Skip mark/rewind in `tryParseTypeArgumentsInExpression` when token isn't `<` by mds-ant · Pull Request #4234 · microsoft/typescript-go

mds-ant · 2026-06-06T22:28:42Z

Context

tryParseTypeArgumentsInExpression is called for every member-access dot and call-expression rest. It currently always saves the full parser/ scanner state, calls reScanLessThanToken, and rewinds when no type argument list is present. reScanLessThanToken only mutates state when the current token is << (splitting it into <). For any other token the mark/rewind pair is a no-op.

This PR

This PR hoists the JS-file check and a </<< token check above p.mark() so the common path returns immediately without copying state. The remaining guard only needs the re-scan, since the JS-file condition is already known to be false.

This PR was assisted by Claude Code.

Performance

goos: darwin
goarch: arm64
pkg: github.com/microsoft/typescript-go/internal/parser
cpu: Apple M1 Pro
                                                      │ /tmp/old.txt │            /tmp/new.txt             │
                                                      │    sec/op    │   sec/op     vs base                │
Parse/empty.ts-10                                        387.7n ± 0%   387.8n ± 0%        ~ (p=1.000 n=25)
Parse/checker.ts-10                                      33.78m ± 0%   29.14m ± 0%  -13.74% (p=0.000 n=25)
Parse/dom.generated.d.ts-10                              15.48m ± 0%   15.43m ± 0%   -0.29% (p=0.000 n=25)
Parse/Herebyfile.mjs-10                                  605.8µ ± 0%   557.3µ ± 0%   -8.01% (p=0.000 n=25)
Parse/jsxComplexSignatureHasApplicabilityError.tsx-10    177.0µ ± 0%   175.9µ ± 0%   -0.62% (p=0.000 n=25)
geomean                                                  465.0µ        443.2µ        -4.69%

                                                      │ /tmp/old.txt │             /tmp/new.txt              │
                                                      │     B/op     │     B/op      vs base                 │
Parse/empty.ts-10                                       1.047Ki ± 0%   1.047Ki ± 0%       ~ (p=1.000 n=25) ¹
Parse/checker.ts-10                                     26.44Mi ± 0%   26.44Mi ± 0%  -0.00% (p=0.001 n=25)
Parse/dom.generated.d.ts-10                             11.08Mi ± 0%   11.08Mi ± 0%       ~ (p=0.863 n=25)
Parse/Herebyfile.mjs-10                                 472.4Ki ± 0%   472.4Ki ± 0%       ~ (p=0.255 n=25)
Parse/jsxComplexSignatureHasApplicabilityError.tsx-10   175.5Ki ± 0%   175.5Ki ± 0%       ~ (p=0.869 n=25)
geomean                                                 484.4Ki        484.4Ki       -0.00%
¹ all samples are equal

                                                      │ /tmp/old.txt │             /tmp/new.txt             │
                                                      │  allocs/op   │  allocs/op   vs base                 │
Parse/empty.ts-10                                         4.000 ± 0%    4.000 ± 0%       ~ (p=1.000 n=25) ¹
Parse/checker.ts-10                                      12.96k ± 0%   12.96k ± 0%       ~ (p=0.138 n=25)
Parse/dom.generated.d.ts-10                              3.255k ± 0%   3.255k ± 0%       ~ (p=1.000 n=25)
Parse/Herebyfile.mjs-10                                  1.772k ± 0%   1.772k ± 0%       ~ (p=1.000 n=25) ¹
Parse/jsxComplexSignatureHasApplicabilityError.tsx-10     196.0 ± 0%    196.0 ± 0%       ~ (p=1.000 n=25) ¹
geomean                                                   567.0         567.0       +0.00%
¹ all samples are equal

…t `<` tryParseTypeArgumentsInExpression is called for every member-access dot and call-expression rest. It currently always saves the full parser/ scanner state, calls reScanLessThanToken, and rewinds when no type argument list is present. reScanLessThanToken only mutates state when the current token is `<<` (splitting it into `<`); for any other token the mark/rewind pair is a no-op. Hoist the JS-file check and a `<`/`<<` token check above p.mark() so the common path returns immediately without copying state. The remaining guard only needs the reScan, since the JS-file condition is already known to be false.

jakebailey · 2026-06-06T22:32:29Z

Do you have a benchmark that would show where this matters? And how often it matters?

mds-ant · 2026-06-06T22:33:55Z

Do you have a benchmark that would show where this matters? And how often it matters?

