[Vm-dev] New Cog VMs available

Eliot Miranda eliot.miranda at gmail.com
Tue Jul 15 19:58:02 UTC 2014

...at http://www.mirandabanda.org/files/Cog/VM/VM.r3041/.

The principal changes are fixing a performance regression in the 3040 VMs,
fixing some Spur bugs in primitiveClone/shallowCopy, and radically
simplifying the Spur become implementation,
trading the elimination of lots of post-become class table scanning for
adding read barriers to class hierarchy method lookup.

CogVM source as per VMMaker.oscog-eem.820/r3041

Put the handling of the cloning of cogged methods in the clone:
removing it from the primitive.  Add it to the pinning clone too.

Specialize the store check trampoline generation.
Move it down to the relevant object representations.

Move setting of isRemembered flag to true into SpurGenerationScavenger>>
remember:.  Inline possibleRootStoreInto: (given that remember: is /not/
inlined.  Call remember directly from the ceStoreCheckTrampoline, and
hence have remember: answer its argument.

Fix localization bug.  Variables in initialize methods were not considered
references (cuz there are excluded).  This caused VMMaker.oscog-eem.816's
extraction of zero/false vars to StackInterpreter>>#initialize to cause
nextPollUsecs to be localized to checkForEventsMayContextSwitch:.

Fix bug in scanClassPostBecome:effects: with new lazy
selector following policy by... throwing it all away.

The read barriers on method lookup (of the methodClass association in super
sends, of the superclass link, of method dictionaries, method dictionary
selectors and methods is cheap.  So replace scanning classes and method
dictionaries in the class table post become with read marriewrs on
superclass and method dictionary etc on lookup.

The read barrier on an object from which we are going to fetch state (such
as a
class or method dictionary) is essentially free on modern machines because
class index and the state very likely share a cache line, and the register
for testing is so cheap compared to memory access.  Further the read
barrier on
selectors is cheap because the method lookup cache is effective in reducing
number of message lookups and because nil entries need no check.

So nuke all the followNecessaryForwardingInMethod: machinery including the
cmUsesMethodClass hack.  Nuke scanClassPostBecome:effects:.

Rip out the forwardingCount: measurement code.  It causes
bad performance regressions (due to failing inlines?)

Clean up, e.g. replace followNonImmediateField:ofObject:
uses with followObjField:ofObject:.

canPinObjects can be inlined.

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