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MP3 Optimization Exploiting Processor Architecture and Using Better Algorithms Mancia Anguita Universidad de Granada J. Manuel Martinez – Lechado Vitelcom Mobile Technology

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Abstract An application’s execution time depends on the processor architecture and clock frequency, the computational complexity of the algorithm, the choice of compiler and optimization options, and it also depends on how well the programmer explicitly and implicitly exploits processor architecture. This article quantifies the influence of these factors for an MP3 decoder through experimental results

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Outline What’s the problem? MP3 decoder overview MP3 decoder implementations Performance comparison Experiment results Conclusion

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What’s the problem? What factors can influence the application’s execution time? Executing processor’s architecture and clock frequency The computational complexity of the algorithm The compiler The programmer’s skill But how much influence do these factors exert on overall performance?

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MP3 decoder overview （ 1 ）

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MP3 decoder overview （ 2 ） Preprocessing Finds frames in the bitstream Extracts their compressed audio data and information Huffman tables, scale factors Requantization Reconstruct the original frequency line samples xr i by using scale factors extracted form preprocessing xr i = sign(is i ) |is i | 4/3 × 2 Cj/4

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MP3 decoder overview （ 3 ） Huffman decoding Huffman encoding is a lossless coding scheme Decoding process is based in several Huffman tables for mapping Huffman code to symbols Total 17 different tables The significant part of the processing handling the compressed audio bitstream Searching Huffman tables

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MP3 decoder overview （ 4 ） Reordering The encoder reorder short blocks to make the Huffman coding more efficiently The decoder reverses this reordering Stereo decoding To exploit redundancies between different stereo channels When using single channel or dual channel, no stereo processing is necessary

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MP3 decoder overview （ 5 ） Alias reduction In the encoder, it is necessary to negate the alias effects of the polyphase filter bank Consist of eight butterfly calculations for each pair of adjacent subbands IMDCT

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MP3 decoder overview （ 6 ） Frequency inversion To compensate for frequency inversions, this stage negate every odd sample in all odd subbands Synthesis polyphase filter bank

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MP3 decoder implementations （ 1 ） Standard version Implement MP3 following documentations Using only the tables specified in the standard Basic version Improving on the standard version Replace some instructions by other with few clock cycles EX : replace floating-point division by multiplicands and some integer multiply instruction by shift Replace computationally intensive library functions with tables Library functions, using special processor instructions, replace slower high-level programmer code Using loop unrolling to improve some loops

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MP3 decoder implementations （ 2 ） SIMD version Improving on the basic version using SIMD extensions MP3 is based on vector operations, so it can achieve benefit from SIMD instructions Requantization, stereo processing, IMDCT, and synthesis filter bank Using SIMD for improving memory initializations and block transfers

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MP3 decoder implementations （ 3 ） Algorithm version Improving basic version with algorithm Synthesis polyphase filter bank Konstantinides’ method reduces the number of operations by transforming the matrixing operation to a 32 DCT and some reorder operation IMDCT Marovich’s method Reduce IMDCT to a fast DCT and some data copying operations Huffman decoding A tree-clustering algorithm can speed up the search process

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MP3 decoder implementations （ 3 ） Algorithm-SIMD version Based on SIMD version combined with the SIMD implementation Using IMDCT and synthesis algorithm and clustering Huffman-decoding

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Performance comparison （ 0 ） Optimization operations

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Performance comparison （ 1 ） O2 Include classical optimizations that are processor independent Include inline function expansion G6 This switch optimizes code for Pentium Pro, PII, and PIII, generating code that is compatible with earlier processors G7 This switch optimizes code for Pentium IV, generating code that is compatible with earlier processors QxK Allow vectorization using the SSE and MMX instruction included in PIII and P4 Arch:SSE Using SSE and cmov instructions

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Performance comparison （ 2 ） Test platform Test MP3 file Note We measure processor clock cycle instead of time, so the result are independent of the processor clock frequency

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Experiment results （ 1 ）

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Experiment results （ 2 ）

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Experiment results （ 3 ）

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Conclusion Exploiting architecture features can be as important as choosing the right algorithms Programmer can exploit architecture features to a higher degree than compiler Optimization choice depends on the application

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