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.)

llvm-svn: 355763

1 files changed, 14 insertions, 10 deletions

diff --git a/libcxx/src/experimental/memory_resource.cpp b/libcxx/src/experimental/memory_resource.cpp
index 977ee2b56ee..22bc12c1005 100644
--- a/libcxx/src/experimental/memory_resource.cpp
+++ b/libcxx/src/experimental/memory_resource.cpp
@@ -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()