Hacker's Delight

6-1. Find First 0-Byte

The need for this function stems mainly from the way character strings are represented in the C language. They have no explicit length stored with them; instead, the end of the string is denoted by an all-0 byte. To find the length of a string, a C program uses the “strlen” (string length) function. This function searches the string, from left to right, for the 0-byte, and returns the number of bytes scanned, not counting the 0-byte.

A fast implementation of “strlen” might load and test single bytes until a word boundary is reached, and then load a word at a time into a register, and test the register for the presence of a 0-byte. On big-endian machines, we want a function that returns the index of the first 0-byte from ...