Fuse local syntax lowerings into one traversal by jakebailey · Pull Request #64539 · microsoft/TypeScript

Jake Bailey (jakebailey) · 2026-09-29T18:04:01Z

I left copilot overnight with the goal of "make our transformers be split but run in a single pass". It produced this as a first checkpoint, doing all of the syntax downleveling in one pass.

Early results imply it's 50% faster with a 35% less memory, but want to see the real benchmarks.

Separate walks for local syntax transformations repeatedly rebuild the same ancestors and allocate separate hoisting environments. Processing these lowerings together keeps operand trees local and avoids revisiting generated nullish coalescing expressions.

Jake Bailey (jakebailey) · 2026-09-29T18:04:18Z

TypeScript Bot (@typescript-bot) perf test this faster

typescript-automation · 2026-09-29T18:04:25Z

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
`perf test this faster`	✅ Started	❌ Results

typescript-automation · 2026-09-29T18:30:10Z

Jake Bailey (@jakebailey), the perf run you requested failed. You can check the log here.

Jake Bailey (jakebailey) · 2026-09-29T18:58:36Z

TypeScript Bot (@typescript-bot) perf test this faster

typescript-automation · 2026-09-29T18:58:42Z

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
`perf test this faster`	✅ Started	👀 Results

typescript-automation · 2026-09-29T19:31:00Z

Jake Bailey (@jakebailey)
The results of the perf run you requested are in!

Here they are:

