneous.  Functions
 * declared without such a specification are defined to throw std::bad_alloc if
 * they fail, and return a non-null pointer otherwise.  We compile without
 * exceptions, so any placement new overload we define that doesn't declare
 * itself as noexcept must therefore avoid generating a null check.  Below is
 * just such an overload.
 *
 * You might think that MOZ_NONNULL might perform the same function, but
 * MOZ_NONNULL isn't supported on all of our compilers, and even when it is
 * supported, doesn't work on all the versions we support.  And even keeping
 * those limitations in mind, we can't put MOZ_NONNULL on the global,
 * standardized placement new function in any event.
 *
 * We deliberately don't add MOZ_NONNULL(3) to tag |p| as non-null, to benefit
 * hypothetical static analyzers.  Doing so makes |MOZ_ASSERT(p)|'s internal
 * test vacuous, and some compilers warn about such vacuous tests.
 */
inline void*
operator new(size_t, mozilla::NotNullTag, void* p)
{
  MOZ_ASSERT(p);
  return p;
}

#endif // mozilla_OperatorNewExtensions_h