Compare names before symbols when matching flow references by jakebailey · Pull Request #64868 · microsoft/TypeScript

Jake Bailey (jakebailey) · 2026-10-19T17:10:32Z

Narrowing a reference walks back through every assignment and variable declaration on its flow path, and asks whether each one assigns the reference. For a declaration target that meant loading the declaration's symbol and looking up its late-bound and merged symbols, which is mostly cache misses on unrelated symbols. In long functions this dominated flow analysis.

A declaration's symbol is named by its identifier, so a reference with a different name can never match it. Comparing names first skips the symbol work for nearly every unrelated declaration.

--checkers 4, n=8, default GC settings:

                  │  before    │              after               │
                  │   sec/op   │   sec/op    vs base              │
Tsc/bluesky         2.767 ± 1%   2.752 ± 2%       ~ (p=0.314 n=8)
Tsc/sentry          10.78 ± 2%   10.76 ± 2%       ~ (p=0.701 n=8)
Tsc/t3code-server   13.50 ± 4%   12.57 ± 1%  -6.85% (p=0.000 n=8)
geomean             7.384        7.194       -2.58%

                  │   before     │               after                │
                  │    rss-B     │    rss-B      vs base              │
Tsc/bluesky         1.183Gi ± 2%   1.181Gi ± 3%       ~ (p=0.878 n=8)
Tsc/sentry          3.549Gi ± 6%   3.481Gi ± 5%       ~ (p=0.721 n=8)
Tsc/t3code-server   2.880Gi ± 2%   2.849Gi ± 2%       ~ (p=0.279 n=8)
geomean             2.296Gi        2.271Gi       -1.06%

                  │   before     │               after                │
                  │    heap-B    │    heap-B     vs base              │
Tsc/bluesky         934.5Mi ± 0%   934.8Mi ± 0%  +0.04% (p=0.031 n=8)
Tsc/sentry          2.608Gi ± 0%   2.608Gi ± 0%       ~ (p=0.505 n=8)
Tsc/t3code-server   2.301Gi ± 0%   2.302Gi ± 0%       ~ (p=0.328 n=8)
geomean             1.763Gi        1.763Gi       +0.03%

Narrowing a reference walks back through every assignment and variable declaration on its flow path, and asks whether each one assigns the reference. For a declaration target that meant loading the declaration's symbol and looking up its late-bound and merged symbols, which is mostly cache misses on unrelated symbols. In long functions this dominated flow analysis. A declaration's symbol is named by its identifier, so a reference with a different name can never match it. Comparing names first skips the symbol work for nearly every unrelated declaration. --checkers 4, n=8, default GC settings: │ before │ after │ │ sec/op │ sec/op vs base │ Tsc/bluesky 2.767 ± 1% 2.752 ± 2% ~ (p=0.314 n=8) Tsc/sentry 10.78 ± 2% 10.76 ± 2% ~ (p=0.701 n=8) Tsc/t3code-server 13.50 ± 4% 12.57 ± 1% -6.85% (p=0.000 n=8) geomean 7.384 7.194 -2.58% │ before │ after │ │ rss-B │ rss-B vs base │ Tsc/bluesky 1.183Gi ± 2% 1.181Gi ± 3% ~ (p=0.878 n=8) Tsc/sentry 3.549Gi ± 6% 3.481Gi ± 5% ~ (p=0.721 n=8) Tsc/t3code-server 2.880Gi ± 2% 2.849Gi ± 2% ~ (p=0.279 n=8) geomean 2.296Gi 2.271Gi -1.06% │ before │ after │ │ heap-B │ heap-B vs base │ Tsc/bluesky 934.5Mi ± 0% 934.8Mi ± 0% +0.04% (p=0.031 n=8) Tsc/sentry 2.608Gi ± 0% 2.608Gi ± 0% ~ (p=0.505 n=8) Tsc/t3code-server 2.301Gi ± 0% 2.302Gi ± 0% ~ (p=0.328 n=8) geomean 1.763Gi 1.763Gi +0.03%

Jake Bailey (jakebailey) · 2026-10-19T17:10:50Z

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

typescript-automation · 2026-10-19T17:10:57Z

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

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

Copilot

🟢 Approval recommended

The narrowly scoped optimization preserves matching behavior, with no blocking issues identified.

