RFR: 8257815: Replace global log2 functions with efficient implementations [v5]
Claes Redestad
redestad at openjdk.java.net
Tue Dec 8 15:14:27 UTC 2020
> This patch replaces the log2 functions in globalDefinitions.hpp with more efficient counterparts in utilities/powerOfTwo.hpp
>
> Naming is hard, but I think the following scheme is reasonable:
>
> - log2i: any integral type. 0-hostile
> - log2i_allow_zero: any integral type. gracefully handles zero (adds a branch)
> - exact_log2i: any integral type. value must be a power of two
>
> I chose log2i rather than log2 to stand out from the log2 functions defined in various standard libraries.
>
> Going through all usage, quite a few uses of log2_long et.c. could be replaced by exact_log2i since they take something that has been checked to be a power of two. Most of the remaining usage seem to be able to use the 0-hostile variant, which avoids a branch.
>
> To sanity check that calculating log2 using count_leading_zeros gives better performance I added a couple of trivial and short-running microbenchmarks to test_powerOfTwo. For small values (<= 1025) the new impl is ~5x faster, with a larger speed-up for larger integer values:
>
> [ RUN ] power_of_2.log2_long_micro
> [ OK ] power_of_2.log2_long_micro (3581 ms)
> [ RUN ] power_of_2.log2_long_small_micro
> [ OK ] power_of_2.log2_long_small_micro (549 ms)
> [ RUN ] power_of_2.log2i_micro
> [ OK ] power_of_2.log2i_micro (259 ms)
> [ RUN ] power_of_2.log2i_small_micro
> [ OK ] power_of_2.log2i_small_micro (113 ms)
>
> I'm not sure if this naive microbenchmark carries its own weight, but it just adds a few seconds and can be useful for quickly checking this performance assumption on other H/W
>
> (Intending this for 17)
Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
Rename log2i ilogi and make it only accept value > 0, rename log2i_allow_zero log2i_graceful, accepting any value
-------------
Changes:
- all: https://git.openjdk.java.net/jdk/pull/1663/files
- new: https://git.openjdk.java.net/jdk/pull/1663/files/6daaaf64..96c51b07
Webrevs:
- full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1663&range=04
- incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1663&range=03-04
Stats: 99 lines in 21 files changed: 15 ins; 6 del; 78 mod
Patch: https://git.openjdk.java.net/jdk/pull/1663.diff
Fetch: git fetch https://git.openjdk.java.net/jdk pull/1663/head:pull/1663
PR: https://git.openjdk.java.net/jdk/pull/1663
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