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Hi Erik,<br>
<br>
<div class="moz-cite-prefix">On 2014-10-08 13:20, Erik Österlund
wrote:<br>
</div>
<blockquote cite="mid:2F82EFA3-F76D-4FDF-832F-8F85F3BB42D0@lnu.se"
type="cite">
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<div>Greetings,</div>
<div><br>
</div>
<div>So here in this email is attached a patch-file containing
my changes since I don't have access to the cr-server.</div>
If anyone could help me make a RFR with a bug-ID with this
changeset, that would be great.
</div>
</blockquote>
<br>
Webrev: <a class="moz-txt-link-freetext" href="http://cr.openjdk.java.net/~stefank/eriko/8059936/webrev.00/">http://cr.openjdk.java.net/~stefank/eriko/8059936/webrev.00/</a><br>
bug: <a class="moz-txt-link-freetext" href="https://bugs.openjdk.java.net/browse/JDK-8059936">https://bugs.openjdk.java.net/browse/JDK-8059936</a><br>
<br>
I ran this through JPRT and got this failure on Solaris x86:<br>
<pre>"hotspot/src/share/vm/memory/specialized_oop_closures.hpp", line 222: Warning: Identifier expected instead of "}".
1 Warning(s) detected.
gmake[8]: *** [abstractCompiler.o] Error 2
gmake[7]: *** [the_vm] Error 2
gmake[6]: *** [fastdebug] Error 2
gmake[5]: *** [generic_build2] Error 2
gmake[4]: *** [fastdebug] Error 2
StefanK
</pre>
<br>
<blockquote cite="mid:2F82EFA3-F76D-4FDF-832F-8F85F3BB42D0@lnu.se"
type="cite">
<div style="word-wrap:break-word">
<div><br>
<div>== Implementation Summary ==</div>
<div><br>
</div>
<div>=== Dispatching To Closures ===</div>
<div>All dispatching to OopClosure go through an all static
class OopClosureDispatcher. It uses SFINAE to find the most
appropriate member function to call by checking certain
conditions of the OopClosureType. For any template
parameterized OopClosureType calls to member function X in
{do_oop, do_metadata, do_klass, do_class_loader_data} it
will attempt the following in this order: 1) Check if the
OopClosureType is manually unspecialized for whatever reason
(currently only used by the abstract classes OopClosure and
ExtendedOopClosure), in that case use virtual call. 2) Check
if OopClosureType (and not a parent) declares X_nv, then use
a non-virtual call to it. Otherwise 3) check if
OopClosureType (and not a parent) declares X, then use a
non-virtual call to X (hence removing need to define both X
and X_nv any more, but still being backward compatible).
Otherwise 3) use a normal virtual call to X.</div>
<div><br>
</div>
<div>The reason why checking that candidates for non-virtual
calls are declared in the concrete OopClosureType and not a
parent is to be safe rather than sorry in detecting
"abstract" base classes that are never used by themselves.
