n. 1. A sequence of one or more distinguished
) characters (or other data items), used to
delimit a piece of data intended to be treated as a unit (the
computer-science literature calls this a `sentinel'). The NUL
(ASCII 0000000) character that terminates strings in C is a fence.
Hex FF is also (though slightly less frequently) used this way.
. 2. [among users of optimizing compilers] Any
technique, usually exploiting knowledge about the compiler, that
blocks certain optimizations. Used when explicit mechanisms are
not available or are overkill. Typically a hack "I call a dummy
procedure there to force a flush of the optimizer's
register-coloring info" can be expressed by the shorter "That's a