[Bug c++/53900] [regression] Too optimistic on a alignment assert
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53900 Benoit Jacob changed: What|Removed |Added CC||jacob.benoit.1 at gmail dot com --- Comment #9 from Benoit Jacob --- (In reply to Andrew Pinski from comment #8) > It is rather a bug in libstdc++ which should be reported separately though > it needs an ABI change to work correctly. Filed bug 65122 for that.
[Bug c++/53900] [regression] Too optimistic on a alignment assert
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53900 --- Comment #8 from Andrew Pinski 2013-01-07 10:21:44 UTC --- (In reply to comment #7) > (In reply to comment #6) > > > __attribute__((aligned(16))) float array[4]; > > > > Explicitly says the array is aligned to 16 bytes which means it is > > undefined by > > the C/C++ standard what happens if you a pointer to the struct which > > contains > > this array. As this pointer will always be aligned by definition of the > > struct > > (and any other use of it is undefined), then by definition > > std::ptrdiff_t(array) will always have the lower 8bits be zero. > > So does that mean that new and std::allocator act undefined for aligned > objects? > Is there anything wrong with Gael's second test case? It is rather a bug in libstdc++ which should be reported separately though it needs an ABI change to work correctly.
[Bug c++/53900] [regression] Too optimistic on a alignment assert
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53900 --- Comment #7 from Christoph Hertzberg 2013-01-07 10:19:13 UTC --- (In reply to comment #6) > > __attribute__((aligned(16))) float array[4]; > > Explicitly says the array is aligned to 16 bytes which means it is undefined > by > the C/C++ standard what happens if you a pointer to the struct which contains > this array. As this pointer will always be aligned by definition of the > struct > (and any other use of it is undefined), then by definition > std::ptrdiff_t(array) will always have the lower 8bits be zero. So does that mean that new and std::allocator act undefined for aligned objects? Is there anything wrong with Gael's second test case?
[Bug c++/53900] [regression] Too optimistic on a alignment assert
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53900 --- Comment #6 from Andrew Pinski 2013-01-07 04:25:38 UTC --- > __attribute__((aligned(16))) float array[4]; Explicitly says the array is aligned to 16 bytes which means it is undefined by the C/C++ standard what happens if you a pointer to the struct which contains this array. As this pointer will always be aligned by definition of the struct (and any other use of it is undefined), then by definition std::ptrdiff_t(array) will always have the lower 8bits be zero.
[Bug c++/53900] [regression] Too optimistic on a alignment assert
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53900 Kacper Kowalik changed: What|Removed |Added CC||xarthisius.kk at gmail dot ||com --- Comment #5 from Kacper Kowalik 2012-12-28 17:47:12 UTC --- (In reply to comment #1) > You tell the compiler array is aligned, so what do you expect? This regression was introduced by: http://gcc.gnu.org/viewcvs?view=revision&revision=172424 2011-04-14 Richard Guenther * tree.h (get_object_alignment_1): Declare. * builtins.c (get_object_alignment_1): Split out worker from ... (get_object_alignment): ... here. * fold-const.c (get_pointer_modulus_and_residue): Use get_object_alignment_1. * gcc.dg/fold-bitand-4.c: Move ... * c-c++-common/fold-bitand-4.c: ... here. Adjust slightly. Could you please verify that it's intended behaviour and/or indicate if provided testcase is a valid/invalid code? Thank you in advance, Kacper Kowalik
[Bug c++/53900] [regression] Too optimistic on a alignment assert
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53900 --- Comment #4 from Gael Guennebaud 2012-07-10 11:09:16 UTC --- Created attachment 27770 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27770 Another demonstration of the issue using std::vector Note that when we declare: __attribute__((aligned(16))) float array[4]; we request GCC to give us an aligned array, and we are not telling gcc that array is aligned that is slightly different. I attached another example demonstrating the problem: a simple declaration like: std::vector vec(5); generates unaligned arrays that are not caught by assertion because it has been removed by gcc 4.7. Outputs are as follow: $ g++-4.7 -m32 alignedassert.cpp && ./a.out ctor 0xffdc58f0 copy 0x9f72008 copy 0x9f72018 copy 0x9f72028 copy 0x9f72038 copy 0x9f72048 ctor 0xffdc5900 g++-4.6 -m32 alignedassert.cpp && ./a.out ctor 0xff8e19e0 copy 0x98e2008 a.out: alignedassert.cpp:18: Foo::Foo(const Foo&): Assertion `(std::ptrdiff_t(array) % std::ptrdiff_t(16))==0' failed. Aborted
[Bug c++/53900] [regression] Too optimistic on a alignment assert
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53900
--- Comment #3 from Richard Guenther 2012-07-09
16:18:58 UTC ---
Well, I suggest you instead do
struct Foo
{
float array[4];
Foo()
{
std::cout << array << "\n";
// check the object is really aligned
if (std::ptrdiff_t(array) & std::ptrdiff_t(0xf)) != 0)
assert (false);
<...code assuming array is aligned...>
}
};
and the compiler will figure out that array is aligned in code dominated
by the alignment check.
[Bug c++/53900] [regression] Too optimistic on a alignment assert
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53900 --- Comment #2 from Gael Guennebaud 2012-07-09 16:12:07 UTC --- The problem is that it is not guaranteed to be effectively aligned, and it is nice to be able to detect when this happens to either abort with a clear message, trigger an exception, or even properly handle this case by disabling at runtime some optimizations. Anyway, if you disagree, I'm sure I'll find a workaround using, e.g., a non inlined intermediate function masking the origin of the pointer.
[Bug c++/53900] [regression] Too optimistic on a alignment assert
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53900 Richard Guenther changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution||INVALID --- Comment #1 from Richard Guenther 2012-07-09 15:23:36 UTC --- You tell the compiler array is aligned, so what do you expect?
