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Concept
Predication (computer architecture)
Summary
In computer architecture, predication is a feature that provides an alternative to conditional transfer of control, as implemented by conditional branch machine instructions. Predication works by having conditional (predicated) non-branch instructions associated with a predicate, a Boolean value used by the instruction to control whether the instruction is allowed to modify the architectural state or not. If the predicate specified in the instruction is true, the instruction modifies the architectural state; otherwise, the architectural state is unchanged. For example, a predicated move instruction (a conditional move) will only modify the destination if the predicate is true. Thus, instead of using a conditional branch to select an instruction or a sequence of instructions to execute based on the predicate that controls whether the branch occurs, the instructions to be executed are associated with that predicate, so that they will be executed, or not executed, based on whether that predicate is true or false.
Vector processors, some SIMD ISAs (such as AVX2 and AVX-512) and GPUs in general make heavy use of predication, applying one bit of a conditional mask vector to the corresponding elements in the vector registers being processed, whereas scalar predication in scalar instruction sets only need the one predicate bit. Where predicate masks become particularly powerful in vector processing is if an array of condition codes, one per vector element, may feed back into predicate masks that are then applied to subsequent vector instructions.
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.
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