On 12-07-29 12:20 PM, Tim Chevalier wrote:
Unfortunately there are some pretty severe performance issues at the moment.
Ragel supports two state machine styles, table-driven and goto-driven. My
backend uses tables, but since Rust doesn't yet support global constant
vectors, I need to malloc the state machine table on every function call. This
results in the [ragel-based url
parser](https://github.com/erickt/ragel/blob/rust/examples/rust/url.rl)
being about
10 times slower than the equivalent table-based parser in OCaml. You can see
the generated code [here](https://gist.github.com/3200980).
There's an open bug on vector constants:
https://github.com/mozilla/rust/issues/571
It's assigned to me (though I haven't done anything on it yet), so
feel free to bug me about it :-)
Yeah, I'm doing this (all the const-exprs) presently. There might be
enough in there now to point ragel at. Constant records and tuples work,
as do const vectors and &-ptrs to consts. No slices yet. Still lots to do.
This sounds like the open bug on labelled break and continue, as Patrick said:
https://github.com/mozilla/rust/issues/2216
which is currently unassigned. As per #217, we decided not to support
general tail calls, but that doesn't mean we can't support tail calls
just for the special labelled break/continue form.
Yeah. I think that might be adequate, at least for building-atop with a
macro or other extension. Assuming we permit a 'sideways again' that
transfers from one labeled loop to another (at the same or higher
nesting level).
The key thing you want is direct branching to your current state (rather
than indirect off a 'current state' variable) and direct jumps from one
state to another; the rest (argument passing etc.) can be encoded at no
cost through local variables. The fast control transfers can't.
I do slightly wonder if the labeled break/again forms will mutate into
general gotos. I hope not.
-Graydon
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