Size new arena chunks from the previous chunk's capacity by jakebailey · Pull Request #65031 · microsoft/TypeScript

Jake Bailey (jakebailey) · 2026-10-29T20:49:38Z

Arena.NewSlice computed the next chunk size from len(a.data), but it grows whenever the request does not fit in the remaining space, which usually happens before the current chunk is full. Doubling the used length rather than the chunk capacity could make the next chunk the same size as, or smaller than, the one being replaced. Arenas that mostly serve slices then kept allocating small chunks and never reached the intended steady-state size.

Base the growth on cap(a.data) so each chunk doubles the previous one up to the size ceiling. Arena.New only grows when len == cap, so it already behaved correctly; it now uses cap as well for consistency.

Thanks Steven (@teamchong) for finding this!

Arena.NewSlice computed the next chunk size from len(a.data), but it grows whenever the request does not fit in the remaining space, which usually happens before the current chunk is full. Doubling the used length rather than the chunk capacity could make the next chunk the same size as, or smaller than, the one being replaced. Arenas that mostly serve slices then kept allocating small chunks and never reached the intended steady-state size. Base the growth on cap(a.data) so each chunk doubles the previous one up to the size ceiling. Arena.New only grows when len == cap, so it already behaved correctly; it now uses cap as well for consistency. Co-authored-by: Steven Chong <25894545+teamchong@users.noreply.github.com>

Jake Bailey (jakebailey) · 2026-10-29T20:49:54Z

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

typescript-automation · 2026-10-29T20:49:59Z

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

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

Jake Bailey (jakebailey) · 2026-10-29T20:50:12Z

This one's definitely an oops on my part

Copilot

🟡 Changes recommended

A regression test is still needed for growth from a partially used arena chunk, since existing assertions do not distinguish backing-capacity growth from separate allocations.

1 open issue

Add regression test for growth from partially used arena chunk · New

What changed in this PR

This PR fixes chunk growth in the native compiler’s arena allocator so slice-heavy workloads can reach the intended chunk size.

Changes:

Base Arena.NewSlice growth on the previous chunk’s capacity rather than its used length.
Use capacity in Arena.New for consistency, without changing its behavior.

File	Description
`tsc/internal/core/arena.go`	Uses chunk capacity when calculating the next allocation size.

_{Review effort: Balanced}

Copilot · 2026-10-29T20:54:57Z

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The existing arena tests only check the slices NewSlice returns, and those checks pass whether a slice is carved from a new chunk or allocated on its own. So they could not catch next-chunk sizing based on the used length instead of the chunk capacity. The new package-internal test starts from a partially used chunk and requests more than the remaining space. It checks that the replacement chunk grows from the previous capacity and that the slice is carved from it.

Copilot

🟢 Approval recommended

No active review issues are reported for the arena chunk growth fix.

No open issues 🎉

1 resolved since last review

Add regression test for growth from partially used arena chunk

_{Review effort: Balanced}

The test asserted the arena's private chunk capacity, pinning one implementation detail rather than observable behavior. It adds maintenance cost without guarding anything users can see, so drop it.

