why use string for this example of appender?

[Steven Schveighoffer]

On 3/31/21 5:32 PM, ludo wrote:
> Hi guys,
> 
> I am working on an old software in D1, which defines at some point an 
> array.d module. See my github file: https://tinyurl.com/5ffbmfvz
> 
> If you go line 347, you see an ArrayBuilder struct, which is supposed to 
> behave exactly like an array but with faster concatenation. The class 
> comment says:
> /**
> * Behaves the same as built-in arrays, except about 6x faster with 
> concatenation at the expense of the base pointer
> * being 4 system words instead of two (16 instead of 8 bytes on a 32-bit 
> system).
> */
> 
> I created a unittest, line 586, to test this statement. You can git 
> clone + dub test, to see the result. On my machine I get:
> *** ArrayBuilder benchmark ***
> 1) Concatenation with std:          745 μs and 7 hnsecs
> 2) Concatenation with Arraybuilder: 236 μs and 8 hnsecs
> 3) Concatenation with Reserve:      583 μs and 5 hnsecs
> 4) Concatenation with Length:       611 μs and 8 hnsecs
> 5) Concatenation with Appender:     418 μs
> 
> In (1) I use standard array concatenation, in (2) I use the ArrayBuilder 
> struct, in (3) I test standard concat with a call to reserve() 
> before-hand, in (4) standard + length= beforehand, and finally I dug out 
> the Appender class in (5).
> 
> None of the D library tools beat the ArrayBuilder class! Note though, 
> that Appender is only two times slower, not 6x slower.
> 
> Can someone explain to me why ArrayBuilder is so fast (or the others so 
> slow)? What is this base pointer being "4 system words instead of 2"? I 
> read the code of ArrayBuilder, but I can not spot any magic trick... Has 
> this anything to do with standard array concat being "safer" 
> (thread-wise, bounds-wise, etc)?

ArrayBuilder should be similar in performance to Appender. I think part 
of the issue with appender could be the ref counted design. Only 1000 
elements is going to show heavily the setup/teardown time of allocation 
of the implementation struct. But that is a guess. You may want to up 
the repetition count to 100 or 1000.

Note your code for appending with length is not doing what you think:

void concat_withLength()
		{
			int[] array;
			array.length = 1000;
			for (int j=0; j<1000; j++)
				array ~= j;
		}

This allocates 1000 elements, and then append 1000 *more* elements.

I think you meant in the loop:

array[j] = j;

This should be the fastest one.

-Steve