[LLVMdev] Is this a bug in clang?

David Vandevoorde
Wed, 20 Apr 2011 18:53:48 -0000

On Apr 20, 2011, at 2:37 PM, John Regehr wrote:

>> It's the evaluation of the expression "i++ + i++" that
result in undefined
>> behavior. If you merely compile a program that contains the expression,
the
>> expression is never evaluated. You need to execute the program to have
the
>> expression be evaluated and behavior be undefined.
>
> I don't think this is correct.  The standard does not make much of a
> distinction between compile time and run time, there is merely a "C
> implementation."
>
>> For example, also, if you do `sizeof(i++ + i++)`, this behavior is
perfectly
>> fine, and not undefined behavior. Because the `i++` is not evaluated.
>
> Probably also wrong.

No, Johannes is right: "undefined behavior" requires evaluation.
Whether than evaluation happens at "compile/translation time" or
"run time" doesn't matter (as you note: the standard's
distinction between the two is implicit at best), but if the offending
expression is e.g. in a function that never gets called, that expression
doesn't cause undefined behavior.

	Daveed