Use feature gate on parallelism

[hyf0]

Related:

• chore: remove rayon on wasm targets rolldown/rolldown#845

Currernt oxc_sourcemap enable using rayon by default and it show improved performance said in #2825 (comment).

However, wasm target doesn't have a good support in rayon. While rayon claims it's ok in some issues, but it's still quite frustrating to debug wasm + rayon. It would be helped if we could disable rayon using feature gate.

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Off the topic, Using feature gate on parallelism should be a principle that apply on all oxc crates that gonna exposed to users when introducing any parallelism in it.

[hyf0]

I also have some confusions on #2825 (comment). While it said it has improved the performance overall, but it doesn't seems to be a suitable strategy for every situation.

I don't know much about these two funcitons:

oxc/crates/oxc_sourcemap/src/encode.rs

Line 11 in 6561392

pub fn encode(sourcemap: &SourceMap) -> Result<String> {

oxc/crates/oxc_sourcemap/src/encode.rs

Line 74 in 6561392

.par_iter()

But it seems that gonna be used in codegen. Then I realized these are gochas in logic

• Rolldown is rendering modules, which means using oxc_codegen, in parallel already, so if this parallelism is enable by default , I'm not sure if it really improve performance in rendering modules while we are already apply parallelism in module level.

• Rolldown will use oxc_sourcemap directly to manipulate the sourcemap in chunk level. Which means apply these functions on modules in chunk in serial.

I suspect that's where it really improve the performance since the benchmark case used in #2825 (comment), which is a huge single chunk.

If we have a case that generate so many mutiple small chunks, I wonder if it would be even a downside to the performance.

However, it still show pentation on the case said #2825 (comment). So using feature gate might not enough, since we need dynamicly decide to use which method depend on the input.

[Boshen]

Let's either:

• remove rayon from this case
• add benchmark to indicate real world scenarios

[hyf0]

Let me try to find a way to benchmark this. On hold for now.

[Boshen]

Next release:

oxc/crates/oxc/Cargo.toml

Lines 43 to 44 in f6daf0b

	sourcemap = ["oxc_sourcemap"]
	sourcemap_concurrent = ["sourcemap", "oxc_sourcemap/concurrent"]