typescript-automation · 2026-10-29T21:43:20Z

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=20}
_Symbols	_88,466	_88,466	_~	_~	_~	_{p=1.000 n=20}
_Types	_79,607	_79,607	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{104,165k (± 0.03%)}	_{104,135k (± 0.03%)}	_~	_104,010k	_104,248k	_{p=0.082 n=20}
_{Memory Allocs}	_{1,263,607 (± 0.01%)}	_{1,263,817 (± 0.01%)}	_{+209 (+ 0.02%)}	_1,263,478	_1,264,524	_{p=0.020 n=20}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Parse Time}	_{0.023s (± 3.50%)}	_{0.023s (± 4.43%)}	_~	_0.019s	_0.027s	_{p=0.494 n=20}
_{Bind Time}	_{0.009s (± 4.42%)}	_{0.009s (± 5.14%)}	_~	_0.007s	_0.011s	_{p=0.136 n=20}
_{Check Time}	_{0.226s (± 0.94%)}	_{0.225s (± 1.24%)}	_~	_0.215s	_0.238s	_{p=0.249 n=20}
_{Emit Time}	_{0.151s (± 1.63%)}	_{0.151s (± 1.16%)}	_~	_0.144s	_0.159s	_{p=0.920 n=20}
_{Total Time}	_{0.413s (± 0.80%)}	_{0.412s (± 0.75%)}	_~	_0.398s	_0.425s	_{p=0.464 n=20}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_{586,512 (± 0.02%)}	_{586,516 (± 0.02%)}	_~	_586,002	_586,854	_{p=1.000 n=20}
_Types	_{241,688 (± 0.00%)}	_{241,687 (± 0.00%)}	_~	_241,683	_241,698	_{p=0.972 n=20}
_{Memory Used}	_{517,926k (± 0.02%)}	_{517,984k (± 0.01%)}	_~	_517,711k	_518,165k	_{p=0.317 n=20}
_{Memory Allocs}	_{7,798,904 (± 0.12%)}	_{7,798,091 (± 0.14%)}	_~	_7,783,933	_7,861,136	_{p=0.478 n=20}
_{Config Time}	_0.009s	_0.009s	_~	_~	_~	_{p=1.000 n=20}
_{Parse Time}	_{0.121s (± 1.54%)}	_{0.124s (± 2.65%)}	_~	_0.111s	_0.141s	_{p=0.233 n=20}
_{Bind Time}	_{0.048s (±10.82%)}	_{0.041s (± 7.19%)}	_{🟩-0.007s (-14.72%)}	_0.036s	_0.059s	_{p=0.025 n=20}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=20}
_{Emit Time}	_{0.853s (± 1.01%)}	_{0.843s (± 0.97%)}	_~	_0.821s	_0.876s	_{p=0.082 n=20}
_{Total Time}	_{1.043s (± 1.25%)}	_{1.027s (± 1.04%)}	_~	_1.004s	_1.075s	_{p=0.071 n=20}
_{arktype - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_685,920	_685,920	_~	_~	_~	_{p=1.000 n=20}
_Types	_939,467	_939,467	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{497,279k (± 0.01%)}	_{497,335k (± 0.01%)}	_~	_497,042k	_497,473k	_{p=0.080 n=20}
_{Memory Allocs}	_{12,106,816 (± 0.01%)}	_{12,107,690 (± 0.01%)}	_~	_12,104,883	_12,112,962	_{p=0.253 n=20}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=20}
_{Parse Time}	_{0.041s (± 2.82%)}	_{0.043s (± 3.91%)}	_~	_0.039s	_0.053s	_{p=0.095 n=20}
_{Bind Time}	_{0.013s (± 6.85%)}	_{0.014s (±10.28%)}	_~	_0.011s	_0.020s	_{p=0.939 n=20}
_{Check Time}	_{0.978s (± 0.55%)}	_{0.975s (± 0.60%)}	_~	_0.952s	_1.004s	_{p=0.239 n=20}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Total Time}	_{1.062s (± 0.44%)}	_{1.062s (± 0.57%)}	_~	_1.041s	_1.091s	_{p=0.587 n=20}
_{class-transformer - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_45,376	_45,376	_~	_~	_~	_{p=1.000 n=20}
_Types	_7,573	_7,573	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{27,835k (± 0.02%)}	_{27,844k (± 0.02%)}	_{+9k (+ 0.03%)}	_27,831k	_27,877k	_{p=0.004 n=20}
_{Memory Allocs}	_{248,709 (± 0.05%)}	_{248,468 (± 0.05%)}	_{-241 (- 0.10%)}	_247,900	_248,848	_{p=0.009 n=20}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Parse Time}	_{0.012s (± 2.98%)}	_{0.011s (± 3.08%)}	_~	_0.009s	_0.012s	_{p=0.177 n=20}
_{Bind Time}	_{0.004s (±13.92%)}	_{0.004s (±11.18%)}	_~	_0.002s	_0.005s	_{p=0.786 n=20}
_{Check Time}	_{0.008s (± 4.16%)}	_{0.008s (± 5.10%)}	_~	_0.006s	_0.009s	_{p=0.304 n=20}
_{Emit Time}	_{0.008s (± 7.52%)}	_{0.009s (± 8.09%)}	_~	_0.007s	_0.013s	_{p=0.185 n=20}
_{Total Time}	_{0.032s (± 2.89%)}	_{0.032s (± 2.49%)}	_~	_0.028s	_0.037s	_{p=0.550 n=20}
