see https://github.com/veorq/cryptocoding/pull/13

select.c

#include <stdint.h>
#include <stdio.h>

/* Unsigned comparisons */
/* Return 1 if condition is true, 0 otherwise */
int ct_isnonzero_u32(uint32_t x)
{
        return (x|-x)>>31;
}

/* Generate a mask: 0xFFFFFFFF if bit != 0, 0 otherwise */
uint32_t ct_mask_u32(uint32_t bit)
{
        return -(uint32_t)ct_isnonzero_u32(bit);
}

/* Conditionally return x or y depending on whether bit is set */
/* Equivalent to: return bit ? x : y */
uint32_t ct_select_u32(uint32_t x, uint32_t y, uint32_t bit)
{
        uint32_t m = ct_mask_u32(bit);
        return (x&m) | (y&~m);
        /* return ((x^y)&m)^y; */
}

unsigned select (unsigned a, unsigned b, unsigned bit)
{
        /* -0 = 0, -1 = 0xff....ff */
        unsigned mask = - bit;
        unsigned ret = mask & (a^b);
        ret = ret ^ a;
        return ret;
}

/* Conditionally return a or b depending on whether bit is set */
/* Equivalent to: return bit ? a : b */
unsigned proposed (unsigned a, unsigned b, unsigned bit)
{
        unsigned isnonzero = (bit | -bit) >> (sizeof(unsigned) * 8 - 1);
        /* -0 = 0, -1 = 0xff....ff */
        unsigned mask = -isnonzero;
        unsigned ret = mask & (b^a);
        ret = ret ^ b;
        return ret;
}


int main(int argc, char *argv[])
{
        for (unsigned i = 0; i <= 2; i++) {
                printf("---\n");
                printf("select       ('a', 'b', %u)=%c\n", i, select('a', 'b', i));
                printf("proposed     ('a', 'b', %u)=%c\n", i, proposed('a', 'b', i));
                printf("ct_select_u32('a', 'b', %u)=%c\n", i, ct_select_u32('a', 'b', i));
        }
        return 0;
}