Count intersection property names with a plain map and slice by jakebailey · Pull Request #64876 · microsoft/TypeScript

Jake Bailey (jakebailey) · 2026-10-19T18:45:55Z

                  │   old.txt   │             new.txt              │
                  │   sec/op    │   sec/op    vs base              │
Tsc/bluesky          2.430 ± 2%   2.383 ± 3%       ~ (p=0.180 n=6)
Tsc/sentry          10.044 ± 1%   9.931 ± 2%       ~ (p=0.180 n=6)
Tsc/t3code-server    8.038 ± 1%   7.904 ± 2%  -1.67% (p=0.026 n=6)
Tsc/mui-docs         6.518 ± 2%   6.138 ± 3%  -5.83% (p=0.002 n=6)
geomean              5.980        5.821       -2.66%

                  │   old.txt    │              new.txt               │
                  │    rss-B     │    rss-B      vs base              │
Tsc/bluesky         1.189Gi ± 3%   1.176Gi ± 4%       ~ (p=0.485 n=6)
Tsc/sentry          3.271Gi ± 4%   3.215Gi ± 6%       ~ (p=0.240 n=6)
Tsc/t3code-server   2.753Gi ± 5%   2.776Gi ± 6%       ~ (p=0.937 n=6)
Tsc/mui-docs        2.362Gi ± 7%   2.314Gi ± 5%       ~ (p=0.699 n=6)
geomean             2.242Gi        2.220Gi       -0.99%

                  │   old.txt    │              new.txt               │
                  │    heap-B    │    heap-B     vs base              │
Tsc/bluesky         915.2Mi ± 0%   914.9Mi ± 0%       ~ (p=0.699 n=6)
Tsc/sentry          2.483Gi ± 0%   2.482Gi ± 0%       ~ (p=0.818 n=6)
Tsc/t3code-server   2.179Gi ± 0%   2.180Gi ± 0%       ~ (p=0.394 n=6)
Tsc/mui-docs        1.732Gi ± 0%   1.732Gi ± 0%       ~ (p=0.310 n=6)
geomean             1.701Gi        1.701Gi       -0.00%

This will conflict with #64815 to some extent

Deciding whether an intersection reduces to never counts how many constituents declare each property name. The ordered map used for this is a general-purpose structure that does more bookkeeping per operation than this hot path needs, and each property costs a lookup and an assignment. On mui-docs this shows up as a few percent of single-checker time. Count into a slice of entries indexed by a plain string map instead. The slice keeps first-occurrence order for the reduction check, and the first constituent's names are inserted without a lookup since property names are already unique within a type. │ old │ new │ │ sec/op │ sec/op vs base │ Tsc/bluesky 3.179 ± 1% 3.133 ± 1% -1.45% (p=0.039 n=6) Tsc/sentry 12.90 ± 7% 12.77 ± 17% ~ (p=0.818 n=6) Tsc/t3code-server 16.85 ± 3% 16.80 ± 3% ~ (p=0.937 n=6) Tsc/mui-docs 8.536 ± 3% 8.152 ± 1% -4.50% (p=0.002 n=6) geomean 8.763 8.604 -1.81% │ old │ new │ │ rss-B │ rss-B vs base │ Tsc/bluesky 1.439Gi ± 2% 1.417Gi ± 1% ~ (p=0.093 n=6) Tsc/sentry 4.566Gi ± 4% 4.504Gi ± 4% ~ (p=0.937 n=6) Tsc/t3code-server 4.488Gi ± 5% 4.344Gi ± 9% ~ (p=0.485 n=6) Tsc/mui-docs 3.535Gi ± 4% 3.391Gi ± 4% ~ (p=0.065 n=6) geomean 3.195Gi 3.114Gi -2.55% │ old │ new │ │ heap-B │ heap-B vs base │ Tsc/bluesky 1.153Gi ± 0% 1.152Gi ± 0% ~ (p=0.394 n=6) Tsc/sentry 3.620Gi ± 0% 3.622Gi ± 0% ~ (p=0.485 n=6) Tsc/t3code-server 3.761Gi ± 0% 3.762Gi ± 0% ~ (p=0.310 n=6) Tsc/mui-docs 2.641Gi ± 0% 2.641Gi ± 0% ~ (p=0.818 n=6) geomean 2.537Gi 2.537Gi +0.01%

Jake Bailey (jakebailey) · 2026-10-19T18:46:04Z

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

typescript-automation · 2026-10-19T18:46:10Z

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

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

Copilot

🟢 Approval recommended

Static review found no blocking issues in this localized refactor; automated validation was not run.

