libstdc++: Prefer posix_memalign for aligned-new [PR113258]

As described in PR libstdc++/113258 there are old versions of tcmalloc
which replace malloc and related APIs, but do not repalce aligned_alloc
because it didn't exist at the time they were released. This means that
when operator new(size_t, align_val_t) uses aligned_alloc to obtain
memory, it comes from libc's aligned_alloc not from tcmalloc. But when
operator delete(void*, size_t, align_val_t) uses free to deallocate the
memory, that goes to tcmalloc's replacement version of free, which
doesn't know how to free it.

If we give preference to the older posix_memalign instead of
aligned_alloc then we're more likely to use a function that will be
compatible with the replacement version of free. Because posix_memalign
has been around for longer, it's more likely that old third-party malloc
replacements will also replace posix_memalign alongside malloc and free.

libstdc++-v3/ChangeLog:

	PR libstdc++/113258
	* libsupc++/new_opa.cc: Prefer to use posix_memalign if
	available.

(cherry picked from commit f50f2ef)

Author: jwakely

libstdc++-v3/libsupc++/new_opa.cc

@@ -46,12 +46,12 @@ using std::bad_alloc;
 using std::size_t;
 extern "C"
 {
-# if _GLIBCXX_HAVE_ALIGNED_ALLOC
+# if _GLIBCXX_HAVE_POSIX_MEMALIGN
+  void *posix_memalign(void **, size_t alignment, size_t size);
+# elif _GLIBCXX_HAVE_ALIGNED_ALLOC
   void *aligned_alloc(size_t alignment, size_t size);
 # elif _GLIBCXX_HAVE__ALIGNED_MALLOC
   void *_aligned_malloc(size_t size, size_t alignment);
-# elif _GLIBCXX_HAVE_POSIX_MEMALIGN
-  void *posix_memalign(void **, size_t alignment, size_t size);
 # elif _GLIBCXX_HAVE_MEMALIGN
   void *memalign(size_t alignment, size_t size);
 # else
@@ -63,13 +63,10 @@ extern "C"
 #endif
 
 namespace __gnu_cxx {
-#if _GLIBCXX_HAVE_ALIGNED_ALLOC
-using ::aligned_alloc;
-#elif _GLIBCXX_HAVE__ALIGNED_MALLOC
-static inline void*
-aligned_alloc (std::size_t al, std::size_t sz)
-{ return _aligned_malloc(sz, al); }
-#elif _GLIBCXX_HAVE_POSIX_MEMALIGN
+// Prefer posix_memalign if available, because it's older than aligned_alloc
+// and so more likely to be provided by replacement malloc libraries that
+// predate the addition of aligned_alloc. See PR libstdc++/113258.
+#if _GLIBCXX_HAVE_POSIX_MEMALIGN
 static inline void*
 aligned_alloc (std::size_t al, std::size_t sz)
 {
@@ -83,6 +80,12 @@ aligned_alloc (std::size_t al, std::size_t sz)
     return ptr;
   return nullptr;
 }
+#elif _GLIBCXX_HAVE_ALIGNED_ALLOC
+using ::aligned_alloc;
+#elif _GLIBCXX_HAVE__ALIGNED_MALLOC
+static inline void*
+aligned_alloc (std::size_t al, std::size_t sz)
+{ return _aligned_malloc(sz, al); }
 #elif _GLIBCXX_HAVE_MEMALIGN
 static inline void*
 aligned_alloc (std::size_t al, std::size_t sz)
@@ -128,7 +131,8 @@ operator new (std::size_t sz, std::align_val_t al)
   if (__builtin_expect (sz == 0, false))
     sz = 1;
 
-#if _GLIBCXX_HAVE_ALIGNED_ALLOC
+#if _GLIBCXX_HAVE_POSIX_MEMALIGN
+#elif _GLIBCXX_HAVE_ALIGNED_ALLOC
 # if defined _AIX || defined __APPLE__
   /* AIX 7.2.0.0 aligned_alloc incorrectly has posix_memalign's requirement
    * that alignment is a multiple of sizeof(void*).