Full width multiplication of two 32-bit numbers.

mult.c

/*
This file contains examples of how to do full-width 32-bit multiplication by dividing each
operand into 16-bit fields.
*/
typedef struct {
    int low;
    int high;
} int_pair_t;


// Extract high and low parts of a 32-bit number without sign extension

#define high_part(x) ((x >> 16) & 0xFFFF)
#define low_part(x) ((x) & 0xFFFF)

// Multiply two 32-bit numbers and return the 64-bit result as two 32-bit numbers.
// It does multiplication assuming that the inputs are unsigned.
int_pair_t umult(int a, int b) {
    int x0 = low_part(a) * low_part(b);
    int x1 = low_part(a) * high_part(b);
    int x2 = high_part(a) * low_part(b);
    int x3 = high_part(a) * high_part(b);

    // This will not carry
    x1 += high_part(x0);
    x1 += low_part(x2);

    // Calculate the high and low parts of the result
    int_pair_t result;
    result.low = low_part(x0) + (low_part(x1) << 16);
    result.high = x3 + high_part(x1) + high_part(x2);

    return result;  
}

// Does signed multiplication of two signed numbers by adjusting the result of
// unsigned multiplication.
int_pair_t smult(int a, int b) {
    int_pair_t result = umult(a, b);

    result.high -= ((a < 0) ? b : 0);
    result.high -= ((b < 0) ? a : 0);

    return result;
}