What?

Calculates log₂ of an unsigned 32 bit integer value with 16bit fraction outputs. (Q16 format)

This algorithm doesn't use any division, loop or branch instructions which makes it fast and constant run time. However, this algorithm heavily relies on CLZ instruction commonly found on desktop / laptop / mobile grade CPUs and ARM Cortex M3, M4, M7 cores.

Input / Output range

• Input must be greater than 0 and must be smaller than (2³² - 1)
• Output is between 0 and ((32 * 65536) - 1)

Reference Implementation

int32_t log2_reference(uint32_t x)
{
  return (int32_t)(log2f(x) * 65536.0);
}

Fundamentals

We are using the following logarithmic identity:

Based on this identiy, we seperate the input value x into a and b values where

• a holds the highest 2^N value smaller or equal to x
• b is (x - a)

After this part,

• log₂(a) can be calculated as N directly
• A lookup table can be used for calculating the rest

Why CLZ is useful?

This core instruction calculates the number of leading zeros in a number in binary form.

Given the following 32-bit number:

0000 0000 0001 0001 1010 0100 0000 1000

... CLZ instruction returns 11

Then, we calculate the locaction of the left-most non zero bit as: 31 - CLZ(x)

Which is equal to the N in the highest 2^N value smaller or equal to input x

Core Algorithm

int32_t log2_fast(uint32_t x)
{ 
  // ...  
  uint32_t xlz = __builtin_clz(x);
  uint32_t xbits = 31 - xlz;
  uint32_t xremainder = x & ~(1 << xbits);
  uint32_t xremainder_big = xremainder << xlz;

  // ...
  uint32_t result = 0;
  result += (xbits << 16);
  result += lutRead(xremainder_big);
  return result;  
}

Error analysis

• In this particular example, 1024 length lookup table is used.
• Size of the lookup table can be increased for reduced error performance. Or, table can be read with linear interpolation for reduced error performance.
  • Maximum error over whole 32bit input range with 1024 length table is 92 / 65536
  • Maximum error over whole 32bit input range with 1024 length table + 16bit interpolation is 2 / 65536

Implementation

• ./scripts/lutgen_log2.py for lookup table generation.
• main.c for the fast and fast & interpolated version of the algorithm.

Licence

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