Hi !

On Mon, May 30, 2016 at 2:39 PM, Ben Coman <btc@openinworld.com> wrote:

On Sun, May 29, 2016 at 10:14 AM, Ben Coman <btc@openinworld.com> wrote:
> Hi Clement, Thanks for your detailed reply. I particularly liked your
> warm up exercises. Goal directed learning is better than general
> browsing.
>
> On Tue, May 24, 2016 at 1:29 AM, Clément Bera <bera.clement@gmail.com> wrote:
>>
>> Hi Ben,
>>
>> The REPL image expects chunk format. Hence you need to write "3 + 4 !"
>>
>> To get warmed-up:
>> 1) Inspect the object memory, then look for the first class table page instance variable. It's an oop referencing an array, try in the simulator to "printOop:" the address of the first class table page that you found. It should print it in the Transcript, the first entries are immediate, in Spur32 SmallInteger/Character/SmallInteger.
>
> The inspector showed a Spur32MMLECoSimulator and classTableFirstPage
> held 16r5311F8. Plugging that into [print oop...] showed...

16r5311F8: a(n) Array
16r52D108 nil 16r15C3A50 class SmallInteger 16r878D70 class
Character 16r15C3A50 class SmallInteger
16r1111DC0 class SmallFloat64 16r52D108 nil 16r52D108 nil 16r52D108 nil
16r52D108 nil 16r52D108 nil 16r52D108 nil 16r52D108 nil
16r52D108 nil 16r52D108 nil 16r52D108 nil 16r52D108 nil
16r87AE60 class Array 16r52D108 nil 16r52D108 nil 16r52D108 nil
16r52D108 nil 16r52D108 nil 16r52D108 nil 16r52D108 nil
16r52D108 nil 16r52D108 nil 16r52D108 nil 16r52D108 nil
16r52D108 nil 16r52D108 nil 16r52D108 nil 16r52D108 nil
16r878C58 class LargeNegativeInteger 16r878C90 class
LargePositiveInteger 16r10AEAE8 class BoxedFloat64 16r879438 class
Message

All the nils I guess are due to the class table being a hash map?

Is there some way from within the simulation to reference an object by
its hex number. For example, to use the size of that array from
within the simulation, something like...

classTableSize := 16r5311F8 objectFromHex size

You got the right result. The the class table is a linked list of pages, each page being an array. The first page, shown here, is reserved for frequently used classes.

Indexes 0-15 are reserved for tagged object.

Indexes 16-32 are reserved for hidden classes. Typically the class table pages are instances of Array, but the use index 16 so the VM know they are hidden.

The rest is for real classes that are frequently used. There are many nils so we have free space for new features. It's not a hash map.

I don't think things like that exists: classTableSize := 16r5311F8 objectFromHex size. For oops debugging features are tied to printing through the simulator instance right now. However there is something like that in the JIT. In the machine code zone we can access part of the bytes as CogMethodSurrogate and its subclasses and in the stack we can do the same for stack pages with the corresponding surrogate. In this case one can do something like:

CogMethodSurrogate at: 16r51578 objectMemory: objectMemory cogit: cogit

And then one can ask the surrogate things like:

surrogate cmRefersToYoung

And it reads the correct bytes for you, in this case answering if the cog method has a reference to a young object.

>
>
>> 2) print the active stack, look for the method's address. Try to print it as an oop, and if it tells you "address in the machine code zone", print the cog method and its machine code instead.

I presume is the active stack is
[print call stack] which produces...

16r1012F8 M MultiByteFileStream(StandardFileStream)>basicNext
16r1E7408: a(n) MultiByteFileStream
16r101334 M UTF8TextConverter>nextFromStream: 16r1EA418: a(n)
UTF8TextConverter
16r10135C M MultiByteFileStream>next 16r1E7408: a(n) MultiByteFileStream
16r10138C I MultiByteFileStream(PositionableStream)>nextChunkNoTag
16r1E7408: a(n) MultiByteFileStream
16r1013B0 I StdioListener>run 16r1E7C98: a(n) StdioListener
16r1013D0 I [] in UndefinedObject>(nil) 16r52D108: a(n) UndefinedObject
16r1013F0 I [] in BlockClosure>newProcess 16r1E7E00: a(n) BlockClosure
---------

[print oop...] 16r1012F8 tells me...
16r1012F8 is in the stack zone

[print cog method for...] 16r1012F8 tells me...
not a method

[print mc/cog frame] says...
Assertion failed
with debugger at CogVMSimulatorLSD(CoInterpreter)>>isMachineCodeFrame:

So I seem to be missing something.

