Global VariablesΒΆ

Global varibles are trivial to implement in LLVM IR:

int variable = 21;

int main()
{
    variable = variable * 2;
    return variable;
}

Becomes:

@variable = global i32 21

define i32 @main() {
    %1 = load i32, i32* @variable  ; load the global variable
    %2 = mul i32 %1, 2
    store i32 %2, i32* @variable   ; store instruction to write to global variable
    ret i32 %2
}

Globals are prefixed with the @ character. You can see that also functions, such as main, are also global variables in LLVM. Please notice that LLVM views global variables as pointers; so you must explicitly dereference the global variable using the load instruction when accessing its value, likewise you must explicitly store the value of a global variable using the store instruction. In that regard LLVM IR is closer to Assembly than C.