The compiler is generally free to allocate general purpose registers in
whatever order it chooses. Reasons for choosing one register before another
usually include compiled instruction size (avoidance of REX prefixes, etc.)
or usage conventions, but somehow haven't included security implications in
the compiled bytecode. Some bytecode is more useful in polymorphic ROP
sequences than others, so it seems prudent to try to avoid that bytecode
when possible.
from Brad (maintainer)
40587658f9