Flip Case

caseflipper.c

#include <stdio.h>

#if 0
unsigned int flip_case(unsigned int c)
{
    unsigned int lower = ((('A' - 1 - c) & (c - 'Z' - 1)) >> 8) & 1;
    unsigned int upper = ((('a' - 1 - c) & (c - 'z' - 1)) >> 8) & 1;

    unsigned int to_lower = (-upper) & ('A' - 'a');
    unsigned int to_upper = (-lower) & ('a' - 'A');

    return c + to_lower + to_upper;
}
#endif

int main()
{
    int c;
    for (c = 0; c < 256; c++) {
        if (c != flip_case(c)) {
            fprintf(stderr, "%.2x %.2x : %c %c\n", c, flip_case(c), c,
                    flip_case(c));
        }
    }
    return 0;
}

flip_case.S

.globl flip_case

flip_case:
    movl    %edi, %eax

    subb    $0x41, %al
    cmpb    $26, %al
    setb    %dl

    addb    $(0x41-0x61), %al
    cmpb    $26, %al
    setb    %dh
    addb    $0x61, %al

    negb    %dh
    negb    %dl

    andb    $(0x61-0x41), %dl
    andb    $(0x41-0x61), %dh

    addb    %dl, %dh
    
    addb    %dh, %al

    movzx    %al, %eax
    
    ret