Is it possible that bitwise left shifts in JavaScript ES6 (<<) become cyclical after surpassing a shift of 63?

From my understanding of the << bitwise left operator in JS (ES6), the void to the right is typically filled with zeros.

However, through observation in both V8 and JSC, it appears that the set bits suddenly reappear when we shift by 64 or more.

(255 << 64).toString(2)
//-> "11111111"

This contradicts my initial expectation, which was that larger shifts would continue to produce only zeros to the right.

I have not found this specific behavior defined in the EcmaScript 2016 documentation for << - could it be that this behavior is undefined for larger shifts?

javascriptv8javascriptcore

Answer №1

In the technical specification outlined in Section 12.8.3.1, it is clearly defined that the number of bits to shift is actually masked:

When dealing with ShiftExpression : ShiftExpression << AdditiveExpression

  1. First, the ShiftExpression is evaluated and stored as lref.
  2. The value of lref is then obtained and assigned to lval.
  3. If there are any issues with this process, it will be handled appropriately.
  4. Next, the AdditiveExpression is evaluated and stored as rref.
  5. The value of rref is then obtained and assigned to rval.
  6. If there are any issues here, they will be addressed.
  7. The lval is converted to a 32-bit integer named lnum.
  8. The rval is converted to an unsigned 32-bit integer named rnum.
  9. A crucial step involves taking the least significant 5 bits of rnum by masking, which is expressed as rnum & 0x1F.
  10. The final result is the left shift of lnum by shiftCount bits, giving a signed 32-bit integer.

It is important to note that when 64 & 0x1F equals 0, there is "no shifting" occurring, which explains the reappearance of bits.

tl;dr

The maximum number of bits that can be shifted is capped at 31, as illustrated by the code snippet below:

function shiftLeft(number, numShift) {
    return number << (numShift % 32);  // equivalent code
}

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