[release/9.0] [mono] Make the compressed interface bitmap feature usable #120154
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…t#119694) * [mono][metadata] Enable compressed interface bitmap by default * Fix build on windows * Fix publishing of compressed bitmap Before this commit we were doing changes on the published data which is problematic for multithreaded environments. We ensure the bitmap is fully processed before publishing it so that we can rely on the data dependency memory ordering constraint for correctness.
…tmap feature (dotnet#119881) * Add option to configure use of compressed interface bitmaps This optimization is disabled by default. Enable it with `MONO_COMPRESSED_INTERFACE_BITMAP=1` * Add support for detecting mismatch with aot images
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Pull Request Overview
This PR enables the compressed interface bitmap feature in Mono by replacing compile-time #ifdef guards with runtime checks based on the mono_opt_compressed_interface_bitmap option. The feature compresses the bitmap that stores implemented interfaces by classes, providing significant memory improvements for applications with many types.
Key changes:
- Replaced compile-time feature guards with runtime option checks
- Added environment variable support for enabling the feature via
MONO_COMPRESSED_INTERFACE_BITMAP=1 - Updated AOT compilation to handle the compressed bitmap flag compatibility
Reviewed Changes
Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.
Show a summary per file
| File | Description |
|---|---|
| src/mono/mono/utils/options-def.h | Added runtime option definition for compressed interface bitmap |
| src/mono/mono/mini/type-checking.c | Replaced compile-time guards with runtime checks for interface bitmap emission |
| src/mono/mono/mini/mini-runtime.c | Made icall registration conditional on runtime option |
| src/mono/mono/mini/aot-runtime.h | Added new AOT file flag and incremented version |
| src/mono/mono/mini/aot-runtime.c | Added compatibility check for compressed bitmap feature |
| src/mono/mono/mini/aot-compiler.c | Set AOT flag when compressed bitmap option is enabled |
| src/mono/mono/metadata/metadata.c | Added environment variable parsing for feature enablement |
| src/mono/mono/metadata/marshal.c | Replaced compile-time conditional with runtime check |
| src/mono/mono/metadata/jit-icall-reg.h | Renamed icall function from generic to compressed-specific |
| src/mono/mono/metadata/class-setup-vtable.c | Updated bitmap allocation logic for runtime option |
| src/mono/mono/metadata/class-internals.h | Updated macros and function declarations for runtime support |
| src/mono/mono/metadata/class-init.c | Added explicit casts and renamed function for compressed bitmap handling |
BrzVlad
added
Servicing-consider
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/ba-g wasm failure unrelated |
steveisok
approved these changes
Oct 1, 2025
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When this option is enabled, the env var
MONO_COMPRESSED_INTERFACE_BITMAP=1is passed, the bitmap that stores the set of implemented interfaces by a class is saved in a compressed format. This has significant memory improvement on applications using a lot of types.Customer Impact
This issue was discovered on a large customer application. The memory usage of these bitmaps was so big in this scenario that the application got killed by iOS.
Regression
The memory increase above the tipping point was caused by more types used after transitioning from xamarin to maui. However, this is not strictly a regression.
Testing
This was tested on the customer application. It was also tested with our own set of tests, it was enabled for a few weeks on main.
Risk
Low. This feature is disabled by default so there shouldn't be any impact on existing applications. This PR adds the possibility to use this optimization, which was slightly bit-rotten and behind a compile time define before.