No open issues 🎉

What changed in this PR

Optimizes flow-reference matching in the native TypeScript checker by avoiding symbol lookups for unrelated declarations.

Changes:

Rejects variable declarations and binding elements with different identifier names before resolving symbols.
Preserves symbol-based matching for remaining cases.

File	Description
`tsc/internal/checker/flow.go`	Adds an early name check when matching declaration targets.

_{Review effort: Balanced}

typescript-automation · 2026-10-19T17:38:50Z

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}	_{113,543k (± 0.06%)}	_{113,517k (± 0.05%)}	_~	_113,408k	_113,727k	_{p=0.504 n=12}
_{Memory Allocs}	_{2,212,516 (± 0.00%)}	_{2,212,674 (± 0.01%)}	_~	_2,212,391	_2,213,242	_{p=0.193 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.040s (± 2.80%)}	_{0.040s (± 3.67%)}	_~	_0.036s	_0.043s	_{p=0.875 n=12}
_{Bind Time}	_{0.014s (± 9.48%)}	_{0.013s (±10.03%)}	_~	_0.011s	_0.018s	_{p=0.907 n=12}
_{Check Time}	_{0.470s (± 0.84%)}	_{0.468s (± 0.63%)}	_~	_0.460s	_0.478s	_{p=0.338 n=12}
_{Emit Time}	_{0.267s (± 2.04%)}	_{0.268s (± 2.46%)}	_~	_0.255s	_0.287s	_{p=0.943 n=12}
_{Total Time}	_{0.798s (± 0.78%)}	_{0.796s (± 1.09%)}	_~	_0.785s	_0.830s	_{p=0.451 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{846,863 (± 0.07%)}	_{846,687 (± 0.07%)}	_~	_845,104	_848,236	_{p=0.887 n=12}
_Types	_{247,033 (± 0.00%)}	_{247,031 (± 0.00%)}	_{-2 (- 0.00%)}	_247,028	_247,034	_{p=0.031 n=12}
_{Memory Used}	_{596,350k (± 0.03%)}	_{596,435k (± 0.02%)}	_~	_596,071k	_596,758k	_{p=0.272 n=12}
_{Memory Allocs}	_{13,148,067 (± 0.10%)}	_{13,146,610 (± 0.07%)}	_~	_13,136,314	_13,183,229	_{p=0.143 n=12}
_{Config Time}	_{0.017s (± 1.95%)}	_{0.017s (± 1.86%)}	_~	_0.016s	_0.017s	_{p=1.000 n=12}
_{Parse Time}	_{0.220s (± 2.61%)}	_{0.229s (± 4.13%)}	_{🔻+0.009s (+ 4.01%)}	_0.212s	_0.269s	_{p=0.043 n=12}
_{Bind Time}	_{0.082s (±12.61%)}	_{0.076s (±16.22%)}	_~	_0.058s	_0.113s	_{p=0.369 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.556s (± 1.61%)}	_{1.574s (± 1.24%)}	_~	_1.526s	_1.614s	_{p=0.285 n=12}
_{Total Time}	_{1.895s (± 1.37%)}	_{1.914s (± 1.07%)}	_~	_1.870s	_1.960s	_{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}	_{2,624,368k (± 0.04%)}	_{2,625,409k (± 0.03%)}	_~	_2,622,669k	_2,627,563k	_{p=0.143 n=12}
_{Memory Allocs}	_{27,699,452 (± 0.07%)}	_{27,720,827 (± 0.05%)}	_~	_27,678,993	_27,742,391	_{p=0.089 n=12}
_{Config Time}	_{0.017s (± 1.07%)}	_{0.017s (± 1.45%)}	_~	_0.016s	_0.017s	_{p=1.000 n=12}
_{Parse Time}	_{0.485s (± 2.75%)}	_{0.469s (± 2.16%)}	_~	_0.441s	_0.486s	_{p=0.124 n=12}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{7.245s (± 0.53%)}	_{7.227s (± 0.53%)}	_~	_7.142s	_7.337s	_{p=0.370 n=12}
