Consider storing R2R images in a separate file

[jonathanpeppers]

Android framework version

net11.0-android (Preview)

Affected platform version

.NET 11

Description

Context:

• Allow R2R for images provided via external_assembly_probe runtime#112934
• Allow using flat layout for component assemblies of composite R2R runtime#115631

When we added ReadyToRun support for Android (CoreCLR), R2R images remain inside .dll files. There was a step where this data was copied to memory and loaded, which may not be optimal.

For CoreCLR (R2R) support on iOS, there was "Platform-Native R2R Envelope" related work — tracked by issue dotnet/runtime#120065. On iOS, PE-based R2R can't work due to page protection restrictions (no JIT entitlement), so R2R native code is emitted into a separate Mach-O (.dylib) file that the OS can load natively.

Key PRs:

PR	Title	Author	Date
#120584	Add initial doc for R2R platform-native image (Mach-O, composite only)	elinor-fung	Oct 2025
#120777	Extract out minimal concerns from PEImageLayout for R2R images	jkoritzinsky	Nov 2025
#121186	Add support for emitting Mach-O R2R images	jkoritzinsky	Nov 2025
#121363	Add support for host callback to get information for platform-native composite R2R images	elinor-fung	Nov 2025
#122511	Reduce the number of sections in R2R PE files	jkoritzinsky	Jan 2026

The integration on the macios side was dotnet/macios#24327, which got a MAUI iOS app running with R2R.

The design doc (readytorun-platform-native-envelope.md) lays out the approach: crossgen2 emits the composite R2R image as a native Mach-O .dylib, and a new get_native_code_data host callback tells the runtime where to find it. This lets the OS dlopen the native code directly rather than needing to copy/map PE data.

Question

Is there anything we can use from this work to improve Android?

Steps to Reproduce

1. dotnet new android
2. dotnet build -c Release uses ReadyToRun by default