Bug 1294537 - introduce a non-null-checking placement operator new; r…

…=sunfish,nbp

The default placement operator new is defined to always require that its
result be null-checked.  A sufficiently smart compiler can remove this
check, but not all compilers are sufficiently smart.  Better to have a
custom placement operator new that will remove null checks in a way
defined by the standard.

js/src/jit/JitAllocPolicy.h

@@ -9,6 +9,7 @@
 
 #include "mozilla/Attributes.h"
 #include "mozilla/GuardObjects.h"
+#include "mozilla/OperatorNewExtensions.h"
 #include "mozilla/TypeTraits.h"
 
 #include "jscntxt.h"
@@ -166,6 +167,13 @@ struct TempObject
                       "Placement new argument type must inherit from TempObject");
         return pos;
     }
+    template <class T>
+    inline void* operator new(size_t nbytes, mozilla::NotNullTag, T* pos) {
+        static_assert(mozilla::IsConvertible<T*, TempObject*>::value,
+                      "Placement new argument type must inherit from TempObject");
+        MOZ_ASSERT(pos);
+        return pos;
+    }        
 };
 
 template <typename T>

mfbt/OperatorNewExtensions.h

@@ -0,0 +1,53 @@
+/* -*- 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"
+#include "mozilla/Attributes.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 do, however, add MOZ_NONNULL here for the potential benefit of static
+ * analysis tools that understand such annotations.
+ */
+MOZ_NONNULL(3)
+inline void*
+operator new(size_t, mozilla::NotNullTag, void* p)
+{
+  MOZ_ASSERT(p);
+  return p;
+}
+
+#endif // mozilla_OperatorNewExtensions_h

mfbt/Vector.h

@@ -17,6 +17,7 @@
 #include "mozilla/MathAlgorithms.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/Move.h"
+#include "mozilla/OperatorNewExtensions.h"
 #include "mozilla/ReentrancyGuard.h"
 #include "mozilla/TemplateLib.h"
 #include "mozilla/TypeTraits.h"
@@ -62,7 +63,7 @@ struct VectorImpl
   MOZ_NONNULL(1)
   static inline void new_(T* aDst, Args&&... aArgs)
   {
-    new(aDst) T(Forward<Args>(aArgs)...);
+    new(KnownNotNull, aDst) T(Forward<Args>(aArgs)...);
   }
 
   /* Destroys constructed objects in the range [aBegin, aEnd). */
@@ -819,7 +820,7 @@ Vector<T, N, AP>::operator=(Vector&& aRhs)
 {
   MOZ_ASSERT(this != &aRhs, "self-move assignment is prohibited");
   this->~Vector();
-  new(this) Vector(Move(aRhs));
+  new(KnownNotNull, this) Vector(Move(aRhs));
   return *this;
 }
 

mfbt/moz.build

@@ -63,6 +63,7 @@ EXPORTS.mozilla = [
     'NotNull.h',
     'NullPtr.h',
     'Opaque.h',
+    'OperatorNewExtensions.h',
     'Pair.h',
     'PodOperations.h',
     'Poison.h',