Optimize `narrowTypeByEquality` and `narrowTypeBySwitchOnDiscriminant` by ahejlsberg · Pull Request #4781 · microsoft/typescript-go

ahejlsberg · 2026-07-28T21:26:48Z

Inspired by the performance gains in #4711, this PR further optimizes union type narrowing in control flow analysis.

ahejlsberg · 2026-07-28T21:27:07Z

typescript-automation · 2026-07-28T21:27:11Z

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

Command	Status	Results
`test top400`	✅ Started	✅ Results
`perf test this faster`	✅ Started	👀 Results

Copilot

Pull request overview

Optimizes primitive-union narrowing during control-flow analysis.

Changes:

Adds fast paths for equality and switch narrowing.
Optimizes type removal and centralizes union membership checks.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
`internal/checker/flow.go`	Adds narrowing fast paths.
`internal/checker/checker.go`	Optimizes union helpers and type removal.

+		filteredType := c.removeType(t, c.getRegularTypeOfLiteralType(valueType))
+		if filteredType != t {
+			return filteredType


+		if !doubleEquals && t.objectFlags&ObjectFlagsPrimitiveUnion != 0 && valueType.flags&TypeFlagsPrimitive != 0 {
+			regularType := c.getRegularTypeOfLiteralType(valueType)
+			if c.unionContainsType(t, regularType) {
+				return regularType
+			}
+		}


+		if t.objectFlags&ObjectFlagsPrimitiveUnion != 0 && discriminantType.flags&TypeFlagsPrimitive != 0 {
+			regularType := c.getRegularTypeOfLiteralType(discriminantType)
+			if c.unionContainsType(t, regularType) {
+				caseType = regularType
+			}
+		}


