Add experimental Kotlin incremental compilation #1316
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erikkerber
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restingbull
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May 29, 2025
| errStream: java.io.PrintStream, | ||
| vararg args: String, | ||
| ): ExitCode { | ||
| System.setProperty("zip.handler.uses.crc.instead.of.timestamp", "true") |
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Do you need to set this, or is this pulled from the K2JVM impl?
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Pulled from BazelK2JVMCompiler. It hits the same internals as the existing implementation on the Kotlin side, and figured if it was still needed there, it may be needed here.
restingbull
reviewed
May 29, 2025
| // The incremental directory is roughly analogous to a Gradle transform cache directory. Must be unique per compilation target. | ||
| val incrementalDirectory = Paths.get("").toAbsolutePath().parent.resolve("_kotlin_incremental/$label/$incrementalId").toFile() | ||
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| val kotlinService = CompilationService.loadImplementation(this.javaClass.classLoader!!) |
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Is it possible/safe to keep the CompilationService implementation around?
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Do you mean simply not loading it every compile task? I'm sure that's safe, yeah.
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Note
This is presented as a POC, but nowhere near merge-ready. In its current state, it may compile your apps and you may see significant build time improvements.
This adds an experimental implementation of incremental compilation via the new "Build Tools API" (BTAPI). The BTAPI itself can replace the direct usage of
K2JVMCompiler, but is also required to leverage incremental compilation.Initial Design
In short:
BazelK2JVMCompilernamedBuildToolsApiCompiler. This invokes the BTAPI.<output base>/execroot/_kotlin_incremental/)build_tools_apito enable building with the new BTAPI (only)incremental_compilationto then add incremental compilationThe rest is fairly trivial usage of the BTAPI, a little of which requires some tinkering and tribal knowledge to know how exactly it is supposed to be used. I did my best to document my knowledge, with brevity.
Performance
The main driver of performance gains are incremental builds that only build a subset of Kotlin files in a target. The incremental data passed in to subsequent builds gives an additional level of work avoidance beyond the action-based caching in Bazel.
I hope to get more comprehensive numbers, but our main test case was a sample incremental build that was ~4x faster in Gradle due to only needing to recompile very small slices of a chain of Kotlin modules. With incremental, Bazel matches what we see with Gradle.
4x isn't a fixed number, it could be greater or smaller depending on how many Kotlin files are in how many dependent targets, and how many files can skip compilation based on the affected classpaths in a build.
TODO
CompilationTask. It works, but not idea.