In ciCallSite::get_context(), is it safe to manipulate a raw oop the way you do
it (with 2 different oops). Can’t it be moved concurrently by the GC?
> Best regards,
> Vladimir Ivanov
> On 4/1/15 11:56 PM, Vladimir Ivanov wrote:
>> HotSpot JITs inline very aggressively through CallSites. The
>> optimistically treat CallSite target as constant, but record a nmethod
>> dependency to invalidate the compiled code once CallSite target changes.
>> Right now, such dependencies have call site class as a context. This
>> context is too coarse and it leads to context pollution: if some
>> CallSite target changes, VM needs to enumerate all nmethods which
>> depends on call sites of such type.
>> As performance analysis in the bug report shows, it can sum to
>> significant amount of work.
>> While working on the fix, I investigated 3 approaches:
>> (1) unique context per call site
>> (2) use CallSite target class
>> (3) use a class the CallSite instance is linked to
>> Considering call sites are ubiquitous (e.g. 10,000s on some octane
>> benchmarks), loading a dedicated class for every call site is an
>> overkill (even VM anonymous).
>> CallSite target class
>> (MethodHandle.form->LambdaForm.vmentry->MemberName.clazz->Class<?>) is
>> also not satisfactory, since it is a compiled LambdaForm VM anonymous
>> class, which is heavily shared. It gets context pollution down, but
>> still the overhead is quite high.
>> So, I decided to focus on (3) and ended up with a mixture of (2) & (3).
>> Comparing to other options, the complications of (3) are:
>> - CallSite can stay unlinked (e.g. CallSite.dynamicInvoker()), so
>> there should be some default context VM can use
>> - CallSite instances can be shared and it shouldn't keep the context
>> class from unloading;
>> It motivated a scheme where CallSite context is initialized lazily and
>> can change during lifetime. When CallSite is linked with an indy
>> instruction, it's context is initialized. Usually, JIT sees CallSite
>> instances with initialized context (since it reaches them through indy),
>> but if it's not the case and there's no context yet, JIT sets it to
>> "default context", which means "use target call site".
>> I introduced CallSite$DependencyContext, which represents a nmethod
>> dependency context and points (indirectly) to a Class<?> used as a context.
>> Context class is referenced through a phantom reference
>> (sun.misc.Cleaner to simplify cleanup). Though it's impossible to
>> extract referent using Reference.get(), VM can access it directly by
>> reading corresponding field. Unlike other types of references, phantom
>> references aren't cleared automatically. It allows VM to access context
>> class until cleanup is performed. And cleanup resets the context to
>> NULL, in addition to invalidating all relevant dependencies.
>> There are 3 context states a CallSite instance can be in:
>> (1) NULL: no depedencies
>> (2) DependencyContext.DEFAULT_CONTEXT: dependencies are stored in
>> call site target class
>> (3) DependencyContext for some class: dependencies are stored on the
>> class DependencyContext instance points to
>> Every CallSite starts w/o a context (1) and then lazily gets one ((2) or
>> (3) depending on the situation).
>> State transitions:
>> (1->3): When a CallSite w/o a context (1) is linked with some indy
>> call site, it's owner is recorded as a context (3).
>> (1->2): When JIT needs to record a dependency on a target of a
>> CallSite w/o a context(1), it sets the context to DEFAULT_CONTEXT and
>> uses target class to store the dependency.
>> (3->1): When context class becomes unreachable, a cleanup hook
>> invalidates all dependencies on that CallSite and resets the context to
>> NULL (1).
>> Only (3->1) requires dependency invalidation, because there are no
>> depedencies in (1) and (2->1) isn't performed.
>> (1->3) is done in Java code (CallSite.initContext) and (1->2) is
>> performed in VM (ciCallSite::get_context()). The updates are performed
>> by CAS, so there's no need in additional synchronization. Other
>> operations on VM side are volatile (to play well with Java code) and
>> performed with Compile_lock held (to avoid races between VM operations).
>> Some statistics:
>> Box2D, latest jdk9-dev
>> - CallSite instances: ~22000
>> - invalidated nmethods due to CallSite target changes: ~60
>> - checked call_site_target_value dependencies:
>> - before the fix: ~1,600,000
>> - after the fix: ~600
>> - dedicated test which excercises different state transitions
>> - jdk/java/lang/invoke, hotspot/test/compiler/jsr292, nashorn
>> Best regards,
>> Vladimir Ivanov
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