typescript-automation · 2026-07-28T21:51:08Z

@ahejlsberg
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	_{81,795 (± 0.02%)}	_{81,829 (± 0.02%)}	_{+35 (+ 0.04%)}	_81,801	_81,858	_{p=0.013 n=6}
_Types	_98,817	_98,817	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{173,358k (± 0.48%)}	_{173,758k (± 0.70%)}	_~	_171,734k	_175,061k	_{p=0.471 n=6}
_{Memory Allocs}	_{2,694,527 (± 0.01%)}	_{2,686,094 (± 0.01%)}	_{-8,433 (- 0.31%)}	_2,685,696	_2,686,313	_{p=0.005 n=6}
_{Config Time}	_{0.000s (±244.70%)}	_{0.000s (±244.70%)}	_~	_0.000s	_0.001s	_{p=1.000 n=6}
_{Parse Time}	_{0.061s (±13.53%)}	_{0.056s (±12.14%)}	_~	_0.049s	_0.069s	_{p=0.259 n=6}
_{Bind Time}	_{0.017s (±13.13%)}	_{0.019s (±34.10%)}	_~	_0.013s	_0.032s	_{p=0.571 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{0.820s (± 0.56%)}	_{0.815s (± 0.73%)}	_~	_0.808s	_0.824s	_{p=0.149 n=6}
_{Total Time}	_{0.899s (± 0.97%)}	_{0.892s (± 1.34%)}	_~	_0.878s	_0.913s	_{p=0.108 n=6}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_{871,302 (± 0.14%)}	_{872,158 (± 0.16%)}	_~	_869,607	_873,509	_{p=0.378 n=6}
_Types	_{263,961 (± 0.00%)}	_{263,962 (± 0.00%)}	_~	_263,960	_263,965	_{p=0.564 n=6}
_{Memory Used}	_{812,489k (± 0.03%)}	_{812,589k (± 0.02%)}	_~	_812,192k	_812,702k	_{p=0.378 n=6}
_{Memory Allocs}	_{13,145,041 (± 0.13%)}	_{13,134,291 (± 0.04%)}	_~	_13,127,967	_13,143,651	_{p=0.128 n=6}
_{Config Time}	_{0.017s (± 2.98%)}	_{0.017s (± 2.38%)}	_~	_0.017s	_0.018s	_{p=0.595 n=6}
_{Parse Time}	_{0.244s (± 4.34%)}	_{0.242s (± 4.26%)}	_~	_0.227s	_0.259s	_{p=0.873 n=6}
_{Bind Time}	_{0.045s (±23.07%)}	_{0.052s (±33.07%)}	_~	_0.040s	_0.079s	_{p=0.370 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{1.896s (± 2.19%)}	_{1.897s (± 1.28%)}	_~	_1.851s	_1.916s	_{p=1.000 n=6}
_{Total Time}	_{2.213s (± 1.63%)}	_{2.219s (± 0.94%)}	_~	_2.180s	_2.241s	_{p=0.810 n=6}
_{mui-docs - native}
_Errors	_{11,275 (± 0.13%)}	_{11,281 (± 0.01%)}	_~	_11,279	_11,282	_{p=0.797 n=6}
_Symbols	_4,447,880	_4,447,880	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,662,215	_1,662,215	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{5,053,044k (± 0.04%)}	_{5,053,470k (± 0.01%)}	_~	_5,052,600k	_5,054,485k	_{p=0.936 n=6}
_{Memory Allocs}	_{51,349,959 (± 0.08%)}	_{51,332,091 (± 0.07%)}	_~	_51,289,014	_51,379,547	_{p=0.689 n=6}
_{Config Time}	_{0.016s (± 3.16%)}	_{0.017s (± 7.26%)}	_~	_0.016s	_0.019s	_{p=0.923 n=6}
_{Parse Time}	_{0.520s (± 2.25%)}	_{0.528s (± 2.37%)}	_~	_0.510s	_0.542s	_{p=0.298 n=6}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{16.709s (± 0.96%)}	_{16.676s (± 0.85%)}	_~	_16.564s	_16.932s	_{p=1.000 n=6}
_{Emit Time}	_{0.470s (± 3.43%)}	_{0.516s (±25.78%)}	_~	_0.456s	_0.788s	_{p=0.688 n=6}
_{Total Time}	_{18.461s (± 1.03%)}	_{18.465s (± 1.18%)}	_~	_18.241s	_18.739s	_{p=0.936 n=6}
_{self-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,394,220	_1,394,220	_~	_~	_~	_{p=1.000 n=6}
_Types	_442,210	_442,210	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{1,611,398k (± 0.31%)}	_{1,605,555k (± 0.33%)}	_{-5,843k (- 0.36%)}	_1,599,587k	_1,615,027k	_{p=0.045 n=6}
_{Memory Allocs}	_{97,100,651 (± 0.07%)}	_{96,794,372 (± 0.06%)}	_{-306,279 (- 0.32%)}	_96,741,613	_96,906,047	_{p=0.005 n=6}
_{Config Time}	_{0.016s (±17.42%)}	_{0.018s (±29.64%)}	_~	_0.010s	_0.025s	_{p=0.465 n=6}
_{Parse Time}	_{0.238s (± 2.87%)}	_{0.237s (± 2.61%)}	_~	_0.230s	_0.246s	_{p=0.688 n=6}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{2.169s (± 0.72%)}	_{2.109s (± 0.41%)}	_{-0.060s (- 2.76%)}	_2.095s	_2.119s	_{p=0.005 n=6}
_{Emit Time}	_{0.299s (± 3.87%)}	_{0.300s (± 3.45%)}	_~	_0.282s	_0.313s	_{p=0.872 n=6}
_{Total Time}	_{28.304s (± 0.39%)}	_{27.507s (± 0.75%)}	_{-0.797s (- 2.82%)}	_27.155s	_27.744s	_{p=0.005 n=6}
_{self-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_337,681	_337,681	_~	_~	_~	_{p=1.000 n=6}