However their derived types are used, but the type passed in
to oop_iterate is not the derived closure type but the
abstract one. So in summary we make sure we make non-virtual
calls to the derived closure types and not the base types.</div>
<div><br>
</div>
<div>=== Dispatching To Klass ===</div>
<div>Two mechanisms are used for retaining the OopClosureType
when dispatching to it using a specific Klass.</div>
<div>Attempt one hopes there is a separation of concern
between finding oops and dispatching to OopClosureType. A
virtual call is made to the Klass to either a) return back
the oop maps or b) identify which Klass it is with a
"DispatchTag".</div>
<div>If oop maps are returned (InstanceKlass, OopArrayKlass,
TypeArrayKlass) then they are iterated over from oop_iterate
where all the type info about the closure is still known. If
this is not supported (InstanceRefKlass,
InstanceMirrorKlass, InstanceClassLoaderKlass), then the
dispatch tag is used to call an inline template method
defined in brand new Instance*Klass.inline.hpp files.</div>
<div><br>
</div>
<div>The new methods in Instance*Klass.inline.hpp are
(non-virtual) template variants of what the old
oop_oop_iterate macros did. They use SFINAE to check if the
OopClosureType expects metadata related calls too (if it's
an ExtendedOopClosure) otherwise does nothing. Note that
oop_iterate now takes an arbitrary type instead of
ExtendedOopClosure, but it uses SFINAE to force template
instantiations only if it's a subtype of ExtendedOopClosure
and otherwise generate compiler errors and similarly on
oop_iterate_no_header, specialization is used and it
explicitly checks the template parameter is of type
OopClosure. If it's an ExtendedOopClosure, it sends the
first ever composite closure type
NoHeaderOopClosure<OopClosureType> to the dispatch
system so that metadata-related calls are never called in
the first place. :)</div>
<div><br>
</div>
<div>Thanks,</div>
<div><br>
</div>
<div>/Erik</div>
</div>
</div>
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<div><br>
<div>
<div>On 01 Oct 2014, at 17:02, Erik Österlund <<a
moz-do-not-send="true"
href="mailto:erik.osterlund@lnu.se">erik.osterlund@lnu.se</a>>
wrote:</div>
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On 01 Oct 2014, at 16:11, Stefan Karlsson <<a
moz-do-not-send="true"
href="mailto:stefan.karlsson@oracle.com">stefan.karlsson@oracle.com</a>>
wrote:</div>
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<br>
On 2014-10-01 15:41, Erik Österlund wrote:<br>
<blockquote type="cite">Hey,<br>
<br>
I have to admit I'm not a huge fan of the current
system for explicitly specializing closures
(removing virtual calls).<br>
<br>
Here's a couple of problems I see with the current
solution:<br>
1. The specialization macros are obviously not
pretty<br>
2. It's awkward to have to remember to explicitly
list the closure as specialized in the macros<br>
3. There are plenty of composite closure types out
there. What I mean by this is when closures are
combined, e.g. one closure is used to filter a
memory range, and if passing the filter, it will
invoke the actual closure, currently resulting in a
virtual call even though the composite structure is
completely known at the call site.<br>
4. Each closure has to have like do_oop, do_oop_v,
do_oop_nv, for both oop types and then a do_oop_work
for joining them. Yuck! Asserts try to check that
the _v and _nv methods do the same thing to combat
programmer mistakes.<br>
<br>
With my alternative template magic solution:<br>
1. You won't have to explicitly specialize wanted
closure types - they are automatically specialized
unless the contrary is explicitly stated.<br>
2. Parameterized composite closure types can be used
without unnecessary virtual call overheads.<br>
3. Only a single do_oop (do_metadata etc) member
function is needed, and hence no need to put asserts
trying to keep _v and _nv synchronized.<br>
4. It is backward compatible and does not require
major refactoring; could transition into this system
step by step. The two systems can even co-exist.<br>
5. It supports an interface where OopClosure is the
interface to oop_iterate, rather than
ExtendedOopClosure. It uses SFINAE to send metadata
info to the closure only if the derived type is an
ExtendedOopClosure, otherwise it simply sends the
oops (do_oop) only. (of course I can remove this if
it's unwanted and we intentionally don't want to
support oop_iterate(OopClosure*) )<br>
<br>
For the interested reader, this is how the old
system worked:<br>
The ugly macros generate overloads of oop_iterate on
oopDesc which uses a virtual call to the Klass (also
using macro generated overloads) to figure out where
the oops are and then call the closure. This step
with the virtual call to the Klass to call the
closure removes any potential for template magic
because template member functions can't be virtual
in C++.<br>
<br>
And this is how my system solves this:<br>
A template oop_iterate (with closure type as
parameter) member function uses a virtual call to
the Klass, but only to acquire information where
oops can be found (and NOT to call the actual
closure too). It then uses static template
polymorphism (CRTP idiom) to invoke the do_oop
method of the corresponding derived closure types
(without virtual calls). This required support from
the Klass implementations. I currently support
object arrays and normal instances. If the Klass
implementation does not support this new scheme, it
simply reverts to a normal virtual call like before.<br>
As a bonus I made a new include file in
utilities/templateIdioms.hpp with some template
magic I needed and which I was missing but could
likely be used in more places in the future.<br>
<br>
Would this change be interesting for the GC group?
