Instead of jumping to locations which store the exact number of bytes, use displacement to move the destination.
In particular the following cleares an area between 8-16 (inclusive) branch-free:
```
movq %r10,(%rdi)
movq %r10,-8(%rdi,%rcx)
```
For instance for rcx of 10 the second line is rdi + 10 - 8 = rdi + 2. Writing 8 bytes starting at that offset overlaps with 6 bytes written previously and writes 2 new.
Provides a nice win for smaller stores. Other ones are erratic depending on the microarchitecture.
General idea taken from NetBSD (restricted use of the trick) and bionic string functions (use for various ranges like in this patch).
{F3898154}