diff --git a/rp2040-hal/Cargo.toml b/rp2040-hal/Cargo.toml
index 342b76c..ecf2ffc 100644
--- a/rp2040-hal/Cargo.toml
+++ b/rp2040-hal/Cargo.toml
@@ -54,3 +54,5 @@ rt = ["rp2040-pac/rt"]
 # embassy-traits = ["embassy_traits", "futures"]
 alloc = []
 rom-func-cache = []
+disable-intrinsics = []
+rom-v2-intrinsics = []
diff --git a/rp2040-hal/src/intrinsics.rs b/rp2040-hal/src/intrinsics.rs
new file mode 100644
index 0000000..dbb67a9
--- /dev/null
+++ b/rp2040-hal/src/intrinsics.rs
@@ -0,0 +1,271 @@
+/// Generate a series of aliases for an intrinsic function.
+macro_rules! intrinsics_aliases {
+    (
+        extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty,
+    ) => {};
+    (
+        unsafe extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty,
+    ) => {};
+
+    (
+        extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty,
+        $alias:ident
+        $($rest:ident)*
+    ) => {
+        #[cfg(all(target_arch = "arm", not(feature = "disable-intrinsics")))]
+        intrinsics! {
+            extern $abi fn $alias( $($argname: $ty),* ) -> $ret {
+                $name($($argname),*)
+            }
+        }
+
+        intrinsics_aliases! {
+            extern $abi fn $name( $($argname: $ty),* ) -> $ret,
+            $($rest)*
+        }
+    };
+
+    (
+        unsafe extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty,
+        $alias:ident
+        $($rest:ident)*
+    ) => {
+        #[cfg(all(target_arch = "arm", not(feature = "disable-intrinsics")))]
+        intrinsics! {
+            unsafe extern $abi fn $alias( $($argname: $ty),* ) -> $ret {
+                $name($($argname),*)
+            }
+        }
+
+        intrinsics_aliases! {
+            unsafe extern $abi fn $name( $($argname: $ty),* ) -> $ret,
+            $($rest)*
+        }
+    };
+}
+
+/// The macro used to define overridden intrinsics.
+///
+/// This is heavily inspired by the macro used by compiler-builtins.  The idea
+/// is to abstract anything special that needs to be done to override an
+/// intrinsic function.  Intrinsic generation is disabled for non-ARM targets
+/// so things like CI and docs generation do not have problems.  Additionally
+/// they can be disabled with the crate feature `disable-intrinsics` for
+/// testing or comparing performance.
+///
+/// Like the compiler-builtins macro, it accepts a series of functions that
+/// looks like normal Rust code:
+///
+///     intrinsics! {
+///         extern "C" fn foo(a: i32) -> u32 {
+///             // ...
+///         }
+///
+///         #[nonstandard_attribute]
+///         extern "C" fn bar(a: i32) -> u32 {
+///             // ...
+///         }
+///     }
+///
+/// Each function can also be decorated with nonstandard attributes to control
+/// additional behaviour:
+///
+/// * `slower_than_default` - indicates that the override is slower than the
+///   default implementation.  Currently this just disables the override
+///   entirely.
+/// * `bootrom_v2` - indicates that the override is only available
+///   on a V2 bootrom or higher.  Only enabled when the feature
+///   `rom-v2-intrinsics` is set.
+/// * `alias` - accepts a list of names to alias the intrinsic to.
+/// * `aeabi` - accepts a list of ARM EABI names to alias to.
