[NativeAOT] Stop linking and shipping libc++ and libunwind

[simonrozsival]

Closes #12139
Closes #12146

Removes libc++_static.a, libc++abi.a and libunwind.a from the NativeAOT link. This is the payoff for the local-string removal work in the rest of this stack: with strings.hh, dynamic_local_string and static_local_string gone, nothing in libnaot-android uses libc++ any more.

Who was actually using the C++ runtime?

Linking with libc++ removed leaves exactly six undefined symbols. Only one of them came from our code:

Symbol	Referenced from
operator new[](size_t)	android-system-shared.cc — ours
operator delete(void*)	gcenv.ee.cpp, UnixNativeCodeManager.cpp
operator delete[](void*)	gcenv.ee.cpp, interoplibinterface_java.cpp
operator new(size_t, nothrow_t const&)	gcenv.ee.cpp, TypeManager.cpp
operator new[](size_t, nothrow_t const&)	gcenv.ee.cpp, RhConfig.cpp
std::nothrow	gcenv.ee.cpp, UnixNativeCodeManager.cpp

The five runtime-owned ones belong to the NativeAOT runtime from dotnet/runtime, and the ILC SDK already ships definitions for all of them in libstdc++compat.a. Our targets were unconditionally removing that archive with the comment "This library conflicts with static libc++" — which is only true while libc++ is linked. With libc++ gone there is no conflict, so we simply stop removing it.

The one symbol that was ours came from a dead code path in monodroid__system_property_get, which is now removed earlier in the stack by #12517 (the PR that makes that path unreachable in the first place).

The result is no shim code at all in this repo, and a link with zero undefined symbols.

Size impact

Default MAUI app (dotnet new maui), net11.0-android, android-arm64, Release, PublishAot=true. Both sides built clean from the same tree.

	before	after	delta
.so	25,229,320	25,029,728	−199,592 (−0.79%)
.so deflated in APK	9,829,143	9,761,291	−67,852 (−0.69%)
APK	14,903,624	14,833,992	−69,632 (−0.47%)

Commits

1. Stop linking libc++ and libunwind — the targets change.
2. Update stale libc++ linker comments.
3. Stop shipping libc++ archives in the runtime packs — see below.

Notes

• This PR depends on [Native] Restore indexed GC temporary peers #12509: before it, the GC bridge used a std::unordered_map, which drags in __next_prime and __libcpp_verbose_abort from libc++ internals. The rest of this stack is already rebased on top of it, so nothing further is needed.
• This only affects NativeAOT. The Mono and CoreCLR runtimes still link libc++, and NativeRuntimeComponents.cs (the unified-runtime archive list) is deliberately untouched.
• libunwind.a drops out with zero undefined symbols — the NativeAOT runtime never referenced it.

Testing

Built, installed and launched a default MAUI app on an API 36 arm64 emulator. Cold start with no crashes.

llvm-nm --undefined-only on the resulting libnaot-android.release-static-release.a reports no operator new/operator delete, __cxa_*, _Unwind_* or __libcpp_* references. The only remaining std:: symbols are string_view appearing in mangled names, which is header-only and carries no runtime dependency.

Not covered: only the arm64 + workload-linker path was exercised locally. The NDK linker path and the x64/arm ABIs rely on CI.

---

Also: stop shipping the archives in the runtime packs

Previously a separate PR stacked directly on this one; folded in here because "stop linking it" and "stop shipping it" are the same change to the reader, and reviewing them apart means reading the same targets twice.

The NativeAOT runtime packs still shipped libc++_static.a, libc++abi.a and libunwind.a even though, after the change above, nothing links them any more.

Why this needs a new item kind

_AndroidNdkRedistributable (in build-tools/scripts/Ndk.targets) tagged NDK files with just two kinds:

• System — libc.so, libdl.so, liblog.so, libm.so, libz.so — shipped to every runtime.
• Toolchain — crtbegin_so.o, crtend_so.o, libc++_static.a, libc++abi.a, libclang_rt.builtins-*.a, libunwind.a — shipped to CoreCLR and NativeAOT, since both do native linking.