_{Emit Time}	_{0.520s (±13.58%)}	_{0.536s (±12.26%)}	_~	_0.416s	_0.660s	_{p=0.809 n=12}
_{Total Time}	_{9.005s (± 0.76%)}	_{8.997s (± 0.60%)}	_~	_8.825s	_9.080s	_{p=0.724 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}	_{1,312,335k (± 0.51%)}	_{1,316,392k (± 0.55%)}	_~	_1,299,858k	_1,336,090k	_{p=0.319 n=12}
_{Memory Allocs}	_{78,231,968 (± 0.12%)}	_{78,294,763 (± 0.06%)}	_~	_78,203,205	_78,421,144	_{p=0.378 n=12}
_{Config Time}	_{0.004s (± 9.04%)}	_{0.004s (±12.64%)}	_~	_0.002s	_0.005s	_{p=0.460 n=12}
_{Parse Time}	_{0.253s (± 1.55%)}	_{0.254s (± 1.99%)}	_~	_0.241s	_0.267s	_{p=0.434 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.674s (± 0.57%)}	_{1.660s (± 0.54%)}	_{-0.014s (- 0.84%)}	_1.636s	_1.681s	_{p=0.018 n=12}
_{Emit Time}	_{0.299s (± 3.57%)}	_{0.302s (± 4.80%)}	_~	_0.270s	_0.334s	_{p=0.898 n=12}
_{Total Time}	_{19.854s (± 0.37%)}	_{19.741s (± 0.59%)}	_~	_19.325s	_19.989s	_{p=0.143 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}	_{247,508k (± 0.04%)}	_{247,494k (± 0.03%)}	_~	_247,290k	_247,660k	_{p=0.590 n=12}
_{Memory Allocs}	_{4,329,543 (± 0.01%)}	_{4,329,384 (± 0.01%)}	_~	_4,328,490	_4,330,503	_{p=0.443 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.114s (± 3.35%)}	_{0.115s (± 5.44%)}	_~	_0.097s	_0.128s	_{p=0.599 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{0.961s (± 0.45%)}	_{0.951s (± 0.46%)}	_{-0.009s (- 0.98%)}	_0.940s	_0.966s	_{p=0.005 n=12}
_{Emit Time}	_{0.148s (± 4.17%)}	_{0.153s (± 6.71%)}	_~	_0.120s	_0.173s	_{p=0.298 n=12}
_{Total Time}	_{1.274s (± 0.64%)}	_{1.276s (± 1.15%)}	_~	_1.242s	_1.318s	_{p=0.660 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}	_{215,213k (± 0.02%)}	_{215,261k (± 0.01%)}	_~	_215,191k	_215,320k	_{p=0.114 n=12}
_{Memory Allocs}	_{1,401,666 (± 0.02%)}	_{1,401,515 (± 0.02%)}	_~	_1,400,773	_1,401,982	_{p=0.410 n=12}
_{Config Time}	_{0.000s (±217.90%)}	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.097s (± 3.25%)}	_{0.099s (± 4.37%)}	_~	_0.089s	_0.108s	_{p=0.599 n=12}
_{Bind Time}	_{0.041s (± 8.59%)}	_{0.042s (± 8.73%)}	_~	_0.035s	_0.051s	_{p=0.787 n=12}
_{Check Time}	_{0.742s (± 0.69%)}	_{0.740s (± 0.76%)}	_~	_0.725s	_0.757s	_{p=0.339 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.895s (± 0.65%)}	_{0.896s (± 0.79%)}	_~	_0.881s	_0.913s	_{p=0.944 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}	_{5,820,990k (± 0.01%)}	_{5,820,941k (± 0.02%)}	_~	_5,818,727k	_5,823,619k	_{p=0.887 n=12}
_{Memory Allocs}	_{59,185,766 (± 0.01%)}	_{59,187,497 (± 0.02%)}	_~	_59,173,901	_59,241,391	_{p=0.671 n=12}
_{Config Time}	_{0.076s (± 0.35%)}	_{0.076s (± 0.24%)}	_~	_0.075s	_0.076s	_{p=1.000 n=12}
