Re: CoreBase toll-free bridging

[David Chisnall]

On 11 Mar 2013, at 08:39, Luboš Doležel <address@hidden> wrote:

> Sure. This is what CFString's struct looks like now (I replaced internal 
> types with what everyone understands):
> 
> stuct __CFString
> {
>    void* isa; // for ObjC support
>    int16_t typeID; // CF type ID - we need to make CFString's typeID constant 
> forever (now it could randomly change)
>    struct
>    {
>        int16_t ro:1; // always set to 1 for constant strings
>        int16_t reserved:7;
>        int16_t info:8; // probably only "hasnull" (0x10) for constant strings
>    };
>    void* data;
>    uint32_t length;
>    uintptr_t hashCode;
> };
> 
> Is this layout something you'd accept as the next constant NSString?

I'd recommend that we have a couple of bits in the flags set for encoding, to 
allow ASCII or UTF-8 (ASCII implies no multibyte characters, so character-index 
lookups are easier).  If we're going to have to live with this for a long time, 
then it might be interesting for variable-length encodings to have a map from 
character indexes to byte indexes.  

I'd rather not have a C bitfield in the definition, because its layout is very 
ABI-specific: An int16_t with fixed bit definitions is much easier to work with.

The other question is how we initialise the isa pointer.  With recent libobjc2 
/ clang, class pointers are exposed as public symbols, so we could just make a 
weak reference to the class, but that would break if GNUstep-base is compiled 
with gcc.  

Alternatively, we could create the normal call to __objc_exec_class() in the 
constructor, but make __objc_exec_class weak and only call it if it is 
non-zero.  This would impose a small startup cost on every compilation unit 
containing CF strings.

Additionally, for short ASCII strings, I'd like to make clang emit GSTinyString 
instances (strings hidden in pointers) on 64-bit platforms.  Do you think that 
this would be a problem for CF?

> Which one exactly? CFSTR() in corebase expands to a regular function call.

Really?  I remember adding support for __builtin___NSStringMakeConstantString() 
a while ago...

David

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