flutter engine binaries for armv7, aarch64, x64

This repo contains flutter engine binaries (in the https://github.com/ardera/flutter-engine-binaries-for-arm filesystem layout) for armv7 and aarch64. The binaries come in 2 variants: generic, and tuned for the Pi 4 CPU.

📦 Downloads

See Releases.

NOTE: Some SDK versions, e.g. 3.19.1 are missing. In most cases, that means that SDK uses the same engine as the older release, so 3.19.0 in this case. The SDK semantic versions are only provided because they're a bit nicer to read than the raw engine commit hashes, but the engine commits are still the "single source of truth"

🛠️ Build Config and Compiler Invocation

Build Config

The engine build is configured with: ¹

$ ./src/flutter/tools/gn \
  --runtime-mode <debug / profile / release> \
  [--unoptimized]
  --target-os linux \
  --linux-cpu <arm / arm64 / x64> \
  [--arm-float-abi hard] \
  --target-dir build \
  --embedder-for-target \
  --disable-desktop-embeddings \
  --no-build-glfw-shell \
  --no-build-embedder-examples \
  --no-goma

After that, the following args are added to the args.gn file for armv7/aarch64 without any CPU-specific tuning:

arm_cpu = "generic"
arm_tune = "generic"

When tuning for pi 3:

arm_cpu = "cortex-a53+nocrypto"
arm_tune = "cortex-a53"

When tuning for pi 4:

arm_cpu = "cortex-a72+nocrypto"
arm_tune = "cortex-a72"

For both armv7 and aarch64, the engine is built against the sysroot provided by the engine build scripts, which is some debian sid sysroot from 2020. (See https://github.com/flutter/buildroot/blob/master/build/linux/sysroot_scripts/install-sysroot.py)

Compiler Invocation

This will result in the clang compiler being invoked with the following args:

artifact	compiler arguments
armv7-generic	`--target=armv7-linux-gnueabihf -mcpu=generic -mtune=generic`
pi4	`--target=armv7-linux-gnueabihf -mcpu=cortex-a72+nocrypto -mtune=cortex-a72`²
pi3	`--target=armv7-linux-gnueabihf -mcpu=cortex-a53+nocrypto -mtune=cortex-a53`³
aarch64-generic	`--target=aarch64-linux-gnu -mcpu=generic -mtune=generic`
pi4-64	`--target=aarch64-linux-gnu -mcpu=cortex-a72+nocrypto -mtune=cortex-a72`²
pi3-64	`--target=aarch64-linux-gnu -mcpu=cortex-a53+nocrypto -mtune=cortex-a53`³
x64-generic	`--target=x86_64-unknown-linux-gnu -mcpu=generic -mtune=generic`

Debug Symbols

Some modifications are made to the engine build scripts so it's always built with -ggdb -fdebug-default-version=4. The debug symbols are then split into a separate file using:

$ objcopy --only-keep-debug libflutter_engine.so libflutter_engine.so.dbgsyms
$ objcopy --strip-debug libflutter_engine.so
$ objcopy --add-gnu-debuglink=libflutter_engine.{debug/profile/release/debug_unopt}.dbgsyms libflutter_engine.so

That means you can just later download the debug symbols when you need them and step through the engine source code.

However, the resulting libflutter_engine.so is ~4MBs larger than one that has all (not only debug symbols) stripped. So, if you want to save a few more megabytes you can strip them using:

objcopy --strip-unneeded libflutter_engine.so

--arm-float-abi hard is only specified when building for armv7, --unoptimized is only specified for unoptimized debug builds. ↩
The CPU of the Raspberry Pi 4 is a Cortex-A72. +nocrypto is specified for -mcpu because the A72 in the Pi 4 is (apparently) the only A72 in the world that doesn't support cryptography instructions: https://github.com/ardera/flutter-ci/issues/3#issuecomment-1272330857 ↩ ↩²
Pi 3 doesn't support cryptography extensions either. ↩ ↩²

ardera/flutter-ci