17 January 1997 Achieving subword parallelism by software reuse of the floating-point data path
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Abstract
Many increasingly important applications, such as video compression, graphics, or other multimedia applications, require only 8 or 16 bit data words. Using the full 64 bit data path available in most computers to perform these low- precision calculations is an inefficient use of resources. Many manufacturers have addressed this problem by introducing new instructions that allow the packing of subword quantities into a full data word. This paper presents a new software-only technique that accomplishes this same objective by packing subword integer quantities into a double precision floating point word. This technique works even in machines that have not been specially modified with new multimedia instructions. While the idea of packing subword integer quantities into a larger integer data word has been proposed before, this technique is unique in packing integer subwords into a single floating point word with a shared exponent. The traditional floating point arithmetic operations of add, subtract, and multiply are used to pack and unpack the subword quantities. Therefore, the algorithm will work on any machine that supports the IEEE double precision floating point arithmetic standard with no machine-specific code required. Furthermore, the methodology can be implemented in a high-level language such as C. In this paper we describe this technique in general and then demonstrate its validity by implementing it in a public domain mpeg decoder application, mpeg_play distributed by the Berkeley Plateau Multimedia Research Group. We achieved an average speed-up of 13.8%. While there is some degradation in quality because calculations are carried out with lower precision, there is no noticeable difference in image appearance. A quantitative comparison of the image quality is presented.
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Daniel F. Zucker, Daniel F. Zucker, Ruby B. Lee, Ruby B. Lee, Michael J. Flynn, Michael J. Flynn, } "Achieving subword parallelism by software reuse of the floating-point data path", Proc. SPIE 3021, Multimedia Hardware Architectures 1997, (17 January 1997); doi: 10.1117/12.263526; https://doi.org/10.1117/12.263526
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