diff --git a/cli/src/build.rs b/cli/src/build.rs
index 3bde999b76..2cbb70d078 100644
--- a/cli/src/build.rs
+++ b/cli/src/build.rs
@@ -98,9 +98,12 @@ pub fn build_file(
 
     let compilation_end = compilation_start.elapsed().unwrap();
 
+    let size = std::fs::metadata(&app_o_file).unwrap().len();
+
     println!(
-        "Finished compilation and code gen in {} ms\n",
-        compilation_end.as_millis()
+        "Finished compilation and code gen in {} ms\n\nProduced a app.o file of size {:?}\n",
+        compilation_end.as_millis(),
+        size,
     );
 
     let cwd = app_o_file.parent().unwrap();
diff --git a/cli/src/repl/gen.rs b/cli/src/repl/gen.rs
index f36672a455..775345182f 100644
--- a/cli/src/repl/gen.rs
+++ b/cli/src/repl/gen.rs
@@ -2,6 +2,7 @@ use crate::repl::eval;
 use bumpalo::Bump;
 use inkwell::context::Context;
 use roc_build::link::module_to_dylib;
+use roc_build::program::FunctionIterator;
 use roc_collections::all::{MutMap, MutSet};
 use roc_fmt::annotation::Formattable;
 use roc_fmt::annotation::{Newlines, Parens};
@@ -111,6 +112,15 @@ pub fn gen_and_eval(src: &[u8], target: Triple, opt_level: OptLevel) -> Result<R
         let module = arena.alloc(roc_gen::llvm::build::module_from_builtins(&context, "app"));
         let builder = context.create_builder();
 
+        // mark our zig-defined builtins as internal
+        use inkwell::module::Linkage;
+        for function in FunctionIterator::from_module(module) {
+            let name = function.get_name().to_str().unwrap();
+            if name.starts_with("roc_builtins") {
+                function.set_linkage(Linkage::Internal);
+            }
+        }
+
         debug_assert_eq!(exposed_to_host.len(), 1);
         let (main_fn_symbol, main_fn_var) = exposed_to_host.iter().next().unwrap();
         let main_fn_symbol = *main_fn_symbol;
diff --git a/compiler/build/src/program.rs b/compiler/build/src/program.rs
index fb5ebafbaa..29ea54e98f 100644
--- a/compiler/build/src/program.rs
+++ b/compiler/build/src/program.rs
@@ -2,6 +2,7 @@ use crate::target;
 use bumpalo::Bump;
 use inkwell::context::Context;
 use inkwell::targets::{CodeModel, FileType, RelocMode};
+use inkwell::values::FunctionValue;
 use roc_gen::llvm::build::{build_proc, build_proc_header, module_from_builtins, OptLevel, Scope};
 use roc_load::file::MonomorphizedModule;
 use roc_mono::layout::LayoutIds;
@@ -70,6 +71,15 @@ pub fn gen_from_mono_module(
     // strip Zig debug stuff
     // module.strip_debug_info();
 
+    // mark our zig-defined builtins as internal
+    use inkwell::module::Linkage;
+    for function in FunctionIterator::from_module(module) {
+        let name = function.get_name().to_str().unwrap();
+        if name.starts_with("roc_builtins") {
+            function.set_linkage(Linkage::Internal);
+        }
+    }
+
     let builder = context.create_builder();
     let (dibuilder, compile_unit) = roc_gen::llvm::build::Env::new_debug_info(module);
     let (mpm, fpm) = roc_gen::llvm::build::construct_optimization_passes(module, opt_level);
@@ -159,3 +169,30 @@ pub fn gen_from_mono_module(
         .write_to_file(&env.module, FileType::Object, &app_o_file)
         .expect("Writing .o file failed");
 }
+
+pub struct FunctionIterator<'ctx> {
+    next: Option<FunctionValue<'ctx>>,
+}
+
+impl<'ctx> FunctionIterator<'ctx> {
+    pub fn from_module(module: &inkwell::module::Module<'ctx>) -> Self {
+        Self {
+            next: module.get_first_function(),
+        }
+    }
+}
+
+impl<'ctx> Iterator for FunctionIterator<'ctx> {
+    type Item = FunctionValue<'ctx>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.next {
+            Some(function) => {
+                self.next = function.get_next_function();
+
+                Some(function)
+            }
+            None => None,
+        }
+    }
+}
diff --git a/compiler/gen/src/llvm/build.rs b/compiler/gen/src/llvm/build.rs
index 110745a880..dbd06bd64c 100644
--- a/compiler/gen/src/llvm/build.rs
+++ b/compiler/gen/src/llvm/build.rs
@@ -386,6 +386,9 @@ pub fn construct_optimization_passes<'a>(
     fpm.add_instruction_combining_pass();
     fpm.add_tail_call_elimination_pass();
 
+    // remove unused global values (e.g. those defined by zig, but unused in user code)
+    mpm.add_global_dce_pass();
+
     let pmb = PassManagerBuilder::create();
     match opt_level {
         OptLevel::Normal => {
diff --git a/compiler/gen/tests/helpers/eval.rs b/compiler/gen/tests/helpers/eval.rs
index 95959224dd..bbe6888c78 100644
--- a/compiler/gen/tests/helpers/eval.rs
+++ b/compiler/gen/tests/helpers/eval.rs
@@ -1,5 +1,6 @@
 use libloading::Library;
 use roc_build::link::module_to_dylib;
+use roc_build::program::FunctionIterator;
 use roc_collections::all::{MutMap, MutSet};
 
 fn promote_expr_to_module(src: &str) -> String {
@@ -154,6 +155,15 @@ pub fn helper<'a>(
 
     let (dibuilder, compile_unit) = roc_gen::llvm::build::Env::new_debug_info(module);
 
+    // mark our zig-defined builtins as internal
+    use inkwell::module::Linkage;
+    for function in FunctionIterator::from_module(module) {
+        let name = function.get_name().to_str().unwrap();
+        if name.starts_with("roc_builtins") {
+            function.set_linkage(Linkage::Internal);
+        }
+    }
+
     // Compile and add all the Procs before adding main
     let env = roc_gen::llvm::build::Env {
         arena: &arena,