diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e2b119c59f..667c3ef007 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -9,6 +9,9 @@ jobs:
     steps:
       - uses: actions/checkout@v1
 
+      - name: Verify compiler/builtin/bitcode/regenerate.sh was run if necessary
+        run: pushd compiler/builtins/bitcode && ./regenerate.sh && git diff --exit-code ../../gen/src/llvm/builtins.bc && popd
+
       - name: Install LLVM
         run: sudo ./ci/install-llvm.sh 10
 
diff --git a/compiler/builtins/bitcode/README.md b/compiler/builtins/bitcode/README.md
index da1dabea44..823c734851 100644
--- a/compiler/builtins/bitcode/README.md
+++ b/compiler/builtins/bitcode/README.md
@@ -17,42 +17,37 @@ The source we'll use to generate the bitcode is in `src/lib.rs` in this director
 To generate the bitcode, `cd` into `compiler/builtins/bitcode/` and run:
 
 ```bash
-$ cargo rustc --release --lib -- --emit=llvm-bc
+$ ./regenerate.sh
 ```
 
-Then look in the root `roc` source directory under `target/release/deps/` for a file
-with a name like `roc_builtins_bitcode-8da0901c58a73ebf.bc` - except
-probably with a different hash before the `.bc`. If there's more than one
-`*.bc` file in that directory, delete the whole `deps/` directory and re-run
-the `cargo rustc` command above to regenerate it.
-
 > If you want to take a look at the human-readable LLVM IR rather than the
 > bitcode, run this instead and look for a `.ll` file instead of a `.bc` file:
 >
 > ```bash
 > $ cargo rustc --release --lib -- --emit=llvm-ir
 > ```
+>
+> Then look in the root `roc` source directory under `target/release/deps/` for a file
+> with a name like `roc_builtins_bitcode-8da0901c58a73ebf.bc` - except
+> probably with a different hash before the `.bc`. If there's more than one
+> `*.bc` file in that directory, delete the whole `deps/` directory and re-run
+> the `cargo rustc` command above to regenerate it.
 
 **Note**: In order to be able to address the bitcode functions by name, they need to be defined with the `#[no_mangle]` attribute.
 
-## Importing the bitcode
-
 The bitcode is a bunch of bytes that aren't particularly human-readable.
 Since Roc is designed to be distributed as a single binary, these bytes
 need to be included in the raw source somewhere.
 
 The `llvm/src/build.rs` file statically imports these raw bytes
-using the [`include_bytes!` macro](https://doc.rust-lang.org/std/macro.include_bytes.html),
-so we just need to move the `.bc` file from the previous step to the correct
-location.
-
+using the [`include_bytes!` macro](https://doc.rust-lang.org/std/macro.include_bytes.html).
 The current `.bc` file is located at:
 
 ```
 compiler/gen/src/llvm/builtins.bc
 ```
 
-...so you want to overwrite it with the new `.bc` file in `target/deps/`
+The script will automatically replace this `.bc` file with the new one.
 
 Once that's done, `git status` should show that the `builtins.bc` file
 has been changed. Commit that change and you're done!
diff --git a/compiler/builtins/bitcode/regenerate.sh b/compiler/builtins/bitcode/regenerate.sh
new file mode 100755
index 0000000000..f4d7a0cd9a
--- /dev/null
+++ b/compiler/builtins/bitcode/regenerate.sh
@@ -0,0 +1,19 @@
+#!/bin/bash
+
+set -euxo pipefail
+
+# Clear out any existing output files. Sometimes if these are there, rustc
+# doesn't generate the .bc file - or we can end up with more than one .bc
+rm -rf ../../../target/release/deps/
+
+# Regenerate the .bc file
+cargo rustc --release --lib -- --emit=llvm-bc
+
+bc_files=$(ls ../../../target/release/deps/*.bc | wc -l)
+
+if [[ $bc_files != 1 ]]; then
+  echo "More than one .bc file was emitted somehow."
+  exit 1;
+fi
+
+cp ../../../target/release/deps/*.bc ../../gen/src/llvm/builtins.bc
diff --git a/compiler/builtins/bitcode/src/lib.rs b/compiler/builtins/bitcode/src/lib.rs
index 34fbe4bc18..57dfd48ba8 100644
--- a/compiler/builtins/bitcode/src/lib.rs
+++ b/compiler/builtins/bitcode/src/lib.rs
@@ -4,6 +4,8 @@
 #![crate_type = "lib"]
 #![no_std]
 
