How should be used LLVM modules? - llvm

Question

I'm using LLVM to convert a user-defined language into bytecode, and I'm not sure to understand how should be used a module.
At the beginning, I thought it was something like the C/C++ object files (to avoid bytecode recompilation of every files when a single file is edited). However, I have found this line into LLVMpy documentation, which seems to say that it is not the case :
Inter-module reference is not possible. That is module A cannot call a function in module B, directly.
Can someone explain why are modules separated from the contexts if we can't have multiple modules for a single context ?

Answer 1

It is possible, but like the .o files you mention, they must first be linked together into a single binary.
Given a pair of bitcode files:
$ llvm-dis a.bc -o -
; ModuleID = 'a.bc'
#0 = global [13 x i8] c"Hello world!\0A"
declare i32 #printf(i8*)
define void #f() {
%1 = call i32 #printf(i8* getelementptr inbounds ([13 x i8]* #0, i64 0, i64 0))
ret void
}
$ llvm-dis b.bc -o -
; ModuleID = 'b.bc'
declare void #f()
define i32 #main() {
call void #f()
ret i32 0
}
This won't work:
$ lli b.bc
LLVM ERROR: Program used external function 'f' which could not be resolved!
But if you link them together, it will:
$ llvm-ld a.bc b.bc -disable-opt -o c
$ llvm-dis c.bc -o -
; ModuleID = 'c.bc'
#0 = global [13 x i8] c"Hello world!\0A"
declare i32 #printf(i8*)
define void #f() {
%1 = call i32 #printf(i8* getelementptr inbounds ([13 x i8]* #0, i64 0, i64 0))
ret void
}
define i32 #main() {
call void #f()
ret i32 0
}
$ lli c.bc
Hello world!