[PATCH 2/5] Don't memcpy non-trivially-copyable types: Make enum_flags triv. copyable
Simon Marchi
simon.marchi@polymtl.ca
Thu Apr 20 03:34:00 GMT 2017
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 âbtrace_insn_s*
> VEC_btrace_insn_s_quick_insert(VEC_btrace_insn_s*, unsigned int, const
> btrace_insn_s*, const char*, unsigned int)â:
> src/gdb/common/vec.h:948:62: error: use of deleted function âvoid*
> memmove(T*, const U*, size_t) [with T = btrace_insn; U = btrace_insn;
> <template-parameter-1-3> = void; size_t = long unsigned int]â
> memmove (slot_ + 1, slot_, (vec_->num++ - ix_) * sizeof (T)); \
> ^
> src/gdb/common/vec.h:436:1: note: in expansion of macro
> âDEF_VEC_FUNC_Oâ
> DEF_VEC_FUNC_O(T) \
> ^
> src/gdb/btrace.h:84:1: note: in expansion of macro âDEF_VEC_Oâ
> DEF_VEC_O (btrace_insn_s);
> ^
> [...]
> src/gdb/common/vec.h:1060:31: error: use of deleted function âvoid*
> memcpy(T*, const U*, size_t) [with T = btrace_insn; U = btrace_insn;
> <template-parameter-1-3> = void; size_t = long unsigned int]â
> sizeof (T) * vec2_->num); \
> ^
> src/gdb/common/vec.h:437:1: note: in expansion of macro
> âDEF_VEC_ALLOC_FUNC_Oâ
> DEF_VEC_ALLOC_FUNC_O(T) \
> ^
> src/gdb/btrace.h:84:1: note: in expansion of macro âDEF_VEC_Oâ
> 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
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