1 open issue

Update PR description to match the map-based implementation · New

What changed in this PR

Optimizes property counting when the checker determines whether an intersection reduces to never, while preserving first-occurrence order.

Changes:

Replaces the ordered map with a name-to-index map and count slice.
Skips lookups for the first constituent’s properties.
Updates counting and workspace-reuse tests.

File	Description
tsc/internal/checker/utilities.go	Implements map-and-slice property counts.
tsc/internal/checker/types_test.go	Adapts counting and reuse assertions.
tsc/internal/checker/checker.go	Uses the new counting operations during intersection reduction.

_{Review effort: Balanced}

+	entries []propertyCount
+	indices map[string]int32


typescript-automation · 2026-10-19T19:14:50Z

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}	_{113,611k (± 0.07%)}	_{113,461k (± 0.04%)}	_{-150k (- 0.13%)}	_113,372k	_113,611k	_{p=0.003 n=12}
_{Memory Allocs}	_{2,212,588 (± 0.00%)}	_{2,212,505 (± 0.01%)}	_~	_2,212,245	_2,212,961	_{p=0.266 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.039s (± 4.10%)}	_{0.040s (± 3.57%)}	_~	_0.037s	_0.044s	_{p=0.480 n=12}
_{Bind Time}	_{0.014s (± 9.58%)}	_{0.014s (± 8.31%)}	_~	_0.011s	_0.017s	_{p=0.830 n=12}
_{Check Time}	_{0.467s (± 0.82%)}	_{0.471s (± 0.64%)}	_~	_0.465s	_0.480s	_{p=0.070 n=12}
_{Emit Time}	_{0.275s (± 2.12%)}	_{0.269s (± 2.63%)}	_~	_0.253s	_0.285s	_{p=0.247 n=12}
_{Total Time}	_{0.802s (± 0.95%)}	_{0.800s (± 0.82%)}	_~	_0.787s	_0.818s	_{p=0.681 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{847,269 (± 0.06%)}	_{847,364 (± 0.08%)}	_~	_845,490	_849,037	_{p=0.843 n=12}
_Types	_{247,031 (± 0.00%)}	_{247,035 (± 0.00%)}	_{+3 (+ 0.00%)}	_247,029	_247,045	_{p=0.016 n=12}
_{Memory Used}	_{596,362k (± 0.02%)}	_{596,381k (± 0.03%)}	_~	_595,851k	_596,863k	_{p=0.799 n=12}
_{Memory Allocs}	_{13,139,205 (± 0.03%)}	_{13,148,114 (± 0.07%)}	_~	_13,134,175	_13,175,783	_{p=0.078 n=12}
_{Config Time}	_{0.017s (± 1.45%)}	_{0.017s (± 1.70%)}	_~	_0.016s	_0.017s	_{p=1.000 n=12}
_{Parse Time}	_{0.221s (± 1.77%)}	_{0.228s (± 2.77%)}	_~	_0.214s	_0.242s	_{p=0.091 n=12}
_{Bind Time}	_{0.081s (±11.65%)}	_{0.074s (±15.34%)}	_~	_0.058s	_0.102s	_{p=0.213 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.561s (± 1.37%)}	_{1.577s (± 1.32%)}	_~	_1.532s	_1.647s	_{p=0.325 n=12}
_{Total Time}	_{1.902s (± 1.39%)}	_{1.916s (± 1.04%)}	_~	_1.880s	_1.971s	_{p=0.434 n=12}
_{mui-docs - native}
_Errors	_11,405	_11,405	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_4,331,983	_4,331,983	_~	_~	_~	_{p=1.000 n=12}
_Types	_1,370,520	_1,370,520	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{2,624,694k (± 0.03%)}	_{2,626,152k (± 0.05%)}	_~	_2,622,356k	_2,629,755k	_{p=0.052 n=12}
_{Memory Allocs}	_{27,713,795 (± 0.08%)}	_{27,713,469 (± 0.05%)}	_~	_27,661,441	_27,738,171	_{p=0.671 n=12}
_{Config Time}	_{0.017s (± 1.70%)}	_{0.017s (± 1.45%)}	_~	_0.016s	_0.017s	_{p=1.000 n=12}
_{Parse Time}	_{0.466s (± 3.53%)}	_{0.478s (± 3.09%)}	_~	_0.440s	_0.513s	_{p=0.202 n=12}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{7.222s (± 0.13%)}	_{6.973s (± 0.62%)}	_{🟩-0.249s (- 3.45%)}	_6.855s	_7.125s	_{p=0.000 n=12}