+mod libm;
+
 /// TODO replace this with a normal Inkwell build_cast call - this was just
 /// used as a proof of concept for getting bitcode importing working!
 #[no_mangle]
@@ -46,202 +48,7 @@ pub fn is_finite_(num: f64) -> bool {
     f64::is_finite(num)
 }
 
-/// Adapted from Rust's libm module, by the Rust core team,
-/// licensed under the Apache License, version 2.0 - https://www.apache.org/licenses/LICENSE-2.0
-/// https://github.com/rust-lang/libm/blob/master/LICENSE-APACHE
-///
-/// Thank you, Rust core team!
-
-// From https://github.com/rust-lang/libm/blob/master/src/math/mod.rs
-#[cfg(not(debug_assertions))]
-macro_rules! i {
-    ($array:expr, $index:expr) => {
-        unsafe { *$array.get_unchecked($index) }
-    };
-    ($array:expr, $index:expr, = , $rhs:expr) => {
-        unsafe {
-            *$array.get_unchecked_mut($index) = $rhs;
-        }
-    };
-    ($array:expr, $index:expr, += , $rhs:expr) => {
-        unsafe {
-            *$array.get_unchecked_mut($index) += $rhs;
-        }
-    };
-    ($array:expr, $index:expr, -= , $rhs:expr) => {
-        unsafe {
-            *$array.get_unchecked_mut($index) -= $rhs;
-        }
-    };
-    ($array:expr, $index:expr, &= , $rhs:expr) => {
-        unsafe {
-            *$array.get_unchecked_mut($index) &= $rhs;
-        }
-    };
-    ($array:expr, $index:expr, == , $rhs:expr) => {
-        unsafe { *$array.get_unchecked_mut($index) == $rhs }
-    };
-}
-
-#[cfg(debug_assertions)]
-macro_rules! i {
-    ($array:expr, $index:expr) => {
-        *$array.get($index).unwrap()
-    };
-    ($array:expr, $index:expr, = , $rhs:expr) => {
-        *$array.get_mut($index).unwrap() = $rhs;
-    };
-    ($array:expr, $index:expr, -= , $rhs:expr) => {
-        *$array.get_mut($index).unwrap() -= $rhs;
-    };
-    ($array:expr, $index:expr, += , $rhs:expr) => {
-        *$array.get_mut($index).unwrap() += $rhs;
-    };
-    ($array:expr, $index:expr, &= , $rhs:expr) => {
-        *$array.get_mut($index).unwrap() &= $rhs;
-    };
-    ($array:expr, $index:expr, == , $rhs:expr) => {
-        *$array.get_mut($index).unwrap() == $rhs
-    };
-}
-
-macro_rules! llvm_intrinsically_optimized {
-    (#[cfg($($clause:tt)*)] $e:expr) => {
-        #[cfg(all(feature = "unstable", $($clause)*))]
-        {
-            if true { // thwart the dead code lint
-                $e
-            }
-        }
-    };
-}
-
-macro_rules! force_eval {
-    ($e:expr) => {
-        unsafe {
-            ::core::ptr::read_volatile(&$e);
-        }
-    };
-}
-
-// From https://github.com/rust-lang/libm/blob/master/src/math/atan.rs
-
-// Clippy fails CI if we don't include overrides below. Since this is copied
-// straight from the libm crate, I figure they had a reason to include
-// the extra percision, so we just silence this warning.
-
-#[allow(clippy::excessive_precision)]
-const ATANHI: [f64; 4] = [
-    4.63647609000806093515e-01, /* atan(0.5)hi 0x3FDDAC67, 0x0561BB4F */
-    7.85398163397448278999e-01, /* atan(1.0)hi 0x3FE921FB, 0x54442D18 */
-    9.82793723247329054082e-01, /* atan(1.5)hi 0x3FEF730B, 0xD281F69B */
-    1.57079632679489655800e+00, /* atan(inf)hi 0x3FF921FB, 0x54442D18 */
-];
-
-#[allow(clippy::excessive_precision)]
-const ATANLO: [f64; 4] = [
-    2.26987774529616870924e-17, /* atan(0.5)lo 0x3C7A2B7F, 0x222F65E2 */
-    3.06161699786838301793e-17, /* atan(1.0)lo 0x3C81A626, 0x33145C07 */
