Declaration emit spends significant CPU in getAliasForSymbolInContainer / getAlternativeContainingModules

[Zzzen]

Summary

While running tsgo -b -f on a large private project, declaration emit spends a significant amount of CPU time in symbol accessibility logic, especially getAlternativeContainingModules and getAliasForSymbolInContainer.

The project source cannot be shared, but I can provide sanitized pprof output and/or raw Go pprof profiles after checking that they do not contain private TypeScript source contents.

Environment

• typescript-go commit: 804eb7a1a1a9b0722af207412921cc025dfbbd5a
• tsgo version: 7.0.0-dev
• Go version: go1.26.0 linux/amd64
• OS: Linux amd64
• Command shape: ./built/local/tsgo -b <private project> -f

Observed performance

Using /usr/bin/time -v:

Elapsed wall time: 43.40s
User time: 196.33s
System time: 10.24s
CPU: 475%
Maximum resident set size: 4,264,424 KB

CPU profile highlights

Profile command:

./built/local/tsgo -b <private project> -f --pprofDir <profile-dir>
go tool pprof -top ./built/local/tsgo <profile-dir>/<pid>-cpuprofile.pb.gz
go tool pprof -top -cum ./built/local/tsgo <profile-dir>/<pid>-cpuprofile.pb.gz

Relevant excerpt:

Duration: 43.09s
Total samples: 208.55s

flat      flat%    cum       cum%
3.00s     1.44%    42.62s    20.44%  github.com/microsoft/typescript-go/internal/checker.(*Checker).getAlternativeContainingModules
1.36s     0.65%    37.27s    17.87%  github.com/microsoft/typescript-go/internal/checker.(*Checker).getAliasForSymbolInContainer
0.15s     0.07%    21.60s    10.36%  github.com/microsoft/typescript-go/internal/checker.(*Checker).getSymbolIfSameReference
0.26s     0.12%    12.96s     6.21%  github.com/microsoft/typescript-go/internal/checker.(*Checker).resolveSymbolEx
7.99s     3.83%     7.99s     3.83%  github.com/microsoft/typescript-go/internal/ast.IsNonLocalAlias

The cumulative profile suggests declaration emit repeatedly checks whether a symbol is accessible through alternative containing modules, and repeatedly scans exports in getAliasForSymbolInContainer.

Allocation profile highlights

Total allocated: ~19.68GB
Maximum RSS from /usr/bin/time: ~4.26GB

Local experiment

I tested a conservative local prototype that caches, per checker/container, the export candidates grouped by the same resolved/merged target symbol. It does not cache the final selected alias directly; it still sorts only the candidates for the currently queried target, to preserve the old selection behavior as closely as possible.

On the same private project:

Before: 43.40s wall, 4,264,424 KB max RSS
After:  37.64s wall, 4,210,296 KB max RSS

That is about a 13% wall time improvement on this workload.

Relevant tests passed locally:

GOCACHE=/tmp/tsgo-go-cache go test ./internal/checker

Question

Would a per-container cache for alias candidates in getAliasForSymbolInContainer, or another cache around this symbol accessibility path, be acceptable? I am happy to share sanitized pprof text output, and possibly raw .pb.gz pprof files if that is useful.

[jakebailey]

The pprof files produced by the published package won't contain anything sensitive; you should be able to share them as-is.

(The only info they'd contain is if you built us from source and didn't do -trimpath and then contain paths to your tsgo checkout; still nothing related to your project.)

[Zzzen]

Hope this helps, and let me know if the matching memory profile would be useful too.

3401843-cpuprofile.pb.gz