On 9/28/2012 5:33 PM, Vitaly Davidovich wrote:
Yeah, the CLR does something similar - there's a type, ValueType,
which is the base class for all structs implicitly. It's just a
moniker though since you can't extend it explicitly (languages like C#
then provide keywords to declare them) and it provides just a handful
of basic methods inherited from object which you can override if need
be. They can implement interfaces, but not extend (or be extended by)
classes. Assignment just does bitwise copying, as well as passing
them as args (unless ref/out is used) or returning them. Assigning or
referring to them via any interface that they implement boxes it.
The really nice thing about them is you can implement (and BCL does)
some lightweight abstractions like enumerators. You can also do
RAII-like things with them; C# actually generates code that refers to
them as-is (not IDisposable) when desugaring using{} blocks. It's
very nice basically :)
my own VM does something vaguely similar as well, but it differs
somewhat from the strategy described (in that value-types are indicated
via a modifier flag and don't require any special treatment at the
bytecode level), and they are not quite as efficient as they could be.
value-classes also exist, and can do RAII-like stuff via
copy-constructors and destructors.
partly this is handled internally with them being objects as before, except:
normal objects have a copyValue method which simply returns the same object;
value-types create a new object which holds a copy of the object (for
value-classes, creates a new instance of the object, calling the
copy-constructor with the old object).
and, similarly:
the dropValue method for normal reference-objects is essentially no-op;
however, for value-types, it will destroy the object (for value-classes,
calling the destructor and then freeing the memory).
note (for my VM, not JVM based): many VM objects are not class
instances... and these methods are VM-internal, and exist separately
from the actual methods declared in a class. some are just weird, such
as toString, which calls a VM internal method, which may in-turn call
the class method if the object is an instance of a class. technically,
the vtable for these internal methods is linked to (indirectly) via the
GC's object header.
I'm wondering why the JVM can't do something like that apart from
having to modify the language spec and thus have VM vendors needing to
implement it. Not downplaying that aspect, but curious what technical
challenges this would present.
I don't really know.
(admittedly, I haven't really done any development on the standard JVM).
I did similar before on my own miniature JVM implementation, partly by
creating a special "Struct" class, and imparting some "magic" to it
(more like that described before).
my own VM's implementation was based on this. underneath, they were
largely built on the same core machinery.
or such...
Sent from my phone
On Sep 28, 2012 5:55 PM, "BGB" <cr88...@gmail.com
<mailto:cr88...@gmail.com>> wrote:
On 9/28/2012 4:10 PM, Vitaly Davidovich wrote:
Since we're in wishful thinking territory now :), the two things
I'd really like are:
1) value/struct types (i.e. avoid heap and be able to pack data
closer together). I don't how much we can rely on EA.
2) more auto-vectorization
I think 2 is being worked on by Vladimir but unclear if there are
any concrete plans for 1. I know John Rose has written about it,
but don't know if anything's actually planned.
yeah, agreed on 1.
I remember reading before of mention of using special signatures
or similar, but I forget the specifics.
I had before floated the idea of if it could be indicated via a
special base-class or interface.
in the latter case, the interface would essentially be "magic",
and tell the VM: "Hey! This thing here is a struct!".
this could sort of work, but would exhibit incorrect behavior on
older JVMs, unless it were done multi-part:
one part, a JVM extension to support built-in structs (indicated
via a special class or interface, as before);
the second part would be providing special
classes/interfaces/methods, which could be used to "implement" the
special struct behavior (could just be "native"?);
the 3rd part would basically be some syntax sugar in Java, mostly
so that the code isn't filled up with nasty looking method calls.
say (extensions):
ValueType interface, provides ability to construct types with
pass-by-value semantics (mostly would be handled specially by
"javac" or similar);
Struct class which implements ValueType, special class, for which
all derived classes are structs;
ValueClass class, which is like struct, but creates classes which
implement pass-by-value semantics.
so, if we have something like:
SomeStruct a, b;
a=new SomeStruct(...);
b=a;
the latter could generate code more like if it were:
b=a.copyValue();
and when they leave scope:
a.dropValue();
b.dropValue();
and:
public struct SomeStruct { ... }
could actually be handled internally more like:
public final class SomeStruct extends Struct { ... }
with the VM realizing that Struct and "Struct.copyValue()" and
similar are magic, with the JIT generating special code to handle
them more efficiently.
or, at least, this is my idle thinking at the moment...
note: unrelated to "java.sql.Struct"...
