[Android] Compress debug sections in coreclr_static.a with -gz=zlib

[davidnguyen-tech]

Description

Reduces libcoreclr_static.a size on Android by compressing DWARF debug sections using -gz=zlib. This adds a single compiler flag for Android builds in eng/native/configurecompiler.cmake.

Results (android-arm64, Release):

Metric	Before	After
libcoreclr_static.a size	702 MB	359 MB
Reduction	—	49%

Debug info is fully preserved (just compressed). The Android NDK linker (lld) and debuggers handle compressed debug sections natively.

Validation

• Clean build: ./build.sh clr.runtime+clr.alljits+host -os android -arch arm64 -c Release — 0 warnings, 0 errors
• Link test: singlefilehost links successfully against the compressed archive
• Also tested -gz=zstd (357 MB) but the gain over zlib is negligible and zlib has broader toolchain compatibility

Consumer compatibility

All consumers of libcoreclr_static.a use the Android NDK toolchain (Clang 18 / lld 18 in NDK r27c), which natively supports compressed debug sections:

Consumer	Toolchain	Verified?
singlefilehost (CoreCLR build)	NDK lld via CMake	✅ Built and linked
AndroidAppBuilder (app packaging)	NDK android.toolchain.cmake	✅ Same NDK toolchain
Functional tests (Android.Device_Emulator.JIT.Static.Test)	Via AndroidAppBuilder	✅ Same pipeline

The NDK linker decompresses SHF_COMPRESSED debug sections transparently during linking. No changes needed on the consumer side.

Context

Analysis of the archive shows debug info accounts for 85–94% of each object file. The vm directory contributes ~412 MB (62%) and jit ~170 MB (26%) of the total.

Further size reductions (e.g., stripping debug info for the shipped NuGet package, or reducing C++ template bloat) are possible but require broader discussion on diagnosability tradeoffs and are out of scope for this PR.

Contributes to #116531

[Copilot]

Pull request overview

This PR reduces the size of the Android libcoreclr_static.a archive by enabling compression of DWARF debug sections during compilation via Clang’s -gz=zlib flag, applied in the shared native compiler configuration.

Changes:

• Add -gz=zlib to Clang compile options when targeting Android to compress debug sections.

[Copilot]

-gz=zlib is added unconditionally for Android/Clang. Since this is a non-trivial compiler flag (and Android builds can use different NDK/Clang versions), please gate it with a compiler-flag support check (e.g., check_c_compiler_flag/check_cxx_compiler_flag) before adding it. That will prevent hard build breaks on toolchains that don’t recognize -gz or the =zlib value.

Suggested change

	add_compile_options(-gz=zlib)
	check_c_compiler_flag("-gz=zlib" C_COMPILER_SUPPORTS_GZ_ZLIB)
	check_cxx_compiler_flag("-gz=zlib" CXX_COMPILER_SUPPORTS_GZ_ZLIB)
	if (C_COMPILER_SUPPORTS_GZ_ZLIB AND CXX_COMPILER_SUPPORTS_GZ_ZLIB)
	add_compile_options(-gz=zlib)
	endif()

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[kotlarmilos]

This looks good assuming that no further actions are needed to read the compressed format. Do we need to preserve runtime debug symbols on Android?

[davidnguyen-tech]

This looks good assuming that no further actions are needed to read the compressed format. Do we need to preserve runtime debug symbols on Android?

There shouldn't be a need for further actions to read the compressed format - added a Consumer compatibility section to the PR description.

If I understand the discussion in #116531 correctly, we don't need to preserve them technically - it's more of a design decision. Let's discuss that in the issue.

[kotlarmilos]

LGTM, thanks! @jkoritzinsky should sign off.