Make ArrayBuilder stack allocate the first 16 entries for less heap allocations

[Henr1k80]

In #120336 I was suggested to add the improvements to the already existing ArrayBuilder<T> and use that in that PR.

The heap allocated buffer will only be allocated if capacity is above the 16 stack allocated capacity.

In cases where there is no conditional adds and capacity is known, it will not make a difference, as the previous internal buffer will just be returned.
But in these cases, an array with correct capacity and an index could just be used instead of the ArrayBuilder<T>.

I have removed the unused Buffer getter.

[stephentoub]

Anywhere ArrayBuilder<T> is used as a field on a class, this is going to increase the size of that class' allocation by 16 Ts.

[Henr1k80]

Yeah, that's a problem.

But it is not used as a field currently. Could there be a warning in <remarks>, or is it a no go?
It could also be made a separate struct if it is useful? E.g. StackArrayBuilder<T>

[Henr1k80]

Could making it a ref struct could solve it for classes?

[jkotas]

runtime/src/coreclr/nativeaot/System.Private.TypeLoader/src/Internal/Runtime/TypeLoader/TypeBuilder.cs

Lines 29 to 33 in b87db0a

	private ArrayBuilder<TypeDesc> _typesThatNeedTypeHandles;
	private ArrayBuilder<InstantiatedMethod> _methodsThatNeedDictionaries;
	private ArrayBuilder<TypeDesc> _typesThatNeedPreparation;

[Henr1k80]

Yeah, that shoots it down @jkotas, at least modifying ArrayBuilder<T>
I did not find these when opening sfx.slnx, so I guess there is more to it than that.

[Henr1k80]

Does it make sense to make a ref struct StackArrayBuilder<T>?

[Henr1k80]

Oh, I can also see that some tests using reflection fails now, probably because of the Buffer get removal...

[Henr1k80]

Ok, I have changed it to a draft proposal of new stack allocating ref structs for building arrays.

One example of a simple fixed max stack size (8), where it fails hard, if you over go over the size.
It will not grow, but it is very simple. But it might need multiple size implementations.
But since it is internal, a size 13 could be made, for minimal stack allocation.

Another one, where growing is allowed, if max expected size is not known, or if growing, beyond expected capacity, is rare .
More complicated (branching-wise) though.
This can be used in the original PR #120336

I have written unit tests, but I do not know where to place them.
I guess this is the complicated part of adding new stuff, as I have seen errors from multiple other places when making a PR.
If you haven't guessed already, I am totally new to contributing to dotnet. Sorry for all the noise.