[LLVMdev] How to cope with 'undefined reference' errors taken the example of building VMkit x86_64 version

[Minas Abrahamyan]

Hi all,

I haven't found this trick anywhere (including llvm/docs/MakefileGuide.html)
so it maybe useful.
This occurs only in 64 bit version of linux, linker of 32 bit system
understands
everything correctly.

Situation:
While building, at link stage, a 64bit version of some executable, with
several archive libraries, say, A.a, B.a, C.a
build fails with some "undefined reference to symbol" errors,
Error occurs in try to link-in some symbol defined in B, which is undefined
symbol in A library.
Linker somehow looks for that symbol only in A library, where it is
undefined, U-symbol, in terms of nm(1).

Solution is to add in USEDLIBS line name of B library immediately after name
of A,  for every such A for which linker gets so confused.

***

Example for VMkit:
USEDLIBS = J3.a Classpath.a J3Compiler.a Allocator.a \
                    Mvm.a MvmCompiler.a $(GCLIB).a CommonThread.a

make output:
<<<
make[1]: Leaving directory `/home/mn/tests/VMkit/vmkit/lib'
make[1]: Entering directory `/home/mn/tests/VMkit/vmkit/tools/vmjc'
llvm[1]: Compiling vmjc.cpp for Debug build
llvm[1]: Linking Debug executable vmjc
/home/mn/tests/VMkit/vmkit/Debug/lib/libGCMmap2.a(gcinit.o): In function
`mvm::Collector::initialise()':
/home/mn/tests/VMkit/vmkit/lib/Mvm/GCMmap2/gcinit.cpp:22: undefined
reference to `GCAllocator::operator new(unsigned long)'
/home/mn/tests/VMkit/vmkit/lib/Mvm/GCMmap2/gcinit.cpp:22: undefined
reference to `GCAllocator::GCAllocator()'
/home/mn/tests/VMkit/vmkit/Debug/lib/libGCMmap2.a(gcinit.o): In function
`mvm::Collector::destroy()':
/home/mn/tests/VMkit/vmkit/lib/Mvm/GCMmap2/gcinit.cpp:42: undefined
reference to `GCAllocator::~GCAllocator()'
/home/mn/tests/VMkit/vmkit/lib/Mvm/GCMmap2/gcinit.cpp:42: undefined
reference to `GCAllocator::operator delete(void*)'
collect2: ld returned 1 exit status
make[1]: *** [/home/mn/tests/VMkit/vmkit/Debug/bin/vmjc] Error 1
make[1]: Leaving directory `/home/mn/tests/VMkit/vmkit/tools/vmjc'
make: *** [all] Error 1
>>>

Solution:
change USEDLIBS to:
USEDLIBS = J3.a Classpath.a J3.a J3Compiler.a Allocator.a \
                    Mvm.a MvmCompiler.a $(GCLIB).a Allocator.a
CommonThread.a

Here J3.a and Allocator.a were repeated.
After change it builds OK.


Regards,
Minas Abrahamyan
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