assumeSafeAppend on slice of static array?

[Steven Schveighoffer via Digitalmars-d]

On Tue, 22 Apr 2014 14:31:07 -0400, H. S. Teoh via Digitalmars-d  
<digitalmars-d at puremagic.com> wrote:

> On Tue, Apr 22, 2014 at 02:24:41PM -0400, Steven Schveighoffer via  
> Digitalmars-d wrote:
>> On Tue, 22 Apr 2014 14:10:30 -0400, H. S. Teoh via Digitalmars-d
>> <digitalmars-d at puremagic.com> wrote:
>>
>> >I'm going through some code and thinking of ways to reduce GC
>> >pressure, and came across a bit that needed to append some items to
>> >an array:
>> >
>> >	T[] args;
>> >	lex.expect("(");
>> >	args ~= parseSingleItem(lex);
>> >	while (!lex.empty) {
>> >		lex.expect(",");
>> >		args ~= parseSingleItem(lex);
>> >	}
>> >	lex.expect(")");
>> >	return computeResult(args);
>> >
>> >Now obviously, in the general case (with arbitrarily many number of
>> >items) some GC allocations will be needed, but the most common
>> >use-cases are actually only 1 or 2 items each time. Allocating lots
>> >of small arrays seem to be rather wasteful, so I thought to use a
>> >static array as a buffer instead.
>> >
>> >The question is, is there a way to take a slice of the static array,
>> >set the length to zero, and append to it with ~= such that when it
>> >runs out of space in the static buffer, it will reallocate a longer
>> >array on the GC heap? Or is this a bad idea?
>>
>> TL;DR: Yes, use Appender :)
>>
>> The reason appending even works is because of the metadata stored in
>> the heap. Obviously, stack frames and fixed-length arrays do not have
>> this data.
>>
>> When that metadata cannot be found, it reallocates because that's the
>> only option.
>>
>> However, Appender, initialized with a static buffer, knows the length
>> of its data, and stores its own capacity, separate from the heap.
> [...]
>
> Unfortunately, the whole point of this exercise was to eliminate GC
> allocations for small arrays -- but since Appender's implementation
> allocates a private Data struct in its ctor, that kinda defeats the
> purpose. For the common case of 1 or 2 items only, it doesn't seem like
> Appender will perform any better, and it will introduce extra GC
> allocations to boot.
>
> :-(

I advocated a long time ago that Appender should have a stack-based  
version.

I still think that's the case. Because really what Appender has as an  
advantage over builtin slices is that it keeps a local copy of the  
capacity, so no heap metadata lookups need to occur. It's not conceptually  
that much added.

Of course, back then, I'm not sure we had @disable this(this), which such  
an appender should have.

-Steve