diff --git a/ptx/src/pass/emit_llvm.rs b/ptx/src/pass/emit_llvm.rs
index 2e8c5fc..5271157 100644
--- a/ptx/src/pass/emit_llvm.rs
+++ b/ptx/src/pass/emit_llvm.rs
@@ -486,7 +486,7 @@ impl<'a, 'input> MethodEmitContext<'a, 'input> {
             ast::Instruction::Cos { data, arguments } => todo!(),
             ast::Instruction::Lg2 { data, arguments } => todo!(),
             ast::Instruction::Ex2 { data, arguments } => todo!(),
-            ast::Instruction::Clz { data, arguments } => todo!(),
+            ast::Instruction::Clz { data, arguments } => self.emit_clz(data, arguments),
             ast::Instruction::Brev { data, arguments } => self.emit_brev(data, arguments),
             ast::Instruction::Popc { data, arguments } => todo!(),
             ast::Instruction::Xor { data, arguments } => todo!(),
@@ -828,6 +828,43 @@ impl<'a, 'input> MethodEmitContext<'a, 'input> {
         };
         Ok(())
     }
+
+    fn emit_clz(
+        &mut self,
+        data: ptx_parser::ScalarType,
+        arguments: ptx_parser::ClzArgs<SpirvWord>,
+    ) -> Result<(), TranslateError> {
+        let llvm_fn = match data.size_of() {
+            4 => c"llvm.ctlz.i32",
+            8 => c"llvm.ctlz.i64",
+            _ => return Err(error_unreachable()),
+        };
+        let type_ = get_scalar_type(self.context, data.into());
+        let pred = get_scalar_type(self.context, ast::ScalarType::Pred);
+        let fn_type = get_function_type(
+            self.context,
+            iter::once(&ast::ScalarType::U32.into()),
+            [Ok(type_), Ok(pred)].into_iter(),
+        )?;
+        let mut fn_ = unsafe { LLVMGetNamedFunction(self.module, llvm_fn.as_ptr()) };
+        if fn_ == ptr::null_mut() {
+            fn_ = unsafe { LLVMAddFunction(self.module, llvm_fn.as_ptr(), fn_type) };
+        }
+        let src = self.resolver.value(arguments.src)?;
+        let false_ = unsafe { LLVMConstInt(pred, 0, 0) };
+        let mut args = [src, false_];
+        self.resolver.with_result(arguments.dst, |dst| unsafe {
+            LLVMBuildCall2(
+                self.builder,
+                fn_type,
+                fn_,
+                args.as_mut_ptr(),
+                args.len() as u32,
+                dst,
+            )
+        });
+        Ok(())
+    }
 }
 
 fn get_pointer_type<'ctx>(