[Development] Feature Freeze Exception: QStringView

Thiago Macieira thiago.macieira at intel.com
Tue Jan 31 19:32:55 CET 2017

On terça-feira, 31 de janeiro de 2017 18:39:43 PST Marc Mutz wrote:
> Q6String will likely have the small-string optimisation, so short strings
> aren't actually COWed, but deep-copied. Passing through QStringView then is
> about the same efficiency as passing through a const QString&.

Not sure you can count on this optimisation. Even my expanded QString is only 
24 bytes on 64-bit, which means that you can store floor((24 - 1) / 2) = 11 
characters in the short string. I don't have my data ready here, but when I 
calculated the histogram for qHash, 22 bytes was very far from the 
overwhelming majority of the strings (I remember 34 bytes was a significant 
mark, I just don't remember if it was 75% or 90%).

My current QString for 32-bit is only 12 bytes, which gives us 5 characters at 
most. That's definitely more overhead than the potential gain.

It would be better to expand it to 32 bytes, even on 32-bit platforms. That's 
15 characters and can be implemented by two SSE moves (or one AVX) on x86, 
versus one SSE plus one GPR on 64-bit or three GPR moves on 32-bit.

That implies we need to expand QVariant to 40 bytes. Fortunately, we don't 
plan on keeping QList the way it is, or it would suffer greatly.

Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center

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