Fastest way to copy sign value among 68k data registers. (Branchless conditional negation of an integer)

Question

I'm looking for an optimized way to change the sign of a value contained in a Data Register using the sign from a value in another Data Register.

Source register will contain either 1 or -1 and destination be always positive therefore all is needed is multiplication. Here's some pseudo code of what I'd do:

MOVE.B #-1,D0
MOVE.W #173,D1
MULS.W D0,D1

After that simple math D1 would carry the sign from D0 and become either -173 or 173. Which is the desired result but MULS takes up to 70 cycles and I'm hoping to save some by finding a trick to somehow only "copy" the sign.

Last word: trick should be branchless as branching is what I'm trying to prevent by copying the sign in the first place.

Thanks in advance for any information or advice.

Answer 1

You could branchlessly select between x and -x , using a bit-hack.

But you can do better: 2's complement negation can be expressed as -x == ~x + 1 , and both the NOT and increment can be expressed in terms of XOR and SUB with -1 . But the same operations with a 0 are no-ops, leaving the value unchanged.

(This trick is often used for 2's complement absolute value, where the 0 or -1 is obtained from arithmetic right shift, x >> 31 , to copy the sign bit to all bits of a register. ie applying your conditional-sign-flip operation to the same number that gave us the sign bit. https://graphics.stanford.edu/~seander/bithacks.html#IntegerAbs )

(TODO: finish this answer with actual 68k XOR and SUB instructions, and a right shift to obtain 0 / -1 from your 1 / -1 input.)