[9] RFR (S): 8074548: Never-taken branches cause repeated deopts in MHs.GWT case

John Rose john.r.rose at oracle.com
Mon Mar 16 18:51:54 UTC 2015

Good fix, thanks.  — John

P.S.  I noticed a typo:  "it's" (contraction of "it is") should be "its" (genitive of "it").

On Mar 16, 2015, at 11:26 AM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
> http://cr.openjdk.java.net/~vlivanov/8074548/webrev.00/
> https://bugs.openjdk.java.net/browse/JDK-8074548
> MethodHandleImpl::profileBoolean doesn't update never-taken branch count when hitting a deopt on it. As a result, for rarely taken branches consequent compilations consider them as never-taken and prune them again causing repeated deopts. It severely affects peak performance.
> The fix is to update MHI::profileBoolean intrinsic to insert a guard and uncommon trap w/ reexecute bit set for never-seen value. Once previously never seen value is encountered, the execution resumes after deopt in MHI::profileBoolean and corresponding count becomes non-zero.
> The guard doesn't add any additional overhead, since it dominates all value usages and all branches on the same value are eliminated.
> Testing: java/lang/invoke, nashorn, octane

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