Are java.lang classes better served by the JVM?
BGB
cr88192 at gmail.com
Fri Sep 28 14:52:16 PDT 2012
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" <headius at headius.com
> <mailto:headius at 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
> <vitalyd at gmail.com <mailto:vitalyd at 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.giulietti at gmail.com
> <mailto:raffaello.giulietti at gmail.com>> wrote:
> >>
> >> On Fri, Sep 28, 2012 at 8:15 PM, Charles Oliver Nutter
> >> <headius at headius.com <mailto:headius at headius.com>> wrote:
> >> > On Fri, Sep 28, 2012 at 10:21 AM, Raffaello Giulietti
> >> > <raffaello.giulietti at gmail.com
> <mailto:raffaello.giulietti at 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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> >
> >
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