_{effect - native}
_Errors	₁₂	₁₂	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_1,235,115	_1,235,115	_~	_~	_~	_{p=1.000 n=20}
_Types	_894,113	_894,113	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{940,524k (± 0.02%)}	_{940,843k (± 0.02%)}	_{+319k (+ 0.03%)}	_940,025k	_941,904k	_{p=0.033 n=20}
_{Memory Allocs}	_{12,670,502 (± 0.02%)}	_{12,674,809 (± 0.05%)}	_~	_12,661,380	_12,721,733	_{p=0.369 n=20}
_{Config Time}	_{0.002s (± 5.09%)}	_0.002s	_~	_~	_~	_{p=1.000 n=20}
_{Parse Time}	_{0.148s (± 1.87%)}	_{0.147s (± 1.87%)}	_~	_0.134s	_0.159s	_{p=0.703 n=20}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Check Time}	_{0.948s (± 0.85%)}	_{0.944s (± 0.71%)}	_~	_0.921s	_0.970s	_{p=0.713 n=20}
_{Emit Time}	_{0.504s (± 1.64%)}	_{0.510s (± 2.80%)}	_~	_0.448s	_0.558s	_{p=0.304 n=20}
_{Total Time}	_{1.693s (± 0.67%)}	_{1.697s (± 0.88%)}	_~	_1.634s	_1.754s	_{p=0.743 n=20}
_{mui-docs - native}
_Errors	_11,405	_11,405	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_3,784,019	_3,784,019	_~	_~	_~	_{p=1.000 n=20}
_Types	_1,355,974	_1,355,974	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{1,838,375k (± 0.07%)}	_{1,836,650k (± 0.07%)}	_~	_1,829,712k	_1,840,495k	_{p=0.091 n=20}
_{Memory Allocs}	_{26,010,421 (± 0.08%)}	_{26,008,769 (± 0.15%)}	_~	_25,883,234	_26,203,108	_{p=0.495 n=20}
_{Config Time}	_0.009s	_{0.009s (± 1.61%)}	_~	_0.008s	_0.009s	_{p=0.487 n=20}
_{Parse Time}	_{0.308s (± 2.90%)}	_{0.302s (± 2.87%)}	_~	_0.264s	_0.348s	_{p=0.654 n=20}
_{Bind Time}	_0.001s	_{0.001s (± 9.93%)}	_~	_0.001s	_0.002s	_{p=1.000 n=20}
_{Check Time}	_{3.298s (± 0.86%)}	_{3.294s (± 1.90%)}	_~	_3.172s	_3.810s	_{p=0.245 n=20}
_{Emit Time}	_{0.259s (± 1.58%)}	_{0.265s (± 1.92%)}	_~	_0.250s	_0.292s	_{p=0.087 n=20}
_{Total Time}	_{4.400s (± 1.25%)}	_{4.382s (± 1.54%)}	_~	_4.246s	_4.933s	_{p=0.414 n=20}
_{sentry - native}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_4,793,245	_4,793,245	_~	_~	_~	_{p=1.000 n=20}
_Types	_2,607,045	_2,607,045	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{2,881,214k (± 0.03%)}	_{2,880,734k (± 0.02%)}	_~	_2,878,901k	_2,883,512k	_{p=0.429 n=20}
_{Memory Allocs}	_{75,232,711 (± 0.16%)}	_{75,146,442 (± 0.12%)}	_~	_74,816,262	_75,754,285	_{p=0.383 n=20}
_{Config Time}	_{0.026s (± 0.74%)}	_{0.026s (± 1.26%)}	_~	_0.025s	_0.028s	_{p=0.716 n=20}
_{Parse Time}	_{0.476s (± 3.03%)}	_{0.477s (± 5.49%)}	_~	_0.428s	_0.668s	_{p=0.457 n=20}
_{Bind Time}	_{0.170s (±11.18%)}	_{0.188s (±11.13%)}	_~	_0.136s	_0.248s	_{p=0.387 n=20}
_{Check Time}	_{6.717s (± 0.45%)}	_{6.704s (± 0.78%)}	_~	_6.593s	_7.060s	_{p=0.256 n=20}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Total Time}	_{7.438s (± 0.33%)}	_{7.447s (± 0.71%)}	_~	_7.319s	_7.797s	_{p=0.678 n=20}
_{social-app - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_1,883,074	_1,883,074	_~	_~	_~	_{p=1.000 n=20}
_Types	_1,158,168	_1,158,168	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{972,575k (± 0.02%)}	_{972,491k (± 0.01%)}	_~	_972,017k	_972,788k	_{p=0.355 n=20}
_{Memory Allocs}	_{13,561,162 (± 0.11%)}	_{13,571,600 (± 0.24%)}	_~	_13,517,767	_13,843,030	_{p=0.904 n=20}
_{Config Time}	_{0.009s (± 9.86%)}	_{0.008s (± 5.34%)}	_~	_0.007s	_0.012s	_{p=0.767 n=20}
_{Parse Time}	_{0.165s (± 3.94%)}	_{0.163s (± 3.11%)}	_~	_0.152s	_0.194s	_{p=0.723 n=20}
_{Bind Time}	_{0.050s (±11.98%)}	_{0.045s (± 7.65%)}	_~	_0.040s	_0.070s	_{p=0.571 n=20}
