Request for review (S): 6833879 Assigning positive zero is ignored when old value is negative zero

[Tom Rodriguez]

I think it's just an illegal identity transform.  We have another  
similar one 6796786 that I looked at.  In that case we transform - (a  
- b) into b - a but that gives the wrong zero when a == b.

tom

On Apr 29, 2009, at 10:59 AM, Changpeng Fang wrote:

> http://cr.openjdk.java.net/~cfang/6833879/webrev.00/
>
> Problem:
> For the following case:
> public static double normalize(float v) {
>       if (v == 0.0f) v = 0.0f;
>       return v;
> }
> If -0.0 is passed, 0.0 is expected to be returned (it is the case  
> for the interpreter). However,
> PhiNode::Identity optimized away the "if and assignment" statement  
> and the server vm
> return -0.0.
>
> Proposed Solution:
> Don't perform this kind of identity optimization if either true path  
> or false path is constant
> 0.0 or (-0.0)
>
> Open Issue:
> When the true path or false path is not constant, it may still be  
> 0.0 (-0.0) at run time.
> Do we have to turn off this identity optimization completely  for  
> float  and double (except
> for constant non-zeros)?
> Tests: Passed the test case in the CR report.
>
> Thanks,
>
> Changpeng