Allow using flat layout for component assemblies of composite R2R

[elinor-fung]

Allow using flat layout for component assemblies of composite R2R. The component assemblies don't have native code, so we can just use their flat layout without loading / mapping them to virtual addresses. For scenarios where we can't just map from a file for the load - for example, external data (Android), this avoids copying all the data of component assemblies.

• Skip emitting empty sections in R2R images
  • When we had no data for a section, we'd emit a single-byte section. Avoid including the section altogether
• Use read-only section for R2R headers
  • This was using writeable data on non-Windows. We do the mapping ourselves and don't need this to be writeable on any platform. The way we emit R2R PE images, the read-only data goes into .text, so this should not incur the penalty of an extra section.
• Relax checks for component assembly initialization to stop requiring a loaded layout
• Handle getting the owner composite name for a flat layout of a component assembly of composite R2R

[elinor-fung]

/azp run runtime-coreclr crossgen2 outerloop

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[Copilot]

Pull Request Overview

This PR enables using a flat layout for component assemblies in composite ReadyToRun images by avoiding unnecessary mapping for assemblies without native code, while also improving section handling. Key changes include:

• Changing the logic for resolving the owner composite executable name in flat layouts.
• Adjusting the relocation and mapping behavior based on whether the image is a component assembly.
• Refactoring section serialization in the R2RPEBuilder to use a new SerializedSectionData structure and streamline section output.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/coreclr/vm/readytoruninfo.cpp	Revised owner composite name resolution and modified relocation checks for component assemblies.
src/coreclr/vm/peimagelayout.cpp	Updated condition to load images into virtual addresses only when needed.
src/coreclr/tools/aot/ILCompiler.ReadyToRun/ObjectWriter/SectionBuilder.cs	Added helper to check if a section has content.
src/coreclr/tools/aot/ILCompiler.ReadyToRun/ObjectWriter/R2RPEBuilder.cs	Refactored section data handling and section serialization logic; made the class sealed.
src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/HeaderNode.cs	Updated section mapping for R2R headers to use read‑only sections on non‑Windows.

[Copilot]

The change replaces a runtime fallback that ensured a non-empty BlobBuilder with a Debug.Assert, which only triggers in debug builds. In a release build, if _sectionBuilder.SerializeSection unexpectedly returns null or an empty blob, this could lead to invalid output; consider explicitly handling or guarding against null/empty extraData in production.

Suggested change

	Debug.Assert(extraData != null);
	Debug.Assert(extraData != null);
	if (extraData == null || extraData.Count == 0)
	{
	throw new InvalidOperationException($"SerializeSection returned a null or empty BlobBuilder for section '{name}'.");
	}

[elinor-fung]

crossgen2-comparison failure exists in main - filed #115661

[elinor-fung]

For the AndroidSampleApp with composite R2R, this cuts the amount we copy/use for its images by ~3.4MB (from ~8.3MB copied to ~4.9MB). I'm testing on emulator and did not see a difference in measured startup time.

[jkotas]

The component assemblies don't have native code, so we can just use their flat layout without loading / mapping them to virtual addresses.

Do we still generate extra index sections like ReadyToRunSectionType::AvailableTypes for the component assemblies and are we able to use these extra index sections at runtime? (I am not able to tell by just looking at the code.)

[elinor-fung]

My understanding is that OwnerCompositeExecutable should be the one and only section for component assemblies. That is what I saw when I was looking at dumped out R2R info for component assemblies and that is what we call out in the R2R format doc for the OwnerCompositeExecutable section. I also didn't see any other section types when I was looking at RewriteComponentFile.