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

[Ben Langmuir]

> On Feb 25, 2015, at 11:58 AM, John McCall <rjmccall at apple.com> wrote:
> 
>> On Feb 25, 2015, at 8:14 AM, Ben Langmuir <blangmuir at apple.com> wrote:
>>> 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.
>> 
> 
> I’m not going to manually verify the ReST syntax, but the text looks great, thanks.

Thanks! r230534
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