diff --git a/src/intrinsic/mod.rs b/src/intrinsic/mod.rs
index db9f32bad5a..f50d096e2e8 100644
--- a/src/intrinsic/mod.rs
+++ b/src/intrinsic/mod.rs
@@ -405,43 +405,15 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tc
             | sym::saturating_sub => {
                 match int_type_width_signed(args[0].layout.ty, self) {
                     Some((width, signed)) => match name {
-                        sym::ctlz | sym::cttz => {
-                            let func = self.current_func();
-                            let then_block = func.new_block("then");
-                            let else_block = func.new_block("else");
-                            let after_block = func.new_block("after");
-
-                            let arg = args[0].immediate();
-                            let result = func.new_local(None, self.u32_type, "zeros");
-                            let zero = self.cx.gcc_zero(arg.get_type());
-                            let cond = self.gcc_icmp(IntPredicate::IntEQ, arg, zero);
-                            self.llbb().end_with_conditional(None, cond, then_block, else_block);
-
-                            let zero_result = self.cx.gcc_uint(self.u32_type, width);
-                            then_block.add_assignment(None, result, zero_result);
-                            then_block.end_with_jump(None, after_block);
-
-                            // NOTE: since jumps were added in a place
-                            // count_leading_zeroes() does not expect, the current block
-                            // in the state need to be updated.
-                            self.switch_to_block(else_block);
-
-                            let zeros = match name {
-                                sym::ctlz => self.count_leading_zeroes(width, arg),
-                                sym::cttz => self.count_trailing_zeroes(width, arg),
-                                _ => unreachable!(),
-                            };
-                            self.llbb().add_assignment(None, result, zeros);
-                            self.llbb().end_with_jump(None, after_block);
-
-                            // NOTE: since jumps were added in a place rustc does not
-                            // expect, the current block in the state need to be updated.
-                            self.switch_to_block(after_block);
-
-                            result.to_rvalue()
+                        sym::ctlz => self.count_leading_zeroes(width, args[0].immediate()),
+
+                        sym::ctlz_nonzero => {
+                            self.count_leading_zeroes_nonzero(width, args[0].immediate())
+                        }
+                        sym::cttz => self.count_trailing_zeroes(width, args[0].immediate()),
+                        sym::cttz_nonzero => {
+                            self.count_trailing_zeroes_nonzero(width, args[0].immediate())
                         }
-                        sym::ctlz_nonzero => self.count_leading_zeroes(width, args[0].immediate()),
-                        sym::cttz_nonzero => self.count_trailing_zeroes(width, args[0].immediate()),
                         sym::ctpop => self.pop_count(args[0].immediate()),
                         sym::bswap => {
                             if width == 8 {
@@ -876,179 +848,175 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
         self.gcc_int_cast(result, result_type)
     }
 
-    fn count_leading_zeroes(&mut self, width: u64, arg: RValue<'gcc>) -> RValue<'gcc> {
-        // TODO(antoyo): use width?
-        let arg_type = arg.get_type();
-        let result_type = self.u32_type;
-        let arg = if arg_type.is_signed(self.cx) {
-            let new_type = arg_type.to_unsigned(self.cx);
-            self.gcc_int_cast(arg, new_type)
+    fn count_zeroes(&mut self, width: u64, arg: RValue<'gcc>, count_leading: bool) -> RValue<'gcc> {
+        // if arg is 0, early return 0, else call count_leading_zeroes_nonzero or count_trailing_zeroes_nonzero
+        let func = self.current_func();
+        let then_block = func.new_block("then");
+        let else_block = func.new_block("else");
+        let after_block = func.new_block("after");
+
+        let result = func.new_local(None, self.u32_type, "zeros");
+        let zero = self.cx.gcc_zero(arg.get_type());
+        let cond = self.gcc_icmp(IntPredicate::IntEQ, arg, zero);
+        self.llbb().end_with_conditional(None, cond, then_block, else_block);
+
+        let zero_result = self.cx.gcc_uint(self.u32_type, width);
+        then_block.add_assignment(None, result, zero_result);
+        then_block.end_with_jump(None, after_block);
+
+        // NOTE: since jumps were added in a place count_xxxxing_zeroes_nonzero() does not expect,
+        // the current block in the state need to be updated.
