[cfe-dev] [RFC] Warning for large Objective C runtime encodings (-Wobjc-encodings-larger-than=)

Nat! via cfe-dev cfe-dev at lists.llvm.org
Tue Dec 11 12:44:42 PST 2018


On 11.12.18 05:06, Dave MacLachlan via cfe-dev wrote:
> Hey esteemed colleagues,
>
> We recently noticed that we were generating some incredibly large 
> Objective C runtime encodings in some of our apps that are using 
> Objective C++ and passing C++ types in Objective C methods, or have 
> C++ types as instance variables. Templates are especially notorious 
> for generating huge amounts of Objective C runtime information. I did 
> a write up on it here:
>
> https://medium.com/@dmaclach/objective-c-encoding-and-you-866624cc02de
>
> but am looking for a better solution than my hacky "smart pointers". 
> As a first step I wanted to have a warning I could control to 
> understand where my problem spots are.
>
> I would appreciate any thoughts on
>
> https://reviews.llvm.org/D55544
>
> It is my first real foray into clang coding, so I expect my 
> implementation is naïve.
>
> Tracking bug:
>
> https://bugs.llvm.org/show_bug.cgi?id=38346
>
> Cheers,
> Dave
>

Hi

This is a an idea, that may or may not be useful, depending on your 
circumstances.

What you could do is turn off the "bool EncodeBlockParameters, bool 
EncodeClassNames, bool EncodePointerToObjCTypedef" (especially the 
latter) flags passed to ASTContext::getObjCEncodingForTypeImpl, 
depending on a newly introduced compiler flag in `Options.td`. The 
resulting encodings are much smaller yet remain compatible. The amount 
of code out there that actually correctly introspects C++ type @encodes 
is probably extremely close to zero.

Usually the decision to use extended or traditional @encode is made at 
the runtime level. My runtime 
(https://github.com/mulle-objc/mulle-objc-runtime) uses extended format, 
but I don't support C++ anyway :) I could see a warning or compiler 
option like this as being useful, especially if the type is used in a 
lot of methods. But then this could also be part of the runtime...

I also can't help but comment, that mixing C++ and Objective-C gets you 
the worst of both worlds. :)

Ciao

    Nat!




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