RFR: 8254231: Implementation of Foreign Linker API (Incubator) [v18]

David Holmes dholmes at openjdk.java.net
Wed Nov 11 07:24:07 UTC 2020


On Tue, 10 Nov 2020 14:16:22 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> This patch contains the changes associated with the first incubation round of the foreign linker access API incubation
>> (see JEP 389 [1]). This work is meant to sit on top of the foreign memory access support (see JEP 393 [2] and associated pull request [3]).
>> 
>> The main goal of this API is to provide a way to call native functions from Java code without the need of intermediate JNI glue code. In order to do this, native calls are modeled through the MethodHandle API. I suggest reading the writeup [4] I put together few weeks ago, which illustrates what the foreign linker support is, and how it should be used by clients.
>> 
>> Disclaimer: the pull request mechanism isn't great at managing *dependent* reviews. For this reasons, I'm attaching a webrev which contains only the differences between this PR and the memory access PR. I will be periodically uploading new webrevs, as new iterations come out, to try and make the life of reviewers as simple as possible.
>> 
>> A big thank to Jorn Vernee and Vladimir Ivanov - they are the main architects of all the hotspot changes you see here, and without their help, the foreign linker support wouldn't be what it is today. As usual, a big thank to Paul Sandoz, who provided many insights (often by trying the bits first hand).
>> 
>> Thanks
>> Maurizio
>> 
>> Webrev:
>> http://cr.openjdk.java.net/~mcimadamore/8254231_v1/webrev
>> 
>> Javadoc:
>> 
>> http://cr.openjdk.java.net/~mcimadamore/8254231_v1/javadoc/jdk/incubator/foreign/package-summary.html
>> 
>> Specdiff (relative to [3]):
>> 
>> http://cr.openjdk.java.net/~mcimadamore/8254231_v1/specdiff_delta/overview-summary.html
>> 
>> CSR:
>> 
>> https://bugs.openjdk.java.net/browse/JDK-8254232
>> 
>> 
>> 
>> ### API Changes
>> 
>> The API changes are actually rather slim:
>> 
>> * `LibraryLookup`
>>   * This class allows clients to lookup symbols in native libraries; the interface is fairly simple; you can load a library by name, or absolute path, and then lookup symbols on that library.
>> * `FunctionDescriptor`
>>   * This is an abstraction that is very similar, in spirit, to `MethodType`; it is, at its core, an aggregate of memory layouts for the function arguments/return type. A function descriptor is used to describe the signature of a native function.
>> * `CLinker`
>>   * This is the real star of the show. A `CLinker` has two main methods: `downcallHandle` and `upcallStub`; the first takes a native symbol (as obtained from `LibraryLookup`), a `MethodType` and a `FunctionDescriptor` and returns a `MethodHandle` instance which can be used to call the target native symbol. The second takes an existing method handle, and a `FunctionDescriptor` and returns a new `MemorySegment` corresponding to a code stub allocated by the VM which acts as a trampoline from native code to the user-provided method handle. This is very useful for implementing upcalls.
>>    * This class also contains the various layout constants that should be used by clients when describing native signatures (e.g. `C_LONG` and friends); these layouts contain additional ABI classfication information (in the form of layout attributes) which is used by the runtime to *infer* how Java arguments should be shuffled for the native call to take place.
>>   * Finally, this class provides some helper functions e.g. so that clients can convert Java strings into C strings and back.
>> * `NativeScope`
>>   * This is an helper class which allows clients to group together logically related allocations; that is, rather than allocating separate memory segments using separate *try-with-resource* constructs, a `NativeScope` allows clients to use a _single_ block, and allocate all the required segments there. This is not only an usability boost, but also a performance boost, since not all allocation requests will be turned into `malloc` calls.
>> * `MemorySegment`
>>   * Only one method added here - namely `handoff(NativeScope)` which allows a segment to be transferred onto an existing native scope.
>> 
>> ### Safety
>> 
>> The foreign linker API is intrinsically unsafe; many things can go wrong when requesting a native method handle. For instance, the description of the native signature might be wrong (e.g. have too many arguments) - and the runtime has, in the general case, no way to detect such mismatches. For these reasons, obtaining a `CLinker` instance is a *restricted* operation, which can be enabled by specifying the usual JDK property `-Dforeign.restricted=permit` (as it's the case for other restricted method in the foreign memory API).
>> 
>> ### Implementation changes
>> 
>> The Java changes associated with `LibraryLookup` are relative straightforward; the only interesting thing to note here is that library loading does _not_ depend on class loaders, so `LibraryLookup` is not subject to the same restrictions which apply to JNI library loading (e.g. same library cannot be loaded by different classloaders).
>> 
>> As for `NativeScope` the changes are again relatively straightforward; it is an API which sits neatly on top of the foreign meory access API, providing some kind of allocation service which shares the same underlying memory segment(s), and turns an allocation request into a segment slice, which is a much less expensive operation. `NativeScope` comes in two variants: there are native scopes for which the allocation size is known a priori, and native scopes which can grow - these two schemes are implemented by two separate subclasses of `AbstractNativeScopeImpl`.
>> 
>> Of course the bulk of the changes are to support the `CLinker` downcall/upcall routines. These changes cut pretty deep into the JVM; I'll briefly summarize the goal of some of this changes - for further details, Jorn has put together a detailed writeup which explains the rationale behind the VM support, with some references to the code [5].
>> 
>> The main idea behind foreign linker is to infer, given a Java method type (expressed as a `MethodType` instance) and the description of the signature of a native function (expressed as a `FunctionDescriptor` instance) a _recipe_ that can be used to turn a Java call into the corresponding native call targeting the requested native function.
>> 