_Types	_199,559	_199,559	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{320,591k (± 0.03%)}	_{320,730k (± 0.03%)}	_{+139k (+ 0.04%)}	_320,603k	_320,857k	_{p=0.045 n=6}
_{Memory Allocs}	_{4,676,102 (± 0.02%)}	_{4,661,662 (± 0.01%)}	_{-14,440 (- 0.31%)}	_4,661,179	_4,662,410	_{p=0.005 n=6}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=6}
_{Parse Time}	_{0.114s (± 3.94%)}	_{0.114s (± 2.42%)}	_~	_0.111s	_0.119s	_{p=0.868 n=6}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{1.282s (± 1.43%)}	_{1.222s (± 0.77%)}	_{🟩-0.059s (- 4.63%)}	_1.212s	_1.234s	_{p=0.005 n=6}
_{Emit Time}	_{0.115s (± 7.36%)}	_{0.122s (± 6.51%)}	_~	_0.113s	_0.137s	_{p=0.108 n=6}
_{Total Time}	_{1.569s (± 1.28%)}	_{1.508s (± 1.02%)}	_{🟩-0.061s (- 3.92%)}	_1.489s	_1.530s	_{p=0.005 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}	_{128,877k (± 0.01%)}	_{128,903k (± 0.02%)}	_{+27k (+ 0.02%)}	_128,886k	_128,958k	_{p=0.031 n=6}
_{Memory Allocs}	_{177,864 (± 0.24%)}	_{177,424 (± 0.07%)}	_~	_177,263	_177,534	_{p=0.093 n=6}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=6}
_{Parse Time}	_{0.102s (± 4.48%)}	_{0.105s (± 5.77%)}	_~	_0.096s	_0.115s	_{p=0.294 n=6}
_{Bind Time}	_{0.037s (±11.07%)}	_{0.041s (± 7.23%)}	_~	_0.037s	_0.044s	_{p=0.147 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{0.143s (± 3.78%)}	_{0.149s (± 4.04%)}	_~	_0.139s	_0.155s	_{p=0.173 n=6}
_{vscode - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_7,454,611	_7,454,611	_~	_~	_~	_{p=1.000 n=6}
_Types	_2,602,333	_2,602,328	_{-5 (- 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory Used}	_{5,025,743k (± 0.02%)}	_{5,025,215k (± 0.01%)}	_~	_5,024,804k	_5,026,304k	_{p=0.128 n=6}
_{Memory Allocs}	_{38,097,508 (± 0.09%)}	_{38,027,882 (± 0.13%)}	_{-69,626 (- 0.18%)}	_38,000,252	_38,126,979	_{p=0.045 n=6}
_{Config Time}	_{0.072s (± 6.03%)}	_{0.073s (±12.22%)}	_~	_0.066s	_0.090s	_{p=0.872 n=6}
_{Parse Time}	_{0.919s (± 3.59%)}	_{0.940s (± 5.39%)}	_~	_0.887s	_1.032s	_{p=0.689 n=6}
_{Bind Time}	_{0.183s (±29.58%)}	_{0.161s (± 2.11%)}	_~	_0.158s	_0.167s	_{p=0.416 n=6}
_{Check Time}	_{8.422s (± 0.85%)}	_{8.368s (± 0.39%)}	_~	_8.340s	_8.426s	_{p=0.173 n=6}
_{Emit Time}	_{2.442s (± 7.18%)}	_{2.559s (± 4.78%)}	_~	_2.453s	_2.743s	_{p=0.423 n=6}
_{Total Time}	_{12.060s (± 0.96%)}	_{12.118s (± 0.97%)}	_~	_11.959s	_12.270s	_{p=0.471 n=6}
_{webpack - native}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_193,769	_193,769	_~	_~	_~	_{p=1.000 n=6}
_Types	₃₄₀	₃₄₀	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{229,991k (± 0.16%)}	_{230,177k (± 0.08%)}	_~	_229,846k	_230,377k	_{p=0.336 n=6}
_{Memory Allocs}	_{974,035 (± 0.79%)}	_{977,511 (± 0.68%)}	_~	_968,848	_987,982	_{p=0.471 n=6}
_{Config Time}	_{0.012s (±17.55%)}	_{0.010s (±14.55%)}	_~	_0.009s	_0.012s	_{p=0.157 n=6}
_{Parse Time}	_{0.149s (± 4.26%)}	_{0.144s (± 4.29%)}	_~	_0.133s	_0.150s	_{p=0.228 n=6}
_{Bind Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=6}
_{Emit Time}	_{0.041s (±22.05%)}	_{0.041s (±13.95%)}	_~	_0.036s	_0.048s	_{p=0.686 n=6}
_{Total Time}	_{0.202s (± 3.12%)}	_{0.196s (± 4.87%)}	_~	_0.179s	_0.203s	_{p=0.297 n=6}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,064,618	_1,064,618	_~	_~	_~	_{p=1.000 n=6}
_Types	_392,776	_392,776	_~	_~	_~	_{p=1.000 n=6}
_{Memory Used}	_{637,894k (± 0.02%)}	_{638,074k (± 0.02%)}	_{+181k (+ 0.03%)}	_637,936k	_638,195k	_{p=0.045 n=6}
_{Memory Allocs}	_{4,993,724 (± 0.25%)}	_{4,989,485 (± 0.06%)}	_~	_4,984,899	_4,994,096	_{p=1.000 n=6}
_{Config Time}	_0.005s	_0.005s	_~	_~	_~	_{p=1.000 n=6}
_{Parse Time}	_{0.133s (± 2.56%)}	_{0.126s (± 3.09%)}	_{🟩-0.007s (- 5.51%)}	_0.121s	_0.131s	_{p=0.016 n=6}
_{Bind Time}	_{0.038s (±27.93%)}	_{0.032s (±26.82%)}	_~	_0.024s	_0.046s	_{p=0.195 n=6}
_{Check Time}	_{1.233s (± 0.61%)}	_{1.243s (± 0.78%)}	_~	_1.234s	_1.254s	_{p=0.173 n=6}
_{Emit Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{1.415s (± 0.54%)}	_{1.411s (± 0.65%)}	_~	_1.397s	_1.422s	_{p=0.421 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:

