Hi Ben,
On Sun, Jun 12, 2016 at 10:36 AM, Ben Coman btc@openinworld.com wrote:
On Sun, Jun 12, 2016 at 10:59 PM, Clément Bera bera.clement@gmail.com wrote:
Hi again,
On Sun, Jun 12, 2016 at 10:44 AM, Clément Bera bera.clement@gmail.com
wrote:
Hi Ben,
I'm glad you're now looking into the JIT. If you have some blog or
something, please write an experience report about you looking into the simulator. It's helpful for us to have noise around the VM.
Cool. I'll have a go.
On Sun, Jun 12, 2016 at 8:35 AM, Ben Coman btc@openinworld.com wrote:
I am stepping for the first time through the CogVM, having [set break selector...] forkAt: After stepping in a few times I get to #activateCoggedNewMethod. CogVMSimulatorLSB(CoInterpreter)>>dispatchOn:in: CogVMSimulatorLSB(CoInterpreter)>>sendLiteralSelector1ArgBytecode CogVMSimulatorLSB(CoInterpreter)>>commonSendOrdinary CogVMSimulatorLSB(CoInterpreter)>>insternalExecuteNewMethod CogVMSimulatorLSB(CoInterpreter)>>activateCoggedNewMethod
Here from the code at the top. methodHeader := self rawHeaderOf: newMethod. self assert: (self isCogMethodReference: methodHeader). cogMethod := self cCoerceSimple: methodHeader to: #'CogMethod *'. methodHeader := cogMethod methodHeader.
I guess methodHeader's double assignment above is related to the machine code frame having two addresses as Clement described...
Errr... I don't really fancy the way you say it but I think yes that's
it.
A method can have 2 addresses, the address of the bytecoded version in
the heap and the address of its jitted version in the machine code zone. In the machine code frame printing, the simulator displays the 2 addresses. But the frame has a single pointer to the method.
So what you're looking at is the dispatch logic from the bytecoded
method to the jitted method. When the JIT compiles a bytecoded method to machine code, it replaces the bytecoded method compiled method header (first literal) by a pointer to the jitted version. The machine code version of the method keeps the compiled method header, so accessing it is different in methods compiled to machine code and methods not compiled to machine code.
#rawHeaderOf: answers the first literal of the bytecoded method which
is a pointer to the jitted version of the method if the method has a jitted version, else is the compiled method header. In the code you show, the VM ensures the method has a jitted version with the assertion, hence the compiled method header is fetched from the jitted version.
I think I've got it. So upon JITing, CompiledMethod and its literals and bytecodes don't move. Only its bytecodeHeader is manipulated and re-purposed.
Before JIT... compiledMethod := { bytecodeHeader, literals, bytecodes }. byteCodeHeader := compiledMethod at: 1
After JIT something like... cogMethod := { cogMethodHeader, bytecodeHeader, machineCode } compiledMethod := { pointerTo_cogMethod, literals, bytecodes }. rawHeader := compiledMethod at: 1 cogMethodHeader := dereferenced(rawHeader) at: 1.
Right. When a method is cogged (jitter) its header is set to point to the Cog method (machine code method), and the actual header is stashed inside the Cog method. This is invisible to the image because only the objectAt: primitive accesses CompiledMethod literals and this primitive checks. In the VM all points where methodHeader is accessed must check for a normal method (header is a SmallInteger) and a cogged method (header is not a SmallInteger).
On Mon, May 30, 2016 at 4:12 PM, Clément Bera <
bera.clement@gmail.com> wrote:
> 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.
> 16r102BDD0 => in the heap. This is the bytecode version of the
method. You can print it using [print oop...]
This time... [print ext head frame] ==> 16r101214 M BlockClosure>forkAt: 16r2FC420: a(n) BlockClosure 16r101210: method: 16rBBF0 16rC4E948 16rC4E948: a(n)
CompiledMethod
self rawHeaderOf: newMethod ==> 16rBBF0 So the "raw header" is the cogged method.
Looking at the output below, the space ship operator <-> seems to link between cogged method headers like a call stack, except #forkAt: calls #newProcess which calls #asContext
[print cog method for...] 16rBBF0 ==> 16rBBF0 <-> 16rBC80: method: 16rC4E948 selector: 16r6CC798 forkAt:
[print cog method for...] 16rBC80 ==> 16rBC80 <-> 16rBEA8: method: 16rC51970 prim 19 selector: 16r6D1620
newProcess
[print cog method for...] 16rBEA8 ==> 16rBEA8 <-> 16rBF28: method: 16rC518C0 selector: 16r76A600
asContext
However the links don't seem to go back up the call stack but forward, to statements to be executed in the future. So I am confused?
Yeah it's the jitted version of the method header address, then <->,
then the jitted method entry point address, the bytecode version address, selector address.