_{Check Time}	_{1.563s (± 1.07%)}	_{1.545s (± 0.76%)}	_~	_1.486s	_1.598s	_{p=0.155 n=20}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Total Time}	_{1.803s (± 1.23%)}	_{1.778s (± 0.82%)}	_~	_1.715s	_1.826s	_{p=0.126 n=20}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_1,184,972	_1,184,972	_~	_~	_~	_{p=1.000 n=20}
_Types	_438,963	_438,963	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{1,168,772k (± 0.54%)}	_{1,164,850k (± 0.16%)}	_~	_1,154,601k	_1,171,387k	_{p=0.904 n=20}
_{Memory Allocs}	_{47,686,127 (± 0.13%)}	_{47,679,034 (± 0.09%)}	_~	_47,522,296	_47,861,864	_{p=1.000 n=20}
_{Config Time}	_{0.002s (±12.90%)}	_{0.002s (±18.20%)}	_~	_0.000s	_0.002s	_{p=0.561 n=20}
_{Parse Time}	_{0.142s (± 2.11%)}	_{0.140s (± 1.65%)}	_~	_0.130s	_0.150s	_{p=0.551 n=20}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Check Time}	_{0.816s (± 0.49%)}	_{0.826s (± 1.38%)}	_~	_0.807s	_0.909s	_{p=0.344 n=20}
_{Emit Time}	_{0.143s (± 2.00%)}	_{0.145s (± 2.48%)}	_~	_0.138s	_0.164s	_{p=0.355 n=20}
_{Total Time}	_{9.488s (± 0.58%)}	_{9.555s (± 0.90%)}	_~	_9.355s	_10.018s	_{p=0.245 n=20}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_263,511	_263,511	_~	_~	_~	_{p=1.000 n=20}
_Types	_196,116	_196,116	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{224,583k (± 0.02%)}	_{224,627k (± 0.02%)}	_~	_224,509k	_224,778k	_{p=0.274 n=20}
_{Memory Allocs}	_{2,401,534 (± 0.03%)}	_{2,401,474 (± 0.01%)}	_~	_2,400,066	_2,402,611	_{p=0.583 n=20}
_{Config Time}	_{0.000s (±208.60%)}	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Parse Time}	_{0.050s (± 3.67%)}	_{0.051s (± 4.06%)}	_~	_0.041s	_0.059s	_{p=0.497 n=20}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Check Time}	_{0.388s (± 0.59%)}	_{0.389s (± 1.01%)}	_~	_0.376s	_0.416s	_{p=0.743 n=20}
_{Emit Time}	_{0.049s (± 4.77%)}	_{0.048s (± 4.69%)}	_~	_0.043s	_0.056s	_{p=0.866 n=20}
_{Total Time}	_{0.516s (± 0.85%)}	_{0.516s (± 0.80%)}	_~	_0.503s	_0.534s	_{p=0.920 n=20}
_{t3code-server - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_4,350,661	_4,350,661	_~	_~	_~	_{p=1.000 n=20}
_Types	_3,040,644	_3,040,644	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{2,433,686k (± 0.03%)}	_{2,434,379k (± 0.02%)}	_~	_2,432,336k	_2,437,015k	_{p=0.076 n=20}
_{Memory Allocs}	_{37,376,012 (± 0.15%)}	_{37,368,824 (± 0.08%)}	_~	_37,287,351	_37,529,218	_{p=0.429 n=20}
_{Config Time}	_{0.003s (± 6.00%)}	_{0.003s (±12.36%)}	_~	_0.002s	_0.006s	_{p=1.000 n=20}
_{Parse Time}	_{0.247s (± 1.82%)}	_{0.248s (± 2.06%)}	_~	_0.230s	_0.280s	_{p=0.899 n=20}
_{Bind Time}	_{0.086s (± 9.60%)}	_{0.090s (±11.88%)}	_~	_0.076s	_0.174s	_{p=0.162 n=20}
_{Check Time}	_{7.235s (± 0.54%)}	_{7.230s (± 0.44%)}	_~	_7.143s	_7.392s	_{p=1.000 n=20}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Total Time}	_{7.581s (± 0.49%)}	_{7.583s (± 0.41%)}	_~	_7.493s	_7.732s	_{p=0.941 n=20}
_{tanstack-router - native}
_Errors	₁	₁	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_571,756	_571,756	_~	_~	_~	_{p=1.000 n=20}
_Types	_397,689	_397,689	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{331,409k (± 0.01%)}	_{331,454k (± 0.01%)}	_~	_331,281k	_331,648k	_{p=0.183 n=20}
_{Memory Allocs}	_{4,391,218 (± 0.15%)}	_{4,384,960 (± 0.08%)}	_~	_4,377,098	_4,402,241	_{p=0.149 n=20}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=20}
_{Parse Time}	_{0.039s (± 3.71%)}	_{0.040s (± 2.92%)}	_~	_0.036s	_0.044s	_{p=0.237 n=20}
_{Bind Time}	_{0.013s (±11.37%)}	_{0.013s (± 8.93%)}	_~	_0.011s	_0.020s	_{p=0.505 n=20}
_{Check Time}	_{0.568s (± 0.59%)}	_{0.572s (± 0.57%)}	_~	_0.559s	_0.586s	_{p=0.255 n=20}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Total Time}	_{0.640s (± 0.65%)}	_{0.645s (± 0.67%)}	_~	_0.626s	_0.666s	_{p=0.143 n=20}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_231,616	_231,616	_~	_~	_~	_{p=1.000 n=20}