_{Parse Time}	_{1.829s (± 1.58%)}	_{1.852s (± 2.55%)}	_~	_1.745s	_1.975s	_{p=0.468 n=12}
_{Bind Time}	_{0.683s (±11.44%)}	_{0.657s (±14.94%)}	_~	_0.512s	_0.893s	_{p=0.660 n=12}
_{Check Time}	_{15.596s (± 1.05%)}	_{15.578s (± 1.19%)}	_~	_15.210s	_15.885s	_{p=0.266 n=12}
_{Emit Time}	_{5.619s (± 6.18%)}	_{5.398s (± 5.68%)}	_~	_4.586s	_6.201s	_{p=0.219 n=12}
_{Total Time}	_{23.898s (± 1.07%)}	_{23.648s (± 0.91%)}	_~	_22.877s	_24.239s	_{p=0.078 n=12}
_{webpack - native}
_Errors	₅₉₈	₅₉₈	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,398,279	_1,398,279	_~	_~	_~	_{p=1.000 n=12}
_Types	_670,934	_670,934	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{885,610k (± 0.01%)}	_{885,755k (± 0.02%)}	_~	_885,389k	_886,368k	_{p=0.266 n=12}
_{Memory Allocs}	_{6,928,614 (± 0.03%)}	_{6,926,996 (± 0.02%)}	_~	_6,925,541	_6,935,063	_{p=0.114 n=12}
_{Config Time}	_0.009s	_0.009s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.309s (± 2.39%)}	_{0.323s (± 2.84%)}	_{🔻+0.014s (+ 4.47%)}	_0.299s	_0.348s	_{p=0.027 n=12}
_{Bind Time}	_{0.079s (± 6.81%)}	_{0.078s (± 5.58%)}	_~	_0.070s	_0.090s	_{p=0.831 n=12}
_{Check Time}	_{2.967s (± 0.41%)}	_{2.860s (± 0.24%)}	_{🟩-0.107s (- 3.61%)}	_2.843s	_2.877s	_{p=0.000 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{3.402s (± 0.41%)}	_{3.308s (± 0.46%)}	_{-0.094s (- 2.77%)}	_3.260s	_3.349s	_{p=0.000 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}	_{471,811k (± 0.02%)}	_{471,917k (± 0.03%)}	_~	_471,608k	_472,207k	_{p=0.160 n=12}
_{Memory Allocs}	_{4,220,158 (± 0.10%)}	_{4,222,590 (± 0.11%)}	_~	_4,212,608	_4,233,632	_{p=0.590 n=12}
_{Config Time}	_{0.003s (±11.61%)}	_{0.003s (± 8.64%)}	_~	_0.002s	_0.003s	_{p=0.640 n=12}
_{Parse Time}	_{0.138s (± 2.93%)}	_{0.135s (± 3.08%)}	_~	_0.126s	_0.149s	_{p=0.246 n=12}
_{Bind Time}	_{0.044s (±16.00%)}	_{0.044s (±17.20%)}	_~	_0.033s	_0.068s	_{p=1.000 n=12}
_{Check Time}	_{1.090s (± 0.87%)}	_{1.082s (± 0.47%)}	_~	_1.068s	_1.096s	_{p=0.193 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.284s (± 1.04%)}	_{1.273s (± 0.76%)}	_~	_1.249s	_1.298s	_{p=0.181 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 17:10

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

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

github-project-automation Bot moved this to Not started in PR Backlog Oct 19, 2026

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 17:11

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Compare names before symbols when matching flow references - #64868

Compare names before symbols when matching flow references#64868
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