[Vm-dev] Order of evaluation bug with in lined to:do: in both Opal and Squeak compilers.

[Eliot Miranda]

On Thu, Apr 20, 2017 at 4:41 PM, Eliot Miranda <eliot.miranda at gmail.com>
wrote:

> Hi All,
>
>     just stumbled across a bytecode compiler bug that's in both the Squeak
> compiler and the Opal compiler.  I'm told by Clément that Opal mimics the
> bug to avoid crashing legacy code.  So there may be places that depend on
> this bug.  It would be good to eliminate the dependencies and the bug.
>
> For illustration look at the to:do: loop in the ifNil: arm in
> Context>>privRefresh:
>
> Context>>privRefresh
> "Reinitialize the receiver so that it is in the state it was at its
> creation."
> closureOrNil
> ifNotNil:
> [pc := closureOrNil startpc.
> self stackp: closureOrNil numArgs + closureOrNil numCopiedValues.
> 1 to: closureOrNil numCopiedValues do:
> [:i | self tempAt: closureOrNil numArgs + i put: (closureOrNil at: i)]]
> ifNil:
> [pc := method initialPC.
> self stackp: method numTemps.
> method numArgs+1 to: method numTemps do:
> [:i | self tempAt: i put: nil]]
>
>
> This should evaluate method numArgs + 1 then method numTemps.  If it were
> written as a non-in-lined (method numArgs+1 to: method numTemps) do: [:i|
> self tempAt: i put: nil] then the [self tempAt: i put: nil] block would be
> created next.  But the bytecode for the inlined version is
>
> self stackp: method numTemps.
> 63 <70> self
> 64 <84 40 03> pushRcvr: 3
> 67 <D6> send: numTemps
> 68 <E1> send: stackp:
> 69 <87> pop
>
> iLimit := method numTemps
> 70 <84 40 03> pushRcvr: 3
> 73 <D6> send: numTemps
> 74 <69> popIntoTemp: 1
>
> i := method numArgs + 1
> 75 <84 40 03> pushRcvr: 3
> 78 <D2> send: numArgs
> 79 <76> pushConstant: 1
> 80 <B0> send: +
> 81 <81 40> storeIntoTemp: 0 (squeak) <69> popIntoTemp: 1 (pharo)
> 83 <10> pushTemp: 0
> 84 <11> pushTemp: 1
> 85 <B4> send: <=
> 86 <AC 0B> jumpFalse: 99
>
> There is a second bug in the Squeak bytecode; storeIntoTemp: is used to
> load i whereas it should be popIntoTemp:.  It was this second bug that
> alerted me to the order-of-evaluation bug.
>

The second bug (Squeak's use of storeIntoTemp:) is actually only a poor
implementation of the value/effect distinction through the inlined
ifNil:ifNotNil:.  Because ifNil:ifNotNil: has a value (albeit one that is
discarded) the Squeak compiler generates a storeIntoTemp: to p[reserve the
value of the to:do: lop, which is the initial index.  So the bug is not
within the generation of the to:do: (the only bug there being the
order-of-evaluation one).  The bug is actually outside; the loop should be
being generated for effect but is being evaluated for value.

On the order of evaluation bug, does anyone have any memory of which
methods depended on this bug?

_,,,^..^,,,_
> best, Eliot
>



-- 
_,,,^..^,,,_
best, Eliot
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