Yes, I'll add a benchmark before marking the PR ready for review!

Copilot

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Optimizes speculative parsing of type arguments in expressions by adding early-exit checks before saving parser state.

Changes:

Adds cheap preconditions to avoid mark/rewind when parsing can’t possibly start.
Skips speculative parsing in JavaScript files and when current token isn’t </<<.

jakebailey · 2026-06-08T19:57:20Z

@typescript-bot perf test this faster

typescript-automation · 2026-06-08T19:57:25Z

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

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

typescript-automation · 2026-06-08T20:20:45Z

@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	₄	₄	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_{81,789 (± 0.06%)}	_{81,780 (± 0.04%)}	_~	_81,747	_81,841	_{p=0.575 n=6}
_Types	_98,821	_98,821	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{180,282k (± 0.84%)}	_{180,799k (± 0.37%)}	_~	_180,243k	_182,147k	_{p=0.298 n=6}
_{Memory Allocs}	_{1,768,115 (± 0.01%)}	_{1,767,929 (± 0.01%)}	_~	_1,767,684	_1,768,209	_{p=0.298 n=6}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Parse Time}	_{0.061s (± 7.41%)}	_{0.055s (± 2.75%)}	_{🟩-0.006s (-10.63%)}	_0.052s	_0.056s	_{p=0.019 n=6}
_{Bind Time}	_{0.017s (±23.53%)}	_{0.018s (±24.43%)}	_~	_0.012s	_0.022s	_{p=0.517 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{0.846s (± 1.55%)}	_{0.858s (± 1.34%)}	_~	_0.843s	_0.873s	_{p=0.199 n=6}
_{Total Time}	_{0.925s (± 1.68%)}	_{0.932s (± 1.43%)}	_~	_0.914s	_0.948s	_{p=0.471 n=6}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_{884,445 (± 0.08%)}	_{884,952 (± 0.07%)}	_~	_883,994	_885,564	_{p=0.173 n=6}
_Types	_{263,954 (± 0.00%)}	_{263,954 (± 0.00%)}	_~	_263,952	_263,956	_{p=0.801 n=6}
_{Memory Used}	_{831,344k (± 0.10%)}	_{831,605k (± 0.13%)}	_~	_830,565k	_833,642k	_{p=0.810 n=6}
_{Memory Allocs}	_{6,696,574 (± 0.23%)}	_{6,704,581 (± 0.36%)}	_~	_6,681,438	_6,734,120	_{p=0.689 n=6}
_{Config Time}	_{0.017s (± 2.98%)}	_{0.018s (± 6.85%)}	_~	_0.017s	_0.020s	_{p=0.923 n=6}
_{Parse Time}	_{0.241s (± 5.45%)}	_{0.237s (± 8.79%)}	_~	_0.214s	_0.263s	_{p=0.471 n=6}
_{Bind Time}	_{0.041s (±26.86%)}	_{0.051s (±44.06%)}	_~	_0.035s	_0.092s	_{p=1.000 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{1.858s (± 1.62%)}	_{1.857s (± 2.37%)}	_~	_1.795s	_1.929s	_{p=0.936 n=6}
_{Total Time}	_{2.168s (± 0.98%)}	_{2.173s (± 2.25%)}	_~	_2.113s	_2.257s	_{p=1.000 n=6}
_{mui-docs - native}
_Errors	_{11,244 (± 0.05%)}	_{11,245 (± 0.02%)}	_~	_11,241	_11,247	_{p=0.867 n=6}
_Symbols	_4,179,255	_4,179,255	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,526,525	_1,526,525	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{4,866,664k (± 0.05%)}	_{4,867,325k (± 0.04%)}	_~	_4,863,992k	_4,869,992k	_{p=0.689 n=6}
_{Memory Allocs}	_{94,114,094 (±14.81%)}	_{88,383,296 (±10.22%)}	_~	_78,976,372	_104,244,348	_{p=0.575 n=6}
_{Config Time}	_{0.017s (± 7.42%)}	_{0.017s (± 5.07%)}	_~	_0.016s	_0.018s	_{p=0.753 n=6}
_{Parse Time}	_{1.127s (±19.83%)}	_{1.013s (±12.10%)}	_~	_0.914s	_1.244s	_{p=0.230 n=6}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{16.602s (± 0.48%)}	_{16.501s (± 0.69%)}	_~	_16.372s	_16.643s	_{p=0.173 n=6}
