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The compiled code is unloaded by iterating the AST out of which the compiled code
came. The problem is that C++ supports static initialization. For example:
int a = 5; on the global scope means set the value of a to 5 before the program
starts. To support that clang implicitly generates functions that enforce the
expected initialization order. That can become easily very complex. Let's see
a more "real-life" example:
int f() { printf("I am f()"); return 0; }
int a = f();
Besides the code for a and f clang will emit:
define internal void @__cxx_global_var_init() section "__TEXT,__StaticInit,regular,pure_instructions" {
entry:
%0 = call i32 @__cxa_atexit(void (i8*)* bitcast (void (%"struct.cling::runtime::internal::__trigger__cxa_atexit"*)* @_ZN5cling7runtime8internal21__trigger__cxa_atexitD1Ev to void (i8*)*), i8* getelementptr inbounds (%"struct.cling::runtime::internal::__trigger__cxa_atexit"* @_ZN5cling7runtime8internal1SE, i32 0, i32 0), i8* @__dso_handle) #1
ret void
}
define internal void @_GLOBAL__I_a() section "__TEXT,__StaticInit,regular,pure_instructions" {
entry:
call void @__cxx_global_var_init()
ret void
}
and more importantly:
define internal void @__cxx_global_var_init1() section "__TEXT,__StaticInit,regular,pure_instructions" {
entry:
%call = call i32 @_Z1fv() ; THAT IS A REFERENCE TO f()
store i32 %call, i32* undef, align 4
ret void
}
So when iterating the AST and trying to remove f(), we cannot because it is still
referenced by __cxx_global_var_init1. However we know this is not an issue,
because that __cxx_global_var_init1 was meant to be executed only once and that
already happened. So in first approximation we could drop the references of f
in __cxx_global_var_init1 and remove the rest.