-    1.39033110312309984516e-17, /* atan(1.5)lo 0x3C700788, 0x7AF0CBBD */
-    6.12323399573676603587e-17, /* atan(inf)lo 0x3C91A626, 0x33145C07 */
-];
-
-#[allow(clippy::excessive_precision)]
-const AT: [f64; 11] = [
-    3.33333333333329318027e-01,  /* 0x3FD55555, 0x5555550D */
-    -1.99999999998764832476e-01, /* 0xBFC99999, 0x9998EBC4 */
-    1.42857142725034663711e-01,  /* 0x3FC24924, 0x920083FF */
-    -1.11111104054623557880e-01, /* 0xBFBC71C6, 0xFE231671 */
-    9.09088713343650656196e-02,  /* 0x3FB745CD, 0xC54C206E */
-    -7.69187620504482999495e-02, /* 0xBFB3B0F2, 0xAF749A6D */
-    6.66107313738753120669e-02,  /* 0x3FB10D66, 0xA0D03D51 */
-    -5.83357013379057348645e-02, /* 0xBFADDE2D, 0x52DEFD9A */
-    4.97687799461593236017e-02,  /* 0x3FA97B4B, 0x24760DEB */
-    -3.65315727442169155270e-02, /* 0xBFA2B444, 0x2C6A6C2F */
-    1.62858201153657823623e-02,  /* 0x3F90AD3A, 0xE322DA11 */
-];
-
-#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
-pub fn fabs(x: f64) -> f64 {
-    // On wasm32 we know that LLVM's intrinsic will compile to an optimized
-    // `f64.abs` native instruction, so we can leverage this for both code size
-    // and speed.
-    llvm_intrinsically_optimized! {
-        #[cfg(target_arch = "wasm32")] {
-            return unsafe { ::core::intrinsics::fabsf64(x) }
-        }
-    }
-    f64::from_bits(x.to_bits() & (u64::MAX / 2))
-}
-
 #[no_mangle]
-#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
 pub fn atan_(x: f64) -> f64 {
-    let mut x = x;
-    let mut ix = (x.to_bits() >> 32) as u32;
-    let sign = ix >> 31;
-    ix &= 0x7fff_ffff;
-    if ix >= 0x4410_0000 {
-        if x.is_nan() {
-            return x;
-        }
-
-        let z = ATANHI[3] + f64::from_bits(0x0380_0000); // 0x1p-120f
-        return if sign != 0 { -z } else { z };
-    }
-
-    let id = if ix < 0x3fdc_0000 {
-        /* |x| < 0.4375 */
-        if ix < 0x3e40_0000 {
-            /* |x| < 2^-27 */
-            if ix < 0x0010_0000 {
-                /* raise underflow for subnormal x */
-                force_eval!(x as f32);
-            }
-
-            return x;
-        }
-
-        -1
-    } else {
-        x = fabs(x);
-        if ix < 0x3ff30000 {
-            /* |x| < 1.1875 */
-            if ix < 0x3fe60000 {
-                /* 7/16 <= |x| < 11/16 */
-                x = (2. * x - 1.) / (2. + x);
-                0
-            } else {
-                /* 11/16 <= |x| < 19/16 */
-                x = (x - 1.) / (x + 1.);
-                1
-            }
-        } else if ix < 0x40038000 {
-            /* |x| < 2.4375 */
-            x = (x - 1.5) / (1. + 1.5 * x);
-            2
-        } else {
-            /* 2.4375 <= |x| < 2^66 */
-            x = -1. / x;
-            3
-        }
-    };
-
-    let z = x * x;
-    let w = z * z;
-    /* break sum from i=0 to 10 AT[i]z**(i+1) into odd and even poly */
-    let s1 = z * (AT[0] + w * (AT[2] + w * (AT[4] + w * (AT[6] + w * (AT[8] + w * AT[10])))));
-    let s2 = w * (AT[1] + w * (AT[3] + w * (AT[5] + w * (AT[7] + w * AT[9]))));
-
-    if id < 0 {
-        return x - x * (s1 + s2);
-    }
-
-    let z = i!(ATANHI, id as usize) - (x * (s1 + s2) - i!(ATANLO, id as usize) - x);
-
-    if sign != 0 {
-        -z
-    } else {
-        z
-    }
+    libm::atan(x)
 }
diff --git a/compiler/builtins/bitcode/src/libm.rs b/compiler/builtins/bitcode/src/libm.rs
