Re: CoreBase toll-free bridging

[Luboš Doležel]

On Mon, 11 Mar 2013 08:13:45 +0000, David Chisnall wrote:
> I'd certainly be interested in improving the NSString structure.  I
> put out a request for this a while ago, but got no feedback.

Great, so let's do it now :-)

> There are a few problems, the most significant is that GNUstep
> periodically does isa comparisons to determine if something is a
> constant string.  Check the code for comparisons against
> NXConstantStringClass.  These would need fixing, and we'd also have a
> transition period where we had two constant string classes.

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?

> Note that, to support CF code in GNUstep, clang exposes a builtin
> function in C code for creating constant string objects.

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

> Adding a hash field to NSConstantString is something I would
> definitely support.  For efficiency, there is a load of stuff in
> GNUstep that replaces NSConstantString instances with GSString
> instances at load time because caching the hash significantly 
> improves
> performance when the hashes are stored in collections.
>
> We don't have to make the hash computation part of the ABI, we can
> simply reserve a flag to indicate 'hash calculated.'  The hash method
> would then check this flag and return the hash field if it is set, or
> compute it if it isn't.

Fine! So I guess we would kill two birds with one stone - remove some 
speed hacks and make it bridging-ready.

--
Luboš Doležel