In that case I could prepare a patch (and perhaps
add support to the other Klass implementations). :)<br>
I would also need some help to check if this works
on your wide range of platforms and compilers etc
(only checked the assembly output for my own setup).<br>
</blockquote>
<br>
Yes, please provide a patch!<br>
<br>
I've had a patch to start solving some of these
problems for a long time:<br>
<a moz-do-not-send="true"
href="http://cr.openjdk.java.net/%7Estefank/prototype/oop_iterate_dispatch/webrev.00/">http://cr.openjdk.java.net/~stefank/prototype/oop_iterate_dispatch/webrev.00/</a><br>
<br>
one problem that I haven't solved in my patch is how
to pass down the OopClosureType past the
Klass::oop_iterate virtual calls. See oop.inline.hpp:<br>
template <bool nv, typename OopClosureType><br>
inline int oopDesc::oop_iterate(OopClosureType* blk) {<br>
CONCRETE_KLASS_DO_AND_RETURN(klass(),
oop_oop_iterate<nv>(this,blk));<br>
}<br>
<br>
and klass.inline.hpp for the dispatch code:<br>
<br>
+#define CONCRETE_KLASS_DO_AND_RETURN(the_klass, todo)
\<br>
+ do { \<br>
+ switch ((the_klass)->dispatch_tag()) { \<br>
+ case Klass::_instance:
return
InstanceKlass::cast(the_klass)->todo; \<br>
+ case Klass::_instance_ref:
return
InstanceRefKlass::cast(the_klass)->todo; \<br>
+ case Klass::_instance_mirror: return
InstanceMirrorKlass::cast(the_klass)->todo; \<br>
+ case Klass::_instance_class_loader: return
InstanceClassLoaderKlass::cast(the_klass)->todo; \<br>
+ case Klass::_obj_array:
return
ObjArrayKlass::cast(the_klass)->todo; \<br>
+ case Klass::_type_array:
return
TypeArrayKlass::cast(the_klass)->todo; \<br>
+ default: fatal("Incorrect dispatch index");
return 0; \<br>
+ } \<br>
+ } while (false)<br>
+<br>
</div>
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<br>
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Yeah, I was faced with the same problem. It's impossible
in C++ to pass template arguments to virtual member
functions. (Note however that it's possible to have
template parameterized /types/ with virtual member
functions, so with your choice of using enums with a
dispatch tag I think it should be possible to have a
KlassDispatchProxy<ClosureType> cl;
cl.get_and_dispatch(obj); where KlassDispatchProxy type
(rather than Klass type) is picked using the enum and
get_and_dispatch is a virtual method for the
parameterized type).</div>
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<br>
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However, my choice was to instead recognize finding oop
maps and dispatching to the closure as two fundamentally
separate concerns. So instead I implemented a new
virtual Klass member function to only get the oop maps
(if supported) of an oop (for InstanceKlass simply
return the array of oop map blocks, for ObjArrayKlass
simply return an oop map dummy with the interval of oops
for the array). This way, I can get the oop maps with a
virtual call without having to know the ClosureType
needed for dispatching, and then when I have the oop
maps, simply dispatc (partially using SFINAE to dispatch
the stuff the closure expects to receive; depending on
if it's an ExtendedOopClosure or not).</div>
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<br>
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I'm glad this is something useful. Currently I only
enable this for InstanceKlass and ObjArrayKlass,
otherwise it reverts to standard virtual calls. Will
prepare a patch with a few more Klass implementations to
make it a bit more complete. :)</div>
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<br>
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/Erik</div>
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thanks,<br>
StefanK<br>
<br>
<br>
<blockquote type="cite"><br>
Cheers!<br>
<br>
/Erik</blockquote>
</div>
</blockquote>
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