new file mode 100644
index 0000000000..67cc4d942d
--- /dev/null
+++ b/compiler/builtins/bitcode/src/libm.rs
@@ -0,0 +1,199 @@
+/// Adapted from Rust's libm module, by the Rust core team,
+/// licensed under the Apache License, version 2.0 - https://www.apache.org/licenses/LICENSE-2.0
+/// https://github.com/rust-lang/libm/blob/master/LICENSE-APACHE
+///
+/// Thank you, Rust core team!
+
+// From https://github.com/rust-lang/libm/blob/master/src/math/mod.rs
+#[cfg(not(debug_assertions))]
+macro_rules! i {
+    ($array:expr, $index:expr) => {
+        unsafe { *$array.get_unchecked($index) }
+    };
+    ($array:expr, $index:expr, = , $rhs:expr) => {
+        unsafe {
+            *$array.get_unchecked_mut($index) = $rhs;
+        }
+    };
+    ($array:expr, $index:expr, += , $rhs:expr) => {
+        unsafe {
+            *$array.get_unchecked_mut($index) += $rhs;
+        }
+    };
+    ($array:expr, $index:expr, -= , $rhs:expr) => {
+        unsafe {
+            *$array.get_unchecked_mut($index) -= $rhs;
+        }
+    };
+    ($array:expr, $index:expr, &= , $rhs:expr) => {
+        unsafe {
+            *$array.get_unchecked_mut($index) &= $rhs;
+        }
+    };
+    ($array:expr, $index:expr, == , $rhs:expr) => {
+        unsafe { *$array.get_unchecked_mut($index) == $rhs }
+    };
+}
+
+#[cfg(debug_assertions)]
+macro_rules! i {
+    ($array:expr, $index:expr) => {
+        *$array.get($index).unwrap()
+    };
+    ($array:expr, $index:expr, = , $rhs:expr) => {
+        *$array.get_mut($index).unwrap() = $rhs;
+    };
+    ($array:expr, $index:expr, -= , $rhs:expr) => {
+        *$array.get_mut($index).unwrap() -= $rhs;
+    };
+    ($array:expr, $index:expr, += , $rhs:expr) => {
+        *$array.get_mut($index).unwrap() += $rhs;
+    };
+    ($array:expr, $index:expr, &= , $rhs:expr) => {
+        *$array.get_mut($index).unwrap() &= $rhs;
+    };
+    ($array:expr, $index:expr, == , $rhs:expr) => {
+        *$array.get_mut($index).unwrap() == $rhs
+    };
+}
+
+macro_rules! llvm_intrinsically_optimized {
+    (#[cfg($($clause:tt)*)] $e:expr) => {
+        #[cfg(all(feature = "unstable", $($clause)*))]
+        {
+            if true { // thwart the dead code lint
+                $e
+            }
+        }
+    };
+}
+
+macro_rules! force_eval {
+    ($e:expr) => {
+        unsafe {
+            ::core::ptr::read_volatile(&$e);
+        }
+    };
+}
+
+// From https://github.com/rust-lang/libm/blob/master/src/math/atan.rs
+
+// Clippy fails CI if we don't include overrides below. Since this is copied
+// straight from the libm crate, I figure they had a reason to include
+// the extra percision, so we just silence this warning.
+
+#[allow(clippy::excessive_precision)]
+const ATANHI: [f64; 4] = [
+    4.63647609000806093515e-01, /* atan(0.5)hi 0x3FDDAC67, 0x0561BB4F */
+    7.85398163397448278999e-01, /* atan(1.0)hi 0x3FE921FB, 0x54442D18 */
+    9.82793723247329054082e-01, /* atan(1.5)hi 0x3FEF730B, 0xD281F69B */
+    1.57079632679489655800e+00, /* atan(inf)hi 0x3FF921FB, 0x54442D18 */
+];
+
+#[allow(clippy::excessive_precision)]
+const ATANLO: [f64; 4] = [
+    2.26987774529616870924e-17, /* atan(0.5)lo 0x3C7A2B7F, 0x222F65E2 */
+    3.06161699786838301793e-17, /* atan(1.0)lo 0x3C81A626, 0x33145C07 */
+    1.39033110312309984516e-17, /* atan(1.5)lo 0x3C700788, 0x7AF0CBBD */