_Types	_181,195	_181,195	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{193,535k (± 0.01%)}	_{193,540k (± 0.01%)}	_~	_193,414k	_193,691k	_{p=0.784 n=20}
_{Memory Allocs}	_{1,318,222 (± 0.02%)}	_{1,318,129 (± 0.02%)}	_~	_1,317,567	_1,319,311	_{p=0.560 n=20}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Parse Time}	_{0.044s (± 2.46%)}	_{0.044s (± 2.32%)}	_~	_0.039s	_0.047s	_{p=0.751 n=20}
_{Bind Time}	_{0.020s (± 3.74%)}	_{0.022s (± 5.32%)}	_{🔻+0.002s (+ 7.33%)}	_0.017s	_0.027s	_{p=0.016 n=20}
_{Check Time}	_{0.302s (± 1.63%)}	_{0.295s (± 1.02%)}	_{-0.006s (- 2.12%)}	_0.287s	_0.308s	_{p=0.014 n=20}
_{Emit Time}	_{0.000s (±208.60%)}	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Total Time}	_{0.373s (± 1.59%)}	_{0.367s (± 0.73%)}	_~	_0.357s	_0.381s	_{p=0.074 n=20}
_{vscode - native}
_Errors	₁₁	₁₁	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_7,501,886	_7,501,886	_~	_~	_~	_{p=1.000 n=20}
_Types	_4,415,089	_4,415,089	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{5,204,554k (± 0.01%)}	_{5,205,091k (± 0.03%)}	_~	_5,201,693k	_5,214,949k	_{p=0.947 n=20}
_{Memory Allocs}	_{56,944,039 (± 0.01%)}	_{56,947,851 (± 0.02%)}	_~	_56,923,113	_56,995,184	_{p=0.429 n=20}
_{Config Time}	_{0.033s (± 0.63%)}	_{0.033s (± 0.63%)}	_~	_0.032s	_0.033s	_{p=1.000 n=20}
_{Parse Time}	_{0.878s (± 1.67%)}	_{0.882s (± 1.02%)}	_~	_0.838s	_0.923s	_{p=0.973 n=20}
_{Bind Time}	_{0.309s (± 7.05%)}	_{0.303s (± 7.12%)}	_~	_0.287s	_0.499s	_{p=0.416 n=20}
_{Check Time}	_{7.614s (± 0.58%)}	_{7.623s (± 0.36%)}	_~	_7.424s	_7.694s	_{p=0.673 n=20}
_{Emit Time}	_{2.662s (± 4.69%)}	_{2.629s (± 3.46%)}	_~	_2.530s	_3.404s	_{p=0.588 n=20}
_{Total Time}	_{11.552s (± 0.92%)}	_{11.525s (± 0.75%)}	_~	_11.408s	_12.269s	_{p=0.815 n=20}
_{webpack - native}
_Errors	₆₁₆	₆₁₆	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_917,188	_917,188	_~	_~	_~	_{p=1.000 n=20}
_Types	_636,859	_636,859	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{693,556k (± 0.02%)}	_{693,609k (± 0.01%)}	_~	_693,034k	_694,073k	_{p=0.449 n=20}
_{Memory Allocs}	_{6,499,181 (± 0.03%)}	_{6,496,659 (± 0.03%)}	_~	_6,490,719	_6,503,097	_{p=0.063 n=20}
_{Config Time}	_0.004s	_0.004s	_~	_~	_~	_{p=1.000 n=20}
_{Parse Time}	_{0.138s (± 3.07%)}	_{0.142s (± 3.63%)}	_~	_0.119s	_0.159s	_{p=0.143 n=20}
_{Bind Time}	_{0.044s (±11.06%)}	_{0.047s (±11.17%)}	_~	_0.036s	_0.072s	_{p=0.392 n=20}
_{Check Time}	_{1.138s (± 1.04%)}	_{1.128s (± 0.87%)}	_~	_1.103s	_1.197s	_{p=0.122 n=20}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Total Time}	_{1.348s (± 1.01%)}	_{1.345s (± 0.90%)}	_~	_1.316s	_1.415s	_{p=0.804 n=20}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=20}
_Symbols	_640,490	_640,490	_~	_~	_~	_{p=1.000 n=20}
_Types	_387,984	_387,984	_~	_~	_~	_{p=1.000 n=20}
_{Memory Used}	_{350,913k (± 0.02%)}	_{351,025k (± 0.02%)}	_{+111k (+ 0.03%)}	_350,755k	_351,256k	_{p=0.030 n=20}
_{Memory Allocs}	_{3,994,408 (± 0.07%)}	_{3,992,294 (± 0.06%)}	_~	_3,982,534	_4,002,681	_{p=0.461 n=20}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=20}
_{Parse Time}	_{0.062s (± 3.03%)}	_{0.061s (± 2.78%)}	_~	_0.055s	_0.068s	_{p=0.423 n=20}
_{Bind Time}	_{0.023s (±11.18%)}	_{0.022s (± 9.82%)}	_~	_0.018s	_0.033s	_{p=0.659 n=20}
_{Check Time}	_{0.438s (± 2.26%)}	_{0.435s (± 1.58%)}	_~	_0.414s	_0.471s	_{p=0.909 n=20}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=20}
_{Total Time}	_{0.528s (± 1.85%)}	_{0.524s (± 1.40%)}	_~	_0.501s	_0.564s	_{p=0.524 n=20}

