Set linkage of __crystal_* funs to internal (#15439)

Unlike the usual `fun` for which we might expect to have external linkage,
the `__crystal_*` funs are an internal implementation detail to abstract calls
to stdlib from the codegen pass (e.g. `__crystal_once`, `__crystal_raise`, ...).

When compiling to a single module, we can mark them to have internal
linkage, so they're only considered inside the object file. We already do
that for all globals and `def`.

spec/compiler/codegen/fun_spec.cr

@@ -0,0 +1,21 @@
+require "../../spec_helper"
+
+describe "Codegen: fun" do
+  it "sets external linkage by default" do
+    mod = codegen(<<-CRYSTAL, inject_primitives: false, single_module: false)
+    fun foo; end
+    fun __crystal_foo; end
+    CRYSTAL
+    mod.functions["foo"].linkage.should eq(LLVM::Linkage::External)
+    mod.functions["__crystal_foo"].linkage.should eq(LLVM::Linkage::External)
+  end
+
+  it "sets internal linkage to __crystal_ funs when compiling to single module" do
+    mod = codegen(<<-CRYSTAL, inject_primitives: false, single_module: true)
+    fun foo; end
+    fun __crystal_foo; end
+    CRYSTAL
+    mod.functions["foo"].linkage.should eq(LLVM::Linkage::External)
+    mod.functions["__crystal_foo"].linkage.should eq(LLVM::Linkage::Internal)
+  end
+end

spec/spec_helper.cr

@@ -221,9 +221,9 @@ def prepare_macro_call(macro_body, flags = nil, &)
   {program, a_macro, call}
 end
 
-def codegen(code, inject_primitives = true, debug = Crystal::Debug::None, filename = __FILE__)
+def codegen(code, *, inject_primitives = true, single_module = false, debug = Crystal::Debug::None, filename = __FILE__)
   result = semantic code, inject_primitives: inject_primitives, filename: filename
-  result.program.codegen(result.node, single_module: false, debug: debug)[""].mod
+  result.program.codegen(result.node, single_module: single_module, debug: debug)[""].mod
 end
 
 private def new_program

src/compiler/crystal/codegen/codegen.cr

@@ -283,6 +283,7 @@ module Crystal
       ret_type = @llvm_typer.llvm_return_type(@main_ret_type)
       main_type = LLVM::Type.function([llvm_context.int32, llvm_context.void_pointer.pointer], ret_type)
       @main = @llvm_mod.functions.add(MAIN_NAME, main_type)
+      @main.linkage = LLVM::Linkage::Internal if @single_module
       @fun_types = { {@llvm_mod, MAIN_NAME} => main_type }
 
       if @program.has_flag?("msvc")

src/compiler/crystal/codegen/fun.cr

@@ -395,6 +395,14 @@ class Crystal::CodeGenVisitor
       context.fun.call_convention = call_convention
     end
 
+    if @single_module && mangled_name.starts_with?("__crystal_")
+      # FIXME: macos ld fails to link when the personality fun is internal; it
+      # might work with lld so we might want to check the linker?
+      unless @program.has_flag?("darwin") && mangled_name.starts_with?("__crystal_personality")
+        context.fun.linkage = LLVM::Linkage::Internal
+      end
+    end
+
     i = 0
     args.each do |arg|
       param = context.fun.params[i + offset]