[Android] Use ELF TLS for Android API level 29+

[hctim]

Summary

Use ELF TLS by default instead of emulated TLS for Android build targets when the API level is > 29.

Android Q features full TLS support. See release notes here and implementation details here.

[eugenis]

Test please.

[rprichard]

Android's ELF TLS requires lld, but the NDK doesn't default to lld yet. (I believe the NDK uses bfd on arm64 and gold everywhere else.) When the NDK Clang driver targets 29/Q+, the NDK could generate executables or shared objects using ELF TLS that don't load on 29/Q.

For reference, the linker quirks are:

• x86-32 bfd uses an obsolete TLS relocation (R_386_TLS_TPOFF32) for relaxation
• arm{32,64} bfd can discard the overalignment placeholder section from crtbegin.c
• gold uses STB_LOCAL TLS/section symbols that need special handling in soinfo::relocate

[brad]

I happened to come across. What about revisiting this now that the NDK has had LLD as the default since r22? Which is approaching 2 years old.

[danalbert]

+1, but like @eugenis says, it needs a test.

llvm/include/llvm/ADT/Triple.h

public:
 
   /// Tests whether the target supports comdat
   bool supportsCOMDAT() const {
     return !isOSBinFormatMachO();
   }
 
   /// Tests whether the target uses emulated TLS as default.
   bool hasDefaultEmulatedTLS() const {
-    return isAndroid() || isOSOpenBSD() || isWindowsCygwinEnvironment();
+    // Android NDK version 29 introduced ELF TLS.
+    return (isAndroid() && isAndroidVersionLT(29)) || isOSOpenBSD() ||
+           isWindowsCygwinEnvironment();
   }
 
   /// @}
   /// @name Mutators
   /// @{
 
   /// setArch - Set the architecture (first) component of the triple
   /// to a known type.