[Vm-dev] VM Maker: VMMaker.oscog-cb.1271.mcz

[Eliot Miranda]

Hi Clément,

    but what happens if the counter trips on a #==?  There are two cases, with and without the callback installed.  AFAICT the code will continue after the jump and so won't reflect the values compared.  Can you explain what actually happens?

Eliot (phone)

On Apr 29, 2015, at 7:33 AM, commits at source.squeak.org wrote:

> 
> Eliot Miranda uploaded a new version of VMMaker to project VM Maker:
> http://source.squeak.org/VMMaker/VMMaker.oscog-cb.1271.mcz
> 
> ==================== Summary ====================
> 
> Name: VMMaker.oscog-cb.1271
> Author: cb
> Time: 29 April 2015, 10:33:32.261 am
> UUID: d1d8e36f-01bc-4d38-b9a2-6a5188c5e3cc
> Ancestors: VMMaker.oscog-eem.1270
> 
> In fact in genJumpIf we always reload the counterReg after the trampoline after the jump back to retry. No need to add logic there.
> 
> Fix the bug that #== was marked as mapped whereas it should not thanks to Eliot.
> 
> The SistaSimulator runs without assertion failures. I'm going to try to recompile it now.
> 
> =============== Diff against VMMaker.oscog-eem.1270 ===============
> 
> Item was removed:
> - ----- Method: SistaStackToRegisterMappingCogit class>>generatorTableFrom: (in category 'class initialization') -----
> - generatorTableFrom: anArray
> -    "Override to replace the unmapped, non-counting inlined #== with a mapped counting inlined #==."
> -    | table |
> -    table := super generatorTableFrom: anArray.
> -    table object do:
> -        [:descriptor|
> -         descriptor generator == #genSpecialSelectorEqualsEquals ifTrue:
> -            [descriptor
> -                isMapped: true;
> -                isMappedInBlock: true;
> -                needsFrameFunction: nil]].
> -    ^table!
> 
> Item was changed:
>  ----- Method: SistaStackToRegisterMappingCogit>>genJumpIf:to: (in category 'bytecode generator support') -----
>  genJumpIf: boolean to: targetBytecodePC
>      "The heart of performance counting in Sista.  Conditional branches are 6 times less
>       frequent than sends and can provide basic block frequencies (send counters can't).
>       Each conditional has a 32-bit counter split into an upper 16 bits counting executions
>       and a lower half counting untaken executions of the branch.  Executing the branch
>       decrements the upper half, tripping if the count goes negative.  Not taking the branch
>       decrements the lower half.  N.B. We *do not* eliminate dead branches (true ifTrue:/true ifFalse:)
>       so that scanning for send and branch data is simplified and that branch data is correct."
>      <inline: false>
>      | desc ok counterAddress countTripped retry counterReg |
>      <var: #ok type: #'AbstractInstruction *'>
>      <var: #desc type: #'CogSimStackEntry *'>
>      <var: #retry type: #'AbstractInstruction *'>
>      <var: #countTripped type: #'AbstractInstruction *'>
> 
>      (coInterpreter isOptimizedMethod: methodObj) ifTrue: [ ^ super genJumpIf: boolean to: targetBytecodePC ].
> 
>      self ssFlushTo: simStackPtr - 1.
>      desc := self ssTop.
>      self ssPop: 1.
>      desc popToReg: TempReg.
> 
>      "We prefer calleeSaved to avoid saving it across the trap trip trampoline"
>      counterReg := self allocateRegPreferringCalleeSavedNotConflictingWith: 0. 
>      retry := self Label.
>      self 
>          genExecutionCountLogicInto: [ :cAddress :countTripBranch | 
>              counterAddress := cAddress. 
>              countTripped := countTripBranch ] 
>          counterReg: counterReg.
>      counterIndex := counterIndex + 1.
> 
>      "Cunning trick by LPD.  If true and false are contiguous subtract the smaller.
>       Correct result is either 0 or the distance between them.  If result is not 0 or
>       their distance send mustBeBoolean."
>      self assert: (objectMemory objectAfter: objectMemory falseObject) = objectMemory trueObject.
>      self annotate: (self SubCw: boolean R: TempReg) objRef: boolean.
>      self JumpZero: (self ensureFixupAt: targetBytecodePC - initialPC).
> 
>      self genFallsThroughCountLogicCounterReg: counterReg counterAddress: counterAddress.
> 
>      self CmpCq: (boolean == objectMemory falseObject
>                      ifTrue: [objectMemory trueObject - objectMemory falseObject]
>                      ifFalse: [objectMemory falseObject - objectMemory trueObject])
>          R: TempReg.
>      ok := self JumpZero: 0.
>      self MoveCq: 0 R: counterReg. "if counterReg is 0 this is a mustBeBoolean, not a counter trip."
> 
> -    self flag: 'Hi Clément.  You can''t just save things to the Smalltalk stack.  You can /only/ save things that execution expects to be there on a context''s stack, because this frame may get mapped to a context object and then back, and gc''ed etc.  The counter reg does not contain an object so is a complete no-no on the Smalltalk stack.  On the C stack in the trampoline is OK, not on the Smalltalk stack in method execution.  So instead of saving and restoring the counterReg around the call, something we can''t do, we can reload it after the call'.
> -    false ifTrue:
> -        ["If counterReg is caller saved then save it"
> -        (self register: counterReg isInMask: callerSavedRegMask) ifTrue: [ self PushR: counterReg ]].
> - 
>      countTripped jmpTarget:
>          (self CallRT: (boolean == objectMemory falseObject
>                          ifTrue: [ceSendMustBeBooleanAddFalseTrampoline]
>                          ifFalse: [ceSendMustBeBooleanAddTrueTrampoline])).
>                          
>      "If we're in an image which hasn't got the Sista code loaded then the ceCounterTripped:
>       trampoline will return directly to machine code, returning the boolean.  So the code should
>       jump back to the retry point. The trampoline makes sure that TempReg has been reloaded."
>      self annotateBytecode: self Label.
> 
> -    self flag: 'see above'.
> -    false ifTrue:
> -        ["If counterReg is caller saved then restore it"
> -        (self register: counterReg isInMask: callerSavedRegMask) ifTrue: [ self PopR: counterReg ]].
> - 
> -    "Note we /can't/ save and restore the counterReg's contents around the call since the stack can
> -     only contain what an interpreted context's stack would contain at the corresponding point.  The
> -     counter is not an object, so can't be written to the stack. Hence we reload it after the call."
> -    self MoveAw: counterAddress R: counterReg.
> - 
>      self Jump: retry.
> +    
>      ok jmpTarget: self Label.
>      ^0!
>