I restarted the simulator and this time...
[print call stack...]
16r1012F8 M MultiByteFileStream(StandardFileStream)>basicNext
16r2A1BA8: a(n) MultiByteFileStream
16r101334 M UTF8TextConverter>nextFromStream: 16r2A2188: a(n)
UTF8TextConverter
16r10135C M MultiByteFileStream>next 16r2A1BA8: a(n) MultiByteFileStream
16r10138C I MultiByteFileStream(PositionableStream)>nextChunkNoTag
16r2A1BA8: a(n) MultiByteFileStream
16r1013B0 I StdioListener>run 16r2A1B00: a(n) StdioListener
16r1013D0 I [] in UndefinedObject>(nil) 16r52D108: a(n) UndefinedObject
16r1013F0 I [] in BlockClosure>newProcess 16r2A1690: a(n) BlockClosure
----------

[print oop...] 16r1012F8
16r1012F8 is in the stack zone

[print oop...] 16r2A1BA8
16r2A1BA8: a(n) MultiByteFileStream
16r2A2740 '????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????'
16r1 =0 (16r0) 16r1 =0 (16r0) 16r52D108 nil
16r52D108 nil 16r52D118 false 16r9AA1E8 #stdin 16r24CC18 a ByteArray
16r2A2728 '?' 16r52D108 nil 16r2A2188 an UTF8TextConverter
16r6DF1D8 #lf
16r52D128 true
-----------

Now after doing 3+4! several times,
[print call stack...] produces...
16r101300 M MultiByteFileStream(StandardFileStream)>basicNext
16r2A1BA8: a(n) MultiByteFileStream
16r10133C M UTF8TextConverter>nextFromStream: 16r2A2188: a(n)
UTF8TextConverter
16r101364 M MultiByteFileStream>next 16r2A1BA8: a(n) MultiByteFileStream
16r10138C M MultiByteFileStream(PositionableStream)>nextChunkNoTag
16r2A1BA8: a(n) MultiByteFileStream
16r1013B0 I StdioListener>run 16r2A1B00: a(n) StdioListener
16r1013D0 I [] in UndefinedObject>(nil) 16r52D108: a(n) UndefinedObject
16r1013F0 I [] in BlockClosure>newProcess 16r2A1690: a(n) BlockClosure

btw, What is the meaning of the M and I in the second column? I
notice that 16r10138C has changed from an I to a M.

Also the address associated with basicNext changed from 16r1012F8 to
16r101300. Can some meaning be inferred from that?

Some explanations are needed here :-)

The M or I at the beginning of the printing are for 'Interpreted frame' or 'Machine code frame'.

When you do [print call stack], you print the list of stack frame in the current stack. For example,

16r101300 M MultiByteFileStream(StandardFileStream)>basicNext

means that:

- the stack frame address in the stack zone is 16r101300

- the machine code version of the method is executed in this frame (M and not I).
- the receiver has the type MultiByteFileStream

- the stack frame on top of the stack is the activation for the method StandardFileStream>>basicNext

Now what you tried to do is to print the frame as a method, and that won't work (It's not obvious and my exercise was not very precised, sorry).

You can use [print frame ...] and put the frame's hex to print it. Alternatively, asyou usually want the top (a.k.a. head) frame, you can directly use [print ext head frame] if it's a machine code frame.

That should print something like that (I print a random frame here):

16r103160: arg1: 16r8239 =16668(16r411C)

16r10315C: arg2: 16r825D =16686(16r412E)

16r103158: arg3: 16r2E21C0 =a(n) Point

16r103154: arg4: 16r9BC480 =a(n) StrikeFont

16r103150: arg5: 16r1 =0(16r0)

16r10314C: caller ip: 16r564E0=353504

16r103148: saved fp: 16r103184=1061252

16r103144: method: 16r51578 16r102BDD0 16r102BDD0: a(n) CompiledMethod

16r103144: mcfrm flags: 16r0 numArgs: 6 noContext notBlock

16r103140: context: 16r52D108 nil

16r10313C: receiver: 16r2E0078 a GrafPort

16r103138: stck: 16r2E0078 a GrafPort

16r103134: stck: 16r1A115D0

Now that you've print the frame, you can see the method addresses in this line:

16r103144: method: 16r51578 16r102BDD0 16r102BDD0: a(n) CompiledMethod.

This is a machine code frame, so the method has two addresses:

16r51578 => in generated method, so you need to use [disassembleMethod/trampoline...] and write down the hex to see the disassembly. (Toggle Transcript first and open a large Transcript if you do that).

16r102BDD0 => in the heap. This is the bytecode version of the method. You can print it using [print oop...]

Ok ! One last warm-up exercise:

3) When the simulator has started and the REPL window has popped up, select [single step]. Then enter something in the REPL window and execute it. Once done, do [report recent instructions]. You should be able to see in the Transcript the last 100 machine instruction with the register state in-between each instruction.

I think I should write down those exercise as a blog post...

cheers -ben