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.

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)

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

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