Sent from my phone
On Sep 28, 2012 3:59 PM, "Charles Oliver Nutter"
<head...@headius.com <mailto:head...@headius.com>> wrote:
Now what we need is a way to inject new intrinsics into the
JVM, so I
can make an asm version of something and tell hotspot "no no, use
this, not the JVM bytecode" :)
- Charlie
On Fri, Sep 28, 2012 at 11:53 AM, Vitaly Davidovich
<vita...@gmail.com <mailto:vita...@gmail.com>> wrote:
> Yup, it would have to do extensive pattern matching
otherwise. C/C++
> compilers do the same thing (I.e. have intimate knowledge
of stdlib calls
> and may optimize more aggressively or replace code with
intrinsic
> altogether).
>
> In this case, jit uses the bsf x86 assembly instruction
whereas hand rolled
> "copy version" generates asm pretty much matching the java
code.
>
> Sent from my phone
>
> On Sep 28, 2012 2:42 PM, "Raffaello Giulietti"
> <raffaello.giulie...@gmail.com
<mailto:raffaello.giulie...@gmail.com>> wrote:
>>
>> On Fri, Sep 28, 2012 at 8:15 PM, Charles Oliver Nutter
>> <head...@headius.com <mailto:head...@headius.com>> wrote:
>> > On Fri, Sep 28, 2012 at 10:21 AM, Raffaello Giulietti
>> > <raffaello.giulie...@gmail.com
<mailto:raffaello.giulie...@gmail.com>> wrote:
>> >> I'm not sure that we are speaking about the same thing.
>> >>
>> >> The Java source code of numberOfTrailingZeros() is
exactly the same in
>> >> Integer as it is in MyInteger. But, as far as I
understand, what
>> >> really runs on the metal upon invocation of the Integer
method is not
>> >> JITted code but something else that probably makes use
of CPU specific
>> >> instructions. This code is built directly into the JVM
and need not
>> >> bear any resemblance with the code that would have been
produced by
>> >> JITting the bytecode.
>> >
>> > Regardless of whether the method is implemented in Java
or not, the
>> > JVM "knows" native/intrinsic/optimized versions of many
java.lang core
>> > methods. numberOfTrailingZeros is one such method.
>> >
>> > Here, the JVM is using its intrinsified version rather
than the JITed
>> > version, presumably because the intrinsified version is
pre-optimized
>> > and faster than what the JVM JIT can do for the JVM
bytecode version.
>> >
>> > system ~/projects/jruby-ruby $ java -XX:+PrintCompilation
>> > -XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining Blah
>> > 65 1 java.lang.String::hashCode (55 bytes)
>> > 78 2 Blah::doIt (5 bytes)
>> > 78 3 java.lang.Integer::numberOfTrailingZeros (79
>> > bytes)
>> > @ 1
>> > java.lang.Integer::numberOfTrailingZeros (79 bytes)
(intrinsic)
>> > 79 1 % Blah::main @ 2 (29 bytes)
>> > @ 9 Blah::doIt (5 bytes)
inline (hot)
>> > @ 1
>> > java.lang.Integer::numberOfTrailingZeros (79 bytes)
(intrinsic)
>> > @ 15 Blah::doIt (5 bytes)
inline (hot)
>> > @ 1
>> > java.lang.Integer::numberOfTrailingZeros (79 bytes)
(intrinsic)
>> >
>> > system ~/projects/jruby-ruby $ cat Blah.java
>> > public class Blah {
>> > public static int value = 0;
>> > public static void main(String[] args) {
>> > for (int i = 0; i < 10_000_000; i++) {
>> > value = doIt(i) + doIt(i * 2);
>> > }
>> > }
>> >
>> > public static int doIt(int i) {
>> > return Integer.numberOfTrailingZeros(i);
>> > }
>> > }
>> > _______________________________________________
>>
>>
>> Yes, this is what Vitaly stated and what happens behind
the curtains.
>>
>> In the end, this means there are no chances for the rest
of us to
>> implement better Java code as a replacement for the
intrinsified
>> methods.
>>
>> For example, the following variant is about 2.5 times
*faster*,
>> averaged over all integers, than the JITted original
method, the one
>> copied verbatim! (Besides, everybody would agree that it
is more
>> readable, I hope.)
>>
>> But since the Integer version is intrinsified, it still
runs about 2
>> times slower than that (mysterious) code.
>>
>> public static int numberOfTrailingZeros(int i) {
>> int n = 0;
>> for (; n < 32 && (i & 1 << n) == 0; ++n);
>> return n;
>> }
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