Bitwise shift precision

Question

I'm trying to port a Python library to Javascript, but stumbled on a numerical difference between the two..

With Python, 1057618395136 >> 16 == 16137976 With Javascript, 1057618395136 >> 16 == 16120

I read the the integer precision in Javascript is 53 bits, Math.pow(2, 53) == 9007199254740991), which is still in the range of the bifshift operation im trying to do.. What am i missing?

Pointy · Accepted Answer · 2016-10-17 19:31:04Z

4

The JavaScript bitwise operators truncate the floating-point values down to 32-bit integers. Thus, though it is true that a 64-bit floating point value can hold 53-bit integers, you cannot take advantage of all 53 bits with shift operators.

To shift right by 16 bits in JavaScript, you can divide by 65536.

answered Oct 17, 2016 at 19:31

Pointy

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2 Comments

Martijnh Over a year ago

Thanks, that makes sense! Out of curiosity (i know such micro optimizations are neglectable), is there a difference in performance between the bitshift and the division?

Pointy Over a year ago

@Martijnh well it's possible that the JavaScript runtime recognizes the fact that you're dividing by a constant that's a power of 2. Most modern hardware (aside from low-end phones) can probably do 64-bit shift operations. Whether any actual implementations do that, I don't know.

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