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build(android): debug APKs ship release-profile rust

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Cargo's debug profile is not "slower" for this library — it is unusable, and
it invalidated every on-device performance test to date: RustCrypto's AES-GCM
compiles to generic-array iterator closures with per-byte precondition checks
instead of ARMv8 hardware AES. Profiled live on a phone (simpleperf, 62k
samples): ~800 µs of user CPU per 1.4 KB packet — the receive pump pinned
above a full core yet only draining ~1,400 pkt/s of a 1,775 pkt/s (20 Mbps)
stream, 2.3 MB standing in the kernel socket buffer, the latency-bound flush
firing every 2 s forever. With release rust in the same debug APK: pump at
~12 % of a core, socket queue zero, no flushes, 2800x1260@120 streaming clean.

preDebugBuild now depends on cargoNdkRelease; `-PrustDebug` opts back into a
debug-profile native build for sessions that actually step through Rust.
Kotlin debuggability is unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
Enrico Bühler
2026-07-07 08:07:42 +02:00
parent 08ab2b6bee
commit 17685ff73b
1 changed files with 11 additions and 1 deletions
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clients/android/kit/build.gradle.kts
+11 -1
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@@ -110,8 +110,18 @@ afterEvaluate {
// screenshot unit tests render Compose on the JVM and never load libpunktfunk_android.so), so
// CI/local screenshot runs don't need the Rust toolchain or NDK. The native build stays wired
// for every normal APK/AAR build.
//
// DEBUG APKs SHIP RELEASE RUST. Cargo's debug profile is not "a bit slower" for this library —
// it is unusable: the AES-GCM data-plane decrypt runs through generic-array iterator closures
// with per-byte UB checks instead of ARMv8 hardware AES. Profiled live on a phone (simpleperf):
// ~800 µs of user CPU per 1.4 KB packet, the receive pump pinned over a full core yet unable to
// drain a 20 Mbps stream — every debug-APK on-device test was silently benchmarking unoptimized
// crypto, not the streaming pipeline. Kotlin debuggability is untouched (the APK is still a
// debug build); only the cargo profile changes. `-PrustDebug` restores a debug-profile native
// build for the rare session that actually steps through Rust.
if (!project.hasProperty("skipRustBuild")) {
tasks.named("preDebugBuild").configure { dependsOn(cargoNdkDebug) }
val debugRust = if (project.hasProperty("rustDebug")) cargoNdkDebug else cargoNdkRelease
tasks.named("preDebugBuild").configure { dependsOn(debugRust) }
tasks.named("preReleaseBuild").configure { dependsOn(cargoNdkRelease) }
}
}