[Bug libstdc++/55727] better support for dynamic allocation of over-aligned types

[Bug libstdc++/55727] better support for dynamic allocation of over-aligned types

[kretz at kde dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55727

--- Comment #1 from Matthias Kretz <kretz at kde dot org> 2012-12-18 08:53:24 UTC ---
Created attachment 28992
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28992
simple testcase for std::vector

[Bug libstdc++/55727] New: better support for dynamic allocation of over-aligned types

[kretz at kde dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55727

             Bug #: 55727
           Summary: better support for dynamic allocation of over-aligned
                    types
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: kretz@kde.org


Created attachment 28991
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28991
simple testcase for new

Hello,

I just tested the alignas support in GCC 4.8 and wondered how far the C++11
spec defines behavior of dynamic allocation of over-aligned types. Because,
with GCC 4.8, dynamic allocation via new or std::allocator (e.g. in
std::vector) results in unaligned placement. I quote the relevant parts of the
spec below, because I believe GCC could be much more helpful to developers.

I believe a must-have is that if dynamic allocation of an over-aligned type
would result in misalignment, the allocatio fails (3.11.9, 17.6.3.5.6). Perfect
would be a compile-time failure, which is theoretically possible. Runtime
errors resulting from misaligned allocation are often hard to debug, therefore
it would be *very* helpful to get an early warning about misalignment.

As I understand it, the C++ spec allows for GCC to also just do the right thing
automatically, which would be my preferred solution. So when I'd do:
struct alignas(128) Foo { int x; };
Foo *f = new Foo;
I'd (per default) get a pointer f that is aligned as requested (without having
to overload new/delete). The same would be very important for std::vector<Foo>.
Right now it does not even suffice to reimplement new/delete inside Foo to make
std::vector<Foo> work.

C++11 spec quotes:

3.9.2.3 "Pointers to over-aligned types (3.11) have no special representation,
but their range of valid values is restricted by the extended alignment
requirement. This International Standard specifies only two ways of obtaining
such a pointer: taking the address of a valid object with an over-aligned type,
and using one of the runtime pointer alignment functions. An implementation may
provide other means of obtaining a valid pointer value for an over-aligned
type.

3.11.9 "a request for runtime allocation of dynamic storage for which the
requested alignment cannot be honored shall be treated as an allocation
failure"

5.3.4.1 "It is implementation-defined whether over-aligned types are supported
(3.11)."

5.3.4.14 "The block of storage is assumed to be appropriately aligned and of
the requested size."

17.6.3.5.6 "If the alignment associated with a specific over-aligned type is
not supported by an allocator, instantiation of the allocator for that type may
fail. The allocator also may silently ignore the requested alignment. [ Note:
Additionally, the member function allocate for that type may fail by throwing
an object of type std::bad_alloc. — end note"

20.6.9.1.5 "Returns: A pointer to the initial element of an array of storage of
size n * sizeof(T), aligned appropriately for objects of type T. It is
implementation-defined whether over-aligned types are supported (3.11)."

[Bug libstdc++/55727] better support for dynamic allocation of over-aligned types

[kretz at kde dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55727

--- Comment #2 from Matthias Kretz <kretz at kde dot org> 2012-12-18 09:11:41 UTC ---
(In reply to comment #0)
> Right now it does not even suffice to reimplement new/delete inside Foo to make
> std::vector<Foo> work.

Sorry, this statement seems to be wrong. The issue that I actually have is of
the form:

from my library:
  struct alignas(32) A { ... }; //overloads new/delete for correct alignment

different library:
  template<typename T> struct B { T x; };

application code:
  std::vector<B<A>>

The members of the vector are now misaligned because obviously the new/delete
overloads in A are not used. But the alignment information from A is passed on
to B so it could be used in the allocator of std::vector.

[Bug libstdc++/55727] better support for dynamic allocation of over-aligned types

[kretz at kde dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55727

--- Comment #3 from Matthias Kretz <kretz at kde dot org> 2012-12-18 13:26:21 UTC ---
(In reply to comment #2)
> (In reply to comment #0)
> > Right now it does not even suffice to reimplement new/delete inside Foo to make
> > std::vector<Foo> work.
> 
> Sorry, this statement seems to be wrong.

After more investigation I see that my original statement was right. The
default allocator uses ::operator new(size_t) instead of T::operator new - and
obviously it has to use the global one because allocation and construction are
split into two functions.

I just created an allocator class that uses posix_memalign and __alignof to
implement an allocator as I'd like to have for a proof of concept.

If you are interested, I can patch the new_allocator class to support
over-alignment (while still using operator new). Would you be interested?

[Bug libstdc++/55727] better support for dynamic allocation of over-aligned types

[kretz at kde dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55727

--- Comment #4 from Matthias Kretz <kretz at kde dot org> 2012-12-18 18:20:00 UTC ---
Created attachment 29002
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29002
support for over-aligned types in new_allocator

I finished my allocator to fix the issue and it was easy to port it over to
new_allocator. So here's the patch.

Questions:
1. I use sizeof(void*) for the natural alignment or alignof(std::max_align_t)
for C++11. Is there a better way to do this?^[
2. I'm not sure you like the way I abuse enum for compile-time constants. May I
use constexpr, or would that have to be ifdefed?
3. What's the proper integer type to use for pointer manipulation? I'd really
like to write __p &= ~__alignment_mask, but I don't see how to not breaking
strict-aliasing with this.

Note that new_allocator will compile to the same code as long as alignof(_Tp1)
is <= alignof(std::max_align_t). The extra code to handle alignment will only
kick in for over-aligned types - and those want the extra complexity.

[Bug libstdc++/55727] better support for dynamic allocation of over-aligned types

[kretz at kde dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55727

Matthias Kretz <kretz at kde dot org> changed:

           What    |Removed                     |Added
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  Attachment #29002|0                           |1
        is obsolete|                            |

--- Comment #5 from Matthias Kretz <kretz at kde dot org> 2012-12-18 18:30:43 UTC ---
Created attachment 29003
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29003
updated the patch

[Bug libstdc++/55727] better support for dynamic allocation of over-aligned types

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55727

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler at
                   |                            |googlemail dot com

--- Comment #6 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-12-19 11:56:32 UTC ---
(In reply to comment #0)
I suggest to also read the proposal

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3396.htm

It points out some general problems with allocation of over-aligned data as of
the current specification and suggests a language extension for better support.