NDK link-time optimization fails missing llvmgold.so, but contains llvmgold.dylib

[JakeWharton]

I'm attempting to add -flto for link-time optimization to my cFlags and ldFlags (using the new Gradle experimental plugin) but receiving a failure to load LLVMgold.so at link time:

/Users/jw/android-sdk/ndk-bundle/toolchains/aarch64-linux-android-4.9/prebuilt/darwin-x86_64/lib/gcc/aarch64-linux-android/4.9.x/../../../../aarch64-linux-android/bin/ld: /Users/jw/android-sdk/ndk-bundle/toolchains/llvm/prebuilt/darwin-x86_64/bin/../lib64/LLVMgold.so: error loading plugin: dlopen(/Users/jw/android-sdk/ndk-bundle/toolchains/llvm/prebuilt/darwin-x86_64/bin/../lib64/LLVMgold.so, 2): image not found

A quick look into the NDK reveals that the lib64/ folder contains an LLVMgold.dylib file, but not the .so:

$ find ~/android-sdk/ndk-bundle -name 'LLVMgold*'
/Users/jw/android-sdk/ndk-bundle/toolchains/llvm/prebuilt/darwin-x86_64/lib64/LLVMgold.dylib

Is the .so missing, or is there a different way to enable LTO that uses this dynamic lib instead?

[JakeWharton]

This is using clang 3.8.

[DanAlbert]

@stephenhines do we just configure something assuming everything is Linux when we build clang, or does upstream clang actually have the wrong? Maybe it's trying .so because the target is Linux?

[stephenhines]

We do only target baseline Linux for our configuration (multiple layers of config.h), so perhaps this is a misconfiguration issue. I am not sure whether we will update the existing configuration logic now that we are switching to a more upstream-like build process.

[stephenhines]

https://android-review.googlesource.com/233868 should be the fix for this. In the meantime, you might be able to workaround this by symlinking to a .so filename (i.e. LLVMGold.so -> LLVMGold.dylib in that directory).

[JakeWharton]

Thanks!

That got me slightly farther along, but the linker now fails with an inability to recognize the format of the .o:

/Users/jw/dev/square/duktape-android/duktape/build/intermediates/objectFiles/arm64-v8aReleaseSharedLibrary/duktapeMainC/72j2t6goef75gvdzb76w4b6so/duktape.o: file not recognized: File format not recognized
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

If I had to guess it's because libLTO.so isn't included in the NDK? I'm not sure if that's something that usually comes standard with a native toolchain like this or not (vastly inexperienced with building native libraries).

[kneth]

I have the same issue. Or to be more specific:

• With ARM64, I get the same error message
• With ARM v7a, I get "file format not recognized" for the .o files
• With x86, I can compile and link, but the produced .so file does not work as the dynamic linker on my emulated device cannot find any methods

[suikki]

On Windows if I specify -flto when using clang I get:

LLVMgold.so: could not load plugin library: unable to load dll

[DanAlbert]

The Windows issue is also covered by the fix @stephenhines posted. Given the "file format not recognized" issues though, that probably isn't sufficient.

[thanm]

@JakeWharton, @kneth -- are you 100% sure that the linker being used for your build is ld.gold and not ld.bfd? Looking at the default for prebuilts on the platform it looks like "ld" defaults to gold for linux but bfd for darwin/aarch64.

[kneth]

I have tried with R12 today - using Linux (well, Ubuntu 14.04 under vagrant/OS X). I am able to compile and link but something is wrong with the produced .so files. The dynamic linker cannot find any symbols.

[DoDoENT]

After creating symlinks as @stephenhines suggested, I now get following linker errors:

/Users/dodo/android-sdks/android-ndk-r12/toolchains/aarch64-linux-android-4.9/prebuilt/darwin-x86_64/lib/gcc/aarch64-linux-android/4.9.x/../../../../aarch64-linux-android/bin/ld.gold: error: ./obj/local/arm64-v8a/libMyStaticLib.a: no archive symbol table (run ranlib)

for every static library in my project and

/var/folders/fc/38fp3j9x3dd74ng4fl8jhjtr0000gn/T/lto-llvm-2b6578.o:ld-temp.o:function myFunction: error: undefined reference to 'mySymbol'

for every single symbol in my entire code.

When disabling LTO or when using GCC, project compiles without errors.

Something is obviously wrong with clang.

[DanAlbert]

I've seen this happen before when ndk-build wrongly uses the system's ar (specifically on Darwin because it's BSD ar instead of GNU ar) instead of the one we ship in the NDK. Can you check that the correct ar was invoked for creating libMyStaticLib.a? I've certainly fixed bugs in ndk-build for that in the past.

[DoDoENT]

I can confirm that toolchains/aarch64-linux-android-4.9/prebuilt/darwin-x86_64/bin/aarch64-linux-android-ar was used for creating all my static libs (checked with ndk-build -n).

This happens with both r12, r12b and r11c. When GCC is used, LTO works.