[Bug]: qt6-qtdeclarative 6.8.0 fails to build for non-aarch64 architectures

[Biswa96]

Problem description

qt6-qtdeclarative 6.8.0 fails to build for non-aarch64 architectures. Here is the linker error.

ld.lld: error: undefined symbol: __tls_get_addr
>>> referenced by tasktree.cpp
>>>               lto.tmp:(Tasking::ExecutionContextActivator::activateTaskTree(Tasking::RuntimeContainer*))
>>> referenced by tasktree.cpp
>>>               lto.tmp:(Tasking::ExecutionContextActivator::activateTaskTree(Tasking::RuntimeContainer*))
>>> referenced by tasktree.cpp
>>>               lto.tmp:(Tasking::ExecutionContextActivator::activateTaskTree(Tasking::RuntimeContainer*))
>>> referenced 20 more times
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

Full log can be found in https://github.com/termux/termux-packages/actions/runs/11242534272

What steps will reproduce the bug?

build it.

What is the expected behavior?

should build.

System information

n/a

[fornwall]

Possibly related: android/ndk#2069

[twaik]

Adding -femulated-tls to build and link flags did not work, having the same error.

[twaik]

tasktree.cpp is a part of $PREFIX/lib/libQt6ExamplesAssetDownloader.a which is a part of qt6-qtbase. Probably it should be rebuilt with -femulated-tls but I am not so sure about this. I am even not sure if femulated-tls should be applied to CFLAGS/CXXFLAGS, LDFLAGS or both.

[truboxl]

I feel that somewhere injected -flto. Try add -flto in LDFLAGS.

[twaik]

Seems like injecting flto to ldflags works locally, but I tried only x86_64 build

[fornwall]

I am even not sure if femulated-tls should be applied to CFLAGS/CXXFLAGS, LDFLAGS or both.

Agreed, I don't think femulated-tls should be used. My current understanding is that NDK should take care of using emulated TLS when targeting API level 28 or lower, https://android.googlesource.com/platform/bionic/+/HEAD/android-changes-for-ndk-developers.md#elf-tls-available-for-api-level-29:

Android supports ELF TLS starting at API level 29. Since NDK r26, clang will automatically enable ELF TLS for minSdkVersion 29 or higher. Otherwise, the existing emutls implementation (which uses pthread_key_create() behind the scenes) will continue to be used. This means that convenient C/C++ thread-local syntax is available at any API level; at worst it will perform similarly to “roll your own” thread locals using pthread_key_create() but at best you'll get the performance benefit of ELF TLS, and the NDK will take care of the details.

And indeed, as indicated above, the issue might be that flto needs to be used both when compiling and linking for the above to work (having it only for compilation was the issue in android/ndk#2069, and might be the issue here as well).

[fornwall]

Seems like injecting flto to ldflags works locally, but I tried only x86_64 build

Verified a 32-bit arm build as well locally. Created the #21739 PR with it, let's see a complete CI run.