_{Emit Time}	_{0.440s (± 1.71%)}	_{0.551s (±32.09%)}	_~	_0.435s	_0.783s	_{p=0.629 n=6}
_{Total Time}	_{18.853s (± 1.17%)}	_{18.733s (± 1.15%)}	_~	_18.430s	_19.081s	_{p=0.575 n=6}
_{self-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,396,711	_1,396,711	_~	_~	_~	_{p=1.000 n=6}
_Types	_442,164	_442,164	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{1,647,560k (± 0.40%)}	_{1,649,522k (± 0.27%)}	_~	_1,644,566k	_1,657,254k	_{p=0.298 n=6}
_{Memory Allocs}	_{57,506,660 (± 0.15%)}	_{57,529,270 (± 0.10%)}	_~	_57,438,659	_57,583,857	_{p=0.575 n=6}
_{Config Time}	_{0.017s (±52.33%)}	_{0.021s (±31.43%)}	_~	_0.014s	_0.033s	_{p=0.295 n=6}
_{Parse Time}	_{0.288s (± 2.86%)}	_{0.278s (± 7.37%)}	_~	_0.252s	_0.307s	_{p=0.471 n=6}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{2.347s (± 1.08%)}	_{2.336s (± 0.89%)}	_~	_2.305s	_2.364s	_{p=0.471 n=6}
_{Emit Time}	_{0.216s (± 4.11%)}	_{0.220s (± 3.05%)}	_~	_0.210s	_0.228s	_{p=0.470 n=6}
_{Total Time}	_{29.750s (± 0.73%)}	_{29.803s (± 1.39%)}	_~	_29.333s	_30.430s	_{p=0.936 n=6}
_{self-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_338,176	_338,176	_~	_~	_~	_{p=1.000 n=6}
_Types	_199,522	_199,522	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{332,123k (± 0.02%)}	_{332,109k (± 0.03%)}	_~	_331,970k	_332,239k	_{p=0.936 n=6}
_{Memory Allocs}	_{2,557,457 (± 0.03%)}	_{2,557,295 (± 0.04%)}	_~	_2,556,285	_2,558,794	_{p=0.575 n=6}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=6}
_{Parse Time}	_{0.131s (± 3.35%)}	_{0.130s (± 4.48%)}	_~	_0.125s	_0.140s	_{p=0.418 n=6}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{1.402s (± 1.37%)}	_{1.387s (± 0.80%)}	_~	_1.365s	_1.396s	_{p=0.054 n=6}
_{Emit Time}	_{0.079s (± 8.72%)}	_{0.088s (±10.33%)}	_{🔻+0.009s (+11.18%)}	_0.079s	_0.099s	_{p=0.030 n=6}
_{Total Time}	_{1.668s (± 0.72%)}	_{1.660s (± 0.67%)}	_~	_1.647s	_1.674s	_{p=0.336 n=6}
_{ts-pre-modules - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_97,488	_97,488	_~	_~	_~	_{p=1.000 n=6}
_Types	₃₅₆	₃₅₆	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{133,412k (± 0.02%)}	_{133,391k (± 0.01%)}	_~	_133,355k	_133,404k	_{p=0.199 n=6}
_{Memory Allocs}	_{193,606 (± 0.17%)}	_{193,521 (± 0.23%)}	_~	_193,152	_194,376	_{p=0.471 n=6}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=6}
_{Parse Time}	_{0.119s (± 4.68%)}	_{0.113s (± 4.00%)}	_~	_0.110s	_0.122s	_{p=0.062 n=6}
_{Bind Time}	_{0.035s (±12.37%)}	_{0.038s (±13.95%)}	_~	_0.029s	_0.044s	_{p=0.336 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{0.158s (± 4.71%)}	_{0.155s (± 4.86%)}	_~	_0.143s	_0.164s	_{p=0.689 n=6}
_{vscode - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_6,561,237	_6,561,237	_~	_~	_~	_{p=1.000 n=6}
_Types	_2,306,257	_2,306,257	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{4,436,438k (± 0.03%)}	_{4,436,446k (± 0.02%)}	_~	_4,435,531k	_4,437,750k	_{p=0.936 n=6}
_{Memory Allocs}	_{31,806,669 (± 0.09%)}	_{31,812,124 (± 0.09%)}	_~	_31,783,865	_31,862,770	_{p=0.575 n=6}
_{Config Time}	_{0.065s (± 9.23%)}	_{0.068s (± 9.23%)}	_~	_0.063s	_0.079s	_{p=0.421 n=6}
_{Parse Time}	_{0.835s (± 4.38%)}	_{0.850s (± 4.80%)}	_~	_0.797s	_0.920s	_{p=0.471 n=6}
