Small Buffer Optimization for string and friends

[Manu]

On 9 April 2012 17:55, Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org>wrote:

> On 4/9/12 4:21 AM, Manu wrote:
>
>> After thinking on it a bit, I'm becoming a little worried about this
>> move for 2 rarely considered reasons:
>> Using lowering to a template, debug(/unoptimised) performance will
>> probably get a lot slower, which is really annoying. And
>> debugging/stepping might become considerably more annoying too, if every
>> time I press F11 (step in) over a function call that happens to receive
>> an arg from an array, the debugger then steps into the array templates
>> index operator... We'd be no better off than with STL, unless the
>> language has clever ways of hiding this magic from the debugger too, and
>> optimising/inlining the index even in debug builds...? But this is the
>> built-in array, and not a library we can optionally not use.
>>
>
> I agree. So we have the counterarguments:
>
> 1. Lowering would treat array primitives as sheer D code, subject to
> refusal of inlining. That means worse performance.
>
> 2. Unless the compiler takes special measures, source-level debuggers will
> trace through core, uninteresting code for array operations.
>
> 3. There are patterns that attempt to optimize by e.g. using .ptr, but end
> up pessimizing code because they trigger multiple memory allocations.


Indeed. I don't think the small array optimisation would benefit us in any
way that could even come close to balancing the loss. I don't think it
would benefit us much at all regardless, since we've already proven the
need for a string class anyway, and we can make such small-string
optimisations already.
I am very wary of removing a fundamental language primitive, that is, the
basic ability to express an array with absolutely no frills.

This idea only really benefits utf-8 strings, so why not just make a real
d-string class in phobos and be done with it? People complain about UTF
issues with raw strings, and this is what you intend to do anyway to
implement the optimisation.
d-string could perhaps be made to appear as a first-class language feature
via the 'string' keyword, and lower to the library in the way you describe.
Just don't blanket this across all arrays in general.
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