[patch] libstdc++/66017 Avoid bad casts and fix alignment of _Rb_tree_node<long long>::_M_storage

[Jonathan Wakely]

On 22/05/15 16:29 +0200, Jakub Jelinek wrote:
>On Fri, May 22, 2015 at 03:15:10PM +0100, Jonathan Wakely wrote:
>> --- a/libstdc++-v3/include/ext/aligned_buffer.h
>> +++ b/libstdc++-v3/include/ext/aligned_buffer.h
>> @@ -31,21 +31,23 @@
>>
>>  #pragma GCC system_header
>>
>> -#if __cplusplus >= 201103L
>> -# include <type_traits>
>> -#else
>> +#if __cplusplus < 201103L
>>  # include <bits/c++0x_warning.h>
>>  #endif
>>
>> +#include <cstddef>
>> +
>>  namespace __gnu_cxx
>>  {
>>    template<typename _Tp>
>>      struct __aligned_buffer
>> -    : std::aligned_storage<sizeof(_Tp), std::alignment_of<_Tp>::value>
>>      {
>> -      typename
>> -	std::aligned_storage<sizeof(_Tp), std::alignment_of<_Tp>::value>::type
>> -	_M_storage;
>> +      // Target macro ADJUST_FIELD_ALIGN can produce different alignment for
>> +      // types when used as class members. __aligned_buffer is intended for
>> +      // use as a class member, so align the buffer as for a class member.
>> +      struct _Tp2 { _Tp _M_t; };
>> +
>> +      alignas(alignof(_Tp2)) unsigned char _M_storage[sizeof(_Tp)];
>
>Is alignof(_Tp2) always the same as alignof(_Tp2::_M_t) on all targets
>(I mean, won't some target align the structure more than its only field)?

Hmm, maybe. I don't know.

>Wouldn't it be safer to use alignof(_Tp2::_M_t) here?

Yes.

>Though, apparently that is a GNU extension, so you'd need to use __alignof__
>instead.

Yes, that's what I did in an earlier version of the patch, so I'll go
back to that.