Avoid hashing keys in active mapper instantiation caches by jakebailey · Pull Request #64871 · microsoft/TypeScript

Jake Bailey (jakebailey) · 2026-10-19T17:40:33Z

Every instantiation under an active mapper built and hashed a 128-bit key before probing the mapper's cache, even when the mapper had just been pushed and its cache was necessarily empty. Most keys carry no alias, so the type ID alone identifies the entry.

Measured with --checkers 4 (n=8):

                      │ before.txt │            after.txt             │
                      │   sec/op   │   sec/op    vs base              │
    Tsc/bluesky         2.756 ± 3%   2.742 ± 3%       ~ (p=0.366 n=8)
    Tsc/sentry          11.20 ± 3%   11.19 ± 4%       ~ (p=0.742 n=8)
    Tsc/t3code-server   10.84 ± 3%   10.72 ± 2%  -1.10% (p=0.038 n=8)
    geomean             6.942        6.902       -0.58%

                      │  before.txt  │             after.txt              │
                      │    rss-B     │    rss-B      vs base              │
    Tsc/bluesky         1.190Gi ± 1%   1.182Gi ± 3%       ~ (p=0.798 n=8)
    Tsc/sentry          3.438Gi ± 7%   3.518Gi ± 6%       ~ (p=0.645 n=8)
    Tsc/t3code-server   2.852Gi ± 2%   2.879Gi ± 3%       ~ (p=0.721 n=8)
    geomean             2.268Gi        2.287Gi       +0.86%

                      │  before.txt  │             after.txt              │
                      │    heap-B    │    heap-B     vs base              │
    Tsc/bluesky         934.9Mi ± 0%   934.8Mi ± 0%       ~ (p=0.959 n=8)
    Tsc/sentry          2.607Gi ± 0%   2.609Gi ± 0%       ~ (p=0.382 n=8)
    Tsc/t3code-server   2.303Gi ± 0%   2.302Gi ± 0%       ~ (p=0.083 n=8)
    geomean             1.763Gi        1.763Gi       -0.00%

Every instantiation under an active mapper built and hashed a 128-bit key before probing the mapper's cache, even when the mapper had just been pushed and its cache was necessarily empty. Most keys carry no alias, so the type ID alone identifies the entry. Measured with --checkers 4 (n=8): │ before.txt │ after.txt │ │ sec/op │ sec/op vs base │ Tsc/bluesky 2.756 ± 3% 2.742 ± 3% ~ (p=0.366 n=8) Tsc/sentry 11.20 ± 3% 11.19 ± 4% ~ (p=0.742 n=8) Tsc/t3code-server 10.84 ± 3% 10.72 ± 2% -1.10% (p=0.038 n=8) geomean 6.942 6.902 -0.58% │ before.txt │ after.txt │ │ rss-B │ rss-B vs base │ Tsc/bluesky 1.190Gi ± 1% 1.182Gi ± 3% ~ (p=0.798 n=8) Tsc/sentry 3.438Gi ± 7% 3.518Gi ± 6% ~ (p=0.645 n=8) Tsc/t3code-server 2.852Gi ± 2% 2.879Gi ± 3% ~ (p=0.721 n=8) geomean 2.268Gi 2.287Gi +0.86% │ before.txt │ after.txt │ │ heap-B │ heap-B vs base │ Tsc/bluesky 934.9Mi ± 0% 934.8Mi ± 0% ~ (p=0.959 n=8) Tsc/sentry 2.607Gi ± 0% 2.609Gi ± 0% ~ (p=0.382 n=8) Tsc/t3code-server 2.303Gi ± 0% 2.302Gi ± 0% ~ (p=0.083 n=8) geomean 1.763Gi 1.763Gi -0.00%

Copilot

🟢 Approval recommended

The focused optimization preserves cache behavior, with no blocking issues identified.

No open issues 🎉

What changed in this PR

Optimizes the compiler’s active mapper instantiation cache to reduce unnecessary key hashing.

