On Thu, 22 Oct 2020 17:04:34 GMT, Maurizio Cimadamore <mcimadam...@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 one > additional commit since the last revision: > > Fix whitespaces Changes requested by ihse (Reviewer). make/modules/java.base/gensrc/GensrcScopedMemoryAccess.gmk line 148: > 146: > 147: $(DEST): $(BUILD_TOOLS_JDK) $(SCOPED_MEMORY_ACCESS_TEMPLATE) > $(SCOPED_MEMORY_ACCESS_BIN_TEMPLATE) > 148: $(MKDIR) -p $(SCOPED_MEMORY_ACCESS_GENSRC_DIR) Please use `$(call MakeDir, $(SCOPED_MEMORY_ACCESS_GENSRC_DIR))` instead. make/modules/java.base/gensrc/GensrcScopedMemoryAccess.gmk line 34: > 32: SCOPED_MEMORY_ACCESS_TEMPLATE := > $(SCOPED_MEMORY_ACCESS_SRC_DIR)/X-ScopedMemoryAccess.java.template > 33: SCOPED_MEMORY_ACCESS_BIN_TEMPLATE := > $(SCOPED_MEMORY_ACCESS_SRC_DIR)/X-ScopedMemoryAccess-bin.java.template > 34: DEST := $(SCOPED_MEMORY_ACCESS_GENSRC_DIR)/ScopedMemoryAccess.java `DEST` is a very generic and not really informative name. Maybe `SCOPED_MEMORY_ACCESS_GENSRC_DEST` to fit in with the rest of the names? And/or, maybe, to cut down on the excessive length, shorten `SCOPED_MEMORY_ACCESS` to `SMA` in all variables. make/modules/java.base/gensrc/GensrcScopedMemoryAccess.gmk line 26: > 24: # > 25: > 26: GENSRC_SCOPED_MEMORY_ACCESS := This variable does not seem to be used. A left-over from previous iterations? Also, please cut down a bit on the consecutive empty lines. ------------- PR: https://git.openjdk.java.net/jdk/pull/634
