Re: New ABI NSConstantString

[David Chisnall]

On 8 Apr 2018, at 10:55, Richard Frith-Macdonald <address@hidden> wrote:
> 
> 
> 
>> On 6 Apr 2018, at 11:00, David Chisnall <address@hidden> wrote:
>> 
>> It would probably help catch more bugs if we made use of NSString’s 
>> class-cluster nature more in -base.  I have just fixed a bug in GSString 
>> where we were checking one object matched a particular class before 
>> dereferencing the _flags ivar of the other.  I caught this because the other 
>> was a GSTinyString, which is almost never a valid pointer.
> 
> Possibly, but performance *is* an issue here.  The NSString code was 
> rewritten some years ago (moving away from them use of class cluster 
> features) as a result of extensive profiling of real-world applications which 
> were running too slow, precisely because NSString methods are very heavily 
> used in real apps.  At the time somethjing like 20% of the CPU was wasted in 
> method dispatch overheads (the -characterAtIndex: method is one of the 
> cluster primitives and a major culprit) but there were also performance 
> issues due to buffer allocation and copying of internal representations.  The 
> changes made a substantial improvement in general performance as well as 
> causing multipler orders of magnitude improvement in a few pathological cases.

I agree that we should be improving performance for critical code, but 
unfortunately it appears that we have done so at the expense of correctness in 
a number of places.  As per my other email, 
-rangeOfComposedCharacterSequenceAtIndex: appears to give the wrong results in 
almost every nontrivial case, and is unfortunately one of the primitive methods 
for a lot of things.

I also note that a lot of the NSString method implementations are not well 
optimised.  In a number of places, -characterAtIndex: is called repeatedly, 
when -getCharacters:range: is normally significantly more efficient.  The ICU 
UText interface provides something very similar to -getCharacters:range: as its 
primitive method (a callback that fills a buffer with UTF-16 characters) and 
has some carefully optimised routines.

David