Re: [PATCH 1/3] rust: maple_tree: add MapleTree

[Matthew Wilcox <willy@infradead.org>]

On Sat, Jul 26, 2025 at 01:23:22PM +0000, Alice Ryhl wrote:
> +struct ma_state rust_helper_MA_STATE(struct maple_tree *mt, unsigned long start, unsigned long end)
> +{
> +	MA_STATE(mas, mt, start, end);
> +	return mas;
> +}

This seems very inefficient.  Returning a struct larger than two words
(on x86 anyway) means that the compiler implements this as:

void rust_helper_MA_STATE(struct ma_state *masp, ...)
{
	MA_STATE(mas, mt, start, end);
	*masp = mas;
}

so that's about 72 bytes being memcpy'd per access to the maple tree.
Sure, it's stack, so it's cache hot, but surely we can implement
the equivalent of MA_STATE in Rust and see a significant performance
win, at least on read operations.