Changes:

Skips cache lookup and key construction for newly pushed mappers.
Uses type IDs directly for alias-free keys, retaining hashing for aliased types.

File	Description
`tsc/internal/checker/checker.go`	Streamlines mapper cache lookups and adds optimized key generation.

_{Review effort: Balanced}

Jake Bailey (jakebailey) · 2026-10-19T17:45:54Z

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

typescript-automation · 2026-10-19T17:46:00Z

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

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

typescript-automation · 2026-10-19T18:18:14Z

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}	_{113,521k (± 0.07%)}	_{113,532k (± 0.07%)}	_~	_113,391k	_113,772k	_{p=0.767 n=12}
_{Memory Allocs}	_{2,212,535 (± 0.00%)}	_{2,212,560 (± 0.00%)}	_~	_2,212,245	_2,212,832	_{p=0.787 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.039s (± 3.96%)}	_{0.040s (± 2.84%)}	_~	_0.036s	_0.043s	_{p=0.181 n=12}
_{Bind Time}	_{0.013s (± 9.04%)}	_{0.014s (± 9.81%)}	_~	_0.011s	_0.018s	_{p=0.407 n=12}
_{Check Time}	_{0.471s (± 0.93%)}	_{0.468s (± 0.67%)}	_~	_0.460s	_0.477s	_{p=0.297 n=12}
_{Emit Time}	_{0.267s (± 2.35%)}	_{0.268s (± 1.89%)}	_~	_0.256s	_0.282s	_{p=0.560 n=12}
_{Total Time}	_{0.796s (± 1.16%)}	_{0.797s (± 0.73%)}	_~	_0.783s	_0.815s	_{p=0.579 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{847,336 (± 0.09%)}	_{847,078 (± 0.06%)}	_~	_846,015	_848,350	_{p=0.514 n=12}
_Types	_{247,033 (± 0.00%)}	_{247,032 (± 0.00%)}	_~	_247,030	_247,041	_{p=0.197 n=12}
_{Memory Used}	_{596,504k (± 0.02%)}	_{596,432k (± 0.02%)}	_~	_596,168k	_596,823k	_{p=0.514 n=12}
_{Memory Allocs}	_{13,141,629 (± 0.04%)}	_{13,141,423 (± 0.04%)}	_~	_13,135,332	_13,166,204	_{p=0.378 n=12}
_{Config Time}	_{0.017s (± 1.45%)}	_{0.017s (± 1.70%)}	_~	_0.016s	_0.017s	_{p=1.000 n=12}
_{Parse Time}	_{0.226s (± 2.72%)}	_{0.224s (± 3.46%)}	_~	_0.208s	_0.241s	_{p=0.809 n=12}
_{Bind Time}	_{0.073s (±13.84%)}	_{0.081s (±17.09%)}	_~	_0.058s	_0.113s	_{p=0.619 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.574s (± 1.50%)}	_{1.549s (± 1.38%)}	_~	_1.490s	_1.590s	_{p=0.124 n=12}
_{Total Time}	_{1.911s (± 1.50%)}	_{1.886s (± 1.22%)}	_~	_1.817s	_1.937s	_{p=0.272 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}	_{2,625,370k (± 0.06%)}	_{2,625,610k (± 0.07%)}	_~	_2,622,009k	_2,631,093k	_{p=0.671 n=12}
_{Memory Allocs}	_{27,710,899 (± 0.07%)}	_{27,707,576 (± 0.09%)}	_~	_27,659,623	_27,787,079	_{p=0.630 n=12}
_{Config Time}	_{0.020s (± 0.90%)}	_0.020s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.567s (± 2.69%)}	_{0.587s (± 2.37%)}	_~	_0.557s	_0.621s	_{p=0.080 n=12}
_{Bind Time}	_{0.003s (±26.03%)}	_{0.003s (±11.61%)}	_~	_0.002s	_0.003s	_{p=0.680 n=12}
