RFR (S): 8148994: Replacing MH::invokeBasic with a direct call breaks LF customization
vladimir.x.ivanov at oracle.com
Fri Feb 5 16:44:06 UTC 2016
Recompilation is usually expensive and if the call site hasn't been
warmed up enough to be inlined, it is a clue it will likely never be.
Actually, this bug is really a corner case. LF customization matters in
cases when there's no inlining possible (e.g., MH::invokeExact() on a
receiver which is not a compile-time constant). Instead of jumping
between numerous tiny nmethods for every LF in the chain, it allows to
generate a single nmethod for the whole chain. There's no need in
customization when inlining happens and customization doesn't help
inlining as well.
In GWT case, uncommon trap is generated for non-taken branch (so causes
a recompilation if it is ever taken), but a cold branch (<30
invocations) is compiled into a call. It could be beneficial to
recompile the branch if it becomes really hot, but it happens rarely and
requires additional profiling.
I vaguely remember I experimented with that, but it severely affected
warmup and didn't give any peak performance improvements (usually, on
On 2/4/16 8:13 PM, Remi Forax wrote:
> Hi Vladimir,
> Perhaps a stupid question, but why LF customization doesn't trigger a recompilation if invokeBasic is already compiled into a direct call ?
> ----- Mail original -----
>> De: "Vladimir Ivanov" <vladimir.x.ivanov at oracle.com>
>> À: "hotspot compiler" <hotspot-compiler-dev at openjdk.java.net>
>> Envoyé: Jeudi 4 Février 2016 17:32:33
>> Objet: Re:  RFR (S): 8148994: Replacing MH::invokeBasic with a direct call breaks LF customization
>> FYI I was asked to keep the tests intact until Jigsaw is integrated.
>> Removed test changes from the webrev (updated in place) and filed
>> JDK-8149040  to clean up the tests later.
>> Best regards,
>> Vladimir Ivanov
>>  https://bugs.openjdk.java.net/browse/JDK-8149040
>> On 2/4/16 6:51 PM, Vladimir Ivanov wrote:
>>> JDK-8072008  introduced an optimization to bypass linker calls in
>>> generated code when target method can't be inlined.
>>> Unfortunately, it doesn't work well with MH::invokeBasic(), because it
>>> clashes with LambdaForm customization: if a call is not inlined, it is
>>> important for performance to update (customize) corresponding LambdaForm
>>> on-the-fly. MH::invokeBasic() linker does MH -> LF -> Method* ->
>>> _from_compiled_entry traversal on every invocation, while direct call
>>> points right to the _from_compiled_entry. So, any LambdaForm updates
>>> aren't visible until recompilation.
>>> The fix is to keep MH::invokeBasic linker, so up-to-date LambdaForm
>>> instance is used on every invocation.
>>> Also, cleaned up relevant tests a bit.
>>> Testing: octane, JPRT.
>>> Best regards,
>>> Vladimir Ivanov
>>>  https://bugs.openjdk.java.net/browse/JDK-8072008
>>> "Emit direct call instead of linkTo* for recursive indy/MH.invoke*
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