Reduce overhead of enabling -Ystatistics #6234
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The implementation trick of using an AlmostFinalValue to have zero cost for the "isEnabled" check in the common case has a small flaw: the switchpoint is tripped _every_ time stats is enabled, rather than just on the first time. This discards a swathe of JIT compiled code each time a Global is started with `-Ystatistics`. This commit avoids tripping the switchpoint redundantly. Performance: ``` ⚡ for extra in "-Ystatistics:_" ""; do for v in 2.12.5-bin-91649d1-SNAPSHOT 2.12.4; do echo $v $extra; sbt 'set scalaVersion in compilation := "'$v'"' 'hot -psource=scalap -f1 -wi 5 -i 3 -pextraArgs='$extra | egrep 'HotScalacBenchmark.compile\s'; done; done 2.12.5-bin-91649d1-SNAPSHOT -Ystatistics:_ [info] HotScalacBenchmark.compile a8c43dc -Ystatistics:_ false scalap sample 33 973.523 ± 23.389 ms/op 2.12.4 -Ystatistics:_ [info] HotScalacBenchmark.compile a8c43dc -Ystatistics:_ false scalap sample 12 2921.333 ± 177.831 ms/op 2.12.5-bin-91649d1-SNAPSHOT [info] HotScalacBenchmark.compile a8c43dc false scalap sample 38 811.846 ± 13.436 ms/op 2.12.4 [info] HotScalacBenchmark.compile a8c43dc false scalap sample 38 820.814 ± 17.809 ms/op ``` There is still more overhead than I would like, and it might still make sense to move a few stats back into the "hot" category.
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Damn! I’m sorry my new iPhone got crazy and I mistakenly pressed “Merge pull request” when I wanted to say LGTM. Sorry for that — if someone takes any issue wIth the PR please feel free to revert and submit the PR again. |
jvican
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Jan 13, 2018
The latest scalac master includes scala/scala#6234 which makes the overhead of statistics smaller.
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The implementation trick of using an AlmostFinalValue to have zero
cost for the "isEnabled" check in the common case has a small flaw:
the switchpoint is tripped every time stats is enabled, rather
than just on the first time. This discards a swathe of JIT compiled
code each time a Global is started with
-Ystatistics.This commit avoids tripping the switchpoint redundantly.
Performance:
There is still more overhead than I would like, and it might still make
sense to move a few stats back into the "hot" category.