+        self.switch_to_block(else_block);
+
+        let zeros = if count_leading {
+            self.count_leading_zeroes_nonzero(width, arg)
         } else {
-            arg
+            self.count_trailing_zeroes_nonzero(width, arg)
         };
-        let arg_type = arg.get_type();
-        let count_leading_zeroes =
-            // TODO(antoyo): write a new function Type::is_compatible_with(&Type) and use it here
-            // instead of using is_uint().
-            if arg_type.is_uchar(self.cx) || arg_type.is_ushort(self.cx) || arg_type.is_uint(self.cx) {
-                "__builtin_clz"
-            }
-            else if arg_type.is_ulong(self.cx) {
-                "__builtin_clzl"
-            }
-            else if arg_type.is_ulonglong(self.cx) {
-                "__builtin_clzll"
-            }
-            else if width == 128 {
-                // Algorithm from: https://stackoverflow.com/a/28433850/389119
-                let array_type = self.context.new_array_type(None, arg_type, 3);
-                let result = self.current_func()
-                    .new_local(None, array_type, "count_loading_zeroes_results");
-
-                let sixty_four = self.const_uint(arg_type, 64);
-                let shift = self.lshr(arg, sixty_four);
-                let high = self.gcc_int_cast(shift, self.u64_type);
-                let low = self.gcc_int_cast(arg, self.u64_type);
-
-                let zero = self.context.new_rvalue_zero(self.usize_type);
-                let one = self.context.new_rvalue_one(self.usize_type);
-                let two = self.context.new_rvalue_from_long(self.usize_type, 2);
-
-                let clzll = self.context.get_builtin_function("__builtin_clzll");
-
-                let first_elem = self.context.new_array_access(None, result, zero);
-                let first_value = self.gcc_int_cast(self.context.new_call(None, clzll, &[high]), arg_type);
-                self.llbb()
-                    .add_assignment(self.location, first_elem, first_value);
-
-                let second_elem = self.context.new_array_access(self.location, result, one);
-                let cast = self.gcc_int_cast(self.context.new_call(self.location, clzll, &[low]), arg_type);
-                let second_value = self.add(cast, sixty_four);
-                self.llbb()
-                    .add_assignment(self.location, second_elem, second_value);
-
-                let third_elem = self.context.new_array_access(self.location, result, two);
-                let third_value = self.const_uint(arg_type, 128);
-                self.llbb()
-                    .add_assignment(self.location, third_elem, third_value);
-
-                let not_high = self.context.new_unary_op(self.location, UnaryOp::LogicalNegate, self.u64_type, high);
-                let not_low = self.context.new_unary_op(self.location, UnaryOp::LogicalNegate, self.u64_type, low);
-                let not_low_and_not_high = not_low & not_high;
-                let index = not_high + not_low_and_not_high;
-                // NOTE: the following cast is necessary to avoid a GIMPLE verification failure in
-                // gcc.
-                // TODO(antoyo): do the correct verification in libgccjit to avoid an error at the
-                // compilation stage.
-                let index = self.context.new_cast(self.location, index, self.i32_type);
-
-                let res = self.context.new_array_access(self.location, result, index);
-
-                return self.gcc_int_cast(res.to_rvalue(), result_type);
-            }
-            else {
-                let count_leading_zeroes = self.context.get_builtin_function("__builtin_clzll");
-                let arg = self.context.new_cast(self.location, arg, self.ulonglong_type);
-                let diff = self.ulonglong_type.get_size() as i64 - arg_type.get_size() as i64;
-                let diff = self.context.new_rvalue_from_long(self.int_type, diff * 8);
-                let res = self.context.new_call(self.location, count_leading_zeroes, &[arg]) - diff;
-                return self.context.new_cast(self.location, res, result_type);
-            };
-        let count_leading_zeroes = self.context.get_builtin_function(count_leading_zeroes);
-        let res = self.context.new_call(self.location, count_leading_zeroes, &[arg]);
-        self.context.new_cast(self.location, res, result_type)
+        self.llbb().add_assignment(None, result, zeros);
+        self.llbb().end_with_jump(None, after_block);
+
+        // NOTE: since jumps were added in a place rustc does not
+        // expect, the current block in the state need to be updated.