>> This inference scheme can be defined in a pretty straightforward fashion by looking at the various ABI specifications (for instance, see [6] for the SysV ABI, which is the one used on Linux/Mac). The various `CallArranger` classes, of which we have a flavor for each supported platform, do exactly that kind of inference.
>> 
>> For the inference process to work, we need to attach extra information to memory layouts; it is no longer sufficient to know e.g. that a layout is 32/64 bits - we need to know whether it is meant to represent a floating point value, or an integral value; this knowledge is required because floating points are passed in different registers by most ABIs. For this reason, `CLinker` offers a set of pre-baked, platform-dependent layout constants which contain the required classification attributes (e.g. a `Clinker.TypeKind` enum value). The runtime extracts this attribute, and performs classification accordingly.
>> 
>> A native call is decomposed into a sequence of basic, primitive operations, called `Binding` (see the great javadoc on the `Binding.java` class for more info). There are many such bindings - for instance the `Move` binding is used to move a value into a specific machine register/stack slot. So, the main job of the various `CallingArranger` classes is to determine, given a Java `MethodType` and `FunctionDescriptor` what is the set of bindings associated with the downcall/upcall.
>> 
>> At the heart of the foreign linker support is the `ProgrammableInvoker` class. This class effectively generates a `MethodHandle` which follows the steps described by the various bindings obtained by `CallArranger`. There are actually various strategies to interpret these bindings - listed below:
>> 
>> * basic intepreted mode; in this mode, all bindings are interpreted using a stack-based machine written in Java (see `BindingInterpreter`), except for the `Move` bindings. For these bindings, the move is implemented by allocating a *buffer* (whose size is ABI specific) and by moving all the lowered values into positions within this buffer. The buffer is then passed to a piece of assembly code inside the VM which takes values from the buffer and moves them in their expected registers/stack slots (note that each position in the buffer corresponds to a different register). This is the most general invocation mode, the more "customizable" one, but also the slowest - since for every call there is some extra allocation which takes place.
>> 
>> * specialized interpreted mode; same as before, but instead of interpreting the bindings with a stack-based interpreter, we generate a method handle chain which effectively interprets all the bindings (again, except `Move` ones).
>> 
>> * intrinsified mode; this is typically used in combination with the specialized interpreted mode described above (although it can also be used with the Java-based binding interpreter). The goal here is to remove the buffer allocation and copy by introducing an additional JVM intrinsic. If a native call recipe is constant (e.g. the set of bindings is constant, which is probably the case if the native method handle is stored in a `static`, `final` field), then the VM can generate specialized assembly code which interprets the `Move` binding without the need to go for an intermediate buffer. This gives us back performances that are on par with JNI.
>> 
>> For upcalls, the support is not (yet) as advanced, and only the basic interpreted mode is available there. We plan to add support for intrinsified modes there as well, which should considerably boost perfomances (probably well beyond what JNI can offer at the moment, since the upcall support in JNI is not very well optimized).
>> 
>> Again, for more readings on the internals of the foreign linker support, please refer to [5].
>> 
>> #### Test changes
>> 
>> Many new tests have been added to validate the foreign linker support; we have high level tests (see `StdLibTest`) which aim at testing the linker from the perspective of code that clients could write. But we also have deeper combinatorial tests (see `TestUpcall` and `TestDowncall`) which are meant to stress every corner of the ABI implementation. There are also some great tests (see the `callarranger` folder) which test the various `CallArranger`s for all the possible platforms; these tests adopt more of a white-box approach - that is, instead of treating the linker machinery as a black box and verify that the support works by checking that the native call returned the results we expected, these tests aims at checking that the set of bindings generated by the call arranger is correct. This also mean that we can test the classification logic for Windows, Mac and Linux regardless of the platform we're executing on.
>> 
>> Some additional microbenchmarks have been added to compare the performances of downcall/upcall with JNI.
>> 
>> [1] - https://openjdk.java.net/jeps/389
>> [2] - https://openjdk.java.net/jeps/393
>> [3] - https://git.openjdk.java.net/jdk/pull/548
>> [4] - https://github.com/openjdk/panama-foreign/blob/foreign-jextract/doc/panama_ffi.md
>> [5] - http://cr.openjdk.java.net/~jvernee/docs/Foreign-abi%20downcall%20intrinsics%20technical%20description.html
>
> Maurizio Cimadamore has updated the pull request incrementally with 10 additional commits since the last revision:
> 
>  - Merge pull request #7 from JornVernee/Additional_Review_Comments
>    
>    Additional review comments
>  - Revert System.java changes
>  - Set copyright year for added files to 2020
>  - Check result of AttachCurrentThread
>  - Sort includes alphabetically
>  - Relax ret_addr_offset() assert
>  - Extra space after if
>  - remove excessive asserts in ProgrammableInvoker::invoke_native
>  - Remove os::is_MP() check
>  - remove blank line in thread.hpp

Updates seem fine to me. Thanks.

Adding approval for hotspot parts.

src/hotspot/share/prims/universalUpcallHandler.cpp line 55:

> 53:     JavaVM_ *vm = (JavaVM *)(&main_vm);
> 54:     jint result = vm->functions->AttachCurrentThread(vm, (void**) &p_env, nullptr);
> 55:     guarantee(result == JNI_OK, "Could not attach thread for upcall. JNI error code: %d", result);

I'm assuming you don't have a mechanism for conveying an error back to the original entry point used by the native thread? Attaching an existing thread should only fail if we run out of C-Heap, so we're on the brink of aborting anyway, but still the guarantee here is not ideal.

-------------

Marked as reviewed by dholmes (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/634


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