[phobos] slice vs. append

[Andrei Alexandrescu]

On 03/22/2010 10:16 AM, Steve Schveighoffer wrote:
> I don't think you understand the purpose of the cache.  It does not
> store arrays, it stores BlkInfo's.  This is information from the GC
> about where a memory block starts, its length, and what flags it was
> created with.  Appending in-place to an array does not change the
> cached blockinfo.

Yes, but at the end of the day, you do have a primitive that tells you 
whether a given array extends to the end of a given block and 
consequently is ok to append to in-place. Correct?

> First, setting length to be smaller already does what you propose,
> except it does not look anything up in the cache.

You told me that for best results I need to do:

arr.length = 0;
arr.assumeSafeAppend();

The last call makes the system assume that I saved no subarray of arr 
that I'm interested in keeping. No?

> Second, slicing is a way way more common operation than setting the
> length, and you would be changing this common operation that does 2
> reads and 2 writes to a function call with a cache search.  Any gain
> you think you get from removing the block from the cache would be
> overshadowed by the bloating of the slice operation itself.

I'd guess you and David are right.


Andrei