Logged In: YES 
user_id=593130

I understand that you got advice to post, but I strongly doubt that 
core Python code is involved for three reasons.

1. On the Unix I used, bus errors were much rarer and esoteric 
than segmentation violations (from bad pointers).  But maybe 
BSD-derived OSX is different.

2. The only difference between fun1 that works and fun3 that 
doesn't, seems to be how the arg in computed.  The receiving 
function only gets a value (which it copies) and does not know its 
history.

3. You report that upgrading the compiler fixes the problem.  (So 
why not just do that?)

If you want to experiment more, redo fun1 and fun3 with 'value' 
replaced by its literal value.  Then redo again with value = 
534.30000000000007 instead of 411 and change arg in fun1 to 
value/1.3 instead of 1.3*value.

About stack trace: On unix (of 15 years ago), program crashes 
usually resulted in a file called 'core' added to either the startup 
or current working directory.  A debugger program could extract 
a stack trace, which was more readable if the executable still had 
the symbol table attached and not stripped.  I don't know 
equivalent for OSX, but I found it very helpful for debugging.