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

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

[Jonathan Wakely]

Does anybody see any problem with making this change, so that we avoid
the problem described in the PR?

-- >8 --

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.
---
 libstdc++-v3/libsupc++/new_opa.cc | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/libstdc++-v3/libsupc++/new_opa.cc b/libstdc++-v3/libsupc++/new_opa.cc
index 8326b7497fe..35606e1c1b3 100644
--- a/libstdc++-v3/libsupc++/new_opa.cc
+++ b/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*).
-- 
2.43.0

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

[Jonathan Wakely]

On Tue, 9 Jan 2024 at 22:00, Jonathan Wakely wrote:
>
> Does anybody see any problem with making this change, so that we avoid
> the problem described in the PR?

Pushed to trunk. We should backport this too.


>
> -- >8 --
>
> 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.
> ---
>  libstdc++-v3/libsupc++/new_opa.cc | 26 +++++++++++++++-----------
>  1 file changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/libstdc++-v3/libsupc++/new_opa.cc b/libstdc++-v3/libsupc++/new_opa.cc
> index 8326b7497fe..35606e1c1b3 100644
> --- a/libstdc++-v3/libsupc++/new_opa.cc
> +++ b/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*).
> --
> 2.43.0
>