feat(mangler): reuse variable names

[sapphi-red]

Changed the mangler to reuse variable names where possible.

This will reduce the code size as shorter variable names can be used in more places. But requires global information and limits parallelism in a single file and requires more memory.

[sapphi-red]

• feat(mangler): reuse variable names #8562 👈 (View in Graphite)
• main

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[codspeed-hq]

CodSpeed Performance Report

Merging #8562 will degrade performances by 72.24%

_{Comparing 01-17-feat_mangler_reuse_variable_names (1b9026f) with main (007e8c0)}

Summary

❌ 3 regressions
✅ 29 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	01-17-feat_mangler_reuse_variable_names	Change
❌	mangler[antd.js]	6 ms	12.4 ms	-51.65%
❌	mangler[react.development.js]	138.8 µs	263.9 µs	-47.42%
❌	mangler[typescript.js]	9.6 ms	34.5 ms	-72.24%

[sapphi-red]

The perf regression looks really bad, but if you take a step back and see the overall minifier timings, it's not that big regression.

antd: 196.8ms (6.4ms is 3.3%)
react: 1.8ms (125.1µs is 7.0%)
typescript: 344.6ms (24.9ms is 7.2%)

[overlookmotel]

Just to raise one other concern:

The addition of scope_id field to Reference will cause difficulties in transformer. In transformer, we update semantic data as we mutate the AST, to keep AST + semantic in sync.

This is quite difficult already, and if Reference has a scope_id field, we'd need to update that field too whenever we move an IdentifierReference into a different scope, or e.g. wrap some code in an IIFE (which introduces a new scope). Both of these are fairly common operations in the transformer.

The changes to TraverseScoping::create_bound_reference and create_unbound_reference in this PR aren't correct - often we are creating a Reference in a different scope from the current one.

FYI: The bigger picture is: Currently we run semantic analysis multiple times during the pipeline. The hope (well certainly my hope) is that we can eventually make all stages of the pipeline keep semantic data in sync with AST changes. Then we'll be able to remove these extra semantic passes, which will be a large perf improvement.

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The performance issue needs to be looked at first. But if we decide that it's worth it (very possibly it is), I would suggest the following:

• Split the introduction of Reference::scope_id field into a separate PR.
• Add checks for correctness of that field to the transformer conformance checker (which checks semantic data is correct after transform), and see how many errors we have due to incorrect Reference::scope_id.
• Assess how much work required to handle updating this extra state in transformer.

It will of course be achievable with some effort, but it'd be useful to understand how much effort before we go ahead. Maybe it's easier than I think!

cc @Dunqing

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Last thing: I'm not very familiar with the mangler. Could you possibly give a brief explanation of how this algorithm works? I'm wondering if any other way to achieve the effect without the Reference::scope_id field.