+///
+macro_rules! intrinsics {
+    () => {};
+
+    (
+        #[slower_than_default]
+        $(#[$($attr:tt)*])*
+        extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty {
+            $($body:tt)*
+        }
+
+        $($rest:tt)*
+    ) => {
+        // Not exported, but defined so the actual implementation is
+        // considered used
+        #[allow(dead_code)]
+        fn $name( $($argname: $ty),* ) -> $ret {
+            $($body)*
+        }
+
+        intrinsics!($($rest)*);
+    };
+
+    (
+        #[bootrom_v2]
+        $(#[$($attr:tt)*])*
+        extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty {
+            $($body:tt)*
+        }
+
+        $($rest:tt)*
+    ) => {
+        // Not exported, but defined so the actual implementation is
+        // considered used
+        #[cfg(not(feature = "rom-v2-intrinsics"))]
+        #[allow(dead_code)]
+        fn $name( $($argname: $ty),* ) -> $ret {
+            $($body)*
+        }
+
+        #[cfg(feature = "rom-v2-intrinsics")]
+        intrinsics! {
+            $(#[$($attr)*])*
+            extern $abi fn $name( $($argname: $ty),* ) -> $ret {
+                $($body)*
+            }
+        }
+
+        intrinsics!($($rest)*);
+    };
+
+    (
+        #[alias = $($alias:ident),*]
+        $(#[$($attr:tt)*])*
+        extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty {
+            $($body:tt)*
+        }
+
+        $($rest:tt)*
+    ) => {
+        intrinsics! {
+            $(#[$($attr)*])*
+            extern $abi fn $name( $($argname: $ty),* ) -> $ret {
+                $($body)*
+            }
+        }
+
+        intrinsics_aliases! {
+            extern $abi fn $name( $($argname: $ty),* ) -> $ret,
+            $($alias) *
+        }
+
+        intrinsics!($($rest)*);
+    };
+
+    (
+        #[alias = $($alias:ident),*]
+        $(#[$($attr:tt)*])*
+        unsafe extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty {
+            $($body:tt)*
+        }
+
+        $($rest:tt)*
+    ) => {
+        intrinsics! {
+            $(#[$($attr)*])*
+            unsafe extern $abi fn $name( $($argname: $ty),* ) -> $ret {
+                $($body)*
+            }
+        }
+
+        intrinsics_aliases! {
+            unsafe extern $abi fn $name( $($argname: $ty),* ) -> $ret,
+            $($alias) *
+        }
+
+        intrinsics!($($rest)*);
+    };
+
+    (
+        #[aeabi = $($alias:ident),*]
+        $(#[$($attr:tt)*])*
+        extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty {
+            $($body:tt)*
+        }
+
+        $($rest:tt)*
+    ) => {
+        intrinsics! {
+            $(#[$($attr)*])*
+            extern $abi fn $name( $($argname: $ty),* ) -> $ret {
+                $($body)*
+            }
+        }
+
+        intrinsics_aliases! {
+            extern "aapcs" fn $name( $($argname: $ty),* ) -> $ret,
+            $($alias) *
+        }
+
+        intrinsics!($($rest)*);
+    };
+
+    (
+        $(#[$($attr:tt)*])*
+        extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty {
+            $($body:tt)*
+        }
+
+        $($rest:tt)*
+    ) => {
+        #[cfg(all(target_arch = "arm", not(feature = "disable-intrinsics")))]
+        $(#[$($attr)*])*
+        extern $abi fn $name( $($argname: $ty),* ) -> $ret {
+            $($body)*
+        }
+
+        #[cfg(all(target_arch = "arm", not(feature = "disable-intrinsics")))]
+        mod $name {
+            #[no_mangle]
+            $(#[$($attr)*])*
+            pub extern $abi fn $name( $($argname: $ty),* ) -> $ret {
+                super::$name($($argname),*)
+            }
+        }
+
