Add optimized longest_match for Power processors #459
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Optimized functions for Power will make use of GNU indirect functions, an extension to support different implementations of the same function, which can be selected during runtime. This will be used to provide optimized functions for different processor versions. Since this is a GNU extension, we placed the definition of the Z_IFUNC macro under `contrib/gcc`. This can be reused by other archs as well. Author: Matheus Castanho <msc@linux.ibm.com> Author: Rogerio Alves <rcardoso@linux.ibm.com>
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nmoinvaz
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Apr 19, 2022
| * bytes where LSB == 0 is the same as counting the length of the match. | ||
| */ | ||
| #ifdef __LITTLE_ENDIAN__ | ||
| asm volatile("vctzlsbb %0, %1\n\t" : "=r" (len) : "v" (vc)); |
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The assembly in both versions is identical. Is this intended?
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Actually I was wrong. One letter off.
This commit introduces an optimized version of the longest_match function for Power processors. It uses VSX instructions to match 16 bytes at a time on each comparison, instead of one by one. Author: Matheus Castanho <msc@linux.ibm.com>
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Hello again,
This optimization uses VSX vector (SIMD) instructions to try to match multiple bytes at the same time during the search for the longest match. A vector load + comparison (16 bytes) has just a small overhead if compared to their regular versions, so the optimized longest_match tries to match as many bytes as possible on every comparison.
This PR shares 1 commit with #457 and #458, which can be removed if either one gets merged first. It also uses GNU indirect functions to choose which function version (optimized or default) to run on the first call to longest_match during runtime.
To test the performance improvement, we used Chromium's zlib_bench.cc with input files from jsnell/zlib-bench.
The results below show compression throughput in MB/s using RAW deflate, for all compression levels:
pngpixels
jpeg
executable
html