Le 22/10/2010 10:43, John Rose a écrit :
> Based partly on our discussions at the Summit about "live constants", and
> also based on the likely requirements of Project Lambda, the JSR 292 EG is
> likely to allow any single invokedynamic instruction to pass one or more
> extra constant values into the bootstrap method invocation.
>
> Here is the current thinking. Language implementors, please tell us if we
> are missing anything.
>
> We call these "static arguments", in contrast to the normal "dynamic
> arguments" that are received on every method call. For invokedynamic, the
> dynamic arguments are received as if by 'invokeExact' on the method handle
> bound to the invokedynamic instruction instance, by the BSM. The BSM
> decides, once at link time, which method handle to choose based on the static
> arguments.
>
> There are three standard static arguments always passed to the BSM:
> 1. an indication of the caller class (note: this is likely to change to a
> MethodHandles.Lookup capability)
> 2. a String naming the method apparently being called
> 3. a MethodType indicating the dynamic arguments and return value types
>
> The String and MethodType are extracted from the NameAndType constant at the
> invokedynamic site.
>
> The invokedynamic instruction points to a constant pool entry that looks like
> this:
>
> struct InvokeDynamic_info {
> u1 tag; // always CONSTANT_InvokeDynamic = 18
> u2 bsm_index; // ref to CONSTANT_MethodHandle
> u2 descr_index; // ref to CONSTANT_NameAndType
> u2 argc; // count of optional static arguments
> u2 argv[argc]; // refs to anything 'ldc' can refer to (int, long, float,
> double, class, method handle, method type)
> }
>
> If we take this path, we will switch to the tag '18', to reduce confusion
> when old and new class files are mixed.
>
> The existing tag '17' for the no-extra-args format will drop out of use and
> be illegal in JDK7 FCS.
>
> Depending on the value of argc, the BSM will be invoked in one of three ways:
> if (argc = 0) binding = bsm.invokeGeneric(lookup, name, type);
> if (argc = 1) binding = bsm.invokeGeneric(lookup, name, type, (Object)
> argv[0]);
> if (argc> 1) binding = bsm.invokeGeneric(lookup, name, type, (Object[])
> argv);
>
> Note that the BSM, since it is derived from a CONSTANT_MethodHandle, can only
> be a "direct method handle", a pointer to a Java method. It cannot be
> adapted (e.g., as a spreader or collector). But in user-visible code, it
> would be reasonable to express a typical BSM as an overloaded method, whose
> third overloading takes a varargs array:
> MethodHandle myBSM(MethodHandles.Lookup look, String name, MethodType
> type);
> MethodHandle myBSM(MethodHandles.Lookup look, String name, MethodType
> type, Object arg);
> MethodHandle myBSM(MethodHandles.Lookup look, String name, MethodType
> type, Object... args);
>
And also:
CallSite myBSM(MethodHandles.Lookup look, String name, MethodType type);
CallSite myBSM(MethodHandles.Lookup look, String name, MethodType type,
Object arg);
CallSite myBSM(MethodHandles.Lookup look, String name, MethodType type,
Object... args);
A bootstrap method can return a call site (CallSite) or a method handle
(MethodHandle).
The idea is to return a CallSite if the target method can change and a
MethodHandle
if the target of an invokedynamic never change.
> It is natural to ask why we are using varargs, when we could just specify
> that the extra static arguments could be passed positionally. The simple
> answer is positional arguments are of limited use, but a varargs array can be
> used to encode very rich and useful BSM arguments.
>
> Since very few Java methods take more than 10 parameters, allowing up to 255
> extra arguments is not very interesting. (Actually the limit would be 251
> non-long non-double arguments, since there are three to start with, plus the
> BSM itself.) Writing a BSM which takes (say) 100 arguments would be silly.
> (Note that BSMs cannot be collectArguments adapters; they have to be simple
> JVM methods or constructors.) And a related one that takes 99 arguments
> would have to be a completely distinct method. It is clear that any large
> number of arguments has to be passed in an array. So let's pass them all in
> a trailing varargs parameter.
>
> Will users want more than a couple of extra static arguments? I think so.
> It will provide a way to bind interesting specifications directly into the
> classfile, without cumbersome bytecode-based construction. Examples:
> - a serialized AST structure, built from a mix of strings and method
> handles, to be interpreted
> - complex application-defined constants, such as lists or sets
> - similarly, templates for partly-constant data structures (the
> invokedynamic builds a factory method for the template)
> - vtables (i.e., maps of names to method handles)
>
> All of these things can be created by executable bytecodes in<clinit>, but
> implementors will (in many cases) be able to create them more compactly from
> series of constants. For example, a list of integer values will occupy 2+1+4
> bytes per element if encoded as a sequence of static arguments. (The '2' is
> the argv element; the 1+4 is the CONSTANT_Integer.) Using<clinit> style
> bytecodes, the same element will require (1+2+3+1)+1+4 bytes, where the
> parenthesized numbers stand for a sequence of "aload buf; bipush J; ldc N;
> aastore". (This sequence stores the element into an object array, which is
> going to be passed to something like Arrays.asList.) The ratio is 7 to 11.
> For integer values which repeat, the ratio is closer to 2 to 6.
>
> There is a limit to this technique, of course, since the constant pool has
> only 65535 constants. But this limit is shared with the<clinit> style
> technique.
>
> A key use case for one or two BSM arguments is closure construction for
> Project Lambda. Here, an extra static argument can specify a private
> synthetic method which gives the body (code, not data) of the closure. The
> data parts are normal dynamic arguments. The BSM produces a factory function
> which (efficiently) binds the data values to the statically specified closure
> body. A second BSM argument might be the SAM type intended for the closure.
> (That could also be inferred from the MethodType.)
>
> Another key use case is an invokedynamic instruction that implements an
> arbitrary live constant, by linking the call site (of zero arguments) to a
> method handle which always returns the desired constant.
> (MethodHandles.constant will do this.) The only missing bit is the
> serialized data behind the live constant. Again, allowing an essentially
> unbounded array gives implementors the right degree (I think) of flexibility.
>
> If, instead of constants, we want templated values (think Groovy strings like
> "hello, $name"), the statically determined structure of the value can be
> expressed in static arguments to an invokedynamic, with the inserted values
> ("$name") passed on the stack. The BSM produces a factory function which
> builds the desired result. The BSM might use a templating engine to
> partially evaluate the static structure, so that the dynamically changing
> parts can be combined in at full speed.
>
> (A useful thing missing here is substructure sharing: What if two
> invokedynamic instructions need almost the same static arguments? This can
> be dealt in user code, with via a static table created in<clinit> or a
> similar method. Shared values can be referred to by small integers assigned
> by the language backend. In essence, the components we are proposing help
> language implementors to build better versions of constant pools and vtables,
> with compactness and efficiency similar to the corresponding native
> structures.)
>
> In conclusion: It is true that most use cases for BSM arguments will only
> need one or two extra arguments. But if we allow an array of strings,
> integer, method handles, etc., with a reasonable length, suddenly our
> language implementor friends have a flexible and natural way to use for
> encoding the "serialized" version of their live constants.
>
> So, let's not just one or two static arguments, and not a useless 251,
> either, but rather a useful 65535.
>
> (I'd go for a larger number, 2**31-1, but it would not mesh with the other
> 16-bit numbers in the class file format. That's got to be fixed in a big
> format revision, another day.)
>
> -- John
>
Rémi
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