count_bits

Live Demo

The count_bits function returns the number of bits set to 1 in an integer value, also known as the population count or Hamming weight.

Signature

bwsl

count_bits :: (int x) -> int {...}
count_bits :: (uint x) -> int {...}

Parameters

Parameter	Type	Description
`x`	`int` or `uint`	The value to count bits in

Return Value

Returns the number of 1 bits in x.

Example

bwsl

pipeline VisibilityCount {
  pass "Main" {
    compute "Main" [64, 1, 1] {
    // Count visible objects in bitmask
    uint visibilityMask = loadVisibilityMask(tileId);
    int visibleCount = count_bits(visibilityMask);

    // Allocate space for visible objects
    int startIndex = atomic_add(totalVisible[0], visibleCount);

    // Process visible objects
    processVisible(tileId, startIndex, visibilityMask);
    }
  }
}

Common Use Cases

Counting Set Flags

bwsl

// Count active flags
int activeCount = count_bits(flagBitmask);

Voting

bwsl

// Count lanes with condition true
uint ballot = waveBallot(condition);
int trueCount = count_bits(ballot);

Sparse Data

bwsl

// Count non-zero elements in packed data
int elementCount = count_bits(occupancyMask);

Hamming Distance

bwsl

// Count differing bits between two values
int difference = count_bits(a ^ b);

Prefix Sum Building Block

bwsl

// Count bits before position for exclusive prefix
uint mask = (1u << position) - 1u;
int prefixCount = count_bits(value & mask);

Hardware Support

Most modern GPUs have a dedicated instruction for population count, making this operation very efficient.

Ballot + Count

Combining wave ballot with count_bits is a common pattern to count how many lanes meet a condition.

Known issue: uint in WebGL output

The GLSL ES output for count_bits on a uint currently fails to compile in WebGL, because GLSL's bitCount returns int (BWSL#213). Other targets are unaffected. For WebGL, pass an int until this is fixed.

Compiled Output

When compiled to GLSL:

glsl

bitCount(x)

When compiled to HLSL:

hlsl

countbits(x)

When compiled to Metal:

metal

popcount(x)

When compiled to SPIR-V:

Uses OpBitCount instruction.

Test count_bits in a live shader

Live Demo

Signature

Parameters

Return Value

Example

Common Use Cases

Counting Set Flags

Voting

Sparse Data

Hamming Distance

Prefix Sum Building Block

Compiled Output

See Also