[Pharo-project] doesNotUnderstand:/createActualMessageTo: for zero-argument sends appears to imply the need for an extra stack slot in methods

Eliot Miranda eliot.miranda at gmail.com
Fri May 6 13:39:04 EDT 2011

Hi All,

    could you check my reasoning here.  I think I've found a serious bug in
the VM with doesNotUnderstand: processing for zero-argument sends and the
calculation of the necessary stack size in MethodNode>>generate:* (senders
of CompiledMethod>>needsFrameSize:).  The bug causes a push beyond the end
of a context, overwriting the header of the following object, when an MNU
occurs for a zero-argument send when the stack is full.  If you look at
Interpreter>>createActualMessageTo: it does the following to push the
message created as the argument of the doesNotUnderstand:

        self pop: argumentCount thenPush: message

If argumentCount is zero this pushes an item on the stack.  If the stack is
full at this point (the receiver of the not-understood message is the last
item in the context) then the push will be out of bound of the
activeContext's stack and overwrite the first word of the following object's

We observe this with a Newspeak application at Cadence and see a subsequent
VM crash.

1. am I right?

2. is this an old bug?

3. did this, or did this not, affect Smalltalk-80 v2 and e.g. the Dorado
implementation, and if so why (e.g. was the message of an MNU not pushed
onto the stack, and instead MNUs activated by special code?, or was the
activeContext not on the heap? or...)

4. what's the right fix?  It seems to me that the right fix is simply to add
one to the needed stack depth, with an explanatory comment, but in senders
of CompiledMethod>>needsFrameSize: or in CompiledMethod>>needsFrameSize:?
 Trying to avoid pushing the argument of an MNU is going to involve lots of
changes to the VM and will be tricky and likely have a bug tail.

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