Add optimization for Adler32 checksum for Power processors #458
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FYI this PR uses the same base commit as #457 to add base code for Power optimizations. When either one gets accepted, the other can be rebased to remove the first commit from the PR. |
added 3 commits
March 11, 2020 10:55
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 <[email protected]> Author: Rogerio Alves <[email protected]>
This commit implements a Power (POWER8+) vector optimization for Adler32 checksum using VSX (vector) instructions. The VSX adler32 checksum is up to 10x fast than the adler32 baseline code. Author: Rogerio Alves <[email protected]>
This commit add tests for adler32 vector optimization for Power (POWER8+). Author: Rogerio Alves <[email protected]>
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Hi,
This PR introduces a optimization for Adler32 checksum for POWER8+ processors that uses VSX (vector) instructions.
If adler32 do 1 byte at time on the first iteration s1 is s1_0 (_n means iteration n) is the initial value of adler, at beginning _0 is 1 unless adler initial value is different than 1. So s1_1 = s1_0 + c[0] after
the first calculation. For the next iteration s1_2 = s1_1 + c[1] and so on. Hence, for iteration N, s1_N = s1_(N-1) + c[N] is the value of s1 on after iteration N. Therefore, for s2, s2_N = s2_0 + Ns1_N + Nc[0] + N-1*c[1] + ... + c[N] In a more general way:
s1_N = s1_0 + sum(i=1 to N)*c[i]
s2_N = s2_0 + N*s1 + sum (i=1 to N)(N-i+1)*c[i]
Where s1_N, s2_N are the values for s1, s2 after N iterations. So if we can process N-byte at time we can obtain adler32 checksum for N-byte at once. Since VSX can support 16-byte vector instructions, we can process 16-byte at time using N = 16 we have:
s1 = s1_16 = s1_0 + sum(i=1 to 16)c[i]
s2 = s2_16 = s2_0 + 16*s1 + sum(i=1 to 16)(16-i+1)*c[i]
The VSX version starts to improve the performance for buffers with size >= 64. The performance is up to 10x better than Adler32 version from adler32 non-vectorized version (average cpu time in ns on 100000 iterations):
For buffer with length <= than 64 the performance is almost the same of
the non-vectorized implementation (with a small performance degradation in some cases):