Branchless signum

Author:	Wojciech Muła
Added on:	2010-04-01

Problem: calculate value of sign(x):

• -1 when x < 0
• 0 when x = 0,
• +1 when x > 0.

My solution do not involve any hardware specific things like ALU flags nor special instructions — just plain AND, OR, shifts.

; input: eax = X

movl %eax, %ebx
sarl $31, %eax  // eax = -1 if X less then zero, 0 otherwise

andl $0x7fffffff, %ebx
addl $0x7fffffff, %ebx // MSB is set if any lower bits were set
shrl $31, $ebx  // eax = +1 if X greater then zero, 0 otherwise

orl %ebx, %eax  // eax = result

C99 implementation:

int32_t sign(int32_t x) {
        int32_t y;
        y = (x & 0x7fffffff) + 0x7fffffff;
        return (x >> 31) | ((uint32_t)y >> 31);
}