mul24 − Fast integer function to multiply 24−bit integer values.
mul24 multiplies two 24−bit integer values x and y. x and y are 32−bit integers but only the low 24−bits are used to perform the multiplication. mul24 should only be used when values in x and y are in the range [−223, 223−1] if x and y are signed integers and in the range [0, 224−1] if x and y are unsigned integers. If x and y are not in this range, the multiplication result is implementation−defined.
Fast integer functions can be used for optimizing performance of kernels. We use the generic type name gentype to indicate that the function can take int, int2, int3, int4, int8, int16, uint, uint2, uint3, uint4, uint8, or uint16 as the type for the arguments.
OpenCL Specification 
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page 256, section 6.12.3 - Integer Functions