Branch predication

In computer science, predication is an architectural feature that provides an alternative to conditional transfer of control, implemented by machine instructions such as conditional branch, conditional call, conditional return, and branch tables. Predication works by executing instructions from both paths of the branch and only permitting those instructions from the taken path to modify architectural state. The instructions from the taken path are permitted to modify architectural state because they have been associated (predicated) with a predicate, a Boolean value used by the instruction to control whether the instruction is allowed to modify the architectural state or not. In computer science, predication is an architectural feature that provides an alternative to conditional transfer of control, implemented by machine instructions such as conditional branch, conditional call, conditional return, and branch tables. Predication works by executing instructions from both paths of the branch and only permitting those instructions from the taken path to modify architectural state. The instructions from the taken path are permitted to modify architectural state because they have been associated (predicated) with a predicate, a Boolean value used by the instruction to control whether the instruction is allowed to modify the architectural state or not. Most computer programs contain conditional code, which will be executed only under specific conditions depending on factors that cannot be determined beforehand, for example depending on user input. As the majority of processors simply execute the next instruction in a sequence, the traditional solution is to insert branch instructions that allow a program to conditionally branch to a different section of code, thus changing the next step in the sequence. This was sufficient until designers began improving performance by implementing instruction pipelining, a method which is slowed down by branches. For a more thorough description of the problems which arose, and a popular solution, see branch predictor. Luckily, one of the more common patterns of code that normally relies on branching has a more elegant solution. Consider the following pseudocode: On a system that uses conditional branching, this might translate to machine instructions looking similar to: With predication, all possible branch paths are coded inline, but some instructions execute while others do not. The basic idea is that each instruction is associated with a predicate (the word here used similarly to its usage in predicate logic) and that the instruction will only be executed if the predicate is true. The machine code for the above example using predication might look something like this: Besides eliminating branches, less code is needed in total, provided the architecture provides predicated instructions. While this does not guarantee faster execution in general, it will if the dosomething and dosomethingelse blocks of code are short enough. Predication's simplest form is partial predication, where the architecture has conditional move or conditional select instructions. Conditional move instructions write the contents of one register over another only if the predicate's value is true, whereas conditional select instructions choose which of two registers has its contents written to a third based on the predicate's value. A more generalized and capable form is full predication. Full predication has a set of predicate registers for storing predicates (which allows multiple nested or sequential branches to be simultaneously eliminated) and most instructions in the architecture have a register specifier field to specify which predicate register supplies the predicate. The main purpose of predication is to avoid jumps over very small sections of program code, increasing the effectiveness of pipelined execution and avoiding problems with the cache. It also has a number of more subtle benefits: 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: