[cfe-commits] [PATCH] Smarter implicit copy-construction/copy-assignment.

[John McCall]

On Feb 5, 2013, at 6:45 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Tue, Feb 5, 2013 at 5:31 PM, Lang Hames <lhames at gmail.com> 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.
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