+        self.switch_to_block(after_block);
+
+        result.to_rvalue()
     }
 
-    fn count_trailing_zeroes(&mut self, _width: u64, arg: RValue<'gcc>) -> RValue<'gcc> {
-        let arg_type = arg.get_type();
+    fn count_zeroes_nonzero(
+        &mut self,
+        width: u64,
+        arg: RValue<'gcc>,
+        count_leading: bool,
+    ) -> RValue<'gcc> {
+        // Pre-condition: arg is guaranteed to not be 0 by caller
+        fn use_builtin_function<'a, 'gcc, 'tcx>(
+            builder: &mut Builder<'a, 'gcc, 'tcx>,
+            builtin: &str,
+            arg: RValue<'gcc>,
+            arg_type: gccjit::Type<'gcc>,
+            expected_type: gccjit::Type<'gcc>,
+        ) -> RValue<'gcc> {
+            let arg = if arg_type != expected_type {
+                builder.context.new_cast(builder.location, arg, expected_type)
+            } else {
+                arg
+            };
+            let builtin = builder.context.get_builtin_function(builtin);
+            let res = builder.context.new_call(builder.location, builtin, &[arg]);
+            builder.context.new_cast(builder.location, res, builder.u32_type)
+        }
+
+        // TODO(antoyo): use width?
         let result_type = self.u32_type;
+        let mut arg_type = arg.get_type();
         let arg = if arg_type.is_signed(self.cx) {
-            let new_type = arg_type.to_unsigned(self.cx);
-            self.gcc_int_cast(arg, new_type)
+            arg_type = arg_type.to_unsigned(self.cx);
+            self.gcc_int_cast(arg, arg_type)
         } else {
             arg
         };
-        let arg_type = arg.get_type();
-        let (count_trailing_zeroes, expected_type) =
-            // TODO(antoyo): write a new function Type::is_compatible_with(&Type) and use it here
-            // instead of using is_uint().
-            if arg_type.is_uchar(self.cx) || arg_type.is_ushort(self.cx) || arg_type.is_uint(self.cx) {
-                // NOTE: we don't need to & 0xFF for uchar because the result is undefined on zero.
-                ("__builtin_ctz", self.cx.uint_type)
-            }
-            else if arg_type.is_ulong(self.cx) {
-                ("__builtin_ctzl", self.cx.ulong_type)
-            }
-            else if arg_type.is_ulonglong(self.cx) {
-                ("__builtin_ctzll", self.cx.ulonglong_type)
-            }
-            else if arg_type.is_u128(self.cx) {
-                // Adapted from the algorithm to count leading zeroes from: https://stackoverflow.com/a/28433850/389119
-                let array_type = self.context.new_array_type(None, arg_type, 3);
-                let result = self.current_func()
-                    .new_local(None, array_type, "count_loading_zeroes_results");
-
-                let sixty_four = self.gcc_int(arg_type, 64);
-                let shift = self.gcc_lshr(arg, sixty_four);
-                let high = self.gcc_int_cast(shift, self.u64_type);
-                let low = self.gcc_int_cast(arg, self.u64_type);
-
-                let zero = self.context.new_rvalue_zero(self.usize_type);
-                let one = self.context.new_rvalue_one(self.usize_type);
-                let two = self.context.new_rvalue_from_long(self.usize_type, 2);
-
-                let ctzll = self.context.get_builtin_function("__builtin_ctzll");
-
-                let first_elem = self.context.new_array_access(self.location, result, zero);
-                let first_value = self.gcc_int_cast(self.context.new_call(self.location, ctzll, &[low]), arg_type);
-                self.llbb()
-                    .add_assignment(self.location, first_elem, first_value);
-
-                let second_elem = self.context.new_array_access(self.location, result, one);
-                let second_value = self.gcc_add(self.gcc_int_cast(self.context.new_call(self.location, ctzll, &[high]), arg_type), sixty_four);
-                self.llbb()
-                    .add_assignment(self.location, second_elem, second_value);
-
-                let third_elem = self.context.new_array_access(self.location, result, two);
-                let third_value = self.gcc_int(arg_type, 128);
-                self.llbb()
-                    .add_assignment(self.location, third_elem, third_value);
-
-                let not_low = self.context.new_unary_op(self.location, UnaryOp::LogicalNegate, self.u64_type, low);
-                let not_high = self.context.new_unary_op(self.location, UnaryOp::LogicalNegate, self.u64_type, high);
-                let not_low_and_not_high = not_low & not_high;
-                let index = not_low + not_low_and_not_high;
-                // NOTE: the following cast is necessary to avoid a GIMPLE verification failure in
-                // gcc.
