// B ~ (z0, z1, z2), such that the product leading digit has the exponent -61.
	// Note, exp >= -53 since x >= PI4 and exp < 971 for maximum float64.
	digit, bitshift := uint(exp+61)/64, uint(exp+61)%64
	z0 := (mPi4[digit] << bitshift) | (mPi4[digit+1] >> (64 - bitshift))
	z1 := (mPi4[digit+1] << bitshift) | (mPi4[digit+2] >> (64 - bitshift))