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libmp: timingsafe sign flip for small power-of-two negative divisor for mpdiv()

--- a/sys/src/libmp/port/mpdiv.c

+++ b/sys/src/libmp/port/mpdiv.c

@@ -24,12 +24,11 @@

 	if(divisor->top == 1 && (divisor->p[0] & divisor->p[0]-1) == 0){

 		vlong r = (vlong)dividend->sign * (dividend->p[0] & divisor->p[0]-1);

 		if(quotient != nil){

-			sign = divisor->sign;

 			for(s = 0; ((divisor->p[0] >> s) & 1) == 0; s++)

 				;

 			mpright(dividend, s, quotient);

-			if(sign < 0 && quotient->top != 0)

-				quotient->sign ^= -2;

+			if(divisor->sign < 0)

+				quotient->sign ^= (-mpmagcmp(quotient, mpzero) >> 31) << 1;

 		}

 		if(remainder != nil){

 			remainder->flags |= dividend->flags & MPtimesafe;