-                // TODO(antoyo): do the correct verification in libgccjit to avoid an error at the
-                // compilation stage.
-                let index = self.context.new_cast(self.location, index, self.i32_type);
-
-                let res = self.context.new_array_access(self.location, result, index);
-
-                return self.gcc_int_cast(res.to_rvalue(), result_type);
-            }
-            else {
+        // TODO(antoyo): write a new function Type::is_compatible_with(&Type) and use it here
+        // instead of using is_uint().
+        if arg_type.is_uchar(self.cx) || arg_type.is_ushort(self.cx) || arg_type.is_uint(self.cx) {
+            let builtin = if count_leading { "__builtin_clz" } else { "__builtin_ctz" };
+            use_builtin_function(self, builtin, arg, arg_type, self.cx.uint_type)
+        } else if arg_type.is_ulong(self.cx) {
+            let builtin = if count_leading { "__builtin_clzl" } else { "__builtin_ctzl" };
+            use_builtin_function(self, builtin, arg, arg_type, self.cx.uint_type)
+        } else if arg_type.is_ulonglong(self.cx) {
+            let builtin = if count_leading { "__builtin_clzll" } else { "__builtin_ctzll" };
+            use_builtin_function(self, builtin, arg, arg_type, self.cx.uint_type)
+        } else if width == 128 {
+            // arg is guaranteed to not be 0, so either its 64 high or 64 low bits are not 0
+            // __buildin_clzll is UB when called with 0, so call it on the 64 high bits if they are not 0,
+            // else call it on the 64 low bits and add 64. In the else case, 64 low bits can't be 0
+            // because arg is not 0.
+            // __buildin_ctzll is UB when called with 0, so call it on the 64 low bits if they are not 0,
+            // else call it on the 64 high bits and add 64. In the else case, 64 high bits can't be 0
+            // because arg is not 0.
+
+            let result = self.current_func().new_local(None, result_type, "count_zeroes_results");
+
+            let cz_then_block = self.current_func().new_block("cz_then");
+            let cz_else_block = self.current_func().new_block("cz_else");
+            let cz_after_block = self.current_func().new_block("cz_after");
+
+            let low = self.gcc_int_cast(arg, self.u64_type);
+            let sixty_four = self.const_uint(arg_type, 64);
+            let shift = self.lshr(arg, sixty_four);
+            let high = self.gcc_int_cast(shift, self.u64_type);
+
+            let (first, second, builtin) = if count_leading {
+                (low, high, self.context.get_builtin_function("__builtin_clzll"))
+            } else {
+                (high, low, self.context.get_builtin_function("__builtin_ctzll"))
+            };
+
+            let zero_64 = self.const_uint(self.u64_type, 0);
+            let cond = self.gcc_icmp(IntPredicate::IntNE, second, zero_64);
+            self.llbb().end_with_conditional(self.location, cond, cz_then_block, cz_else_block);
+            self.switch_to_block(cz_then_block);
+
+            let result_128 =
+                self.gcc_int_cast(self.context.new_call(None, builtin, &[second]), result_type);
+
+            cz_then_block.add_assignment(self.location, result, result_128);
+            cz_then_block.end_with_jump(self.location, cz_after_block);
+
+            self.switch_to_block(cz_else_block);
+            let count_more_zeroes =
+                self.gcc_int_cast(self.context.new_call(None, builtin, &[first]), result_type);
+            let sixty_four_result_type = self.const_uint(result_type, 64);
+            let count_result_type = self.add(count_more_zeroes, sixty_four_result_type);
+            cz_else_block.add_assignment(self.location, result, count_result_type);
+            cz_else_block.end_with_jump(self.location, cz_after_block);
+            self.switch_to_block(cz_after_block);
+            result.to_rvalue()
+        } else {
+            let byte_diff = self.ulonglong_type.get_size() as i64 - arg_type.get_size() as i64;
+            let diff = self.context.new_rvalue_from_long(self.int_type, byte_diff * 8);
+            let ull_arg = self.context.new_cast(self.location, arg, self.ulonglong_type);
+
+            let res = if count_leading {
+                let count_leading_zeroes = self.context.get_builtin_function("__builtin_clzll");
+                self.context.new_call(self.location, count_leading_zeroes, &[ull_arg]) - diff
+            } else {
                 let count_trailing_zeroes = self.context.get_builtin_function("__builtin_ctzll");
-                let arg_size = arg_type.get_size();
-                let casted_arg = self.context.new_cast(self.location, arg, self.ulonglong_type);
-                let byte_diff = self.ulonglong_type.get_size() as i64 - arg_size as i64;
-                let diff = self.context.new_rvalue_from_long(self.int_type, byte_diff * 8);
                 let mask = self.context.new_rvalue_from_long(arg_type, -1); // To get the value with all bits set.
