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Removing a GlobalValue from the module might make other values (referenced only
by the one being removed) available for removal. Those should go as well. The
initial implementation is a bit greedy - it removes more: i.e dtors because they
are not referenced (because IncrementalExecutor takes care of them in an custom
way)
When the machine code was generated the JIT retains a lock to the global values
so they don't go out of sync. In the cases of circular references like:
int g();
int f() {g();}
int g() {f();}
we cannot count on the removal order to get rid of the uses. We need to use the
replaceAllUsesWith. This is however tricky because the JIT already generated the
code for f and g and it doesn't make sense to replace anything with anything else.
Thus in order to support that we need to hack one of the JIT callbacks which
prevents the replace of values of already emitted code.
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.
