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kotlinlang

Does anyone have any advice for how to reduce the size of my final JS output? I'm worried I might be writing my code in a way that accidentally prevents DCE from working without realizing it, for example.

(I'm writing both a C4W app and some libraries -- not sure if the way I'm writing the libraries could be improved)

A great place to start would be if I could get some sort of report about what code is being kept alive vs what's getting eliminated. I'm paranoid some stuff is sticking around that I don't expect.

I recall there was a compiler flag to report DCE for kjs. I tried to find it quickly, but it's inconvenient to do on the smartphone. I'll look for it later on a laptop.

No rush! Thanks! Now that I know there's a compiler flag, I can look into it too later.

The flag I recalled was <https://youtrack.jetbrains.com/issue/KT-45059/KJS-IR-Add-possibility-for-runtime-diagnostics-of-DCE-result|https://youtrack.jetbrains.com/issue/KT-45059/KJS-IR-Add-possibility-for-runtime-diagnostics-of-DCE-result>

But it's only for runtime diagnostics, not like a log of the DCE actions/decisions.

Perhaps another flag <https://github.com/JetBrains/kotlin/blob/master/compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JSCompilerArguments.kt#L199|https://github.com/JetBrains/kotlin/blob/master/compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JSCompilerArguments.kt#L199> could be helpful

There is a compiler option `-Xir-dce-print-reachability-info`
Then DCE will print to stdout what and why was marked as reachable.
```from.fqn -&gt; to.fqn // comment```


<@U0BMF6PDJ> <@U02AB5P2XU6> I struggle to make it working on a basic Compose for Web project. I configured:
```js(IR) {
	browser()
	binaries.executable()
	compilations.all {
		kotlinOptions {
			freeCompilerArgs += "-Xir-dce-print-reachability-info"
			freeCompilerArgs.forEach(::println)
		}
	}
}```
Then I run `./gradlew clean compileProductionExecutableKotlinJs` but I see no reachability info on the standard output. What did I miss?

(Ah glad to hear you're digging into this -- I'm still a bit backlogged and never got a chance)

AFAIR `compilations.all {`  inside `js`/`wasm` block affects only first fase of compilation  — klib generation.

Also, it turned out there is a bug so if you use `-Xir-dce-print-reachability-info` you also need restore `-Xir-dce` (cc <@UAG4QQZB6>)

Final snippet could look like:
```
tasks.withType&lt;org.jetbrains.kotlin.gradle.tasks.Kotlin2JsCompile&gt;().configureEach {
    kotlinOptions.freeCompilerArgs += listOf(
        "-Xir-dce", 
        "-Xir-dce-print-reachability-info"
    )
}```


you find printed result in `kotlin-daemon.*.log` inside your `$TMPDIR`

alternatively you can pass  `-Pkotlin.compiler.execution.strategy=in-process` to the gradle so you will get compiler stdout right into your terminal

Tested, it works as expected with this configuration.