_{Check Time}	_{8.594s (± 0.21%)}	_{8.593s (± 0.63%)}	_~	_8.473s	_8.774s	_{p=0.192 n=12}
_{Emit Time}	_{0.700s (± 9.88%)}	_{0.650s (±12.93%)}	_~	_0.497s	_0.785s	_{p=0.164 n=12}
_{Total Time}	_{10.815s (± 0.66%)}	_{10.758s (± 0.79%)}	_~	_10.509s	_10.909s	_{p=0.139 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}	_{1,314,690k (± 0.59%)}	_{1,311,651k (± 0.52%)}	_~	_1,300,979k	_1,336,741k	_{p=0.410 n=12}
_{Memory Allocs}	_{78,296,066 (± 0.05%)}	_{78,228,132 (± 0.11%)}	_~	_77,874,740	_78,356,818	_{p=0.410 n=12}
_{Config Time}	_{0.004s (± 8.27%)}	_{0.004s (± 6.39%)}	_~	_0.003s	_0.004s	_{p=1.000 n=12}
_{Parse Time}	_{0.252s (± 1.93%)}	_{0.252s (± 1.88%)}	_~	_0.241s	_0.264s	_{p=0.619 n=12}
_{Bind Time}	_{0.000s (±217.90%)}	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.668s (± 0.48%)}	_{1.660s (± 0.46%)}	_~	_1.643s	_1.689s	_{p=0.086 n=12}
_{Emit Time}	_{0.310s (± 5.03%)}	_{0.297s (± 5.15%)}	_~	_0.266s	_0.339s	_{p=0.340 n=12}
_{Total Time}	_{19.806s (± 0.52%)}	_{19.669s (± 0.73%)}	_~	_19.291s	_19.938s	_{p=0.370 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}	_{247,562k (± 0.02%)}	_{247,533k (± 0.02%)}	_~	_247,413k	_247,698k	_{p=0.514 n=12}
_{Memory Allocs}	_{4,329,862 (± 0.01%)}	_{4,329,842 (± 0.01%)}	_~	_4,328,943	_4,330,990	_{p=0.875 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.114s (± 5.62%)}	_{0.115s (± 5.14%)}	_~	_0.104s	_0.130s	_{p=0.966 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{0.961s (± 0.64%)}	_{0.958s (± 0.70%)}	_~	_0.944s	_0.976s	_{p=0.486 n=12}
_{Emit Time}	_{0.146s (± 6.54%)}	_{0.151s (± 5.09%)}	_~	_0.131s	_0.173s	_{p=0.504 n=12}
_{Total Time}	_{1.279s (± 0.74%)}	_{1.278s (± 0.77%)}	_~	_1.249s	_1.299s	_{p=0.766 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}	_{215,232k (± 0.02%)}	_{215,242k (± 0.02%)}	_~	_215,098k	_215,370k	_{p=0.671 n=12}
_{Memory Allocs}	_{1,401,433 (± 0.01%)}	_{1,401,694 (± 0.02%)}	_~	_1,401,241	_1,402,346	_{p=0.078 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.095s (± 3.85%)}	_{0.097s (± 3.54%)}	_~	_0.089s	_0.106s	_{p=0.354 n=12}
_{Bind Time}	_{0.041s (±11.21%)}	_{0.040s (± 8.26%)}	_~	_0.033s	_0.051s	_{p=0.809 n=12}
_{Check Time}	_{0.741s (± 0.69%)}	_{0.740s (± 0.87%)}	_~	_0.727s	_0.757s	_{p=0.639 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.892s (± 0.44%)}	_{0.892s (± 0.97%)}	_~	_0.873s	_0.912s	_{p=0.680 n=12}
_{vscode - native}
_Errors	₃₉₆	₃₉₆	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_12,386,876	_12,386,876	_~	_~	_~	_{p=1.000 n=12}
_Types	_4,049,733	_4,049,733	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{5,821,839k (± 0.02%)}	_{5,821,463k (± 0.02%)}	_~	_5,819,511k	_5,824,730k	_{p=0.478 n=12}
_{Memory Allocs}	_{59,185,798 (± 0.01%)}	_{59,190,729 (± 0.02%)}	_~	_59,178,341	_59,237,100	_{p=0.799 n=12}
