The question about how to refer an element in a pointer array by a dynamic index by IR

Now I am trying to use LLVM to write a simulator and take LLVM as IR
to dynamic translate machine code,
I encounter some issues when try to write some IR to express writting
a value to the reigster.
The situation is described as the following description.

write_reg is my function for writing "v" to the register indicated by
"index". ptr_gpr is the struct pointer for my general register file.

write_reg(cpu_t *cpu, uint32_t index, Value *v,BasicBlock *bb ){
       Value **regs = cpu->ptr_gpr;
}

If index do not need to do any runtime calculation, I can write the
following sentence to fulfil my requirement.

new StoreInst(v, regs[index], bb);

But if I need to do some runtime calculation for index such as
modular. How to deal that?
1. Use IR to calculate index by some
"BinaryOperator::Create(Instruction::Add, a, b, "", bb)" sentences.
2. Use the above index to refer the register in "regs".
Should I use GEP in second step ? And how to use it?

Thanks
MK

Dear Michael,

I didn't see a response to your email on llvmdev, so I thought I'd take a shot at answering it.

Michael.Kang wrote:

Now I am trying to use LLVM to write a simulator and take LLVM as IR
to dynamic translate machine code,
I encounter some issues when try to write some IR to express writting
a value to the reigster.
The situation is described as the following description.

write_reg is my function for writing "v" to the register indicated by
"index". ptr_gpr is the struct pointer for my general register file.

write_reg(cpu_t *cpu, uint32_t index, Value *v,BasicBlock *bb ){
       Value **regs = cpu->ptr_gpr;
}
  
So, if I understand correctly, you are generating the code for write_reg() at run-time, and you're wanting to know how to generate code that indexes into your register file structure, correct?

This basically boils down to a "How do I use GetElementPtr?" question. To answer that, I'll just redirect you to the GetElementPtr document (http://llvm.org/docs/GetElementPtr.html). That document describes, in detail, how to use the GEP instruction.

You may also want to write some example C code that does the indexing operation you want and then compile it to LLVM assembly using llvm-gcc (llvm-gcc -emit-llvm -S file.c, I believe). That'll give you an example of the LLVM code that you want to generate.

-- John T.