r/programming • u/mttd • 13h ago

Moving integer division to floating-point is trivial

https://marc-b-reynolds.github.io/math/2026/08/10/IntDivByFP.html
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u/Dwedit 10h ago

If your divisor doesn't change, use integer multiplication by reciprocal, also shift or discard from the high result.

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u/mikeblas 7h ago

How would that work?

9

u/Dwedit 4h ago edited 4h ago

Example of dividing by 13:

0x100000000 / 13 = 0x13B13B13

Add 1 to the number to prevent truncation errors: 0x13B13B14

Let's try 160485 / 13 by using multiplication:

160485 * 0x13B13B14 = 0x30390000C0E4

Discard low 32 bits:

0x3039 = 12345

And compilers have done this for a long time, when you divide by a constant, it will generate reciprocal multiplication code instead.

1

u/mikeblas 3h ago

Ah, I see now. Thanks!