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4-operation form is preferred over 3-operation because it breaks a long dependency chain, thus allowing a superscalar processor to execute more operations in parallel. The idea was taken from: http://www.zorinaq.com/papers/md5-amd64.html AMD Athlon(tm) II X3 450 Processor, x86_64 $ for i in $(seq 1 4); do ./avutil_md5_test2; done size: 1048576 runs: 1024 time: 5.821 +- 0.019 size: 1048576 runs: 1024 time: 5.822 +- 0.019 size: 1048576 runs: 1024 time: 5.841 +- 0.018 size: 1048576 runs: 1024 time: 5.821 +- 0.018 $ for i in $(seq 1 4); do ./avutil_md5_test2; done size: 1048576 runs: 1024 time: 5.646 +- 0.019 size: 1048576 runs: 1024 time: 5.646 +- 0.018 size: 1048576 runs: 1024 time: 5.642 +- 0.019 size: 1048576 runs: 1024 time: 5.641 +- 0.019 Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
FFmpeg README ------------- 1) Documentation ---------------- * Read the documentation in the doc/ directory in git. You can also view it online at http://ffmpeg.org/documentation.html 2) Licensing ------------ * See the LICENSE file. 3) Build and Install -------------------- * See the INSTALL file.