Editing Predication (computer architecture) (section)

==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 system|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:<ref name="Fisher04">{{cite book |first1=Joseph A. |last1=Fisher |first2=Paolo |last2=Faraboschi |first3=Cliff |last3=Young |year=2004 |title=Embedded Computing — A VLIW Approach to Architecture, Compilers, and Tools |page=172 |chapter-url=https://books.google.com/books?id=DbOa0p1wRD4C&pg=PA172 |chapter=4.5.2 Predication § Predication in the Embedded Domain |isbn=9780080477541 |publisher=Elsevier}}</ref>
*Predication complicates the hardware by adding levels of [[control unit|logic]] to critical [[datapath|paths]] and potentially degrades clock speed.
*A predicated block includes cycles for all operations, so shorter [[control-flow graph|paths]] may take longer and be penalized.
*Predication is not usually speculated and causes a longer dependency chain. For ordered data this translates to a performance loss compared to a predictable branch.<ref>{{cite web |last1=Cordes |first1=Peter |title=assembly - How does Out of Order execution work with conditional instructions, Ex: CMOVcc in Intel or ADDNE (Add not equal) in ARM |url=https://stackoverflow.com/a/50960323 |website=Stack Overflow |quote=Unlike with control dependencies (branches), they don't predict or speculate what the flags will be, so a cmovcc instead of a jcc can create a loop-carried dependency chain and end up being worse than a predictable branch. [https://stackoverflow.com/questions/50959808 gcc optimization flag -O3 makes code slower than -O2] is an example of that.}}</ref>

Predication is most effective when paths are balanced or when the longest path is the most frequently executed,<ref name="Fisher04"/> but determining such a path is very difficult at compile time, even in the presence of [[profiling (computer programming)|profiling information]].