Brian Carroll (Nov 06 2023 at 08:23): Brian Carroll (Nov 06 2023 at 08:23):Hey folks, I just opened a PR to remove an optimisation from the Wasm dev backend but wanted to get feedback on the idea here too.
While Brendan was working on the Zig 0.11 PR, the last issue was a tricky one in Wasm, and I commented that it seemed to be in the most complex part of the Wasm backend. I was actually a bit afraid to touch it.
But that code is mainly concerned with a size optimisation, which we could do without.
So the PR is to delete the optimisation and all of the book-keeping code we need for it.
The diff is +320 -910 which is a nice simplification.
Brian Carroll (Nov 06 2023 at 08:23):In theory this makes the generated code more bloated. But it deletes complex code that I'm afraid of. And I have not actually been able to measure the bloat!
Brian Carroll (Nov 06 2023 at 08:24):Disk space for all test_gen binaries with optimisation: 759,592 bytes
Disk space for all test_gen binaries without optimisation: 759,688 bytes
So that's just 96 bytes.
Brian Carroll (Nov 06 2023 at 08:24):This is weirdly small though.
Brian Carroll (Nov 06 2023 at 08:25):There's an example in the PR body for 1 + 2 + 3 where the optimisation removes 10 instructions, at 2 bytes each, which is 20 bytes. And that's just 1 test out of 1250.
Brian Carroll (Nov 06 2023 at 08:36):Some technical background:
Each Wasm function can store local variables in one of 3 places:
We need to choose one of these 3 places to store each Symbol in the monomorphic IR.
Currently on the main branch, we use all 3 in various situations.
Brian Carroll (Nov 06 2023 at 08:36):The PR removes (1) as an option.
It turns out to be more complicated than you'd expect to track symbols in the stack machine. Wasm has structured control flow with nested block scopes. So there are instructions like loop rather than just "jump" or "goto". You can't access a value in a higher scope.
Brian Carroll (Nov 06 2023 at 08:38):After this PR we no longer track what's stored in the stack machine.
Instead we do a much more direct translation of the mono IR.
We translate each let in the IR to:
let the value stack machine is empty. Brian Carroll (Nov 06 2023 at 08:40):The drawback is that we are not taking advantage of the stack machine, so we get inefficient code.
The advantage is that we are more directly translating the mono IR so our Rust code is much simpler and easier to maintain.
Brian Carroll (Nov 06 2023 at 08:41):There doesn't seem to be much speed advantage, at least in the test_gen tests.
The total runtime for cargo test-gen-wasm goes from 7.428s to 7.412s (based on hyperfine with 10 runs).
Brian Carroll (Nov 06 2023 at 08:41):So the main argument is simplicity/maintainability.
Brian Carroll (Nov 06 2023 at 08:45):Frankly, I don't want to be the only one who is able to make changes in this code. I've had less time for Roc in the past year and I'm not sure how my availability is going to fluctuate in the future. So getting rid of something very complex that we don't really need, seems like a good idea.
Brian Carroll (Nov 06 2023 at 08:53):If we decide that we need the size optimisation, it would be much simpler to implement it after generating the instructions for a function. We could probably combine it into an existing pass. I think it would be a lot less code and easier to understand/maintain/debug.
Anton (Nov 06 2023 at 09:12):Sounds like a wise decision!
Richard Feldman (Nov 06 2023 at 11:54):Agreed! :smiley:
Richard Feldman (Nov 06 2023 at 11:54):Thanks @Brian Carroll, I think that's a really good call!
Last updated: Jul 06 2025 at 12:14 UTC