_{Bind Time}	_{0.151s (±36.07%)}	_{0.158s (±40.83%)}	_~	_0.127s	_0.290s	_{p=0.296 n=6}
_{Check Time}	_{8.469s (± 0.35%)}	_{8.439s (± 0.65%)}	_~	_8.379s	_8.541s	_{p=0.128 n=6}
_{Emit Time}	_{2.240s (± 7.80%)}	_{2.245s (± 8.80%)}	_~	_1.927s	_2.532s	_{p=0.689 n=6}
_{Total Time}	_{11.778s (± 0.88%)}	_{11.778s (± 1.27%)}	_~	_11.632s	_12.052s	_{p=0.689 n=6}
_{webpack - native}
_Errors	₅₈₃	₅₈₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_717,611	_717,611	_~	_~	_~	_{p=1.000 n=6}
_Types	_339,294	_339,294	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{564,788k (± 0.02%)}	_{564,853k (± 0.02%)}	_~	_564,742k	_565,093k	_{p=0.423 n=6}
_{Memory Allocs}	_{4,074,948 (± 0.36%)}	_{4,082,484 (± 0.35%)}	_~	_4,058,848	_4,096,646	_{p=0.471 n=6}
_{Config Time}	_{0.010s (±25.70%)}	_{0.011s (±26.74%)}	_~	_0.007s	_0.013s	_{p=0.868 n=6}
_{Parse Time}	_{0.147s (± 3.31%)}	_{0.152s (± 3.25%)}	_~	_0.143s	_0.156s	_{p=0.077 n=6}
_{Bind Time}	_{0.031s (±32.81%)}	_{0.032s (±31.31%)}	_~	_0.022s	_0.046s	_{p=0.748 n=6}
_{Check Time}	_{1.085s (± 0.76%)}	_{1.086s (± 1.86%)}	_~	_1.062s	_1.117s	_{p=0.936 n=6}
_{Emit Time}	_0.001s	_{0.001s (±34.96%)}	_~	_0.001s	_0.002s	_{p=0.405 n=6}
_{Total Time}	_{1.287s (± 0.96%)}	_{1.295s (± 0.76%)}	_~	_1.283s	_1.310s	_{p=0.296 n=6}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,060,815	_1,060,815	_~	_~	_~	_{p=1.000 n=6}
_Types	_384,533	_384,533	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{638,509k (± 0.02%)}	_{638,577k (± 0.01%)}	_~	_638,511k	_638,729k	_{p=0.630 n=6}
_{Memory Allocs}	_{5,164,443 (± 0.04%)}	_{5,169,152 (± 0.08%)}	_{+4,709 (+ 0.09%)}	_5,164,334	_5,175,014	_{p=0.031 n=6}
_{Config Time}	_{0.005s (±11.05%)}	_{0.005s (±17.48%)}	_~	_0.003s	_0.005s	_{p=0.752 n=6}
_{Parse Time}	_{0.142s (± 6.62%)}	_{0.143s (± 2.43%)}	_~	_0.138s	_0.146s	_{p=1.000 n=6}
_{Bind Time}	_{0.035s (±23.54%)}	_{0.032s (±16.87%)}	_~	_0.026s	_0.041s	_{p=0.469 n=6}
_{Check Time}	_{1.281s (± 0.48%)}	_{1.284s (± 0.52%)}	_~	_1.274s	_1.292s	_{p=0.336 n=6}
_{Emit Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{1.468s (± 0.59%)}	_{1.468s (± 0.88%)}	_~	_1.448s	_1.483s	_{p=0.748 n=6}

System info unknown

Hosts

native

Scenarios

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

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Developer Information:

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mds-ant marked this pull request as ready for review June 6, 2026 23:39

Copilot AI review requested due to automatic review settings June 6, 2026 23:39

Copilot AI reviewed Jun 6, 2026

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RyanCavanaugh added the No linked issue This PR doesn't say what bug it fixes label Jun 8, 2026

RyanCavanaugh added this to the Possible Improvement milestone Jun 8, 2026

DanielRosenwasser reviewed Jun 8, 2026

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Skip mark/rewind in `tryParseTypeArgumentsInExpression` when token isn't `<`#4234

Skip mark/rewind in `tryParseTypeArgumentsInExpression` when token isn't `<`#4234
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mds-ant:perf/parser-typeargs-early

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