llvm dev - Apr 2011 - [LLVMdev] Is this a bug in clang?

> 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.

John

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

> No, Johannes is right: "undefined behavior" requires evaluation.
Undefined behavior does not require evaluation.  For example, undefined 
behavior occurs when "A nonempty source file does not end in a new-line 
character which is not immediately preceded by a backslash character or 
ends in a partial preprocessing token or comment."

John