feat: set default SWC minimizer passes to 2 to reduce bundle size

[chenjiahan]

What is passes

The maximum number of times to run compress. In some cases, more than one pass leads to further compressed code. Keep in mind more passes will take more time.

• https://swc.rs/docs/configuration/minification#jscminifycompress
• https://terser.org/docs/options/#compress-options

Why increase passes

Considering that Rspack is already quite fast, we should allocate more budget for bundle size optimization.

Bundle Size:

• Increasing passes can reduce output size by 0-6%, with optimization results depending on user code
• Optimization effects are similar between 2 and 3 passes, with diminishing returns beyond 3 passes

Build Time:

• For simple cases, increasing passes adds 3-5% to overall build time
• For complex cases, increasing passes adds 10-20% to overall build time

Setting passes to 2 by default offers a good balance of build performance and bundle size optimization.

Test case 1

• Repo: https://github.com/filipsobol/bundle-size-benchmark

• Test config:

output: {
  minify: {
    jsOptions: {
      minimizerOptions: {
        compress: {
          passes: Number(process.env.PASSES) ?? 1,
        },
      },
    },
  },
},
performance: {
  chunkSplit: {
    strategy: "all-in-one",
  },
},

• Build time:

hyperfine --warmup 2 --runs 5 'PASSES=1 node --run rsbuild' 'PASSES=2 node --run rsbuild' 'PASSES=3 node --run rsbuild' 'PASSES=4 node --run rsbuild'

Benchmark 1: PASSES=1 node --run rsbuild
  Time (mean ± σ):      1.581 s ±  0.018 s    [User: 2.705 s, System: 0.698 s]
  Range (min … max):    1.557 s …  1.604 s    5 runs
 
Benchmark 2: PASSES=2 node --run rsbuild
  Time (mean ± σ):      1.716 s ±  0.030 s    [User: 2.925 s, System: 0.786 s]
  Range (min … max):    1.683 s …  1.762 s    5 runs
 
Benchmark 3: PASSES=3 node --run rsbuild
  Time (mean ± σ):      1.870 s ±  0.067 s    [User: 3.126 s, System: 0.837 s]
  Range (min … max):    1.794 s …  1.962 s    5 runs
 
Benchmark 4: PASSES=4 node --run rsbuild
  Time (mean ± σ):      1.998 s ±  0.042 s    [User: 3.323 s, System: 0.894 s]
  Range (min … max):    1.935 s …  2.045 s    5 runs
 
Summary
  PASSES=1 node --run rsbuild ran
    1.09 ± 0.02 times faster than PASSES=2 node --run rsbuild
    1.18 ± 0.04 times faster than PASSES=3 node --run rsbuild
    1.26 ± 0.03 times faster than PASSES=4 node --run rsbuild

• Bundle size:

[passes 1]
Total: 2561.6 kB (gzip: 685.3 kB)

[passes 2] 124.6kB less than the previous value
Total: 2437.0 kB (gzip: 650.8 kB)

[passes 3] 29.2kB less than the previous value
Total: 2407.8 kB (gzip: 642.1 kB)

[passes 4] 8.5kB less than the previous value
Total: 2399.3 kB (gzip: 639.3 kB)

Test case 2

• Repo: https://github.com/web-infra-dev/rspack/tree/main/website

• Test config:

  output: {
    minify: {
      jsOptions: {
        minimizerOptions: {
          compress: {
            passes: Number(process.env.PASSES) ?? 1,
          },
        },
      },
    },
  },

• Build time:

hyperfine --warmup 2 --runs 5 'PASSES=1 node --run build' 'PASSES=2 node --run build' 'PASSES=3 node --run build' 'PASSES=4 node --run build'

Benchmark 1: PASSES=1 node --run build
  Time (mean ± σ):      3.399 s ±  0.165 s    [User: 8.534 s, System: 2.053 s]
  Range (min … max):    3.241 s …  3.654 s    5 runs
 
Benchmark 2: PASSES=2 node --run build
  Time (mean ± σ):      3.539 s ±  0.112 s    [User: 9.080 s, System: 2.199 s]
  Range (min … max):    3.399 s …  3.699 s    5 runs
 
Benchmark 3: PASSES=3 node --run build
  Time (mean ± σ):      3.518 s ±  0.026 s    [User: 9.603 s, System: 2.328 s]
  Range (min … max):    3.491 s …  3.555 s    5 runs
 
Benchmark 4: PASSES=4 node --run build
  Time (mean ± σ):      3.712 s ±  0.114 s    [User: 9.932 s, System: 2.442 s]
  Range (min … max):    3.594 s …  3.885 s    5 runs
 
Summary
  PASSES=1 node --run build ran
    1.04 ± 0.05 times faster than PASSES=3 node --run build
    1.04 ± 0.06 times faster than PASSES=2 node --run build
    1.09 ± 0.06 times faster than PASSES=4 node --run build

• Bundle size:

[passes 1]
Total: 8950.5 kB (gzip: 2489.8 kB)

[passes 2] 14.3kB less than the previous value
Total: 8936.2 kB (gzip: 2485.5 kB)

[passes 3] 5.7kB less than the previous value
Total: 8930.5 kB (gzip: 2483.5 kB)

[passes 4] 0.4kB less than the previous value
Total: 8930.1 kB (gzip: 2483.3 kB)

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[netlify]

✅ Deploy Preview for rspack canceled.

Name	Link
🔨 Latest commit	f92f27c
🔍 Latest deploy log	https://app.netlify.com/sites/rspack/deploys/676d23bd84e3c000083e696b

[codspeed-hq]

CodSpeed Performance Report

Merging #8853 will not alter performance

_{Comparing passes_default_value_1226 (f92f27c) with main (7707956)}

Summary

✅ 3 untouched benchmarks