[Vm-dev] When Cog should deoptimize CM and when its not?
eliot.miranda at gmail.com
Mon Mar 11 22:23:11 UTC 2013
On Fri, Mar 8, 2013 at 2:34 PM, Igor Stasenko <siguctua at gmail.com> wrote:
> Hi, Eliot & all
> I was looking at #become: code
> and did not found the place where it deoptimizing compiled methods to
> throw away the jited version of it (if it has one),
> before even trying to do anything with it.
> i know it pricey, because it should:
> - remove CogMethod from JIT zone
> - walk the stack and look for all activations of it, and replace
> native code instruction pointer with bytecode instruction pointer
> - fix the compiled method's header
> (another alternative, if method has activations, i would fail
> primitive actually, because this is too hard to do things right and
> very dangerous
> to play with, as well as it makes little sense )
> IMO, if we add that, then we could minimize the impact of JIT
> complexity on #become: and make things work
> in same way as in stack vm (which is stable ;).
> And since become scans part/whole heap.. deoptimizing the CM would not
> add too much on top :)
> I don't know if it is there, maybe it is.. then sorry for first part..
> but not the second one:
> objectAt: index put: value
> "Primitive. Store the value argument into a literal in the receiver. An
> index of 2 corresponds to the first literal. Fails if the index is less than 2
> or greater than the number of literals. Answer the value as the result.
> Normally only the compiler sends this message, because only the
> compiler stores values in CompiledMethods. Essential. See Object
> documentation whatIsAPrimitive."
> <primitive: 69>
> self primitiveFailed
> i was playing with metaprogramming , using
> to store/change some value directly in method's literal (yeah.. you
> can kill the messenger and stop reading further ;) .
> And i found that the change is not immediately visible..
> My idea is plain simple:
> - you have a template method, like:
> ^ 'ssssss'
> and then i simply doing things like:
> copy := foo copy literalAt:1 put: 'bar'.
> methodDict at: #bar put: copy
> and when i wanna change the #bar value next time, i don't creating new
> method again, but just modifying
> its literal.
> Things worked perfectly on old interpreted VMs and i used this
> trickery to implement prototypes (where object's properties
> are held by correspoding CM )
> But with Cog, because of its aggressive caching, it no longer working:
> at: aName put: value
> | method |
> (getter methodDict includesKey: aName) ifTrue: [
> method := getter methodDict at: aName.
> method literalAt: 1 put: value.
> method flushCache.
> ] ifFalse: [
> method := getter addReadAccessorFor: aName value: value.
> setter addWriteAccessorFor: aName readAccessor: method.
> aName flushCache.
> ^ value
> here neither "method flushCache. " nor "aName flushCache." helps with it..
> my test still fails.
> But of course, when i click to debug , it magically passes when i
> evaluating very same expression in debugger :)
> Things working only when i always replacing things with new method(s)
> (as i doing under under ifFalse: clause).
> I strongly suggest that <primitive: 69> should always deoptimize CM
> (throw away Cogged method , even if it has high price to pay)
> as well as #flushCache (<primitive: 116>) should also do the same, because
> - JITed code is a cache
> - we should have a means to throw it away when asked so
> The #voidCogVMState flushes all machine code.. but is it possible to
> make more could we have one which flushing only particular one?
It's already there. See primitive 215, primitiveVoidVMStateForMethod,
CompiledMethod>voidCogVMState. Find attached.
> (...in alternative universe, i'd rather look for a way how to
> "deoptimize Jited method in smart way, so when you deoptimizing the
> it won't require scanning stacks to patch instruction pointers, and
> instead mark/make jited code obsolete by:
> - patching the machine code at entry point to immediately switch back
> to interpreter mode of corresponding method (to prevent spawning new
> activations with native code)
> - patching the machine code at every possible return address (call
> sites) to immediately switch to interpreter (to gracefully handle
> existing activations so, there is no need to fix instruction pointers)
> - keep that obsolete CogMethod till next GC.
> Best regards,
> Igor Stasenko.
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