[squeak-dev] Interesting Decompiler bug
Levente Uzonyi
leves at elte.hu
Thu Jun 20 14:09:29 UTC 2013
On Thu, 20 Jun 2013, Nicolas Cellier wrote:
> The bug is that (true ifTrue:[^true] ifFalse:[^false]) won't push any value on the stack (same as emitEffect: rather than emitValue:)
> The decompiler wants to find the argument of self value: ... on the stack and it's not there.
>
> So either we should forbid such construct at compile time
> (emit an 'un-reachable code' error when emitValue: does not emit any value)
I think this is the right thing to do, but the problem is a bit more
complex.
The following doesn't break the decompier, but it won't be decompiled
correctly, because the decompiler avoids creating branches:
decompilerBug2
true ifTrue: [ ^1 ] ifFalse: [ ^1 ].
^2
The decompiler will produce this:
decompilerBug2
true
ifTrue: [^ 1].
^ 1.
^ 2
Which can't be compiled anymore.
The original bug also appears with #caseOf:otherwise: :
decompilerBug3
self foo: (
true
caseOf: { [ false ] -> [ ^1 ] }
otherwise: [ ^1 ])
#caseOf: :
decompilerBug4
self foo: (true caseOf: { [ false ] -> [ ^1 ] })
and #repeat :
decompilerBug5
self foo: [ ^self ] repeat
Levente
>
> Or patch Decompiler like mad to work around...
> (but Decompiler's methods are yet well below usual quality in term of complexity - long methods, many instance variables, incredibly complex state, ...)
>
> I agree, this is going to be a rare problem only for newbies, because an experimented Smalltalker will avoid writing unreachable code.
> So, the third solution is DO NOTHING, or more exactly add a test and an expected failure.
> Like that we'll say next time, hey bad luck, this is a known limitation.
>
> Nicolas
>
>
>
> 2013/6/20 Frank Shearar <frank.shearar at gmail.com>
> On 20 June 2013 02:12, David T. Lewis <lewis at mail.msen.com> wrote:
> > On Thu, Jun 20, 2013 at 01:26:14AM +0200, Nicolas Cellier wrote:
> >> See
> >> http://stackoverflow.com/questions/17129583/smalltalk-collection-is-empty-error-when-saving/17202938#17202938
> >>
> >> This bug was found by a newbie on pharo 1.1, but it's still there in 4.5
> >> trunk image
> >>
> >> Try to decompile this (no matter in which class)
> >>
> >> < aNumberWithUnits
> >> (self compareUnits: aNumberWithUnits)
> >> ifTrue: [self value: ((aNumberWithUnits value) < (self value)
> >> ifTrue: [^true] ifFalse: [^false]).]
> >> ifFalse: [^Error new signal: 'Incompatible unit types.'].
> >>
> >> Thanks to the newbie!
> >> I would just have hated that!
> >> Learning from one's own error is great, but from a dysfunctional system not
> >> so :(
> >
> > Good bug catch! Here is a reduced version of the method that produces the
> > same error when decompiled:
> >
> >
> > Foo>>decompilerBug
> > self value: (true
> > ifTrue: [^ true]
> > ifFalse: [^ false])
>
> I'm guessing that this is because
>
> true
> ifTrue: [^ true]
> ifFalse: [^ false]
>
> decompiles as
>
> true ifTrue: [^ true]
> ^ false
>
> ?
>
> I'm not terribly worried about claims of a "dysfunctional system".
> This bug is so severe that noone has noticed it before! I agree that
> we need to fix the bug, but the original code makes no sense in the
> first place. #value: is unreachable.
>
> frank
>
> > Dave
> >
> >
>
>
>
>
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