_{Emit Time}	_{0.566s (±11.48%)}	_{0.524s (±13.72%)}	_~	_0.417s	_0.661s	_{p=0.352 n=12}
_{Total Time}	_{9.012s (± 0.82%)}	_{8.726s (± 1.21%)}	_{🟩-0.286s (- 3.17%)}	_8.532s	_8.929s	_{p=0.000 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,388,680	_1,388,680	_~	_~	_~	_{p=1.000 n=12}
_Types	_441,862	_441,862	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{1,311,304k (± 0.49%)}	_{1,316,572k (± 0.55%)}	_~	_1,300,268k	_1,343,645k	_{p=0.143 n=12}
_{Memory Allocs}	_{78,204,684 (± 0.14%)}	_{78,244,813 (± 0.10%)}	_~	_77,942,738	_78,401,237	_{p=0.671 n=12}
_{Config Time}	_{0.003s (±12.31%)}	_{0.003s (± 9.48%)}	_~	_0.003s	_0.004s	_{p=1.000 n=12}
_{Parse Time}	_{0.255s (± 2.42%)}	_{0.251s (± 1.44%)}	_~	_0.242s	_0.260s	_{p=0.181 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.679s (± 0.46%)}	_{1.675s (± 0.52%)}	_~	_1.648s	_1.696s	_{p=0.451 n=12}
_{Emit Time}	_{0.290s (± 3.30%)}	_{0.304s (± 3.61%)}	_{🔻+0.015s (+ 5.06%)}	_0.286s	_0.345s	_{p=0.027 n=12}
_{Total Time}	_{19.823s (± 0.63%)}	_{19.908s (± 0.71%)}	_~	_19.415s	_20.223s	_{p=0.319 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}	_{247,491k (± 0.04%)}	_{247,503k (± 0.02%)}	_~	_247,424k	_247,698k	_{p=1.000 n=12}
_{Memory Allocs}	_{4,329,337 (± 0.01%)}	_{4,330,004 (± 0.01%)}	_{+667 (+ 0.02%)}	_4,328,447	_4,330,998	_{p=0.045 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.112s (± 4.22%)}	_{0.114s (± 4.07%)}	_~	_0.099s	_0.124s	_{p=0.400 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{0.965s (± 0.58%)}	_{0.968s (± 0.74%)}	_~	_0.954s	_0.985s	_{p=0.284 n=12}
_{Emit Time}	_{0.146s (± 3.46%)}	_{0.154s (± 4.88%)}	_~	_0.134s	_0.170s	_{p=0.070 n=12}
_{Total Time}	_{1.276s (± 0.74%)}	_{1.294s (± 0.85%)}	_{+0.018s (+ 1.41%)}	_1.271s	_1.322s	_{p=0.027 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}	_{215,252k (± 0.02%)}	_{215,226k (± 0.02%)}	_~	_215,082k	_215,331k	_{p=0.272 n=12}
_{Memory Allocs}	_{1,401,488 (± 0.02%)}	_{1,401,793 (± 0.01%)}	_~	_1,401,399	_1,402,406	_{p=0.052 n=12}
_{Config Time}	_0.000s	_{0.000s (±146.91%)}	_~	_0.000s	_0.001s	_{p=0.478 n=12}
_{Parse Time}	_{0.097s (± 3.48%)}	_{0.098s (± 2.60%)}	_~	_0.093s	_0.105s	_{p=0.467 n=12}
_{Bind Time}	_{0.043s (± 6.32%)}	_{0.042s (± 7.29%)}	_~	_0.032s	_0.051s	_{p=0.639 n=12}
_{Check Time}	_{0.745s (± 0.86%)}	_{0.746s (± 0.57%)}	_~	_0.734s	_0.757s	_{p=0.898 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.900s (± 0.90%)}	_{0.902s (± 0.62%)}	_~	_0.889s	_0.918s	_{p=0.504 n=12}
_{vscode - native}
_Errors	₃₉₆	₃₉₆	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_12,501,426	_12,501,426	_~	_~	_~	_{p=1.000 n=12}
_Types	_4,091,299	_4,091,299	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{5,843,208k (± 0.01%)}	_{5,843,893k (± 0.02%)}	_~	_5,841,588k	_5,846,890k	_{p=0.242 n=12}
_{Memory Allocs}	_{59,416,904 (± 0.01%)}	_{59,424,090 (± 0.02%)}	_~	_59,410,476	_59,471,687	_{p=0.078 n=12}
_{Config Time}	_{0.076s (± 0.48%)}	_{0.076s (± 0.24%)}	_~	_0.076s	_0.077s	_{p=0.590 n=12}
