Hi Eliot,

I am pretty sure this is not a vm issue :)

The mustBeBooleanInMagic: creates a method on the fly and executes it with the

a context as argument.

The "ThisContext" is not the "thisContext", but the argument of the generated method.

This is the bytecode of the generated method:

17 <00> pushRcvr: 0
18 <8F 00 00 02> closureNumCopied: 0 numArgs: 0 bytes 22 to 23
22     <76> pushConstant: 1
23     <7D> blockReturn
24 <8F 00 00 02> closureNumCopied: 0 numArgs: 0 bytes 28 to 29
28     <75> pushConstant: 0
29     <7D> blockReturn
30 <F0> send: ifTrue:ifFalse:
31 <7C> returnTop

The original method is

foo
^ notABool1 ifTrue:[1] ifFalse:[0]

The generated method compiles this part
notABool1 ifTrue:[1] ifFalse:[0]

to bytecode without the jumpFalse optimization.
The problem is that the receiver for the ifTrue:ifFalse send is accessed through

pushRcvr:0, whereas the thisContext is not the context of the original method anymore.

Nicolai

2014-08-10 17:34 GMT+02:00 Eliot Miranda <eliot.miranda@gmail.com>:

Hi Nicolai,

please try and remember to invlude vm-dev when you see a VM issue...

On Sun, Aug 10, 2014 at 7:34 AM, Nicolai Hess <nicolaihess@web.de> wrote:

There is a certain risk, that the mustBeBooleanMagicIn method rewrite can crash the
vm
Issue 13805

Since you're using the Opal compiler ca you add the description of the bytecode (aCompiledMethod symbolic) to the issue? I'll try and take a look soon but want to be sure I'm debugging the relevant code.

The receiver of the rewritten method is the "nonboolean" value receiving the

ifTrue:ifFalse: message. But the rewritten method can include
instvar accessor and self sends of the original method.

We can include more rewrite rules in mustBeBooleanMagicIn like
"rewrite instvar accessing"

    context receiver class instVarNamesAndOffsetsDo: [ :n :o |
        RBParseTreeRewriter new
        replace: n with: ('ThisContext receiver instVarAt: ', o asString);
        executeTree: methodNode.
        ].
"rewrite self sends"
    RBParseTreeRewriter new
        replace: 'self' with: 'ThisContext receiver';
        executeTree: methodNode.

But I don't know if this works for all possible situations. Or if there are
better ways to do the rewriting.

Remember the mirror methods:
thisContext object: o instVarAt:

which doesn't send to o, reaching directly into it.

But what's the point of replacing "self" with "thisContext receiver"? The latter is potentially an extremely slow way of doing the former. The former won't create a context object in a method activation that doesn't need one (doesn't include a block), whereas the latter always will. But they will always yield the same result.

--
curious,
Eliot