System info unknown

Hosts

native

Scenarios

Compiler-Unions - native
angular-1 - native
arktype - native
class-transformer - native
effect - native
mui-docs - native
sentry - native
social-app - native
strada-build-src - native
strada-compiler - native
t3code-server - native
tanstack-router - native
ts-pre-modules - native
vscode - native
webpack - native
xstate-main - native

_Benchmark	_Name	_Iterations
_Current	_pr	₂₀
_Baseline	_baseline	₂₀

Developer Information:

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Ryan Cavanaugh (RyanCavanaugh)

The NewSlice growth calculation correctly needs the previous capacity, not its partially consumed length; New remains equivalent because it only grows at length equal to capacity. Existing arena tests pass. No issues found.

Copilot AI balanced review requested due to automatic review settings October 29, 2026 20:49

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Comment thread tsc/internal/core/arena.go Resolved

Jake Bailey (jakebailey) added the Domain: Performance Reports of unusually slow behavior label Oct 29, 2026

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Remove arena growth white-box test

8480a06

The test asserted the arena's private chunk capacity, pinning one implementation detail rather than observable behavior. It adds maintenance cost without guarding anything users can see, so drop it.

Ryan Cavanaugh (RyanCavanaugh) approved these changes Oct 30, 2026

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github-project-automation Bot moved this from Not started to Needs merge in PR Backlog Oct 30, 2026

Jake Bailey (jakebailey) added this pull request to the merge queue Oct 30, 2026

Merged via the queue into microsoft:main with commit cb5f346 Oct 30, 2026
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Jake Bailey (jakebailey) deleted the perf/arena-growth branch October 30, 2026 14:51

github-project-automation Bot moved this from Needs merge to Done in PR Backlog Oct 30, 2026

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Size new arena chunks from the previous chunk's capacity - #65031

Size new arena chunks from the previous chunk's capacity#65031
Jake Bailey (jakebailey) merged 3 commits into
microsoft:mainfrom
jakebailey:perf/arena-growth

Jake Bailey (jakebailey) commented Oct 29, 2026 •

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Jake Bailey (jakebailey) commented Oct 29, 2026 •

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