llvm dev - Jul 2010 - [LLVMdev] [cfe-dev] conditional operator

Hello Jochen-

Your expectation is incorrect, I'm afraid. Typically this would be expanded
to something like this:

   …
   %cmp = …
   br i1 %cmp, label %ternary.true, label %ternary.false
ternary.true:
   %truevalue = …
   br label %ternary.end
ternary.false:
   %falsevalue = …
   br label %ternary.end
ternary.end:
   %value = phi %type [%truevalue, %ternary.true], [%falsevalue, %ternary.false]

This may then be optimised to a select by later passes, if it's considered
to be
profitable to do so.

The reason for this is that C requires that only the value which is needed
is evaluated. If one of the selected values has a side effect, this becomes
significant.

Alistair

On 20 Jul 2010, at 15:39, Jochen Wilhelmy wrote:
> Hi!
> 
> when using something like
> (a < b) ? a : b
> 
> I would expect that the select instruction is always used.
> But, if a or b are a global variable, a real branch with multiple
> basic blocks is created. Is this a bug or a feature?
> 
> -Jochen
> 
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev

> The reason for this is that C requires that only the value which is needed
> is evaluated. If one of the selected values has a side effect, this becomes
significant.
>   
Ah, I see. But in my case I have only normal variables (local or 
global), no volatile and no functions.
Therefore I wonder why the optimizer misses it in some cases (with globals).

-Jochen

On Tue, Jul 20, 2010 at 9:59 AM, Jochen Wilhelmy <j.wilhelmy at arcor.de>
wrote:
>
>> The reason for this is that C requires that only the value which is
needed
>> is evaluated. If one of the selected values has a side effect, this
becomes significant.
>>
> Ah, I see. But in my case I have only normal variables (local or
> global), no volatile and no functions.
> Therefore I wonder why the optimizer misses it in some cases (with
globals).
It normally gets optimized... can you attach a testcase?

-Eli