gh-115704: Improve DJBX33A hash algorithm #115705
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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool. If this change has little impact on Python users, wait for a maintainer to apply the |
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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool. If this change has little impact on Python users, wait for a maintainer to apply the |
PeterYang12
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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool. If this change has little impact on Python users, wait for a maintainer to apply the |
1 similar comment
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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool. If this change has little impact on Python users, wait for a maintainer to apply the |
Python/pyhash.c
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| hash = hash * 33 * 33 * 33 * 33 + | ||
| p[0] * 33 * 33 * 33 + | ||
| p[1] * 33 * 33 + | ||
| p[2] * 33 + | ||
| p[3]; |
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I think you can directly compute those values (namely the powers of 33), although the compiler should be able to optimize it as well. You could however add a small comment.
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Thank you for your review. Done.
Python/pyhash.c
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| if (len >= 2) { | ||
| if (len > 2) { | ||
| hash = hash * 33 * 33 * 33 + | ||
| p[0] * 33 * 33 + | ||
| p[1] * 33 + | ||
| p[2]; | ||
| } | ||
| else { | ||
| hash = hash * 33 * 33 + p[0] * 33 + p[1]; | ||
| } | ||
| } |
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Does the compiler behaves differently if you use
if (len > 2) { ... }
else if (len == 2) { ... }
else if (len) { ... }instead?
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this PR is quite fundamentally different from the Lemire post/the PHP change. In PHP, the change was done in the implementation of the hash function for arbitrary lengths. In this PR, only the code for computing the hash of bytes with This might still be a worthwhile change, but there should be really strong benchmarking results that show this. |
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I support it only to clean up a little by getting rid of the ugly (IMO) C fall-through code and replacing it with a simple loop. I don't think the compiler will generate anything significantly different. |
Accelerating python hash algorithm by "unoptimizing" it when using DJBX33A as hash algorithm. See Daniel Lemire's blog post: https://lemire.me/blog/2016/07/21/accelerating-php-hashing-by-unoptimizing-it/ Signed-off-by: PeterYang12 <[email protected]>
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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool. If this change has little impact on Python users, wait for a maintainer to apply the |
Accelerating python hash algorithm by "unoptimizing" it when using DJBX33A as hash algorithm.
See Daniel Lemire's blog post:
https://lemire.me/blog/2016/07/21/accelerating-php-hashing-by-unoptimizing-it/
This idea has already been implemented in the PHP interpreter.