[foreign] RFR 8210757: Add binder support for direct native invocation strategy

Maurizio Cimadamore maurizio.cimadamore at oracle.com
Fri Sep 28 11:19:46 UTC 2018




On 28/09/18 12:19, Maurizio Cimadamore wrote:
> This is an updated version of the direct invocation scheme support. 
> Very close to the last one, but there are some minor 
> refactorings/improvements:
> 1) Added a @Stable annotation in DirectNativeInvoker's MH field
> 2) box/unbox routine used by the UniversalXYZ strategies have been 
> moved from NativeInvoker to UniversalNativeInvoker
> 3) I revamped the logic which detects whether fastpath is applicable - 
> now we create the calling sequence first, and we use that to check 
> whether we can fast path it. Some internal benchmark have shown that 
> with a large number of symbols, we were doing a lot of work because we 
> were trying the fastpath always and then, in case of exception 
> fallback to slow path; in such cases we would create calling sequence 
> twice. This new technique might also be more friendly w.r.t. Windows 
> and other ABIs.
> I'd really like to move ahead with this (as this RFR has been out for 
> quite a while now) - if there's no other comments I'll go ahead.
> Maurizio
> On 14/09/18 19:04, Maurizio Cimadamore wrote:
>> Hi,
>> as mentioned in [1], this patch adds binder support for the so called 
>> 'direct' invocation scheme, which allows for greater native 
>> invocation downcall/upcall performances by means of specialized 
>> adapters. The core idea, also described in [1], is to define adapters 
>> of the kind:
>> invokeNative_V_DDDDD
>> invokeNative_V_JDDDD
>> invokeNative_V_JJDDD
>> invokeNative_V_JJJDD
>> invokeNative_V_JJJJD
>> invokeNative_V_JJJJJ
>> Where long arguments come before double arguments (and do this for 
>> each arity e.g. <=5).
>> If all arguments are passed in register, then this reordering doesn't 
>> affect behavior, and greatly limits the number of permutations to be 
>> supported/generated.
>> The downcall part (java to native) is relative straightforward: the 
>> directNativeInvoker.cpp file defines a bunch of native entry points, 
>> one per shape, which cast the input address to a function pointer of 
>> the desired shape, and then call it:
>> jlong NI_invokeNative_J_JD(JNIEnv *env, jobject _unused, jlong addr, 
>> jlong arg0, jdouble arg1) {
>>     return ((jlong (*)(jlong, jdouble))addr)(arg0, arg1);
>> }
>> The upcall business is a little trickier: first, if we are only to 
>> optimize upcalls where argument passing happens in registers, then 
>> it's crucial to note that by the time we get into the assembly stub, 
>> all the registers will have been populated by the native code to 
>> contain the right arguments in the right places. So we can avoid all 
>> the shuffling in the assembly adapter and simply jump onto a C 
>> function that looks like this:
>> long specialized_upcall_helper_J(long l0, long l1, long l2, long l3,
>>                                       double d0, double d1, double 
>> d2, double d3,
>>                                        unsigned int mask, jobject 
>> rec) { ... }
>> Note here that the first 8 arguments are just longs and doubles, and 
>> those will be expected to be in registers, according to the System V 
>> ABI. (In windows, the situation will be a bit different as less 
>> integer registers are available, so this will need some work there).
>> So, to recap, the assembly upcall stub simply 'append' the receiver 
>> object and a 'signature mask' in the last two available C registers 
>> and then jump onto the helper function. The helper function will find 
>> all the desired arguments in the right places - there will be, in the 
>> general case, some unused arguments, but that's fine, after all it 
>> didn't cost anything to us to load them in the first place!
>> Note that we have three helper variants, one for each return type { 
>> long, double, void }. This is required as we need the C helper to 
>> return a value of the right type which will generate the right 
>> assembly sequence to store the result in the right register (either 
>> integer or MMX).
>> So, with three helpers we can support all the shapes with up to 8 
>> arguments. On the Java side we have, of course, to define a 
>> specialized entry point for each shape.
>> All the magic for adapting method handle to and from the specialized 
>> adapters happen in the DirectSignatureShuffler class; this class is 
>> responsible for adapting each argument e.g. from Java to native 
>> value, and then reordering the adapted method handle to match the 
>> order in which arguments are expected by the adapter (e.g. move all 
>> longs in front). The challenge was in having DirectSignatureShuffle 
>> to be fully symmetric - e.g. I did not want to have different code 
>> paths for upcalls and downcalls, so the code tries quite hard to be 
>> parametric in the shuffling direction (java->native or native->java) 
>> - which means that adapters will be applied in one way or in the 
>> inverse way depending on the shuffling direction (and as to whether 
>> we are adapting an argument or a return). Since method handle filters 
>> are composable, it all works out quite beautifully.
>> Note that the resulting, adapted MH is stored in a @Stable field to 
>> tell the JIT to optimize the heck out of it (as if it were a static 
>> constant).
>> This patch contains several other changes - which I discuss briefly 
>> below:
>> * we need to setup a framework in which new invocation strategies can 
>> be plugged in - note that we now have essentially 4 cases:
>> { NativeInvoker, UpcallHandler } x { Universal, Direct }
>> When the code wants e.g. a NativeInvoker, it asks for one to the 
>> NativeInvoker::of factory (UpcallHandler work in a similar way); this 
>> factory will attempt to go down the fast path - if an error occurs 
>> when computing the fast path, the call will fallback to the universal 
>> (slow) path.
>> Most of the changes you see in the Java code are associated to this 
>> refactoring - e.g. all clients of NativeInvoker/UpcallHandler should 
>> now go through the factory
>> * CallbackImplGenerator had a major issue since the new factory for 
>> NativeInvoker wants to bind an address eagerly (this is required e.g. 
>> to be forward compatible with linkToNative backend); which means that 
>> at construction time we have to get the address of the callback, call 
>> the NativeInvoker factory and then stash the target method handle 
>> into a field of the anon callback class. Vlad tells me that fields of 
>> anon classes are always 'trusted' by the JIT, which means they should 
>> be treated as '@Stable' (note that I can't put a @Stable annotation 
>> there, since this code will be spinned in user-land).
>> * There are a bunch of properties that can be set to either force 
>> slow path or force 'direct' path; in the latter case, if an error 
>> occurs when instantiating the direct wrapper, an exception is thrown. 
>> This mode is very useful for testing, and I indeed have tried to run 
>> all our tests with this flag enabled, to see how many places could 
>> not be optimized.
>> * I've also reorganized all the native code in hotspot/prims so that 
>> we have a separate file for each scheme (and so that native Java 
>> methods could be added where they really belong). This should also 
>> help in the long run as it should make adding/removing a given scheme 
>> easier.
>> * I've also added a small test which tries to pass structs of 
>> different sizes, but I will also work on a more complex test which 
>> will stress-test all invocation modes in a more complete fashion. 
>> With respect to testing, I've also done a fastdebug build and ran all 
>> tests with that (as fastdebug catches way many more hotspot assertion 
>> than the product version); everything passed.
>> Webrev:
>> http://cr.openjdk.java.net/~mcimadamore/panama/8210757/
>> I'd like to thank Vladimir Ivanov for the prompt support whenever I 
>> got stuck down the macro assembler rabbit hole :-)
>> Cheers
>> Maurizio
>> [1] - 
>> http://mail.openjdk.java.net/pipermail/panama-dev/2018-September/002652.html

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