_{Parse Time}	_{1.826s (± 2.06%)}	_{1.803s (± 2.00%)}	_~	_1.742s	_1.906s	_{p=0.434 n=12}
_{Bind Time}	_{0.641s (±13.90%)}	_{0.712s (±12.79%)}	_~	_0.527s	_0.923s	_{p=0.198 n=12}
_{Check Time}	_{14.703s (± 0.95%)}	_{14.583s (± 0.99%)}	_{-0.119s (- 0.81%)}	_14.330s	_14.847s	_{p=0.050 n=12}
_{Emit Time}	_{5.322s (± 6.90%)}	_{5.266s (± 6.37%)}	_~	_4.502s	_5.920s	_{p=0.713 n=12}
_{Total Time}	_{22.653s (± 1.32%)}	_{22.531s (± 1.05%)}	_~	_21.993s	_22.995s	_{p=0.590 n=12}
_{webpack - native}
_Errors	₅₉₈	₅₉₈	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,398,302	_1,398,302	_~	_~	_~	_{p=1.000 n=12}
_Types	_670,900	_670,900	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{885,544k (± 0.02%)}	_{885,710k (± 0.02%)}	_~	_885,190k	_885,997k	_{p=0.114 n=12}
_{Memory Allocs}	_{6,928,895 (± 0.04%)}	_{6,928,483 (± 0.03%)}	_~	_6,924,793	_6,936,985	_{p=0.799 n=12}
_{Config Time}	_0.009s	_0.009s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.317s (± 2.37%)}	_{0.322s (± 3.67%)}	_~	_0.295s	_0.349s	_{p=0.451 n=12}
_{Bind Time}	_{0.079s (± 7.54%)}	_{0.077s (± 3.36%)}	_~	_0.070s	_0.085s	_{p=1.000 n=12}
_{Check Time}	_{2.968s (± 0.25%)}	_{2.965s (± 0.50%)}	_~	_2.934s	_3.012s	_{p=0.486 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{3.412s (± 0.33%)}	_{3.411s (± 0.45%)}	_~	_3.364s	_3.445s	_{p=0.989 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,060,810	_1,060,810	_~	_~	_~	_{p=1.000 n=12}
_Types	_389,671	_389,671	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{471,880k (± 0.02%)}	_{471,904k (± 0.02%)}	_~	_471,677k	_472,246k	_{p=0.671 n=12}
_{Memory Allocs}	_{4,221,665 (± 0.10%)}	_{4,222,310 (± 0.08%)}	_~	_4,210,454	_4,229,639	_{p=0.755 n=12}
_{Config Time}	_0.003s	_{0.003s (± 8.64%)}	_~	_0.002s	_0.003s	_{p=0.478 n=12}
_{Parse Time}	_{0.134s (± 3.56%)}	_{0.140s (± 2.86%)}	_~	_0.125s	_0.148s	_{p=0.085 n=12}
_{Bind Time}	_{0.045s (±12.71%)}	_{0.044s (±17.39%)}	_~	_0.033s	_0.065s	_{p=0.527 n=12}
_{Check Time}	_{1.093s (± 0.99%)}	_{1.092s (± 0.93%)}	_~	_1.070s	_1.119s	_{p=0.921 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.285s (± 0.86%)}	_{1.287s (± 1.01%)}	_~	_1.251s	_1.316s	_{p=0.989 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:

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Copilot AI balanced review requested due to automatic review settings October 19, 2026 18:45

github-project-automation Bot added this to PR Backlog Oct 19, 2026

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typescript-automation Bot added Author: Team For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels Oct 19, 2026

Copilot started reviewing on behalf of Jake Bailey (jakebailey) October 19, 2026 18:46

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Comment thread tsc/internal/checker/utilities.go

Comment on lines +937 to +938

entries []propertyCount

indices map[string]int32

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Count intersection property names with a plain map and slice - #64876

Count intersection property names with a plain map and slice#64876
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