Force a global variable using a specific register by Clang/LLVM - c++

Question

I have the code as follows:
// global.cpp
int a = 5;
register unsigned b __asm("r9");
int test()
{
b = 4;
a++;
b = b + 7;
int c = a + b;
return c;
}
I run clang -target arm -c global.cpp -emit-llvm -o global.bc & llc -march=arm -filetype=asm global.bc -o -, but I get the following error:
.text
.syntax unified
.eabi_attribute 67, "2.09" # Tag_conformance
.eabi_attribute 6, 1 # Tag_CPU_arch
.eabi_attribute 8, 1 # Tag_ARM_ISA_use
.eabi_attribute 34, 1 # Tag_CPU_unaligned_access
.eabi_attribute 17, 1 # Tag_ABI_PCS_GOT_use
.eabi_attribute 20, 1 # Tag_ABI_FP_denormal
.eabi_attribute 21, 1 # Tag_ABI_FP_exceptions
.eabi_attribute 23, 3 # Tag_ABI_FP_number_model
.eabi_attribute 24, 1 # Tag_ABI_align_needed
.eabi_attribute 25, 1 # Tag_ABI_align_preserved
.eabi_attribute 38, 1 # Tag_ABI_FP_16bit_format
.eabi_attribute 18, 4 # Tag_ABI_PCS_wchar_t
.eabi_attribute 26, 2 # Tag_ABI_enum_size
.eabi_attribute 14, 0 # Tag_ABI_PCS_R9_use
.file "global.cpp"
LLVM ERROR: Invalid register name "r9".
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
Stack dump:
0. Program arguments: llc -march=arm -filetype=asm global.bc -o -
1. Running pass 'Function Pass Manager' on module 'global.bc'.
2. Running pass 'ARM Instruction Selection' on function '#_Z4testv'
#0 0x00007f345d2fea1a llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0xb12a1a)
#1 0x00007f345d2fc7a4 llvm::sys::RunSignalHandlers() (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0xb107a4)
#2 0x00007f345d2fc8f8 (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0xb108f8)
#3 0x00007f345c39fde0 (/lib64/libc.so.6+0x38de0)
#4 0x00007f345c39fd61 raise (/lib64/libc.so.6+0x38d61)
#5 0x00007f345c389536 abort (/lib64/libc.so.6+0x22536)
#6 0x00007f345d2165e6 llvm::report_fatal_error(llvm::Twine const&, bool) (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0xa2a5e6)
#7 0x00007f345f40b269 (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0x2c1f269)
#8 0x00007f345dbb9000 llvm::SelectionDAGISel::Select_READ_REGISTER(llvm::SDNode*) (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0x13cd000)
#9 0x00007f345dbbc52b llvm::SelectionDAGISel::SelectCodeCommon(llvm::SDNode*, unsigned char const*, unsigned int) (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0x13d052b)
#10 0x00007f345f3fcf21 (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0x2c10f21)
#11 0x00007f345dbb85e8 llvm::SelectionDAGISel::DoInstructionSelection() (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0x13cc5e8)
#12 0x00007f345dbc11b5 llvm::SelectionDAGISel::CodeGenAndEmitDAG() (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0x13d51b5)
#13 0x00007f345dbc4560 llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0x13d8560)
#14 0x00007f345dbc7501 llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0x13db501)
#15 0x00007f345f406014 (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0x2c1a014)
#16 0x00007f345d6c949d (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0xedd49d)
#17 0x00007f345d4503f8 llvm::FPPassManager::runOnFunction(llvm::Function&) (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0xc643f8)
#18 0x00007f345d451971 llvm::FPPassManager::runOnModule(llvm::Module&) (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0xc65971)
#19 0x00007f345d44f70b llvm::legacy::PassManagerImpl::run(llvm::Module&) (/usr/lib/llvm/11/bin/../lib64/libLLVM-11.so+0xc6370b)
#20 0x000056232e849ef6 main (/usr/lib/llvm/11/bin/llc+0xeef6)
#21 0x00007f345c38ae6b __libc_start_main (/lib64/libc.so.6+0x23e6b)
#22 0x000056232e84a6ca _start (/usr/lib/llvm/11/bin/llc+0xf6ca)
Could anyone give me hints how to force global variable b to be stored in register r9 ?
My LLVM & Clang version: 11.0.0
Update:
I want to put a global variable to specific register (R9 in this example) since I am planning to port global register variables feature from ARM to my own back-end (I need global variables to be allocated in registers due to that no stack space for doing this). I just would like to confirm if this functionality can be used on ARM, so I could port this feature to my back-end.

Answer 1

"global variables in registers" is only supported for SP register on ARM. Patches are welcome :)

Answer 2

This post may not directly related to the question. But since I want to fix some global variables to corresponding registers on my own backed, I finally make it works by modifying LLVM via manipulating IR and writing the back-end. The details are as follows:
For the back-end, I refer to Tutorial: Creating an LLVM Backend for
the Cpu0 Architecture and remember to define the getRegisterByName function in XXXISelLowering.cpp.
For IR manipulation, I creating a pass to do that. Mainly, there are 3 steps:
Getting a list of global variable and its corresponding register (via cl:opt)
Iterating over each global variable:
Creating corresponding metadata (MDNode, NamedMDNode)
Initializing via module-level inline assembly (appendModuleInlineAsm)
Creating new instruction using corresponding metadata, which will be used to replace original users of global variable. Using IRBuilder might be easier to create new instruction to replace original one.
Replacing original global variable with corresponding metadata. Note that this step should be done outside the iteration since replacing might break iteration.
I also found that following references help me to learn LLVM:
Getting Started with LLVM Core Libraries
LLVM doxygen (Find the class you interested and look at what functions are provided)
Grep source code directly to learn how to use the class, functions, etc.