On Sun, 2022-04-24 at 16:54 -0400, Mikulas Patocka wrote:
> This patch changes the function hex_to_bin so that it contains no branches
> and no memory accesses.
[]
> +++ linux-2.6/lib/hexdump.c 2022-04-24 18:51:20.000000000 +0200
[]
> + * the next line is similar to the previous one, but we need to decode both
> + * uppercase and lowercase letters, so we use (ch & 0xdf), which converts
> + * lowercase to uppercase
> */
> int hex_to_bin(char ch)
> {
> - if ((ch >= '0') && (ch <= '9'))
> - return ch - '0';
> - ch = tolower(ch);
> - if ((ch >= 'a') && (ch <= 'f'))
> - return ch - 'a' + 10;
> - return -1;
> + return -1 +
> + ((ch - '0' + 1) & (((ch - '9' - 1) & ('0' - 1 - ch)) >> 8)) +
> + (((ch & 0xdf) - 'A' + 11) & ((((ch & 0xdf) - 'F' - 1) & ('A' - 1 - (ch & 0xdf))) >> 8));
probably easier to read using a temporary for ch & 0xdf
int CH = ch & 0xdf;
return -1 +
((ch - '0' + 1) & (((ch - '9' - 1) & ('0' - 1 - ch)) >> 8)) +
((CH - 'A' + 11) & (((CH - 'F' - 1) & ('A' - 1 - CH)) >> 8));