tsc

Comparison Report - baseline..pr

_Metric	_baseline	_pr	_Delta	_Best	_Worst	_p-value
_{Compiler-Unions - native}
_Errors	₄₁	₄₁	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_115,276	_115,276	_~	_~	_~	_{p=1.000 n=12}
_Types	_79,613	_79,613	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{141,867k (± 0.62%)}	_{141,355k (± 0.44%)}	_~	_139,868k	_143,205k	_{p=0.378 n=12}
_{Memory Allocs}	_{2,194,639 (± 0.00%)}	_{2,192,577 (± 0.00%)}	_{-2,062 (- 0.09%)}	_2,192,309	_2,192,874	_{p=0.000 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.040s (± 4.21%)}	_{0.042s (± 3.75%)}	_~	_0.037s	_0.046s	_{p=0.137 n=12}
_{Bind Time}	_{0.013s (±14.31%)}	_{0.012s (±13.11%)}	_~	_0.010s	_0.017s	_{p=0.925 n=12}
_{Check Time}	_{0.492s (± 1.03%)}	_{0.495s (± 0.77%)}	_~	_0.487s	_0.506s	_{p=0.212 n=12}
_{Emit Time}	_{0.266s (± 2.04%)}	_{0.267s (± 2.50%)}	_~	_0.250s	_0.283s	_{p=0.966 n=12}
_{Total Time}	_{0.817s (± 0.91%)}	_{0.822s (± 1.06%)}	_~	_0.800s	_0.845s	_{p=0.272 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{846,143 (± 0.10%)}	_{846,648 (± 0.09%)}	_~	_844,864	_848,376	_{p=0.478 n=12}
_Types	_{249,941 (± 0.00%)}	_{249,941 (± 0.00%)}	_~	_249,937	_249,950	_{p=0.967 n=12}
_{Memory Used}	_{790,897k (± 0.05%)}	_{791,107k (± 0.04%)}	_~	_789,981k	_791,773k	_{p=0.590 n=12}
_{Memory Allocs}	_{12,937,472 (± 0.03%)}	_{12,948,041 (± 0.18%)}	_~	_12,933,196	_13,064,857	_{p=0.590 n=12}
_{Config Time}	_0.016s	_0.016s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.276s (± 1.10%)}	_{0.266s (± 3.41%)}	_~	_0.240s	_0.287s	_{p=0.053 n=12}
_{Bind Time}	_{0.059s (± 0.55%)}	_{0.075s (±19.91%)}	_~	_0.058s	_0.127s	_{p=0.161 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.593s (± 0.60%)}	_{1.619s (± 1.42%)}	_{+0.026s (+ 1.65%)}	_1.565s	_1.683s	_{p=0.046 n=12}
_{Total Time}	_{1.957s (± 0.47%)}	_{1.993s (± 1.74%)}	_~	_1.938s	_2.107s	_{p=0.104 n=12}
_{mui-docs - native}
_Errors	_{11,399 (± 0.03%)}	_{11,400 (± 0.04%)}	_~	_11,375	_11,405	_{p=0.555 n=12}
_Symbols	_4,450,780	_4,450,780	_~	_~	_~	_{p=1.000 n=12}
_Types	_1,441,847	_1,441,847	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{3,194,928k (± 0.04%)}	_{3,195,676k (± 0.05%)}	_~	_3,191,163k	_3,199,767k	_{p=0.443 n=12}
_{Memory Allocs}	_{34,584,477 (± 0.04%)}	_{34,579,310 (± 0.05%)}	_~	_34,536,942	_34,634,703	_{p=0.478 n=12}
_{Config Time}	_{0.015s (± 1.87%)}	_{0.015s (± 1.20%)}	_~	_0.015s	_0.016s	_{p=0.590 n=12}
_{Parse Time}	_{0.571s (± 2.75%)}	_{0.582s (± 3.16%)}	_~	_0.508s	_0.617s	_{p=0.247 n=12}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{8.694s (± 1.13%)}	_{8.593s (± 1.24%)}	_~	_8.414s	_8.869s	_{p=0.128 n=12}
_{Emit Time}	_{0.500s (±15.91%)}	_{0.535s (±13.93%)}	_{🔻+0.035s (+ 7.05%)}	_0.431s	_0.713s	_{p=0.040 n=12}
_{Total Time}	_{10.559s (± 0.42%)}	_{10.510s (± 0.54%)}	_~	_10.377s	_10.669s	_{p=0.117 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,390,229	_1,390,229	_~	_~	_~	_{p=1.000 n=12}
_Types	_441,498	_441,498	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{1,690,503k (± 0.81%)}	_{1,686,372k (± 0.81%)}	_~	_1,660,641k	_1,727,203k	_{p=0.843 n=12}
_{Memory Allocs}	_{92,772,204 (± 0.08%)}	_{92,753,820 (± 0.16%)}	_~	_92,267,622	_92,966,252	_{p=0.551 n=12}
_{Config Time}	_{0.004s (±18.70%)}	_{0.004s (± 9.52%)}	_~	_0.004s	_0.006s	_{p=0.320 n=12}
_{Parse Time}	_{0.217s (± 4.03%)}	_{0.216s (± 4.43%)}	_~	_0.187s	_0.231s	_{p=0.766 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.799s (± 0.48%)}	_{1.805s (± 0.54%)}	_~	_1.778s	_1.827s	_{p=0.298 n=12}
_{Emit Time}	_{0.302s (± 5.68%)}	_{0.307s (± 4.24%)}	_~	_0.267s	_0.334s	_{p=0.580 n=12}
_{Total Time}	_{23.526s (± 0.77%)}	_{23.586s (± 0.60%)}	_~	_23.255s	_23.920s	_{p=0.514 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_334,953	_334,953	_~	_~	_~	_{p=1.000 n=12}
_Types	_197,616	_197,616	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{313,528k (± 0.08%)}	_{313,556k (± 0.12%)}	_~	_313,093k	_315,364k	_{p=0.932 n=12}
_{Memory Allocs}	_{4,628,547 (± 0.01%)}	_{4,629,089 (± 0.01%)}	_~	_4,627,748	_4,630,491	_{p=0.078 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.112s (± 4.38%)}	_{0.117s (± 4.46%)}	_~	_0.104s	_0.126s	_{p=0.246 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.042s (± 0.32%)}	_{1.043s (± 0.51%)}	_~	_1.034s	_1.061s	_{p=0.944 n=12}
_{Emit Time}	_{0.134s (±12.02%)}	_{0.141s (±11.02%)}	_~	_0.094s	_0.165s	_{p=0.173 n=12}
_{Total Time}	_{1.339s (± 1.25%)}	_{1.351s (± 1.21%)}	_~	_1.319s	_1.392s	_{p=0.561 n=12}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_303,834	_303,834	_~	_~	_~	_{p=1.000 n=12}
