diff --git a/liba/Makefile b/liba/Makefile
index 0a5f8a1fc..0abd5ac52 100644
--- a/liba/Makefile
+++ b/liba/Makefile
@@ -76,6 +76,7 @@ tests += $(addprefix liba/test/, \
 # In practice we're always using liba on such a target.
 objs += $(addprefix liba/src/aeabi-rt/, \
   atexit.o \
+  llsl.o \
   llsr.o \
   memclr.o \
   memcpy.o \
diff --git a/liba/src/aeabi-rt/llsl.c b/liba/src/aeabi-rt/llsl.c
new file mode 100644
index 000000000..1a510f689
--- /dev/null
+++ b/liba/src/aeabi-rt/llsl.c
@@ -0,0 +1,9 @@
+/* See the "Run-time ABI for the ARM Architecture", Section 4.2 */
+
+typedef int uint32_t;
+
+long long __aeabi_llsl(long long value, int shift) {
+  uint32_t low = (uint32_t)value << shift;
+  uint32_t high = ((uint32_t)(value >> 32) << shift) | ((uint32_t)value >> (32 - shift));
+  return ((long long)high << 32) | low;
+}