+        // Not exported, but defined so the actual implementation is
+        // considered used
+        #[cfg(not(all(target_arch = "arm", not(feature = "disable-intrinsics"))))]
+        #[allow(dead_code)]
+        fn $name( $($argname: $ty),* ) -> $ret {
+            $($body)*
+        }
+
+        intrinsics!($($rest)*);
+    };
+
+    (
+        $(#[$($attr:tt)*])*
+        unsafe extern $abi:tt fn $name:ident( $($argname:ident: $ty:ty),* ) -> $ret:ty {
+            $($body:tt)*
+        }
+
+        $($rest:tt)*
+    ) => {
+        #[cfg(all(target_arch = "arm", not(feature = "disable-intrinsics")))]
+        $(#[$($attr)*])*
+        unsafe extern $abi fn $name( $($argname: $ty),* ) -> $ret {
+            $($body)*
+        }
+
+        #[cfg(all(target_arch = "arm", not(feature = "disable-intrinsics")))]
+        mod $name {
+            #[no_mangle]
+            $(#[$($attr)*])*
+            pub unsafe extern $abi fn $name( $($argname: $ty),* ) -> $ret {
+                super::$name($($argname),*)
+            }
+        }
+
+        // Not exported, but defined so the actual implementation is
+        // considered used
+        #[cfg(not(all(target_arch = "arm", not(feature = "disable-intrinsics"))))]
+        #[allow(dead_code)]
+        unsafe fn $name( $($argname: $ty),* ) -> $ret {
+            $($body)*
+        }
+
+        intrinsics!($($rest)*);
+    };
+}
diff --git a/rp2040-hal/src/lib.rs b/rp2040-hal/src/lib.rs
index 6f70dc3..47850da 100644
--- a/rp2040-hal/src/lib.rs
+++ b/rp2040-hal/src/lib.rs
@@ -15,6 +15,9 @@ pub use paste;
 
 pub extern crate rp2040_pac as pac;
 
+#[macro_use]
+mod intrinsics;
+
 pub mod adc;
 pub(crate) mod atomic_register_access;
 pub mod clocks;
diff --git a/rp2040-hal/src/rom_data.rs b/rp2040-hal/src/rom_data.rs
index dd46451..4e28aec 100644
--- a/rp2040-hal/src/rom_data.rs
+++ b/rp2040-hal/src/rom_data.rs
@@ -279,6 +279,60 @@ rom_functions! {
     b"WV" unsafe fn wait_for_vector() -> !;
 }
 
+// Various C intrinsics in the ROM
+intrinsics! {
+    #[alias = __popcountdi2]
+    extern "C" fn __popcountsi2(x: u32) -> u32 {
+        popcount32(x)
+    }
+
+    #[alias = __clzdi2]
+    extern "C" fn __clzsi2(x: u32) -> u32 {
+        clz32(x)
+    }
+
+    #[alias = __ctzdi2]
+    extern "C" fn __ctzsi2(x: u32) -> u32 {
+        ctz32(x)
+    }
+
+    // __rbit is only unofficial, but it show up in the ARM documentation,
+    // so may as well hook it up.
+    #[alias = __rbitl]
+    extern "C" fn __rbit(x: u32) -> u32 {
+        reverse32(x)
+    }
+
+    unsafe extern "aapcs" fn __aeabi_memset(dest: *mut u8, n: usize, c: i32) -> () {
+        // Different argument order
+        memset(dest, c as u8, n as u32);
+    }
+
+    #[alias = __aeabi_memset8]
+    unsafe extern "aapcs" fn __aeabi_memset4(dest: *mut u8, n: usize, c: i32) -> () {
+        // Different argument order
+        memset4(dest as *mut u32, c as u8, n as u32);
+    }
+
+    unsafe extern "aapcs" fn __aeabi_memclr(dest: *mut u8, n: usize) -> () {
+        memset(dest, 0, n as u32);
+    }
+
+    #[alias = __aeabi_memclr8]
+    unsafe extern "aapcs" fn __aeabi_memclr4(dest: *mut u8, n: usize) -> () {
+        memset4(dest as *mut u32, 0, n as u32);
+    }
+
+    unsafe extern "aapcs" fn __aeabi_memcpy(dest: *mut u8, src: *const u8, n: usize) -> () {
+        memcpy(dest, src, n as u32);
+    }
+
+    #[alias = __aeabi_memcpy8]
+    unsafe extern "aapcs" fn __aeabi_memcpy4(dest: *mut u8, src: *const u8, n: usize) -> () {
+        memcpy44(dest as *mut u32, src as *const u32, n as u32);
+    }
+}
+
 unsafe fn convert_str(s: *const u8) -> &'static str {
     let mut end = s;
     while *end != 0 {