Short circuit evaluation/expression rearrangement (2nd summary)

[Mark Brader]

>>>> [a + b + c]
>>>> What you DO know:     The addition of a and b will be made before c is
>>>>                       added to the result.
>>> 
>>> More precisely, what you know is that the program will *behave as if*
>>> things were done that way.  In particular, if the compiler can be
>>> sure that the order of evaluation will not affect behavior, it can
>>> use any order it pleases.
>>
>> Still more precisely, what you know is that *if* the program would *not*
>> cause an exception (e.g. overflow) if things were done that way, *then*
>> it will behave *as if* things were done that way.
> 
> No...  How can you distinguish the evaluation of a rearranged
> expression from an undefined evaluation which could be _anything_?

If you can distinguish it, then the compiler can't rearrange it --
*unless* your program, as written, would cause an exception.  If it
*would*, then we have undefined behavior and the compiler *can* rewrite
in any way it pleases -- including a way which would avoid the exception
and produce the answer you wanted, if such a way exists.

-- 
Mark Brader		     "It is impractical for the standard to attempt to
SoftQuad Inc., Toronto	      constrain the behavior of code that does not obey
utzoo!sq!msb, msb at sq.com      the constraints of the standard."  -- Doug Gwyn

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