| 摘要 |
A lookup operation is carried out on a data table by logically dividing the data table into a number of smaller sets of data that can be indexed with a single byte of data. Each set of data consists of two vectors, which constitute the operands for a permute instruction. Only a limited number of bits are required to index into the table during the execution of this instruction. The remaining bits of each index are used as masks into a series of select instructions. The select instruction chooses between two vector components, based on the mask, and places the selected components into a new vector. The mask is generated by shifting one of the higher order bits of the index to the most significant position, and then propagating that bit throughout a byte, for example by means of an arithmetic shift. This procedure is carried out for all of the index bytes in the vector, to generate a select mask. The select mask is then used during a select operation, to choose between the results of permute instructions on different ones of the logically divided sets of data. Multi-byte table entries are retrieved by replicating each index value and adding consecutive values to form multiple consecutive index values that are then used in multiple permute operations.
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