-                let masked = mask & self.context.new_unary_op(self.location, UnaryOp::BitwiseNegate, arg_type, arg);
-                let cond = self.context.new_comparison(self.location, ComparisonOp::Equals, masked, mask);
+                let masked = mask
+                    & self.context.new_unary_op(
+                        self.location,
+                        UnaryOp::BitwiseNegate,
+                        arg_type,
+                        arg,
+                    );
+                let cond =
+                    self.context.new_comparison(self.location, ComparisonOp::Equals, masked, mask);
                 let diff = diff * self.context.new_cast(self.location, cond, self.int_type);
-                let res = self.context.new_call(self.location, count_trailing_zeroes, &[casted_arg]) - diff;
-                return self.context.new_cast(self.location, res, result_type);
+
+                self.context.new_call(self.location, count_trailing_zeroes, &[ull_arg]) - diff
             };
-        let count_trailing_zeroes = self.context.get_builtin_function(count_trailing_zeroes);
-        let arg = if arg_type != expected_type {
-            self.context.new_cast(self.location, arg, expected_type)
-        } else {
-            arg
-        };
-        let res = self.context.new_call(self.location, count_trailing_zeroes, &[arg]);
-        self.context.new_cast(self.location, res, result_type)
+            self.context.new_cast(self.location, res, result_type)
+        }
+    }
+
+    fn count_leading_zeroes(&mut self, width: u64, arg: RValue<'gcc>) -> RValue<'gcc> {
+        self.count_zeroes(width, arg, true)
+    }
+
+    fn count_leading_zeroes_nonzero(&mut self, width: u64, arg: RValue<'gcc>) -> RValue<'gcc> {
+        // Pre-condition: arg is guaranteed to not be 0 by caller, else count_leading_zeros should be used
+        self.count_zeroes_nonzero(width, arg, true)
+    }
+
+    fn count_trailing_zeroes(&mut self, width: u64, arg: RValue<'gcc>) -> RValue<'gcc> {
+        self.count_zeroes(width, arg, false)
+    }
+
+    fn count_trailing_zeroes_nonzero(&mut self, width: u64, arg: RValue<'gcc>) -> RValue<'gcc> {
+        // Pre-condition: arg is guaranteed to not be 0 by caller, else count_trailing_zeros should be used
+        self.count_zeroes_nonzero(width, arg, false)
     }
 
     fn pop_count(&mut self, value: RValue<'gcc>) -> RValue<'gcc> {
diff --git a/tests/lang_tests_common.rs b/tests/lang_tests_common.rs
index 9abe97b1087..983ffc2b640 100644
--- a/tests/lang_tests_common.rs
+++ b/tests/lang_tests_common.rs
@@ -115,7 +115,19 @@ pub fn main_inner(profile: Profile) {
                 }
             }
             match profile {
-                Profile::Debug => {}
+                Profile::Debug => {
+                    if test_target.is_ok() {
+                        // m68k doesn't have lubsan for now
+                        compiler.args(["-C", "llvm-args=sanitize-undefined"]);
+                    } else {
