[11] RFR: 8198418: Invoke LambdaMetafactory::metafactory exactly from the BootstrapMethodInvoker

[Claes Redestad]

Hi,

a small regression to lambda bootstrapping came in with the recent
condy merge, and it took me a while to figure out why.

Before condy, the first three parameters of calls from the BSM invoker
to the six parameter LambdaMetafactory::metafactory were statically
known, so only the fourth through sixth param were dynamically bound
to enforce runtime type checks (MH.invoke -> MH.checkGenericInvoker
-> MH.asType(MT) -> MHI.makePairwiseConvertByEditor -> generates a
slew of filterArguments, rebinds, casting MHs etc).

With condy, the third parameter is now an Object (in reality either a
Class or a MethodType), thus not statically known. This means the
MethodType sent to checkGenericInvoker will have to add a cast for
this param too, thus in makePairwiseConvertByEditor we see an
additional rebind, some additional time spent spinning classes etc.
Effectively increasing the cost of first lambda initialization by a small
amount (a couple of ms).

Here came the realization that much of the static overhead of the
lambda bootstrapping could be avoided altogether since we can
determine and cast arguments statically for the special-but-common
case of LambdaMetafactory::metafactory. By using exact type
information, and even bootstrapMethod.invokeExact, no dynamic
runtime checking is needed, so the time spent in
makePairwiseConvertByEditor is avoided entirely.

This might be a hack, but a hack that removes a large chunk of the
code executed (~75% less bytecode) for the initial lambda bootstrap.
Startup tests exercising lambdas show a 10-15ms improvement - the
static overhead of using lambdas is now just a few milliseconds in total.

Webrev: http://cr.openjdk.java.net/~redestad/8198418/jdk.00/
RFE: https://bugs.openjdk.java.net/browse/JDK-8198418

The patch includes a test for an experimental new metafactory method
that exists only in the amber condy-folding branch. I can easily break it
out and push that directly to amber once this patch syncs up there, but
have tested that keeping it in here does no harm.

Thanks!

/Claes