[PATCH] Allow (Object *)kMyGlobalCFObjc casts without bridging

[Ben Langmuir]

> On Feb 24, 2015, at 2:30 PM, John McCall <rjmccall at apple.com> wrote:
> 
>> On Feb 24, 2015, at 11:14 AM, Ben Langmuir <blangmuir at apple.com> wrote:
>> Hi John,
>> 
>> As we discussed: 
>> 
>> Allow (Object *)kMyGlobalCFObj casts without bridging
>> 
>> Previously we allowed these casts only for constants declared in system
>> headers, which we assume are retain/release-neutral. Now also allow them
>> for constants in user headers, treating them as +0.  Practically, this
>> means that we will now allow:
>> id x = (id)kMyGlobalConst;
>> 
>> But unlike with system headers we cannot mix them with +1 values:
>> id y = (id)(b ? kMyGlobalConst : [Obj newValAtPlusOne]); // error
>> id z = (id)(b ? kSystemGlobalConst: [Obj newValAtPlusOne]); // OK
>> 
>> <extern-global-const-unbridged-cast.patch>
> 
> Yes, this good great, thank you.
> 
> You should also adjust the ARC specification:

Updated patch attached with the spec updates.

> 
> 1. Add "a load from a const non-system global variable of C retainable
> pointer type” to the list of known unretained expressions in the section
> entitled "Conversion to retainable object pointer type of expressions
> with known semantics”.  This should have a “beginning LLVM 3.6” note;
> when it’s available in a released Apple compiler, we can be more specific.

Note the “of C retainable…” bit is part of the context for this list of bullets.  I stole the “beginning LLVM 3.6” revision syntax from elsewhere, so hopefully I got it right. I’m having a bit of trouble with sphinx, but I’ll be sure to check the generated doc before I commit.

> 2. Add a note to the rationale explaining the different treatment of
> system and non-system globals: basically, that it’s reasonable to assume
> that global constants were initialized with true constants (e.g. string
> literals), but user constants might have been initialized with something
> dynamically allocated, using a global initializer.


Thanks,

Ben

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