No luck compiling __thread using ndk clang 3.4/3.5

Question

I am trying to use __thread in this small program without luck. Any idea if this TLS is supported in ndk 10c clang 3.4/3.5? The same program compiles fine with ndk gcc 4.8/4.9 and native clang/gcc compilers.

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Here is the program and compile line -

__thread int counter;
int main () { counter=20; return 0; }

[armeabi] Compile++ thumb: test <= test.cpp
/Users/padlar/android/android-ndk-r10c/toolchains/llvm-3.5/prebuilt/darwin-x86/bin/clang++ -MMD -MP -MF ./obj/local/armeabi/objs/test/test.o.d -gcc-toolchain /Users/padlar/android/android-ndk-r10c/toolchains/arm-linux-androideabi-4.8/prebuilt/darwin-x86 -fpic -ffunction-sections -funwind-tables -fstack-protector -Wno-invalid-command-line-argument -Wno-unused-command-line-argument -no-canonical-prefixes -fno-integrated-as  -target armv5te-none-linux-androideabi -march=armv5te -mtune=xscale -msoft-float -fno-exceptions -fno-rtti -mthumb -Os -g -DNDEBUG -fomit-frame-pointer -fno-strict-aliasing -I/Users/padlar/android/android-ndk-r10c/sources/cxx-stl/stlport/stlport -I/Users/padlar/android/android-ndk-r10c/sources/cxx-stl//gabi++/include -Ijni -DANDROID  -Wa,--noexecstack -Wformat -Werror=format-security -fPIE  -frtti     -I/Users/padlar/android/android-ndk-r10c/platforms/android-19/arch-arm/usr/include -c  jni/test.cpp -o ./obj/local/armeabi/objs/test/test.o
[armeabi] Executable     : test
/Users/padlar/android/android-ndk-r10c/toolchains/llvm-3.5/prebuilt/darwin-x86/bin/clang++ -Wl,--gc-sections -Wl,-z,nocopyreloc --sysroot=/Users/padlar/android/android-ndk-r10c/platforms/android-19/arch-arm -Wl,-rpath-link=/Users/padlar/android/android-ndk-r10c/platforms/android-19/arch-arm/usr/lib -Wl,-rpath-link=./obj/local/armeabi ./obj/local/armeabi/objs/test/test.o /Users/padlar/android/android-ndk-r10c/sources/cxx-stl/stlport/libs/armeabi/thumb/libstlport_static.a -lgcc  -gcc-toolchain /Users/padlar/android/android-ndk-r10c/toolchains/arm-linux-androideabi-4.8/prebuilt/darwin-x86 -no-canonical-prefixes -target armv5te-none-linux-androideabi  -Wl,--no-undefined -Wl,-z,noexecstack -Wl,-z,relro -Wl,-z,now -fPIE -pie  -L/Users/padlar/android/android-ndk-r10c/platforms/android-19/arch-arm/usr/lib -llog -lc -lm -o ./obj/local/armeabi/test
jni/test.cpp:2: error: undefined reference to '__aeabi_read_tp'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [obj/local/armeabi/test] Error 1

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ndk-which objdump -S obj/local/armeabi/objs/test/test.o

obj/local/armeabi/objs/test/test.o:     file format elf32-littlearm
Disassembly of section .text.main:

00000000 <main>:
__thread int counter;
int main () { counter=20; return 0; }
   0:   b580        push    {r7, lr}
   2:   4904        ldr r1, [pc, #16]   ; (14 <main+0x14>)
   4:   f7ff fffe   bl  0 <__aeabi_read_tp>
   8:   1840        adds    r0, r0, r1
   a:   2114        movs    r1, #20
   c:   6001        str r1, [r0, #0]
   e:   2000        movs    r0, #0
  10:   bd80        pop {r7, pc}
  12:   46c0        nop         ; (mov r8, r8)
  14:   00000000    .word   0x00000000

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gcc4.6 disass of the same program

ndk-which objdump -S obj/local/armeabi/objs/test/test.o

obj/local/armeabi/objs/test/test.o:     file format elf32-littlearm
Disassembly of section .text.startup.main:

00000000 <main>:
__thread int counter;
int main () { counter=20; return 0; }
   0:   b508        push    {r3, lr}
   2:   4804        ldr r0, [pc, #16]   ; (14 <main+0x14>)
   4:   4478        add r0, pc
   6:   f7ff fffe   bl  0 <__emutls_get_address>
   a:   2314        movs    r3, #20
   c:   6003        str r3, [r0, #0]
   e:   2000        movs    r0, #0
  10:   bd08        pop {r3, pc}
  12:   46c0        nop         ; (mov r8, r8)
  14:   0000000c    .word   0x0000000c

Answer 1

__thread style TLS (similarly C++11 style thread_local) is not currently supported by the Android linker (we're working on it though). For now you'll have to use pthread_getspecific and pthread_setspecific.

EDIT: Starting with NDK r12, Clang has support for emulated TLS. While the above is still technically true (the linker on the device does not support this), the compiler can replace builtin TLS support with pthread calls on your behalf.

NB: Android still only supports thread_local for objects with trivial destructors until r14. See discussions on https://github.com/android-ndk/ndk/issues/156#issuecomment-231878690 and https://github.com/android-ndk/ndk/issues/216.

Answer 2

Firstly, you should avoid using __thread as it may not be portable.

Here is what gnu gcc says here https://gcc.gnu.org/onlinedocs/gcc-4.8.3/gcc/Thread-Local.html :

The runtime model GCC uses to implement this originates in the IA-64 processor-specific ABI, but has since been migrated to other processors as well. It requires significant support from the linker (ld), dynamic linker (ld.so), and system libraries (libc.so and libpthread.so), so it is not available everywhere.

And from wikipédia http://en.wikipedia.org/wiki/Thread-local_storage#C.2B.2B :

C++0x introduces the thread_local keyword. Aside that, various C++ compiler implementations provide specific ways to declare thread-local variables:

Sun Studio C/C++, IBM XL C/C++, GNU C and Intel C/C++ (Linux systems) use the syntax:

    __thread int number;
Visual C++, Intel C/C++ (Windows systems), Borland C++ Builder and Digital Mars C++ use the syntax:

    __declspec(thread) int number;
 Borland C++ Builder also supports the syntax:

    int __thread number;

So it may work with thread_local key word