Add compilation caching mode to VBCSCompiler server by Copilot · Pull Request #82881 · dotnet/roslyn

Copilot · 2026-03-20T22:24:24Z

Add an opt-in file-system compilation cache to the VBCSCompiler server. When ROSLYN_CACHE_PATH is set (or the use-global-cache feature flag is passed via MSBuild), compilations with identical deterministic keys restore cached outputs instead of re-running CompileAndEmit.

No cache eviction is implemented yet. Side effects like compiler warnings and /reportanalyzer output are not cached.

Co-authored-by: jaredpar <146967+jaredpar@users.noreply.github.com> Agent-Logs-Url: https://github.com/dotnet/roslyn/sessions/9aafb5ec-14d4-43bf-949b-48a8f6f4a409

…ve exception handling Co-authored-by: jaredpar <146967+jaredpar@users.noreply.github.com> Agent-Logs-Url: https://github.com/dotnet/roslyn/sessions/9aafb5ec-14d4-43bf-949b-48a8f6f4a409

src/Compilers/Core/Portable/CommandLine/CommonCompiler.cs

…lation twice Co-authored-by: jaredpar <146967+jaredpar@users.noreply.github.com> Agent-Logs-Url: https://github.com/dotnet/roslyn/sessions/f91b92a0-0da1-4c9e-ad90-69cf4e004cc5

…heUtilities Co-authored-by: jaredpar <146967+jaredpar@users.noreply.github.com> Agent-Logs-Url: https://github.com/dotnet/roslyn/sessions/f91b92a0-0da1-4c9e-ad90-69cf4e004cc5

…ML docs) Co-authored-by: jaredpar <146967+jaredpar@users.noreply.github.com> Agent-Logs-Url: https://github.com/dotnet/roslyn/sessions/5b7ed4ce-497c-4362-b550-b9de0fecbb2d

333fred

Got a couple of files in, haven't done a full review yet, but need to step away for now so publishing what I've said so far.

src/Compilers/Core/Portable/CommandLine/CommonCompiler.cs

src/Compilers/Server/VBCSCompiler/CSharpCompilerServer.cs

src/Compilers/Server/VBCSCompiler/CompilationCache.cs

jjonescz#35

src/Compilers/Core/MSBuildTask/ManagedCompiler.cs

docs/compilers/Design/compiler-output-cache-experiment.md

src/Compilers/Server/VBCSCompiler/CompilationCache.cs

jaredpar

Feel like the passing of environment variables between the processes is adding a bit of complexity we don't need. Wanted to make sure we get to an agreement on that vs. just munging the flag we pass through.

src/Compilers/Core/MSBuildTask/ManagedCompiler.cs

src/Compilers/Server/VBCSCompiler/CompilationCache.cs

RikkiGibson

Done review pass

src/Compilers/Core/Portable/CommandLine/CommonCompiler.cs

src/Compilers/Server/VBCSCompiler/CompilationCache.cs

src/Compilers/Server/VBCSCompiler/CompilerRequestHandler.cs

src/Compilers/Core/MSBuildTaskTests/ManagedCompilerGlobalCacheTests.cs

Move ROSLYN_CACHE_PATH handling to request normalization so cache enablement flows through /features:use-global-cache instead of extra server environment propagation. Update compiler server cache creation to read parsed feature flags, refresh the design doc, and add coverage for existing /features switches in both C# and VB. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

jaredpar

Had a few comments but overall looks good

src/Compilers/Core/Portable/InternalUtilities/CompilerOptionParseUtilities.cs

src/Compilers/Server/VBCSCompiler/CompilationCache.cs

src/Compilers/Core/Portable/CommandLine/CommonCompiler.cs

…unix permissions are good.

src/Compilers/Core/CodeAnalysisTest/FileSystem/PathUtilitiesTests.cs

src/Compilers/Core/Portable/FileSystem/PathUtilities.cs

src/Compilers/Server/VBCSCompiler/CompilationCache.cs

RikkiGibson · 2026-04-03T19:23:50Z

src/Compilers/Server/VBCSCompilerTests/CompilationCacheBehaviorTests.cs

+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task CacheHit_OmitsWarningDiagnosticsFromOutput(bool visualBasic)


At first I thought: no big deal, msbuild already does this for incremental builds, and then thought..

Yeah, if I pass /t:Rebuild, and I do not see any warnings locally, but warnings appear in CI, I am going to feel quite confused/inconvenienced.

This isn't blocking, but, it might be worth looking at serializing the diagnostics for a compilation and including that in a cache also.

This is also documented in the feature doc as a known limitation. I think it's ok to proceed for now.

