assumeSafeAppend on slice of static array?

[H. S. Teoh via Digitalmars-d]

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.

:-(


T

-- 
What are you when you run out of Monet? Baroque.