Hacker's Delight

2-11. Comparison Predicates

A “comparison predicate” is a function that compares two quantities, producing a single bit result of 1 if the comparison is true, and 0 if the comparison is false. Below we show branch-free expressions to evaluate the result into the sign position. To produce the 1/0 value used by some languages (e.g., C), follow the code with a shift right of 31. To produce the −1/0 result used by some other languages (e.g., Basic), follow the code with a shift right signed of 31.

These formulas are, of course, not of interest on machines such as MIPS, the Compaq Alpha, and our model RISC, which have comparison instructions that compute many of these predicates directly, placing a 0/1-valued result in a general purpose register. ...