Branch Predication - Disadvantages

Disadvantages

Predication's primary drawback is in increased encoding space. In typical implementations, every instruction reserves a bitfield for the predicate specifying under what conditions that instruction should have an effect. When available memory is limited, as on embedded devices, this space cost can be prohibitive. However, some architectures such as Thumb-2 are able to avoid this issue (see below). Other detriments are the following:

• Predication complicates the hardware by adding levels of logic to critical paths and potentially degrades clock speed.
• A predicated block includes cycles for all operations, so shorter paths may take longer and be penalized.

Predication is most effective when paths are balanced or when the longest path is the most frequently executed, but determining such a path is very difficult at compile time, even in the presence of profiling information.