AOT code size analysis

The Dart VM's AOT compiler has support for emitting binary size information for all the code that gets generated. This information can then be visualized.

Telling the AOT compiler to generate binary size information

Our AOT compiler accepts an extra --print-instructions-sizes-to=sizes.json flag. If supplied the AOT compiler will emit binary size information for all generated functions to sizes.json.

This flag can be passed to gen_snapshot directly, or to the various wrapper scripts (e.g. pkg/vm/tool/precompiler2):

% tools/build.py -mrelease -ax64 runtime_kernel dart_precompiled_runtime
% pkg/vm/tool/precompiler2 --print-instructions-sizes-to=hello_sizes.json hello.dart hello.dart.aot

In Flutter, pass this argument to flutter build:

% flutter build aot --release --extra-gen-snapshot-options=--print-instructions-sizes-to=hello_sizes.json

Visualizing the information from the binary size json file

To visualize the information emitted by the AOT compiler one can use our binary size analysis tool:

% dart pkg/vm/bin/run_binary_size_analysis.dart hello_sizes.json hello_sizes
Generated file:///.../sdk/hello_sizes/index.html
% chrome hello_sizes/index.html

Comparing the sizes of two AOT builds

To visualize the differences between two AOT builds one can use our binary size comparison tool:

% dart pkg/vm/bin/compare_sizes.dart app-sizes--before.json app-sizes--after.json

+---------+--------+--------------+
| Library | Method | Diff (Bytes) |
+---------+--------+--------------+
...

Object-level data

gen_snapshot also accepts an extra --write-v8-snapshot-profile-to=hello.heapsnapshot flag. If supplied the AOT compiler will emit snapshot size information for all objects in the snapshot to hello.heapsnapshot in V8 snapshot format.

This flag can be passed to gen_snapshot directly, or to the various wrapper scripts (e.g. pkg/vm/tool/precompiler2):

% tools/build.py -mrelease -ax64 runtime_kernel dart_precompiled_runtime
% pkg/vm/tool/precompiler2 --write-v8-snapshot-profile-to=hello.heapsnapshot hello.dart hello.dart.aot

In Flutter, pass this argument to flutter build:

% flutter build aot --release --extra-gen-snapshot-options=--write-v8-snapshot-profile-to=hello.heapsnapshot

This output can be visualized by loading it in the “Memory” tab in Chrome's developer tools, or by loading it into Graph Explorer.