+    6.12323399573676603587e-17, /* atan(inf)lo 0x3C91A626, 0x33145C07 */
+];
+
+#[allow(clippy::excessive_precision)]
+const AT: [f64; 11] = [
+    3.33333333333329318027e-01,  /* 0x3FD55555, 0x5555550D */
+    -1.99999999998764832476e-01, /* 0xBFC99999, 0x9998EBC4 */
+    1.42857142725034663711e-01,  /* 0x3FC24924, 0x920083FF */
+    -1.11111104054623557880e-01, /* 0xBFBC71C6, 0xFE231671 */
+    9.09088713343650656196e-02,  /* 0x3FB745CD, 0xC54C206E */
+    -7.69187620504482999495e-02, /* 0xBFB3B0F2, 0xAF749A6D */
+    6.66107313738753120669e-02,  /* 0x3FB10D66, 0xA0D03D51 */
+    -5.83357013379057348645e-02, /* 0xBFADDE2D, 0x52DEFD9A */
+    4.97687799461593236017e-02,  /* 0x3FA97B4B, 0x24760DEB */
+    -3.65315727442169155270e-02, /* 0xBFA2B444, 0x2C6A6C2F */
+    1.62858201153657823623e-02,  /* 0x3F90AD3A, 0xE322DA11 */
+];
+
+#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
+pub fn fabs(x: f64) -> f64 {
+    // On wasm32 we know that LLVM's intrinsic will compile to an optimized
+    // `f64.abs` native instruction, so we can leverage this for both code size
+    // and speed.
+    llvm_intrinsically_optimized! {
+        #[cfg(target_arch = "wasm32")] {
+            return unsafe { ::core::intrinsics::fabsf64(x) }
+        }
+    }
+    f64::from_bits(x.to_bits() & (u64::MAX / 2))
+}
+
+#[inline(always)]
+#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
+pub fn atan(x: f64) -> f64 {
+    let mut x = x;
+    let mut ix = (x.to_bits() >> 32) as u32;
+    let sign = ix >> 31;
+    ix &= 0x7fff_ffff;
+    if ix >= 0x4410_0000 {
+        if x.is_nan() {
+            return x;
+        }
+
+        let z = ATANHI[3] + f64::from_bits(0x0380_0000); // 0x1p-120f
+        return if sign != 0 { -z } else { z };
+    }
+
+    let id = if ix < 0x3fdc_0000 {
+        /* |x| < 0.4375 */
+        if ix < 0x3e40_0000 {
+            /* |x| < 2^-27 */
+            if ix < 0x0010_0000 {
+                /* raise underflow for subnormal x */
+                force_eval!(x as f32);
+            }
+
+            return x;
+        }
+
+        -1
+    } else {
+        x = fabs(x);
+        if ix < 0x3ff30000 {
+            /* |x| < 1.1875 */
+            if ix < 0x3fe60000 {
+                /* 7/16 <= |x| < 11/16 */
+                x = (2. * x - 1.) / (2. + x);
+                0
+            } else {
+                /* 11/16 <= |x| < 19/16 */
+                x = (x - 1.) / (x + 1.);
+                1
+            }
+        } else if ix < 0x40038000 {
+            /* |x| < 2.4375 */
+            x = (x - 1.5) / (1. + 1.5 * x);
+            2
+        } else {
+            /* 2.4375 <= |x| < 2^66 */
+            x = -1. / x;
+            3
+        }
+    };
+
+    let z = x * x;
+    let w = z * z;
+    /* break sum from i=0 to 10 AT[i]z**(i+1) into odd and even poly */
+    let s1 = z * (AT[0] + w * (AT[2] + w * (AT[4] + w * (AT[6] + w * (AT[8] + w * AT[10])))));
+    let s2 = w * (AT[1] + w * (AT[3] + w * (AT[5] + w * (AT[7] + w * AT[9]))));
+
+    if id < 0 {
+        return x - x * (s1 + s2);
+    }
+
+    let z = i!(ATANHI, id as usize) - (x * (s1 + s2) - i!(ATANLO, id as usize) - x);
+
+    if sign != 0 {
+        -z
+    } else {
+        z
+    }
+}
diff --git a/compiler/gen/src/llvm/builtins.bc b/compiler/gen/src/llvm/builtins.bc
index 9518ecae24..9d937909bb 100644
Binary files a/compiler/gen/src/llvm/builtins.bc and b/compiler/gen/src/llvm/builtins.bc differ