NativeAOT still needs crtbegin_so.o, crtend_so.o and libclang_rt.builtins-*.a, so the Toolchain group cannot just be dropped for NativeAOT.

This adds a third kind, CplusPlus, for the three C++ archives, and ships it only for CoreCLR. Both packaging sites are updated:

• src/native/native.targets — the local bin/<Config>/lib/packs layout.
• build-tools/create-packs/Microsoft.Android.Runtime.proj — the shipped NuGet packs.

Size

Per ABI, removed from the NativeAOT runtime pack:

Archive	Size
libc++_static.a	15,182,348
libc++abi.a	3,125,348
libunwind.a	91,152
Total	18,398,848

Across android-arm, android-arm64 and android-x64 that is roughly 55 MB of pack content. This does not change application size — that is the linker change above — but it shrinks what users restore.

Testing

Deleted each pack directory and regenerated it via _CopyToPackDirs, rather than checking a pack that could still contain stale files.

NativeAOT (android-arm64) — the three archives are gone, and everything NativeAOT links is still present:

crtbegin_so.o  crtend_so.o  libc.so  libclang_rt.builtins-aarch64-android.a
libdl.so  liblog.so  libm.so  libz.so
libnaot-android.debug-static-debug.a  libnaot-android.debug.so
libnaot-android.release-static-release.a  libnaot-android.release.so
libxa-java-interop-release.a

CoreCLR (android-arm64) — all three are still shipped:

crtbegin_so.o  crtend_so.o  libarchive-dso-stub.so  libc.so
libc++_static.a  libc++abi.a  libclang_rt.builtins-aarch64-android.a
libdl.so  liblog.so  libm.so  libunwind.a  libz.so

Mono is unaffected — it only ever received the System kind.

[Copilot]

Pull request overview

This PR advances the NativeAOT size/startup work in #12139 by removing libc++/libunwind from the NativeAOT link, while also refactoring several native runtime paths to avoid local-string helpers and reduce incidental C++ runtime dependencies. It also updates test baselines for resulting native library/package size changes.

Changes:

• Update NativeAOT MSBuild targets to stop linking libc++_static.a, libc++abi.a, and libunwind.a, and to stop removing libstdc++compat.a.
• Refactor multiple native runtime components (Mono + CoreCLR hosts) to use explicit bounded buffers / snprintf / heap fallback helpers instead of local-string types.
• Update .apkdesc test resource baselines to reflect new libmonodroid.so and package sizes.

