AIX/xlclang++: thread_local variables give linker error: undefined symbols - multithreading

Question

I am trying to build a shared object (.so) on AIX, using the IBM xlclang++ compiler (v16.1). The .so uses C++11 concurrency, and I have defined some global variables as thread_local. The linker gives Undefined symbol errors for those variables (and for another symbol: .__pt_atexit_np).
There are also some thread_local static variables inside a function. (See DoSomething below.) The linker does not complain about those, but the above-mentioned symbol .__pt_atexit_np might have something to with this.
This code compiles, links, and runs fine on Windows (as a DLL), both when using MS Visual C++ 2019 and clang++ (v9.0.0).
To enable multithreaded code on AIX (I hoped), I added the compiler/linker options -qthreaded and -qtls=global-dynamic.
// --- In GlobData.h: ---
extern thread_local bool GLOB_bTracingEnabled;
// --- In GlobData.cpp: --- (Error: Undefined symbol)
thread_local bool GLOB_bTracingEnabled;
// --- In Calculations.cpp: --- (Error?)
int DoSomething() {
thread_local static int _staticVar;
// ...
}
#--- In makefile: ---
INCL1=.
CCC=/opt/IBM/xlC/16.1.0/bin/xlclang++
CCFLAGS1=-q64 -qrtti -qthreaded -qtls=global-dynamic -qmkshrobj -bE:MyDLL.exp -DNDEBUG -I$(INCL1)
CCFLAGS=$(CCFLAGS1) -c
LD=/opt/IBM/xlC/16.1.0/bin/xlclang++
LDFLAGS=$(CCFLAGS1)
LIB_FILE=mydll.so
SOURCEDIR1=.
OBJ_FILES=StdAfx.o GlobData.o Calculations.o
VPATH=$(SOURCEDIR1)
.SUFFIXES: .cpp .o
#Make all:
all :: $(LIB_FILE)
#Compile rule:
.cpp.o:
$(CCC) $(CCFLAGS) -o$# $<
#Link rule:
$(LIB_FILE) : $(OBJ_FILES)
$(LD) $(LDFLAGS) -o $# $(OBJ_FILES)
strip -t -X64 $#
Link errors:
ld: 0711-317 ERROR: Undefined symbol: .ZTH20GLOB_bTracingEnabled
ld: 0711-317 ERROR: Undefined symbol: .__pt_atexit_np
ld: 0711-345 Use the -bloadmap or -bnoquiet option to obtain more information.
Compiling/linking with option -bnoquiet (as suggested above) gave the following info (tidied up a bit). (Hoping it's useful for someone...)
/opt/IBM/xlC/16.1.0/bin/xlclang++ -O3 -q64 -qchars=signed -qrtti -qthreaded -qtls=global-dynamic -bnoquiet -qmkshrobj -qmaxmem=-1 -bE:mydll.exp -DNDEBUG -DAIX -DBPC_DLL_EXPORTS -I../Common -I../../../aks/source -I. -o ./mydll.so StdAfx.o GlobData.o Calculations.o [...]
(ld): setopt 64
(ld): halt 4
(ld): setfflag 4
(ld): cdtors 1 all 0 s
(ld): savename ./bpc-urm.so
(ld): filelist 92 2
(ld): setopt noprogram
(ld): noentry
NOENTRY: There is no entry point.
(ld): i /lib/crti_64.o
(ld): i StdAfx.o
(ld): i GlobData.o
(ld): i Calculations.o
[...]
(ld): lib /opt/IBM/xlc/16.1.0/lib/libxlopt.a
(ld): lib /opt/IBM/xlc/16.1.0/lib/libxlipa.a
(ld): lib /opt/IBM/xlc/16.1.0/lib/libxl.a
(ld): lib /usr/lib/libc++.a
(ld): lib /opt/IBM/xlC/16.1.0/lib/libCcore.a
(ld): lib /usr/lib/libpthreads.a
(ld): lib /usr/lib/libm.a
(ld): lib /usr/lib/libatomic.a
(ld): lib /usr/lib/libc.a
LIBRARY: Symbols imported from import file /usr/lib/libc++.a[shr.imp]: 0
LIBRARY: Symbols imported from import file /usr/lib/libc++.a[shr_64.imp]: 1834
LIBRARY: Symbols imported from import file /usr/lib/libc++.a[cxxabi.imp]: 0
LIBRARY: Symbols imported from import file /usr/lib/libc++.a[cxxabi_64.imp]: 354
LIBRARY: Symbols imported from import file /opt/IBM/xlC/16.1.0/lib/libCcore.a[shrcore_32.imp]: 0
LIBRARY: Symbols imported from import file /opt/IBM/xlC/16.1.0/lib/libCcore.a[ansicore_32.imp]: 0
LIBRARY: Symbols imported from import file /opt/IBM/xlC/16.1.0/lib/libCcore.a[shrcore_64.imp]: 131
LIBRARY: Symbols imported from import file /opt/IBM/xlC/16.1.0/lib/libCcore.a[ansicore_64.imp]: 81
LIBRARY: Shared object libpthreads.a[shr_xpg5_64.o]: 346 symbols imported.
