diff --git a/compiler/gen_dev/src/generic64/aarch64.rs b/compiler/gen_dev/src/generic64/aarch64.rs
index 890995c726..6363ce14b5 100644
--- a/compiler/gen_dev/src/generic64/aarch64.rs
+++ b/compiler/gen_dev/src/generic64/aarch64.rs
@@ -510,6 +510,15 @@ impl Assembler<AArch64GeneralReg, AArch64FloatReg> for AArch64Assembler {
         unimplemented!("registers less than not implemented yet for AArch64");
     }
 
+    #[inline(always)]
+    fn is_zero_reg64_reg64(
+        _buf: &mut Vec<'_, u8>,
+        _dst: AArch64GeneralReg,
+        _src: AArch64GeneralReg,
+    ) {
+        unimplemented!("registers is zero not implemented yet for AArch64");
+    }
+
     #[inline(always)]
     fn ret(buf: &mut Vec<'_, u8>) {
         ret_reg64(buf, AArch64GeneralReg::LR)
diff --git a/compiler/gen_dev/src/generic64/mod.rs b/compiler/gen_dev/src/generic64/mod.rs
index 354c545ebc..70431216ca 100644
--- a/compiler/gen_dev/src/generic64/mod.rs
+++ b/compiler/gen_dev/src/generic64/mod.rs
@@ -186,6 +186,8 @@ pub trait Assembler<GeneralReg: RegTrait, FloatReg: RegTrait> {
         src2: GeneralReg,
     );
 
+    fn is_zero_reg64_reg64(buf: &mut Vec<'_, u8>, dst: GeneralReg, src: GeneralReg);
+
     fn ret(buf: &mut Vec<'_, u8>);
 }
 
@@ -500,12 +502,7 @@ impl<
 
         // move return value to dst.
         match ret_layout {
-            Layout::Builtin(Builtin::Int1) => {
-                let dst_reg = self.claim_general_reg(dst)?;
-                ASM::mov_reg64_reg64(&mut self.buf, dst_reg, CC::GENERAL_RETURN_REGS[0]);
-                Ok(())
-            }
-            Layout::Builtin(Builtin::Int64) => {
+            Layout::Builtin(Builtin::Int64 | Builtin::Int1) => {
                 let dst_reg = self.claim_general_reg(dst)?;
                 ASM::mov_reg64_reg64(&mut self.buf, dst_reg, CC::GENERAL_RETURN_REGS[0]);
                 Ok(())
@@ -899,6 +896,23 @@ impl<
         }
     }
 
+    fn build_num_is_zero(
+        &mut self,
+        dst: &Symbol,
+        src: &Symbol,
+        arg_layout: &Layout<'a>,
+    ) -> Result<(), String> {
+        match arg_layout {
+            Layout::Builtin(Builtin::Int64) => {
+                let dst_reg = self.claim_general_reg(dst)?;
+                let src_reg = self.load_to_general_reg(src)?;
+                ASM::is_zero_reg64_reg64(&mut self.buf, dst_reg, src_reg);
+                Ok(())
+            }
+            x => Err(format!("NumIsZero: layout, {:?}, not implemented yet", x)),
+        }
+    }
+
     fn create_struct(
         &mut self,
         sym: &Symbol,
diff --git a/compiler/gen_dev/src/generic64/x86_64.rs b/compiler/gen_dev/src/generic64/x86_64.rs
index 8e542c1c15..a1fcb8b5e5 100644
--- a/compiler/gen_dev/src/generic64/x86_64.rs
+++ b/compiler/gen_dev/src/generic64/x86_64.rs
@@ -1118,6 +1118,12 @@ impl Assembler<X86_64GeneralReg, X86_64FloatReg> for X86_64Assembler {
         setl_reg64(buf, dst);
     }
 
+    #[inline(always)]
+    fn is_zero_reg64_reg64(buf: &mut Vec<'_, u8>, dst: X86_64GeneralReg, src: X86_64GeneralReg) {
+        test_reg64_reg64(buf, src, src);
+        sete_reg64(buf, dst);
+    }
+
     #[inline(always)]
     fn ret(buf: &mut Vec<'_, u8>) {
         ret(buf);
diff --git a/compiler/gen_dev/src/lib.rs b/compiler/gen_dev/src/lib.rs
index 0aff496a0c..a776bc9704 100644
--- a/compiler/gen_dev/src/lib.rs
+++ b/compiler/gen_dev/src/lib.rs
@@ -5,7 +5,7 @@
 use bumpalo::{collections::Vec, Bump};
 use roc_builtins::bitcode::{self, FloatWidth, IntWidth};
 use roc_collections::all::{MutMap, MutSet};
-use roc_module::ident::TagName;
+use roc_module::ident::{ModuleName, TagName};
 use roc_module::low_level::LowLevel;
 use roc_module::symbol::{Interns, Symbol};
 use roc_mono::ir::{
@@ -236,13 +236,24 @@ where
                                 arg_layouts,
                                 ret_layout,
                             )
-                        } else {
+                        } else if func_sym
+                            .module_string(&self.env().interns)
+                            .starts_with(ModuleName::APP)
+                        {
                             let fn_name = LayoutIds::default()
                                 .get(*func_sym, layout)
                                 .to_symbol_string(*func_sym, &self.env().interns);
                             // Now that the arguments are needed, load them if they are literals.
                             self.load_literal_symbols(arguments)?;
                             self.build_fn_call(sym, fn_name, arguments, arg_layouts, ret_layout)
+                        } else {
+                            self.build_run_low_level_dev(
+                                sym,
+                                *func_sym,
+                                arguments,
+                                arg_layouts,
+                                ret_layout,
+                            )
                         }
                     }
 
@@ -460,6 +471,37 @@ where
         }
     }
 
+    // build dev backend-only low-level
+    fn build_run_low_level_dev(
+        &mut self,
+        sym: &Symbol,
+        func_sym: Symbol,
+        args: &'a [Symbol],
+        arg_layouts: &[Layout<'a>],
+        ret_layout: &Layout<'a>,
+    ) -> Result<(), String> {
+        self.load_literal_symbols(args)?;
+        match func_sym {
+            Symbol::NUM_IS_ZERO => {
+                debug_assert_eq!(
+                    1,
+                    args.len(),
+                    "NumIsZero: expected to have exactly one argument"
+                );
+                debug_assert_eq!(
+                    Layout::Builtin(Builtin::Int1),
+                    *ret_layout,
+                    "NumIsZero: expected to have return layout of type I1"
+                );
+                self.build_num_is_zero(sym, &args[0], &arg_layouts[0])
+            }
+            _ => Err(format!(
+                "the function, {:?}, is not yet implemented",
+                func_sym
+            )),
+        }
+    }
+
     /// build_fn_call creates a call site for a function.
     /// This includes dealing with things like saving regs and propagating the returned value.
     fn build_fn_call(
@@ -541,6 +583,14 @@ where
         arg_layout: &Layout<'a>,
     ) -> Result<(), String>;
 
+    /// build_num_is_zero stores the result of `src == 0` into dst.
+    fn build_num_is_zero(
+        &mut self,
+        dst: &Symbol,
+        src: &Symbol,
+        arg_layout: &Layout<'a>,
+    ) -> Result<(), String>;
+
     /// literal_map gets the map from symbol to literal, used for lazy loading and literal folding.
     fn literal_map(&mut self) -> &mut MutMap<Symbol, Literal<'a>>;