Index: compiler-rt/lib/builtins/CMakeLists.txt
===================================================================
--- compiler-rt/lib/builtins/CMakeLists.txt
+++ compiler-rt/lib/builtins/CMakeLists.txt
@@ -580,6 +580,7 @@
 set(avr_SOURCES
   avr/mulqi3.S
   avr/mulhi3.S
+  avr/mulsi3.S
   avr/exit.S
   avr/divmodhi4.S
   avr/udivmodhi4.S
Index: compiler-rt/lib/builtins/avr/mulsi3.S
===================================================================
--- /dev/null
+++ compiler-rt/lib/builtins/avr/mulsi3.S
@@ -0,0 +1,89 @@
+//===------------ mulsi3.S - int32 multiplication -------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// The corresponding C code is something like:
+//
+// unsigned long __mulsi3(unsigned long A, unsigned long B) {
+//   unsigned long S = 0;
+//   do {
+//     if (A & 1)
+//       S += B;
+//     A >>= 1;
+//     B <<= 1;
+//   } while (A != 0);
+//   return S;
+// }
+//
+// __mulsi3 has a special ABI: the calling convention is as usual except that
+// only R26, R27, Rtmp and SREG are clobbered.
+//
+//===----------------------------------------------------------------------===//
+
+    .text
+    .align 2
+
+#if !defined(__AVR_TINY__) // TODO: add support for avrtiny
+
+    .set __tmp_reg__, 0
+    .set __zero_reg__, 1
+
+    .globl __mulsi3
+    .type  __mulsi3, @function
+
+; parameter A is stored in r22-r25
+; parameter B is stored in r18-r21
+; result is stored in r22-r25
+__mulsi3:
+    ; S = 0;
+    clr   r26
+    clr   r27
+    clr   __tmp_reg__
+    ;clr   __zero_reg__ ; not needed (already zero)
+
+1: ; loop
+    ; if (A & 1)
+    ;   S += B;
+    sbrs r22, 0
+    rjmp 2f
+    add r26,          r18
+    adc r27,          r19
+    adc __tmp_reg__,  r20
+    adc __zero_reg__, r21
+
+2:
+    ; A >>= 1;
+    lsr r25
+    ror r24
+    ror r23
+    ror r22
+
+    ; B <<= 1;
+    lsl r18
+    rol r19
+    rol r20
+    rol r21
+
+    ; do { ... } while (A != 0);
+    subi r22, 0
+    sbci r23, 0
+    sbci r24, 0
+    sbci r25, 0
+    brne 1b
+
+3:
+    ; Return the result via r22-r25.
+    mov   r22, r26
+    mov   r23, r27
+    mov   r24, __tmp_reg__
+    mov   r25, __zero_reg__
+    ; Restore __zero_reg__ to 0.
+    clr   __zero_reg__
+    ; Return to parent.
+    ret
+
+#endif // defined(__AVR_TINY__)