[Bug target/69841] Wrong template instantiation in C++11 on armv7l

[jgreenhalgh at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69841

James Greenhalgh <jgreenhalgh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |alan.lawrence at arm dot com,
                   |                            |jakub at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org

--- Comment #3 from James Greenhalgh <jgreenhalgh at gcc dot gnu.org> ---
I'm still confused by this. After coming out of the front end I checked the
DECL_ALIGN for each field of each of the parameters being passed to this
function. I see:

----
f1.ii

std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator
std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>::_M_emplace_hint_unique(std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::const_iterator, _Args&& ...)  
      (struct _Rb_tree * const this
           (decl alignment: 32),
       struct const_iterator __pos
         Fields:
           _M_node (decl alignment: 32)
           _Rb_tree_const_iterator (decl alignment: 8)
           value_type (decl alignment: 8)
           reference (decl alignment: 32)
           pointer (decl alignment: 32)
           iterator (decl alignment: 8)
           iterator_category (decl alignment: 8)
           difference_type (decl alignment: 32)
           _Self (decl alignment: 8)
           _Base_ptr (decl alignment: 32)
           _Link_type (decl alignment: 32)
           (decl alignment: 32, max field alignment: 32),
       const struct piecewise_construct_t & __args#0
           (decl alignment: 32),
       struct tuple & __args#1
           (decl alignment: 32),
       struct tuple & __args#2
           (decl alignment: 32))

f2.ii

std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator
std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>::_M_emplace_hint_unique(std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::const_iterator, _Args&& ...) 
      (struct _Rb_tree * const this
           (decl alignment: 32),
       struct const_iterator __pos
         Fields:
           _M_node (decl alignment: 32)
           _Rb_tree_const_iterator (decl alignment: 8)
           value_type (decl alignment: 64)
           reference (decl alignment: 32)
           pointer (decl alignment: 32)
           iterator (decl alignment: 32)
           iterator_category (decl alignment: 8)
           difference_type (decl alignment: 32)
           _Self (decl alignment: 8)
           _Base_ptr (decl alignment: 32)
           _Link_type (decl alignment: 32)
           (decl alignment: 32, max field alignment: 64),
       const struct piecewise_construct_t & __args#0
           (decl alignment: 32),
       struct tuple & __args#1
           (decl alignment: 32),
       struct tuple & __args#2
           (decl alignment: 32))
---

That is to say, after gimplification we've already decided that the alignment
of the value_type field of the std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>::const_iterator parameter to std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_emplace_hint_unique in f2.ii is 64, whereas in f1.ii we
don't have any extra alignment information.

I know nothing about the C++ front-end and how we could end up in this
situation. I can understand why, given this, we would generate the code we do
for ARM.