Copilot AI and others added 2 commits March 20, 2026 22:20

Add compilation caching to VBCSCompiler via ROSLYN_CACHE_PATH

e4f3b7a

Co-authored-by: jaredpar <146967+jaredpar@users.noreply.github.com> Agent-Logs-Url: https://github.com/dotnet/roslyn/sessions/9aafb5ec-14d4-43bf-949b-48a8f6f4a409

Address code review feedback: fix diff line-ending handling and impro…

38bd47f

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Copilot AI assigned Copilot and jaredpar Mar 20, 2026

Copilot created this pull request from a session on behalf of jaredpar March 20, 2026 22:25 View session

Copilot AI requested a review from jaredpar March 20, 2026 22:25

Copilot finished work on behalf of jaredpar March 20, 2026 22:25

github-actions bot added the Area-Compilers label Mar 20, 2026

jaredpar reviewed Mar 20, 2026

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src/Compilers/Core/Portable/CommandLine/CommonCompiler.cs Outdated Show resolved Hide resolved

Copilot started work on behalf of jaredpar March 20, 2026 22:58 View session

Copilot AI and others added 2 commits March 20, 2026 23:19

Move cache check into RunCore via virtual hooks; avoid creating Compi…

b4b9864

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Add exception handling for GetDeterministicKey call in CompilationCac…

482c6ca

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Copilot AI requested a review from jaredpar March 20, 2026 23:27

Copilot finished work on behalf of jaredpar March 20, 2026 23:27

Copilot started work on behalf of jaredpar March 22, 2026 21:25 View session

Store and restore all compiler outputs in cache (PDB, ref assembly, X…

9ee06da

…ML docs) Co-authored-by: jaredpar <146967+jaredpar@users.noreply.github.com> Agent-Logs-Url: https://github.com/dotnet/roslyn/sessions/5b7ed4ce-497c-4362-b550-b9de0fecbb2d

Copilot finished work on behalf of jaredpar March 22, 2026 21:34

333fred reviewed Mar 24, 2026

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src/Compilers/Server/VBCSCompiler/CSharpCompilerServer.cs Outdated Show resolved Hide resolved

jjonescz added 2 commits March 25, 2026 14:48

Fix launch commands

815f136

Allow using global cache via a feature flag

7abe99e

333fred reviewed Mar 25, 2026

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src/Compilers/Server/VBCSCompiler/CompilationCache.cs Show resolved Hide resolved

src/Compilers/Server/VBCSCompiler/CompilationCache.cs Outdated Show resolved Hide resolved

src/Compilers/Server/VBCSCompiler/CompilationCache.cs Outdated Show resolved Hide resolved

Update caching mechanism to include race protection

7064c6e

jjonescz#35

jjonescz reviewed Mar 26, 2026

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src/Compilers/Core/MSBuildTask/ManagedCompiler.cs Outdated Show resolved Hide resolved

jjonescz and others added 7 commits March 26, 2026 11:38

Allow passing value to the feature flag

cc91672

Update caching mechanism to include race protection.

61ffdcb

Treat missing output files as cache miss

86f7026

Avoid cache fields

fec1323

Log client-side message for cache hits

f28907a

Handle additionalEnvironmentVariables in fast check

c39034e

Avoid partially writing outputs on cache miss

258508c

jjonescz approved these changes Mar 30, 2026

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333fred marked this pull request as ready for review March 30, 2026 16:31

333fred requested a review from a team as a code owner March 30, 2026 16:31

jaredpar requested changes Mar 30, 2026

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Copilot started work on behalf of jaredpar March 30, 2026 18:13 View session

Copilot stopped work on behalf of jaredpar due to an error March 30, 2026 18:13
The session was cancelled by the user.

Copilot AI requested a review from jaredpar March 30, 2026 18:13

RikkiGibson reviewed Mar 30, 2026

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jaredpar approved these changes Mar 31, 2026

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jjonescz reviewed Mar 31, 2026

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src/Compilers/Core/Portable/CommandLine/CommonCompiler.cs Outdated Show resolved Hide resolved

333fred added 4 commits March 31, 2026 10:12

Remove unnecessary check, quote cache path

2ad3c96

Remove unnecessary remarks.

0e602a1

Move to GetTempCachePath helper with logic from the SDK for ensuring …

1305d46

…unix permissions are good.

Re-enable .NET Framework

f9e19bb

jjonescz approved these changes Apr 1, 2026

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jjonescz reviewed Apr 1, 2026

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src/Compilers/Core/CodeAnalysisTest/FileSystem/PathUtilitiesTests.cs Outdated Show resolved Hide resolved

333fred reviewed Apr 1, 2026

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src/Compilers/Core/Portable/FileSystem/PathUtilities.cs Outdated Show resolved Hide resolved

Remove consideration of Path.GetTempPath().

9ba703c

jjonescz approved these changes Apr 2, 2026

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RikkiGibson self-requested a review April 2, 2026 17:14

RikkiGibson approved these changes Apr 3, 2026

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Mark property required

f955706

333fred enabled auto-merge (squash) April 3, 2026 20:48

333fred merged commit ae96a65 into main Apr 3, 2026
23 of 24 checks passed

dotnet-policy-service bot added this to the Next milestone Apr 3, 2026

333fred deleted the copilot/add-caching-mode-to-compiler-server branch April 3, 2026 22:32

dotnet-maestro bot mentioned this pull request Apr 8, 2026

[release/11.0.1xx-preview3] Source code updates from dotnet/roslyn dotnet/dotnet#5858

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dotnet-bot mentioned this pull request Apr 8, 2026

[Automated] PRs inserted in VS build main-11707.340 #83096

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[main] Source code updates from dotnet/roslyn dotnet/dotnet#5962

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Copilot AI commented Mar 20, 2026 •

edited by jjonescz

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