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Resources/Base/BuildReleaseArm64XFormsDotNet.CoreCLR.R8.apkdesc	Update expected APK contents/sizes after native size change.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Resources/Base/BuildReleaseArm64XFormsDotNet.CoreCLR.apkdesc	Update expected APK contents/sizes after native size change.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Resources/Base/BuildReleaseArm64SimpleDotNet.CoreCLR.R8.apkdesc	Update expected APK contents/sizes after native size change.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Resources/Base/BuildReleaseArm64SimpleDotNet.CoreCLR.apkdesc	Update expected APK contents/sizes after native size change.
src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.NativeAOT.targets	Stop linking libc++/libunwind for NativeAOT and keep libstdc++compat.a.
src/native/mono/runtime-base/util.hh	Add joined-path formatting/join helpers and shared helpers include.
src/native/mono/runtime-base/android-system.hh	Add string_view system property overload returning into caller buffer.
src/native/mono/runtime-base/android-system.cc	Implement new buffer-based system property retrieval fallback path.
src/native/mono/monodroid/monodroid-glue.cc	Replace local-string timing “more info” assembly strings with bounded formatting + heap fallback.
src/native/common/runtime-base/timing-internal.cc	Replace property parsing and timing log path composition to use char buffers + join helper.
src/native/common/include/runtime-base/timing-internal.hh	Replace dynamic_local_string formatting with snprintf-based message building and add_more_info overloads.
src/native/clr/runtime-base/util.cc	Replace dynamic_local_string directory creation with manual mutable buffer + heap fallback.
src/native/clr/runtime-base/logger.cc	Switch various path and category parsing to buffer-based helpers and string_view.
src/native/clr/runtime-base/android-system.cc	Replace local-string path building with join helpers; refactor DSO path formatting to explicit buffers.
src/native/clr/runtime-base/android-system-shared.cc	Change system property retrieval to direct buffer writes; remove small-buffer heap shim in __system_property_get wrapper.
src/native/clr/include/runtime-base/util.hh	Add join_paths / format_dso_name helpers; remove path-combine concepts/local-string overloads.
src/native/clr/include/runtime-base/logger.hh	Update set_category signature to accept std::string_view.
src/native/clr/include/runtime-base/android-system.hh	Introduce buffer-based path formatting helpers; refactor primary override dir formatting.
src/native/clr/include/host/pinvoke-override-impl.hh	Build lib*.so pinvoke names via new DSO-name formatter with heap fallback.
src/native/clr/include/host/os-bridge.hh	Add missing include for shared utilities used by the header.
src/native/clr/include/host/host-environment.hh	Refactor XDG path building to use buffer join logic (and environment setup changes).
src/native/clr/include/host/assembly-store.hh	Remove dependency on local-string header.
src/native/clr/host/typemap.cc	Build managed type names via bounded formatting + heap fallback.
src/native/clr/host/host.cc	Avoid temporary local-string allocations for logging/timing type names.
src/native/clr/host/bridge-processing.cc	Add missing includes needed after refactors.
src/native/clr/host/assembly-store.cc	Replace local-string timing messages with bounded snprintf messages.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0
• Review effort level: Lite

[simonrozsival]

Linker evidence for keeping libstdc++compat.a

The reasoning in this PR is now backed by the linker's own extraction trace, rather than by argument. Replaying the app link with ld.lld --why-extract produces this chain:

libbootstrapperdll.o
  --[RhRegisterOSModule]-->        libRuntime.WorkstationGC.a(Runtime.PrivateLibunwind.o)
  --[operator new(size_t, nothrow_t const&)]-->  libstdc++compat.a(stdcppshim.cpp.o)

Two conclusions follow directly:

1. The nothrow-new demand originates in dotnet/runtime, not in our code. It comes from the runtime's own bundled libunwind (Runtime.PrivateLibunwind.o), which the ILC SDK already ships libstdc++compat.a to satisfy. Our targets were unconditionally removing that archive with the comment "This library conflicts with static libc++" — true only while libc++ was linked. Keeping it is the entire fix; no C++ runtime shims of our own are needed.

2. Dropping the NDK's libunwind.a is safe. The runtime carries its own unwinder and never depended on the NDK copy. libRuntime.WorkstationGC.a defines __unw_iterate_dwarf_unwind_cache, UnwindHelpers::StepFrame and UnwindHelpers::GetUnwindProcInfo, and walks DWARF at runtime via dl_iterate_phdr. Stack walking is unaffected.

Symbol-level verification

strings -a on the published .so (llvm-nm is useless here — the shipped .so is stripped, so symbol counts are 0/0 either way):

probe	before	after
libc++abi	2	0
std::bad_alloc	1	0
terminate_handler	2	0
_Unwind_Resume	2	0

Size impact

Default MAUI app, android-arm64, Release, PublishAot:

	before	after	delta
.so	25,229,320	25,029,728	−199,592 (−0.79%)
.so deflated	9,829,143	9,761,291	−67,852 (−0.69%)
APK	14,903,624	14,833,992	−69,632 (−0.47%)

Why there is no CMake change in this PR

src/native/**/CMakeLists.txt only produces a static archive (libnaot-android.release-static-release.a) — ar bundling, no linking, so there is nothing there to stop linking. The application link is driven entirely by Microsoft.Android.Sdk.NativeAOT.targets, which is what this PR changes.