[libcxx-commits] [libcxx] r355763 - LWG 2843 "Unclear behavior of std::pmr::memory_resource::do_allocate()"

[Eric Fiselier via libcxx-commits]

Author: ericwf
Date: Fri Mar  8 16:38:19 2019
New Revision: 355763

URL: http://llvm.org/viewvc/llvm-project?rev=355763&view=rev
Log:
LWG 2843 "Unclear behavior of std::pmr::memory_resource::do_allocate()"

Patch by Arthur O'Dwyer.
Reviewed as https://reviews.llvm.org/D47344

new_delete_resource().allocate(n, a) has basically two permissible results:

* Return an appropriately sized and aligned block.
* Throw bad_alloc.

Before this patch, libc++'s new_delete_resource would do a third and impermissible thing, which was
to return an appropriately sized but inappropriately under-aligned block. This is now fixed.

(This came up while I was stress-testing unsynchronized_pool_resource on my MacBook. If we can't
trust the default resource to return appropriately aligned blocks, pretty much everything breaks.
For similar reasons, I would strongly support just patching __libcpp_allocate directly, but I don't
care to die on that hill, so I made this patch as a <memory_resource>-specific workaround.)

Modified:
    libcxx/trunk/src/experimental/memory_resource.cpp
    libcxx/trunk/www/cxx2a_status.html

Modified: libcxx/trunk/src/experimental/memory_resource.cpp
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/src/experimental/memory_resource.cpp?rev=355763&r1=355762&r2=355763&view=diff
==============================================================================
--- libcxx/trunk/src/experimental/memory_resource.cpp (original)
+++ libcxx/trunk/src/experimental/memory_resource.cpp Fri Mar  8 16:38:19 2019
@@ -25,19 +25,23 @@ _LIBCPP_BEGIN_NAMESPACE_LFTS_PMR
 class _LIBCPP_TYPE_VIS __new_delete_memory_resource_imp
     : public memory_resource
 {
-public:
-    ~__new_delete_memory_resource_imp() = default;
-
-protected:
-    virtual void* do_allocate(size_t __size, size_t __align)
-        { return _VSTD::__libcpp_allocate(__size, __align); /* FIXME */}
+    void *do_allocate(size_t size, size_t align) override {
+#ifdef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+        if (__is_overaligned_for_new(align))
+            __throw_bad_alloc();
+#endif
+        return _VSTD::__libcpp_allocate(size, align);
+    }
 
-    virtual void do_deallocate(void* __p, size_t __n, size_t __align) {
-      _VSTD::__libcpp_deallocate(__p, __n, __align); /* FIXME */
+    void do_deallocate(void *p, size_t n, size_t align) override {
+      _VSTD::__libcpp_deallocate(p, n, align);
     }
 
-    virtual bool do_is_equal(memory_resource const & __other) const _NOEXCEPT
-        { return &__other == this; }
+    bool do_is_equal(memory_resource const & other) const _NOEXCEPT override
+        { return &other == this; }
+
+public:
+    ~__new_delete_memory_resource_imp() override = default;
 };
 
 // null_memory_resource()

Modified: libcxx/trunk/www/cxx2a_status.html
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/www/cxx2a_status.html?rev=355763&r1=355762&r2=355763&view=diff
==============================================================================
--- libcxx/trunk/www/cxx2a_status.html (original)
+++ libcxx/trunk/www/cxx2a_status.html Fri Mar  8 16:38:19 2019
@@ -219,7 +219,7 @@
 	<tr><td><a href="https://wg21.link/LWG2164">2164</a></td><td>What are the semantics of <tt>vector.emplace(vector.begin(), vector.back())</tt>?</td><td>Jacksonville</td><td></td></tr>
 	<tr><td><a href="https://wg21.link/LWG2243">2243</a></td><td><tt>istream::putback</tt> problem</td><td>Jacksonville</td><td>Complete</td></tr>
 	<tr><td><a href="https://wg21.link/LWG2816">2816</a></td><td><tt>resize_file</tt> has impossible postcondition</td><td>Jacksonville</td><td><i>Nothing to do</i></td></tr>
-	<tr><td><a href="https://wg21.link/LWG2843">2843</a></td><td>Unclear behavior of <tt>std::pmr::memory_resource::do_allocate()</tt></td><td>Jacksonville</td><td></td></tr>
+	<tr><td><a href="https://wg21.link/LWG2843">2843</a></td><td>Unclear behavior of <tt>std::pmr::memory_resource::do_allocate()</tt></td><td>Jacksonville</td><td>Complete</td></tr>
 	<tr><td><a href="https://wg21.link/LWG2849">2849</a></td><td>Why does <tt>!is_regular_file(from)</tt> cause <tt>copy_file</tt> to report a "file already exists" error?</td><td>Jacksonville</td><td><i>Nothing to do</i></td></tr>
 	<tr><td><a href="https://wg21.link/LWG2851">2851</a></td><td><tt>std::filesystem</tt> enum classes are now underspecified</td><td>Jacksonville</td><td><i>Nothing to do</i></td></tr>
 	<tr><td><a href="https://wg21.link/LWG2946">2946</a></td><td>LWG 2758's resolution missed further corrections</td><td>Jacksonville</td><td>Complete</td></tr>