Large tuples are a performance footgun

[epage]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

rust version

1.68

winnow version

0.4.0

Minimal reproducible code

(a, b).map(...).parse_next(input)

Steps to reproduce the bug with the above code

TBD

Actual Behaviour

Slow

Expected Behaviour

Fast

Additional Context

See #230

Options

• Reduce the number of tuple elements supported to not encourage bad behavior
• The struct macro from An easier way to discard fields in a tuple #82

[epage]

One thing that further decreases performance are large output types. Surprisingly, I've seen quite big performance improvements by Box-ing large types that are passed though multiple layers of parser code. Of course, boxing by default is a performance footgun and usually makes the parser substantially slower.

In addition to that, tuple parsers should be used carefully as they can run into the same performance issues when multiple large output types are involved or if they simply have too many items in them.

E.g. i replaced something like

(a, b).map(...).parse_next(input)

with

let (input, a) = a.parse_next(input)?;
...
let (input, b) = b.parse_next(input)?;
...
Ok((input, ...))

in quite a few places to resolve performance issues.

I wonder if #251 improved the situation as the compiler is now more likely to optimize the tuple version into the imperative version.

[epage]

@martinohmann when you have more time, could you create a branch where hcl-edit is using tuples so I can do some more analysis of this? I'd like to see how #251 or tuple alternatives may be able to help improve things. Because of the chance of this being fixed in #251, I'm deprioritizing this for now, so no rush.

[martinohmann]

Good idea! I'll try find some tuple cases. The parser is built in a way now that makes bringing these back a bit more involved. But I think I see 1-3 cases that are "easy" to negatively impact performance. Not sure if I can get to it this month or next month. Will ping you once I have a branch.