What Does findLSB Return in GLSL?

In the OpenGL Shading Language (GLSL), the built-in function findLSB returns the zero-based index of the least significant bit set to 1 in an integer, or -1 if no bits are set. This article explains the exact return values of findLSB, how it handles different data types, and edge cases such as zero and negative integers.

Return Values of findLSB

The findLSB function scans the binary representation of an integer from right to left (least significant to most significant bit) and determines the position of the first set bit (1):

• When a set bit is found: It returns an integer ranging from 0 to 31, where 0 represents the least significant bit (\(2^0\)) and 31 represents the most significant bit in a standard 32-bit integer.
• When no set bits are found: If the input value is 0, findLSB returns -1.

Supported Signatures and Types

The function is overloaded to accept both signed and unsigned 32-bit integers, as well as their vector equivalents:

• genIType findLSB(genIType value)
• genIType findLSB(genUType value)

In GLSL, genIType covers int, ivec2, ivec3, and ivec4, while genUType covers uint, uvec2, uvec3, and uvec4. When applied to vector types, the operation executes per-component, returning a vector of signed integers (ivec) containing the results for each corresponding component.

Examples

• findLSB(1) returns 0 (binary ...00000001, bit 0 is set).
• findLSB(16) returns 4 (binary ...00010000, bit 4 is set).
• findLSB(10) returns 1 (binary ...00001010, lowest set bit is bit 1).
• findLSB(0) returns -1 (no bits are set).
• findLSB(-4) returns 2 (in two's complement, -4 is ...11111100, lowest set bit is bit 2).
• findLSB(uvec2(0u, 8u)) returns ivec2(-1, 3).