_{Config Time}	_{0.062s (± 0.39%)}	_{0.062s (± 0.43%)}	_~	_0.061s	_0.063s	_{p=0.627 n=12}
_{Parse Time}	_{1.448s (± 1.91%)}	_{1.466s (± 2.86%)}	_~	_1.386s	_1.626s	_{p=0.630 n=12}
_{Bind Time}	_{0.579s (±13.74%)}	_{0.539s (±13.21%)}	_~	_0.402s	_0.707s	_{p=0.248 n=12}
_{Check Time}	_{13.050s (± 0.94%)}	_{13.078s (± 1.01%)}	_~	_12.854s	_13.398s	_{p=0.977 n=12}
_{Emit Time}	_{4.850s (± 7.19%)}	_{4.770s (± 7.52%)}	_~	_3.801s	_5.406s	_{p=0.799 n=12}
_{Total Time}	_{20.067s (± 1.37%)}	_{19.994s (± 1.40%)}	_~	_19.220s	_20.532s	_{p=0.671 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}	_{885,687k (± 0.02%)}	_{885,754k (± 0.02%)}	_~	_885,303k	_886,198k	_{p=0.671 n=12}
_{Memory Allocs}	_{6,926,710 (± 0.01%)}	_{6,927,880 (± 0.03%)}	_~	_6,923,495	_6,936,369	_{p=0.291 n=12}
_{Config Time}	_{0.008s (± 4.27%)}	_{0.007s (± 4.37%)}	_~	_0.007s	_0.008s	_{p=0.684 n=12}
_{Parse Time}	_{0.250s (± 2.26%)}	_{0.251s (± 3.83%)}	_~	_0.226s	_0.276s	_{p=0.832 n=12}
_{Bind Time}	_{0.062s (± 8.13%)}	_{0.063s (±11.52%)}	_~	_0.055s	_0.087s	_{p=0.414 n=12}
_{Check Time}	_{2.466s (± 0.45%)}	_{2.461s (± 0.39%)}	_~	_2.440s	_2.501s	_{p=0.103 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{2.818s (± 0.34%)}	_{2.813s (± 0.51%)}	_~	_2.778s	_2.854s	_{p=0.504 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}	_{471,926k (± 0.03%)}	_{471,848k (± 0.02%)}	_~	_471,638k	_472,281k	_{p=0.347 n=12}
_{Memory Allocs}	_{4,222,579 (± 0.12%)}	_{4,220,463 (± 0.08%)}	_~	_4,212,645	_4,232,279	_{p=0.843 n=12}
_{Config Time}	_{0.003s (± 6.23%)}	_{0.003s (± 6.23%)}	_~	_0.002s	_0.003s	_{p=1.000 n=12}
_{Parse Time}	_{0.137s (± 3.20%)}	_{0.136s (± 3.56%)}	_~	_0.123s	_0.146s	_{p=0.744 n=12}
_{Bind Time}	_{0.041s (±15.18%)}	_{0.040s (±14.56%)}	_~	_0.034s	_0.066s	_{p=0.896 n=12}
_{Check Time}	_{1.095s (± 1.17%)}	_{1.072s (± 0.90%)}	_{-0.023s (- 2.10%)}	_1.054s	_1.099s	_{p=0.006 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.283s (± 1.05%)}	_{1.258s (± 0.92%)}	_{-0.026s (- 2.00%)}	_1.238s	_1.287s	_{p=0.005 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 17:40

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

typescript-automation Bot assigned Jake Bailey (jakebailey) Oct 19, 2026

typescript-automation Bot added the Author: Team label Oct 19, 2026

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typescript-automation Bot added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Oct 19, 2026

Copilot started reviewing on behalf of Jake Bailey (jakebailey) October 19, 2026 17:41

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Avoid hashing keys in active mapper instantiation caches - #64871

Avoid hashing keys in active mapper instantiation caches#64871
Jake Bailey (jakebailey) wants to merge 1 commit into
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