+                        compiler.args([
+                            "-C",
+                            "llvm-args=sanitize-undefined",
+                            "-C",
+                            "link-args=-lubsan",
+                        ]);
+                    }
+                }
                 Profile::Release => {
                     compiler.args(["-C", "opt-level=3", "-C", "lto=no"]);
                 }
diff --git a/tests/run/int_intrinsics.rs b/tests/run/int_intrinsics.rs
new file mode 100644
index 00000000000..c3d363c8428
--- /dev/null
+++ b/tests/run/int_intrinsics.rs
@@ -0,0 +1,72 @@
+// Compiler:
+//
+// Run-time:
+#![feature(core_intrinsics, intrinsics)]
+#![no_main]
+
+use std::intrinsics::black_box;
+
+#[rustc_intrinsic]
+pub const fn ctlz<T: Copy>(_x: T) -> u32;
+
+#[rustc_intrinsic]
+pub const fn cttz<T: Copy>(_x: T) -> u32;
+
+#[no_mangle]
+extern "C" fn main(_argc: i32, _argv: *const *const u8) -> i32 {
+    macro_rules! check {
+        ($func_name:ident, $input:expr, $expected:expr, $res_ident:ident) => {{
+            $res_ident += 1;
+            if $func_name(black_box($input)) != $expected {
+                return $res_ident;
+            }
+        }};
+    }
+    let mut res = 0;
+    check!(ctlz, 0_u128, 128_u32, res);
+    check!(ctlz, 1_u128, 127_u32, res);
+    check!(ctlz, 0x4000_0000_0000_0000_0000_0000_0000_0000_u128, 1_u32, res);
+    check!(ctlz, 0x8000_0000_0000_0000_0000_0000_0000_0000_u128, 0_u32, res);
+    check!(cttz, 0_u128, 128_u32, res);
+    check!(cttz, 1_u128, 0_u32, res);
+    check!(cttz, 2_u128, 1_u32, res);
+    check!(cttz, 0x8000_0000_0000_0000_0000_0000_0000_0000_u128, 127_u32, res);
+
+    check!(ctlz, 0_u64, 64_u32, res);
+    check!(ctlz, 1_u64, 63_u32, res);
+    check!(ctlz, 0x4000_0000_0000_0000_u64, 1_u32, res);
+    check!(ctlz, 0x8000_0000_0000_0000_u64, 0_u32, res);
+    check!(cttz, 0_u64, 64_u32, res);
+    check!(cttz, 1_u64, 0_u32, res);
+    check!(cttz, 2_u64, 1_u32, res);
+    check!(cttz, 0x8000_0000_0000_0000_u64, 63_u32, res);
+
+    check!(ctlz, 0_u32, 32_u32, res);
+    check!(ctlz, 1_u32, 31_u32, res);
+    check!(ctlz, 0x4000_0000_u32, 1_u32, res);
+    check!(ctlz, 0x8000_0000_u32, 0_u32, res);
+    check!(cttz, 0_u32, 32_u32, res);
+    check!(cttz, 1_u32, 0_u32, res);
+    check!(cttz, 2_u32, 1_u32, res);
+    check!(cttz, 0x8000_0000_u32, 31_u32, res);
+
+    check!(ctlz, 0_u16, 16_u32, res);
+    check!(ctlz, 1_u16, 15_u32, res);
+    check!(ctlz, 0x4000_u16, 1_u32, res);
+    check!(ctlz, 0x8000_u16, 0_u32, res);
+    check!(cttz, 0_u16, 16_u32, res);
+    check!(cttz, 1_u16, 0_u32, res);
+    check!(cttz, 2_u16, 1_u32, res);
+    check!(cttz, 0x8000_u16, 15_u32, res);
+
+    check!(ctlz, 0_u8, 8_u32, res);
+    check!(ctlz, 1_u8, 7_u32, res);
+    check!(ctlz, 0x40_u8, 1_u32, res);
+    check!(ctlz, 0x80_u8, 0_u32, res);
+    check!(cttz, 0_u8, 8_u32, res);
+    check!(cttz, 1_u8, 0_u32, res);
+    check!(cttz, 2_u8, 1_u32, res);
+    check!(cttz, 0x80_u8, 7_u32, res);
+
+    0
+}