Re: [PATCH 2/5] Don't memcpy non-trivially-copyable types: Make enum_flags triv. copyable

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2017-04-12 22:27, Pedro Alves wrote:
> The delete-memcpy-with-non-trivial-types patch exposed many instances
> of this problem:
> 
>   src/gdb/btrace.h: In function ‘btrace_insn_s*
> VEC_btrace_insn_s_quick_insert(VEC_btrace_insn_s*, unsigned int, const
> btrace_insn_s*, const char*, unsigned int)’:
>   src/gdb/common/vec.h:948:62: error: use of deleted function ‘void*
> memmove(T*, const U*, size_t) [with T = btrace_insn; U = btrace_insn;
> <template-parameter-1-3> = void; size_t = long unsigned int]’
>      memmove (slot_ + 1, slot_, (vec_->num++ - ix_) * sizeof (T));    \
> 								^
>   src/gdb/common/vec.h:436:1: note: in expansion of macro 
> ‘DEF_VEC_FUNC_O’
>    DEF_VEC_FUNC_O(T)         \
>    ^
>   src/gdb/btrace.h:84:1: note: in expansion of macro ‘DEF_VEC_O’
>    DEF_VEC_O (btrace_insn_s);
>    ^
> [...]
>   src/gdb/common/vec.h:1060:31: error: use of deleted function ‘void*
> memcpy(T*, const U*, size_t) [with T = btrace_insn; U = btrace_insn;
> <template-parameter-1-3> = void; size_t = long unsigned int]’
> 	  sizeof (T) * vec2_->num);       \
> 				 ^
>   src/gdb/common/vec.h:437:1: note: in expansion of macro 
> ‘DEF_VEC_ALLOC_FUNC_O’
>    DEF_VEC_ALLOC_FUNC_O(T)         \
>    ^
>   src/gdb/btrace.h:84:1: note: in expansion of macro ‘DEF_VEC_O’
>    DEF_VEC_O (btrace_insn_s);
>    ^
> 
> So, VECs (given it's C roots) rely on memcpy/memcpy of VEC elements to
> be well defined, in order to grow/reallocate its internal elements
> array.  This means that we can only put trivially copyable types in
> VECs.  E.g., if a type requires using a custom copy/move ctor to
> relocate, then we can't put it in a VEC (so we use std::vector
> instead).  But, as shown above, we're violating that requirement.
> 
> btrace_insn is currently not trivially copyable, because it contains
> an enum_flags field, and that is itself not trivially copyable.  This
> patch corrects that.
> 
> Note that std::vector relies on std::is_trivially_copyable too to know
> whether it can reallocate its elements with memcpy/memmove instead of
> having to call copy/move ctors and dtors, so if we have types in
> std::vectors that weren't trivially copyable because of enum_flags,
> this will make such vectors more efficient.
> 
> gdb/ChangeLog:
> yyyy-mm-dd  Pedro Alves  <palves@redhat.com>
> 
> 	* common/enum-flags.h (enum_flags): Define copy/move ctors/op= as
> 	defaulted.
> ---
>  gdb/common/enum-flags.h | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/gdb/common/enum-flags.h b/gdb/common/enum-flags.h
> index e63c8a4..bea0ad5 100644
> --- a/gdb/common/enum-flags.h
> +++ b/gdb/common/enum-flags.h
> @@ -120,15 +120,12 @@ public:
>      : m_enum_value ((enum_type) 0)
>    {}
> 
> -  enum_flags (const enum_flags &other)
> -    : m_enum_value (other.m_enum_value)
> -  {}
> -
> -  enum_flags &operator= (const enum_flags &other)
> -  {
> -    m_enum_value = other.m_enum_value;
> -    return *this;
> -  }
> +  /* Define copy/move ctor/op= as defaulted so that enum_flags is
> +     trivially copyable.  */
> +  enum_flags (const enum_flags &other) = default;
> +  enum_flags (enum_flags &&) noexcept = default;
> +  enum_flags &operator= (const enum_flags &other) = default;
> +  enum_flags &operator= (enum_flags &&) = default;

What's the difference between defining these as default, and not 
defining them at all (which I assume will still make the compiler emit 
the default versions)?

Do you want to add a static_assert(is_pod...) for that class?  For now 
we have the VECs that will warn us if it becomes non-POD, but eventually 
they'll be gone.

Simon