LIBRARY: Shared object libatomic.a[libatomic.so.1]: 40 symbols imported.
LIBRARY: Shared object libc.a[shr_64.o]: 3110 symbols imported.
LIBRARY: Shared object libc.a[posix_aio_64.o]: 20 symbols imported.
LIBRARY: Shared object libc.a[aio_64.o]: 18 symbols imported.
LIBRARY: Shared object libc.a[pse_64.o]: 8 symbols imported.
LIBRARY: Shared object libc.a[dl_64.o]: 4 symbols imported.
LIBRARY: Shared object libc.a[pty_64.o]: 1 symbols imported.
LIBRARY: Shared object libc.a[cthread_64.o]: 25 symbols imported.
LIBRARY: Shared object libc.a[uchar_64.o]: 4 symbols imported.
FILELIST: Number of previously inserted files processed: 92
(ld): exports mydll.exp
EXPORTS: Symbols exported: 59
(ld): resolve
RESOLVE: 6667 of 30031 symbols were kept.
(ld): addgl /usr/lib/glink64.o
ADDGL: Glink code added for 116 symbols.
(ld): er full
ld: 0711-318 ERROR: Undefined symbols were found.
The following symbols are in error:
Symbol Inpndx TY CL Source-File(Object-File) OR Import-File{Shared-object}
RLD: Address Section Rld-type Referencing Symbol
----------------------------------------------------------------------------------------------
._ZTH20GLOB_bTracingEnabled [240] ER PR File0.cpp(File0.o)
00000cbc .text R_RBR [54] <.GLOB_EnableTracing#AF26_2>
._ZTH20GLOB_bTracingEnabled [386] ER PR File1.cpp(File1.o)
0000004c .text R_RBR [8] <._ZTW20GLOB_bTracingEnabled>
00001564 .text R_RBR [24] .Namespace2::Function1(...)
0000162c .text R_RBR [24] .Namespace2::Function1(...)
00001690 .text R_RBR [24] .Namespace2::Function1(...)
._ZTH20GLOB_bTracingEnabled [563] ER PR File2.cpp(File2.o)
000013dc .text R_RBR [26] .Class1::Function2(...) const
00001400 .text R_RBR [26] .Class1::Function2(...) const
._ZTH20GLOB_bTracingEnabled [782] ER PR File3.cpp(File3.o)
00001710 .text R_RBR [22] .Class2::Function3(...) const
00001788 .text R_RBR [22] .Class2::Function3(...) const
00001800 .text R_RBR [22] .Class2::Function3(...) const
00001878 .text R_RBR [22] .Class2::Function3(...) const
00001910 .text R_RBR [22] .Class2::Function3(...) const
00001928 .text R_RBR [22] .Class2::Function3(...) const
00001940 .text R_RBR [22] .Class2::Function3(...) const
0000202c .text R_RBR [27] <._ZTW20GLOB_bTracingEnabled>
0000286c .text R_RBR [38] .Class2::Function4(...) const
00003064 .text R_RBR [43] .Class2::Function5(...) const
._ZTH20GLOB_bTracingEnabled [283] ER PR File4.cpp(File4.o)
00000260 .text R_RBR [11] .Class3::Function6(...) const
00000370 .text R_RBR [11] .Class3::Function6(...) const
00000604 .text R_RBR [15] <.IPRA.$Function7(...)>
0000069c .text R_RBR [15] <.IPRA.$Function7(...)>
[...]
.__pt_atexit_np [1033] ER PR Calculations.cpp(Calculations.o)
00003720 .text R_RBR [55] .Namespace1::DoSomething(...)
00003750 .text R_RBR [55] .Namespace1::DoSomething(...)
ER: The return code is 8.
ld: 0711-317 ERROR: Undefined symbol: ._ZTH20GLOB_bTracingEnabled
[...]
ld: 0711-317 ERROR: Undefined symbol: .__pt_atexit_np
make: 1254-004 The error code from the last command is 8.