On Feb 5, 2013, at 6:45 PM, Richard Smith <[email protected]> wrote:
> On Tue, Feb 5, 2013 at 5:31 PM, Lang Hames <[email protected]> wrote:
> Hi John, Richard,
>
> Here's the updated patch for smarter copy-assignment/copy-construction for
> structs with Non-POD members.
>
> Richard - I've added test cases for volatile fields, inner classes, array
> members, pod-like members, bitfields and volatile fields.
> I've also pulled the naming scheme in line with the coding standards. I made
> sure I use ' &' rather than '& ' where appropriate.
>
> John -
>
> 1) Qualifiers are now being grabbed once.
> 2) I now fall back on existing codegen for when GCMode != NonGC.
> 3) Made sure I round-up to the nearest byte when copying bitfields.
> 4) I've tried to address the style issues you raised.
> 5)
>
>
> > + // Bail out on non-POD, not-trivially-constructable members.
> > + if (!FieldType.isPODType(GetCGF().getContext()) &&
> > + (!CE || !CE->getConstructor()->isTrivial()))
> > + return false;
> >
> > If you're calling a trivial copy/move constructor, it does not matter
> > whether the type is POD. Test case: give a field a type with a trivial
> > copy constructor and non-trivial destructor, or more realistically a
> > non-trivial default constructor.
>
> Note that this is the test for bailing out. The POD check needs to be there
> for primitive types, for which CE == 0, otherwise we'd decide that we
> couldn't memcpy primitive fields.
>
> Just because the type is POD, doesn't mean the selected constructor is
> trivial.
The point is that Lang's code was gating on *both*, which is clearly
unnecessary in the case of class types.
> Counterexample:
>
> struct S { S() = default; template<typename T> S(T&&); S(const S&) = default;
> };
> struct U { U(); S s; };
> static_assert(__is_pod(S), "");
> U u = static_cast<U&&>(U()); // static_cast to disable copy elision
>
> S is POD, but U's move constructor calls S's constructor template.
>
> I think this is what you mean:
>
> CE ? CE->getConstructor()->isTrivial() :
> FieldType.isTriviallyCopyableType(Ctx)
Mmm. This would actually drop members of reference type, which really can be
safely memcpy'ed in a copy constructor.
On the other than, it would also drop members of atomic type, which really
shouldn't be memcpy'ed. So there's that.
Lang, I think using Richard's proposal and then specifically opting-in a couple
of extra cases (e.g. reference types) would make a lot of sense.
John.
_______________________________________________
cfe-commits mailing list
[email protected]
http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