Download Benchmarks

typescript-automation · 2026-07-28T22:41:53Z

@ahejlsberg Here are the results of running the top 400 repos with tsc comparing main and refs/pull/4781/merge:

Everything looks good!

Optimize narrowTypeByEquality and narrowTypeBySwitchOnDiscriminant

d37b56a

Copilot AI review requested due to automatic review settings July 28, 2026 21:26

Copilot started reviewing on behalf of ahejlsberg July 28, 2026 21:27 View session

Copilot AI reviewed Jul 28, 2026

View reviewed changes

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Optimize `narrowTypeByEquality` and `narrowTypeBySwitchOnDiscriminant` - #4781

Optimize `narrowTypeByEquality` and `narrowTypeBySwitchOnDiscriminant`#4781
ahejlsberg wants to merge 1 commit into
mainfrom
cfa-narrowing-optimization

ahejlsberg commented Jul 28, 2026

Uh oh!

ahejlsberg commented Jul 28, 2026

Uh oh!

typescript-automation Bot commented Jul 28, 2026 •

edited

Loading

Uh oh!

Copilot AI left a comment

Uh oh!

typescript-automation Bot commented Jul 28, 2026

tsc

Uh oh!

typescript-automation Bot commented Jul 28, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Uh oh!

Conversation

ahejlsberg commented Jul 28, 2026

Uh oh!

ahejlsberg commented Jul 28, 2026

Uh oh!

typescript-automation Bot commented Jul 28, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

typescript-automation Bot commented Jul 28, 2026

tsc

Uh oh!

typescript-automation Bot commented Jul 28, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

typescript-automation Bot commented Jul 28, 2026 •

edited

Loading