Some implementations of VFP use support code to handle denormalized numbers. The performance of such systems, in calculations involving denormalized numbers, is much less than it is in normal calculations.
Flush-to-zero mode replaces denormalized numbers with 0. This does not comply with IEEE 754 arithmetic, but in some circumstances can improve performance considerably.
NEON and VFPv3 flush-to-zero preserves the sign bit. VFPv2 flush-to-zero flushes to +0.
NEON always uses flush-to-zero mode.