Q: How to use LTO when using AOT?

[JCash]

Using Clang, the linker can opt away (-flto) functions that don't seem to be used.

In my example, I produce a library (.a) using dotnet+AoT.
This library is then linked into an executable using C++ and Clang.

The issue is that some functions are removed from the executable are removed, and the the resulting executable will segfault.

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGSEGV
  * frame #0: 0x000000010005e5e5 test`RhpNewArray + 71
    frame #1: 0x0000000100092748 test`_S_P_CoreLib_Internal_Runtime_CompilerHelpers_StartupCodeHelpers__CreateTypeManagers(osModule=<unavailable>, pModuleHeaders=<unavailable>, count=<unavailable>, pClasslibFunctions=<unavailable>, nClasslibFunctions=<unavailable>) at StartupCodeHelpers.cs:110
    frame #2: 0x0000000100092567 test`_S_P_CoreLib_Internal_Runtime_CompilerHelpers_StartupCodeHelpers__InitializeModules(osModule=<unavailable>, pModuleHeaders=<unavailable>, count=<unavailable>, pClasslibFunctions=<unavailable>, nClasslibFunctions=<unavailable>) at StartupCodeHelpers.cs:34
    frame #3: 0x0000000100004300 test`InitializeRuntime() at main.cpp:188:5 [opt]
    frame #4: 0x000000010000bfd5 test`Thread::ReversePInvokeAttachOrTrapThread(ReversePInvokeFrame*) [inlined] Thread::EnsureRuntimeInitialized(this=<unavailable>) at thread.cpp:1219:13 [opt]
    frame #5: 0x000000010000bfad test`Thread::ReversePInvokeAttachOrTrapThread(this=0x0000000100404268, pFrame=0x00007ff7bfefefe0) at thread.cpp:1181:13 [opt]
    frame #6: 0x0000000100079fcb test`_libNativeLibrary_CS__CsAdd(a=<unavailable>, b=<unavailable>) at Library.cs:36
    frame #7: 0x0000000100079e9f test`main(argc=1, argv=0x00007ff7bfeff2f0) at main.cpp:14:47 [opt]
    frame #8: 0x00007ff80366041f dyld`start + 1903

What is the recommended way to fix this issue?
Is there a global function that I can "extern" to make sure the linker cannot remove it?
E.g. if I in main.cpp had something like:

extern "C"  MyGlobalInit() { // Never actually used, but the linker cannot know that
    Some_AOT_Initialize_Function();
}

Or is there some other Clang trick to make sure it doesn't remove certain functions?

I would happily take any ideas on how to solve this.

Regards,
Mathias

I have an example here, which can demonstrate the issue
https://github.com/JCash/csharpextension/tree/lto-error

To build: ./scripts/compile.sh
To run: ./scripts/run.sh

Tagging subscribers to this area: @agocke, @MichalStrehovsky, @jkotas
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Using Clang, the linker can opt away (-flto) functions that don't seem to be used.

In my example, I produce a library (.a) using dotnet+AoT.
This library is then linked into an executable using C++ and Clang.

The issue is that some functions are removed from the executable are removed, and the the resulting executable will segfault.

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGSEGV
  * frame #0: 0x000000010005e5e5 test`RhpNewArray + 71
    frame #1: 0x0000000100092748 test`_S_P_CoreLib_Internal_Runtime_CompilerHelpers_StartupCodeHelpers__CreateTypeManagers(osModule=<unavailable>, pModuleHeaders=<unavailable>, count=<unavailable>, pClasslibFunctions=<unavailable>, nClasslibFunctions=<unavailable>) at StartupCodeHelpers.cs:110
    frame #2: 0x0000000100092567 test`_S_P_CoreLib_Internal_Runtime_CompilerHelpers_StartupCodeHelpers__InitializeModules(osModule=<unavailable>, pModuleHeaders=<unavailable>, count=<unavailable>, pClasslibFunctions=<unavailable>, nClasslibFunctions=<unavailable>) at StartupCodeHelpers.cs:34
    frame #3: 0x0000000100004300 test`InitializeRuntime() at main.cpp:188:5 [opt]
    frame #4: 0x000000010000bfd5 test`Thread::ReversePInvokeAttachOrTrapThread(ReversePInvokeFrame*) [inlined] Thread::EnsureRuntimeInitialized(this=<unavailable>) at thread.cpp:1219:13 [opt]
    frame #5: 0x000000010000bfad test`Thread::ReversePInvokeAttachOrTrapThread(this=0x0000000100404268, pFrame=0x00007ff7bfefefe0) at thread.cpp:1181:13 [opt]
    frame #6: 0x0000000100079fcb test`_libNativeLibrary_CS__CsAdd(a=<unavailable>, b=<unavailable>) at Library.cs:36
    frame #7: 0x0000000100079e9f test`main(argc=1, argv=0x00007ff7bfeff2f0) at main.cpp:14:47 [opt]
    frame #8: 0x00007ff80366041f dyld`start + 1903

What is the recommended way to fix this issue?
Is there a global function that I can "extern" to make sure the linker cannot remove it?
E.g. if I in main.cpp had something like:

extern "C"  MyGlobalInit() { // Never actually used, but the linker cannot know that
    Some_AOT_Initialize_Function();
}

Or is there some other Clang trick to make sure it doesn't remove certain functions?

I would happily take any ideas on how to solve this.

Regards,
Mathias

I have an example here, which can demonstrate the issue
https://github.com/JCash/csharpextension/tree/lto-error

To build: ./scripts/compile.sh
To run: ./scripts/run.sh

Author:	JCash
Assignees:	-
Labels:	untriaged, area-NativeAOT-coreclr
Milestone:	-

[MichalStrehovsky]

-flto relies on LLVM bitcode to be embedded in the executable. It should have no effect on object files that don't have LLVM bitcode in it (i.e. it should have no effect whatsoever on .NET code since we don't compile to LLVM bitcode).

The failure you're seeing looks like #96663. It's possible -flto also enables -dead_strip.