// Because mant >= 2^54, it is enough to choose:
	//     2^(e2+54) >= 10^(-q+prec-1)
	// or q = -mulByLog2Log10(e2+54) + prec - 1
	//
	// The minimal required exponent is -mulByLog2Log10(1025)+18 = -291
	// The maximal required exponent is mulByLog2Log10(1074)+18 = 342
	q := -mulByLog2Log10(e2+54) + prec - 1

	// Now compute mant*(2^e2)*(10^q).
	// Is it an exact computation?