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

[john cuthbertson - Sun Microsystems]

Hi Changpeng,

Is it not possible to compare the unsigned hex patterns for the values 
in t & f using a couple of union and only remove the if and assignment 
iff the hex patterns are equal?

JohnC

Tom Rodriguez wrote:
> 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
>