The cogMethod header is used to store the bytecoded compiled method
header (because it was replaced with a pointer to the cogMethod) and various flags.
Considering further [print cog method for...] 16rBBF0 ==> 16rBBF0 <-> 16rBC80: method: 16rC4E948 selector: 16r6CC798 forkAt:
[print oop...] 16r6CC798 ==> a(n) ByteSymbol nbytes 7 forkAt:
Clement early advised is the bytecode version of the method is this... [print oop...] 16rC4E948 ==> 16rC4E948: a(n) CompiledMethod nbytes 37 16rBBF0 is in generated methods 16r6D1620 #newProcess 16r6CC650 #priority: 16r6CC690 #resume 16r6CC798 #forkAt: 16rAE5490 a ClassBinding #BlockClosure ->
16r0088D618
16rC4E968: 70/112 D0/208 88/136 10/16 E1/225 87/135 D2/210 7C/124 16rC4E970: 28/40 AF/175 BA/186 F3/243 20/32
Now I've been a bit slow on the uptake and only just realised, but to
confirm...
the line 16r6CC798 is the one specifying the method as
BlockClosure>>forkAt:
16r6CC798 is the address of the selector #forkAt:
Sorry I wasn't clear. I wasn't referring to the address itself of the selector - that was just a line reference. My insight I wanted to confirm was that the last oop before the bytecode was... a ClassBinding #BlockClosure -> 16r0088D618 and the next last before that was... #forkAt: 16rAE5490 indicating the output of [print oop...] was method BlockClosure>>forkAt: , while above that line are the methods called by #forkAt: and below it is the bytecode.
Ahhh, actually I just saw this relevant comment in CompiledMethod... "The last literal in a CompiledMethod must be its methodClassAssociation, a binding whose value is the class the method is installed in. The methodClassAssociation is used to implement super sends. If a method contains no super send then its methodClassAssociation may be nil (as would be the case for example of methods providing a pool of inst var accessors). By convention the penultimate literal of a method is either its selector or an instance of AdditionalMethodState. "
So it seems it won't always show the Class>>method, but often will.
For the last two lines, I notice the numbers before the slash (70, 88, 10...) are the method bytecode, but what are the numbers after the slash?
The bytecode in decimal instead of hexa I think.
I checked. You are right. Obvious in hindsight.
Note that you can use the image[level byte code printing machinery on a method in the simulator by using Stackinterpreter>>symbolicMethod:. The text is output in the simulator window's transcript or the system transcript. See "toggle transcript" towards the top of the bottom right hand simulator window's menu.
In #activeCoggedNewMethod: the second assignment to methodHeader ==> 16r208000B
which matches the mthhdr field of the raw header [print cog method header for...] 16rBBF0 ==> BBF0 objhdr: 8000000A000035 nArgs: 1 type: 2 blksiz: 90 method: C4E948 mthhdr: 208000B selctr: 6CC798=#forkAt: blkentry: 0 stackCheckOffset: 5E/BC4E cmRefersToYoung: no cmIsFullBlock: no
What is "type: 2" ?
Haha.
Well when you iterate over the machine code zone you need to know what
the current element you iterate on is. In the machine code zone there can be:
- cog method
- closed PICS
- open PICS
- free space
And now we're adding cog full block method but it's sharing the index
with cog method and have a separated flag :-)
The type tells you what it is. Look at the Literal variables CMFree,
CMClosedPIC, CMOpenPIC, etc .
2 is CMMethod with is a constant. You can improve the printing there
and commit the changes if you feel so.
What did I write here I don't understand myself ? I mean CMMethod = 2,
so type = 2 means the struct you're looking at in the machine code zone is a method and not free space or a PIC.
Ok I have to go I will look at the rest of your mail later.
Let's do this...
Stepping through to Cogit>>ceEnterCogCodePopReceiverReg I notice its protocol is "simulation only" and it calls "simulateEnilopmart:numArgs:
ceEnterCogCodePopReceiverReg"
but I don't see any other implementors of
#ceEnterCogCodePopReceiverReg.
Also there is a pragma <doNotGenerate>.
Obviously the real non-simulated VM works differently, but I can't determine how.
btw, I have noticed that ceEnterCogCodePopReceiverReg ==> 16r10B8 and [print cog method for...] 16r10B8 ==> trampoline ceEnterCogCodePopReceiverReg
Is ceEnterCogCodePopReceiverReg provided by the platform C code?
Well it's in cogitIA32.c. I don't remember where it comes from.
Cool. I had a peek.
Basically in Cog you have specific machine code routines, called
trampolines, that switch from machine code to C code. When trampoline is written backward (Enilopmart) it means that the routine is meant to switch from C code to machine code.
The real VM calls in ceEnterCogCodePopReceiverReg a machine code routine
that does the right thing (register remapped, maybe fp and sp saved, etc) to switch from the C runtime from the C compiler to the machine code runtime executing code generated by the JIT.
