Index: compiler-rt/lib/builtins/CMakeLists.txt
===================================================================
--- compiler-rt/lib/builtins/CMakeLists.txt
+++ compiler-rt/lib/builtins/CMakeLists.txt
@@ -672,6 +672,7 @@
   avr/mulqi3.S
   avr/mulhi3.S
   avr/exit.S
+  avr/udivmodhi4.S
 )
 
 add_custom_target(builtins)
Index: compiler-rt/lib/builtins/avr/udivmodhi4.S
===================================================================
--- /dev/null
+++ compiler-rt/lib/builtins/avr/udivmodhi4.S
@@ -0,0 +1,48 @@
+//===------------ udivmodhi4.S - uint16 div & mod -------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// As described at
+// https://gcc.gnu.org/wiki/avr-gcc#Exceptions_to_the_Calling_Convention,
+// R23:R22 holds the uint16 quotient, and R25:R24 holds the uint16 remainder,
+// while R21/R26/R27 are clobbered.
+//
+//===----------------------------------------------------------------------===//
+
+	.text
+	.align 2
+
+	.globl __udivmodhi4
+	.type  __udivmodhi4, @function
+
+__udivmodhi4:
+	sub     r26, r26
+	sub     r27, r27           ; Initialize the remainder to zero.
+	ldi     r21, 17            ; Only loop 16 cycles for uint16.
+
+__udivmodhi4_loop:
+	adc     r24, r24
+	adc     r25, r25
+	dec     r21
+	breq    __udivmodhi4_end
+	adc     r26, r26
+	adc     r27, r27
+	cp      r26, r22
+	cpc     r27, r23           ; Compare with the divisor.
+	brcs    __udivmodhi4_loop
+	sub     r26, r22
+	sbc     r27, r23           ; Subtract the divisor.
+	rjmp    __udivmodhi4_loop
+
+__udivmodhi4_end:
+	com     r24
+	com     r25
+	mov     r22, r24
+	mov     r23, r25           ; The quotient is returned in R23:R22.
+	mov     r24, r26
+	mov     r25, r27           ; The remainder is returned in in R25:R24.
+	ret