RFR(M): 8200303: C2 should leverage profiling for lookupswitch/tableswitch

Vladimir Kozlov vladimir.kozlov at oracle.com
Tue Mar 27 16:04:47 UTC 2018

Failed to build on SPARC (PCH?):

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On 3/27/18 8:20 AM, Vladimir Kozlov wrote:
> Thank you, Roland, for contributing this.
> Changes look reasonable. I will start testing including performance.
> Thanks,
> Vladimir
> On 3/27/18 7:35 AM, Roland Westrelin wrote:
>> http://cr.openjdk.java.net/~roland/8200303/webrev.00/
>> Over in the Shenandoah project, Aleksey found that:
>> 1) a counted loop with a switch:
>> for (...) {
>>    switch (..) {
>>      ...
>>      default:
>>        throw SomeException();
>>    }
>> }
>> where some cases break out of the loop would not perform as well when
>> loop strip mining is enabled, even if the cases that exit the loop are
>> never taken in practice.
>> Because C2 gives all branches out of a JumpNode the same probability,
>> exiting the loop has a non null probability and GCM computes (wrongly)
>> that scheduling the loop strip mining book keeping logic in the loop is
>> cheaper than out of the loop.
>> 2) Shenandoah write barriers in some of the cases should be hoisted but
>> are not because C2 can't tell that only a single case of the switch is
>> ever hit.
>> http://mail.openjdk.java.net/pipermail/shenandoah-dev/2017-December/004535.html 
>> In the Shenandoah repo, we have a change that makes C2 leverage
>> profiling for switch. Experiments showed that 1) and 2) above are fixed
>> and that some common benchmarks run with parallel gc benefit as well
>> (~+7% on Serial):
>> http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-February/004886.html 
>> The patch I'm proposing here is based on the patch we've been using for
>> a couple months in Shenandoah:
>> - it fixes profile collection in c1 for lookupswitch/tableswitch
>> - it sets profiling information on IfNodes and JumpNodes emitted from
>>    lookupswitch/tableswitch and propagate it after matching so GCM can
>>    take advantage of it
>> - it takes advantage of profiling to find never taken cases and trim
>>    down the cases (or ranges as they're called in the code). A never
>>    taken range can now cause an uncommon trap.
>> and also has some improvements:
>> - if some ranges are a lot more common than others, it might pay off to
>>    check for them one after the other before going to the binary
>>    search. The patch has some logic to evaluate the number of steps in
>>    the binary search and determine whether checking for the most common
>>    case upfront would pay off (from profile data)
>> - the binary search doesn't always keep the tree balanced but instead
>>    picks a mid point that split frequencies in half
>> Roland.

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