_Types	_181,663	_181,663	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{273,891k (± 0.03%)}	_{273,998k (± 0.02%)}	_{+107k (+ 0.04%)}	_273,794k	_274,133k	_{p=0.020 n=12}
_{Memory Allocs}	_{1,618,273 (± 0.01%)}	_{1,618,392 (± 0.01%)}	_~	_1,617,986	_1,618,805	_{p=0.198 n=12}
_{Config Time}	_{0.000s (±113.79%)}	_{0.000s (±217.90%)}	_~	_0.000s	_0.001s	_{p=0.590 n=12}
_{Parse Time}	_{0.098s (± 2.89%)}	_{0.098s (± 4.19%)}	_~	_0.089s	_0.111s	_{p=0.898 n=12}
_{Bind Time}	_{0.035s (± 8.15%)}	_{0.041s (± 9.99%)}	_{🔻+0.006s (+16.19%)}	_0.030s	_0.051s	_{p=0.022 n=12}
_{Check Time}	_{0.825s (± 0.62%)}	_{0.822s (± 0.57%)}	_~	_0.806s	_0.833s	_{p=0.339 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.972s (± 0.64%)}	_{0.975s (± 0.78%)}	_~	_0.958s	_0.994s	_{p=0.722 n=12}
_{vscode - native}
_Errors	₃₇₆	₃₇₆	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_10,054,578	_10,054,578	_~	_~	_~	_{p=1.000 n=12}
_Types	_3,383,028	_3,383,028	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{6,573,290k (± 0.01%)}	_{6,572,450k (± 0.01%)}	_~	_6,569,996k	_6,575,640k	_{p=0.242 n=12}
_{Memory Allocs}	_{50,468,817 (± 0.03%)}	_{50,459,894 (± 0.01%)}	_~	_50,448,262	_50,476,088	_{p=0.713 n=12}
_{Config Time}	_{0.066s (± 0.27%)}	_{0.066s (± 0.49%)}	_~	_0.065s	_0.067s	_{p=0.634 n=12}
_{Parse Time}	_{1.851s (± 2.92%)}	_{1.823s (± 2.60%)}	_~	_1.710s	_1.940s	_{p=0.468 n=12}
_{Bind Time}	_{0.444s (±16.34%)}	_{0.509s (±17.55%)}	_{🔻+0.065s (+14.57%)}	_0.411s	_0.772s	_{p=0.009 n=12}
_{Check Time}	_{13.307s (± 1.01%)}	_{13.114s (± 1.50%)}	_~	_12.707s	_13.426s	_{p=0.504 n=12}
_{Emit Time}	_{3.956s (± 6.87%)}	_{4.541s (±11.87%)}	_~	_3.718s	_5.605s	_{p=0.298 n=12}
_{Total Time}	_{19.725s (± 0.91%)}	_{20.165s (± 1.91%)}	_~	_19.545s	_20.961s	_{p=0.089 n=12}
_{webpack - native}
_Errors	₅₇₇	₅₇₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,278,213	_1,278,213	_~	_~	_~	_{p=1.000 n=12}
_Types	_606,210	_606,210	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{972,842k (± 0.01%)}	_{972,917k (± 0.02%)}	_~	_972,601k	_973,355k	_{p=0.443 n=12}
_{Memory Allocs}	_{6,310,338 (± 0.02%)}	_{6,309,819 (± 0.01%)}	_~	_6,307,943	_6,312,636	_{p=0.630 n=12}
_{Config Time}	_{0.009s (± 2.04%)}	_0.009s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.282s (± 3.03%)}	_{0.277s (± 2.02%)}	_~	_0.264s	_0.290s	_{p=0.311 n=12}
_{Bind Time}	_{0.075s (±17.22%)}	_{0.083s (±19.16%)}	_~	_0.060s	_0.129s	_{p=0.722 n=12}
_{Check Time}	_{1.740s (± 0.73%)}	_{1.750s (± 0.60%)}	_~	_1.717s	_1.776s	_{p=0.192 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{2.140s (± 0.86%)}	_{2.154s (± 1.03%)}	_~	_2.088s	_2.204s	_{p=0.285 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,030,694	_1,030,694	_~	_~	_~	_{p=1.000 n=12}
_Types	_376,396	_376,396	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{599,061k (± 0.01%)}	_{599,127k (± 0.01%)}	_~	_598,920k	_599,442k	_{p=0.370 n=12}
_{Memory Allocs}	_{4,761,868 (± 0.03%)}	_{4,764,106 (± 0.05%)}	_~	_4,758,949	_4,772,984	_{p=0.160 n=12}
_{Config Time}	_{0.003s (± 8.64%)}	_{0.003s (±12.54%)}	_~	_0.002s	_0.003s	_{p=0.371 n=12}
_{Parse Time}	_{0.140s (± 1.71%)}	_{0.148s (± 3.09%)}	_{🔻+0.007s (+ 5.17%)}	_0.136s	_0.161s	_{p=0.009 n=12}
_{Bind Time}	_{0.049s (±14.42%)}	_{0.044s (±17.29%)}	_~	_0.033s	_0.066s	_{p=0.384 n=12}
_{Check Time}	_{1.061s (± 0.48%)}	_{1.067s (± 0.80%)}	_~	_1.051s	_1.086s	_{p=0.504 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.267s (± 0.74%)}	_{1.273s (± 0.78%)}	_~	_1.249s	_1.305s	_{p=0.451 n=12}

System info unknown

Hosts

native

Scenarios

Compiler-Unions - native
angular-1 - native
mui-docs - native
strada-build-src - native
strada-compiler - native
ts-pre-modules - native
vscode - native
webpack - native
xstate-main - native

_Benchmark	_Name	_Iterations
_Current	_pr	₁₂
_Baseline	_baseline	₁₂

Developer Information:

Download Benchmarks

Jake Bailey (jakebailey) · 2026-09-29T19:57:36Z

No change???

Wesley Wigham (weswigham) · 2026-10-01T17:46:03Z

I always thought we'd just update transformers.Chain to, rather than just chaining at the top level, bookkeeping the full transform stack during a full walk and do the walk-fusion in that. That was always my long-term intent, anyway.

typescript-automation Bot assigned Jake Bailey (jakebailey) Sep 29, 2026

typescript-automation Bot added Author: Team For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels Sep 29, 2026

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Fuse local syntax lowerings into one traversal - #64539

Fuse local syntax lowerings into one traversal#64539
Jake Bailey (jakebailey) wants to merge 1 commit into
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