Halting when VM sends a particular message or on assertion failures

This post is mostly a reminder for myself, because each time I need to do it, I forget how it was ūüôā

There are usually 2 cases where I want that the VM halts (breakpoint):

1) When a particular message is being processed.

2) When there is an assertion failure. CogVM has some kind of assertions (conditions) that when evaluated to false, mean something probably went wrong. When this happens, the condition is printed in the console and the line number is shown. For example, if we get this in the console:

(getfp() & STACK_ALIGN_MASK) == STACK_FP_ALIGN_BYTES 41946

It means that the condition evaluated to false. And 41946 is the line number. Great. But how can I put a breakpoint here so that the VM halts?

So….what can we do with CogVM? Of course, we first need to build the VM in “debug mode”. Here you can see how to build the VM, and here how to do it in debug mode.¬†Then we can do something like this (taken from an Eliot’s email)

McStalker.macbuild$ gdb Debug.app
GNU gdb 6.3.50-20050815 (Apple version gdb-1515) (Sat Jan 15 08:33:48 UTC 2011)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin"...<wbr />Reading symbols for shared libraries ................ done
(gdb) break warning
Breakpoint 1 at 0x105e2b: file /Users/eliot/Cog/oscogvm/macbuild/../src/vm/gcc3x-cointerp.c, line 39.
(gdb) run -breaksel initialize ~/Squeak/Squeak4.2/trunk4.2.image
Starting program: /Users/eliot/Cog/oscogvm/macbuild/Debug.app/Contents/MacOS/Croquet -breaksel initialize ~/Squeak/Squeak4.2/trunk4.2.image
Reading symbols for shared libraries .+++++++++++++++..................................................................................... done
Reading symbols for shared libraries . done

Breakpoint 1, warning (s=0x16487c "send breakpoint (heartbeat suppressed)") at /Users/eliot/Cog/oscogvm/macbuild/../src/vm/gcc3x-cointerp.c:39
39              printf("\n%s\n", s);
(gdb) where 5
#0  warning (s=0x16487c "send breakpoint (heartbeat suppressed)") at /Users/eliot/Cog/oscogvm/macbuild/../src/vm/gcc3x-cointerp.c:39
#1  0x0010b490 in interpret () at /Users/eliot/Cog/oscogvm/macbuild/../src/vm/gcc3x-cointerp.c:4747
#2  0x0011d521 in enterSmalltalkExecutiveImplementation () at /Users/eliot/Cog/oscogvm/macbuild/../src/vm/gcc3x-cointerp.c:14103
#3  0x00124bc7 in initStackPagesAndInterpret () at /Users/eliot/Cog/oscogvm/macbuild/../src/vm/gcc3x-cointerp.c:17731
#4  0x00105ec9 in interpret () at /Users/eliot/Cog/oscogvm/macbuild/../src/vm/gcc3x-cointerp.c:1933
(More stack frames follow...)

So the magic line here is “(gdb) break warning” which puts a breakpoint in the warning() function. Automatigally, the assertion failures end up using this function, and therefore, it halts. With this line we achieve 2)

To achieve 1) the key line is “Starting program: /Users/eliot/Cog/oscogvm/macbuild/Debug.app/Contents/MacOS/Croquet -breaksel initialize ~/Squeak/Squeak4.2/trunk4.2.image”. Here with “-breaksel” we can pass a selector as parameter (#initialize in this case). So each time the message #initialize is send, the VM will halt also in the warning function, so you have there all the stack to¬†analyze¬†whatever you want.

I am not 100% sure but the following is what I understood about how it works from Eliot:

So, if I understood correctly, I can put a¬†breakpoint¬†in the function warning() with “break¬†warning”. With the -breaksel¬† parameter you set an instVar with the selector name and size. Then after, anywhere I can send¬† #compilationBreak: selectorOop¬†point: selectorLength¬†¬† and that will magically check whether the selectorOop is the one I passes with -breaksel and if true, it will call warning, who has a¬†breakpoint, hence, I can debug ūüôā¬†¬† AWESOME!!!!¬†¬† Now with CMake I can even generate a xcode project and debug it ūüôā ¬†

That was all. Maybe this was helpful for someone else too.

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