diff --git a/js/src/jit/arm/Assembler-arm.h b/js/src/jit/arm/Assembler-arm.h
index 6b5137090c5..3e42a759122 100644
--- a/js/src/jit/arm/Assembler-arm.h
+++ b/js/src/jit/arm/Assembler-arm.h
@@ -647,9 +647,13 @@ class Imm8 : public Operand2
 
   public:
     static datastore::Imm8mData EncodeImm(uint32_t imm) {
+        // RotateLeft below may not be called with a shift of zero.
+        if (imm <= 0xFF)
+            return datastore::Imm8mData(imm, 0);
+
         // An encodable integer has a maximum of 8 contiguous set bits,
         // with an optional wrapped left rotation to even bit positions.
-        for (int rot = 0; rot < 16; rot++) {
+        for (int rot = 1; rot < 16; rot++) {
             uint32_t rotimm = mozilla::RotateLeft(imm, rot*2);
             if (rotimm <= 0xFF)
                 return datastore::Imm8mData(rotimm, rot);
diff --git a/mfbt/MathAlgorithms.h b/mfbt/MathAlgorithms.h
index 81ec6dad4de..28e41826c45 100644
--- a/mfbt/MathAlgorithms.h
+++ b/mfbt/MathAlgorithms.h
@@ -484,6 +484,11 @@ inline T
 RotateLeft(const T aValue, uint_fast8_t aShift)
 {
   MOZ_ASSERT(aShift < sizeof(T) * CHAR_BIT, "Shift value is too large!");
+  MOZ_ASSERT(aShift > 0,
+             "Rotation by value length is undefined behavior, but compilers "
+             "do not currently fold a test into the rotate instruction. "
+             "Please remove this restriction when compilers optimize the "
+             "zero case (http://blog.regehr.org/archives/1063).");
   static_assert(IsUnsigned<T>::value, "Rotates require unsigned values");
   return (aValue << aShift) | (aValue >> (sizeof(T) * CHAR_BIT - aShift));
 }
@@ -496,6 +501,11 @@ inline T
 RotateRight(const T aValue, uint_fast8_t aShift)
 {
   MOZ_ASSERT(aShift < sizeof(T) * CHAR_BIT, "Shift value is too large!");
+  MOZ_ASSERT(aShift > 0,
+             "Rotation by value length is undefined behavior, but compilers "
+             "do not currently fold a test into the rotate instruction. "
+             "Please remove this restriction when compilers optimize the "
+             "zero case (http://blog.regehr.org/archives/1063).");
   static_assert(IsUnsigned<T>::value, "Rotates require unsigned values");
   return (aValue >> aShift) | (aValue << (sizeof(T) * CHAR_BIT - aShift));
 }