On Tue, 22 Apr 2014 14:31:07 -0400, H. S. Teoh via Digitalmars-d
<[email protected]> 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
<[email protected]> 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