I see its a function pointer... void (*ceEnterCogCodePopReceiverReg)(void)
set by... ceEnterCogCodePopReceiverReg = genEnilopmartForandandforCallcalled(ReceiverResultReg, NoReg, NoReg, 0, "ceEnterCogCodePopReceiverReg");
which is beyond my current level need-to-know. Still useful to fill in the background architecture. This comment comparing trampoline/enilopmart to system-call-like transition was enlightening...
/* An enilopmart (the reverse of a trampoline) is a piece of code that makes the system-call-like transition from the C runtime into generated machine code. The desired arguments and entry-point are pushed on a stackPage's stack. The enilopmart pops off the values to be loaded into registers and then executes a return instruction to pop off the entry-point and jump to it. BEFORE AFTER (stacks grow down) whatever stackPointer -> whatever target address => reg1 = reg1val, etc reg1val pc = target address reg2val stackPointer -> reg3val */
/* Cogit>>#genEnilopmartFor:and:and:forCall:called: */
Right. Trampolines are the machine code routines that call into the Smalltalk (simulator) / C (real VM) run-time support routines. They make a stack switch from the Smalltalk/Cog-machine-code stack to the actual C stack, and pass parameters. Enilopmarts do the reverse. They switch from the actual C stack back into the Smalltalk/Cog-machine-code stack, possibly popping values pushed onto that stack into specific registers, and then executing a return instruction to jump to some machine code address in the machine code zone to start or resume machine-code execution.
In simulation, the C code is simulated by executing Slang as Smalltalk
code and the machine code is simulated using the processor simulator (Bochs for IA32). So it has to be done differently as there is no C stack with register state and stuff. Both trampolines and enilmoparts are simulated with specific code.
Stepping through to simulateCogCodeAt: it called processor singleStepIn:minimumAddress:readOnlyBelow: which called
BochsIA32Alien>>primitiveSingleStepInMemory:minimumAddress:readOnlyBelow:
<primitive: 'primitiveSingleStepInMemoryMinimumAddressReadWrite' module: 'BochsIA32Plugin' error: ec> ^ec == #'inappropriate operation' ifTrue: [self handleExecutionPrimitiveFailureIn: memoryArray minimumAddress: minimumAddress] ifFalse: [self reportPrimitiveFailure]
and the debugger cursor was inside the ifTrue: statement. I found I didn't have bochs installed, but after installing bochs-2.6-2, I go the same result. So could I get some background around this..
Also I'm curious how the simulator seemed to be running a CogVM before bochs was installed. Perhaps since I was not debugging through it, the machine code ran for real rather than being simulated.
No the machine code is always simulated. Bochs was working for sure if
you successfully simulated the image on top of the cog simulator until the display was shown.
If you have a VM from one of Eliot's build (from the Cog blog) the
processor simulators are present as plugins by default. On Mac you can do [show package contents...] and then look at the file inside to check the Bochs Plugin is there. It's not the case on the Pharo VMs so don't use them for CogVM simulation. You don't need to install anything.
Ahhh... I see them now. ./lib/squeak/5.0-3692/BochsX64Plugin ./lib/squeak/5.0-3692/BochsIA32Plugin
The clears my misconception - a lack of understanding the purpose of the primitive failure and a red herring when I saw the Boch's system package wasn't installed.
On normal simulation the simulator goes often in the branch you've just
shown. It means it reached a simulation trap. As for enilmopart that can't be properly simulated, trampolines can't be simulated. So to simulate a trampoline the processor simulator fails a call and the trampoline is done in the simulation code. Look at #handleCallOrJumpSimulationTrap: for example.
Not quite. The trampolines are simulated. The calls the trampolines make can't be simulated. These calls are to illegal addresses and cause traps. The trap handler maps the addresses into the appropriate Smalltalk blocks/methods and invokes them. The same goes for accessing variables in the simulator such as framePointer, stackPointer, instructionPointer etc. These are Smalltalk objects that are instanced variables of the CoInterpreter. They are mapped to illegal addresses in machine code and attempts to access them cause traps and the trap handler maps these to fetch/store the relevant inst var. In the real VM the actual addresses of the variables are used directly.
Ah, so its an 'inappropriate operation' from Bochs' perspective, but from the Simulator's perspective the primitiveFail is a useful condition like the #ensure: "Primitive 198 always fails. The VM uses prim 198 in a context's method as the mark for an ensure:/ifCurtailed: activation." ?
Right. Andreas realised specific primitives that did nothing could be used to mark methods for the VM's benefit, without needing a bit in the method header. Very clever, very economical. A nice idea.
cheers -ben
btw, I bumped into a bit of history... http://www.mirandabanda.org/cogblog/2008/12/12/simulate-out-of-the-bochs/
:-)