logic in Math.nextAfter for handling -0.0 input

[Tom Rodriguez]

If it was tiered you’d need to make sure it was a graal compile, not just a C1 compile.  I wrote at unit test which shows we are currently doing it wrong.  I’ll push that along with the fix.

tom

On Jun 23, 2014, at 2:07 PM, Deneau, Tom <tom.deneau at amd.com> wrote:

> Hmm, I tried running with -vm graal and running enough times so Math.nextAfter would get compiled and it was still getting the right answer but maybe I did something wrong.
>  
> -- Tom
>  
> From: Tom Rodriguez [mailto:tom.rodriguez at oracle.com] 
> Sent: Monday, June 23, 2014 4:04 PM
> To: Deneau, Tom
> Cc: graal-dev at openjdk.java.net
> Subject: Re: logic in Math.nextAfter for handling -0.0 input
>  
> I think graal does this in general so it’s not an HSAIL specific problem.  From AddFloatNode.java:
>  
>  @Override
>     public Node canonical(CanonicalizerTool tool) {
>         …
>         } else if (y().isConstant()) {
>             Constant c = y().asConstant();
>             if ((c.getKind() == Kind.Float && c.asFloat() == 0.0f) || (c.getKind() == Kind.Double && c.asDouble() == 0.0)) {
>                 return x();
>             }
>         }
>         return this;
>     }
>  
> FloatSubNode has similar problems.  I’ll delete the relevant code.
>  
> tom
>  
>  
> On Jun 23, 2014, at 1:33 PM, Deneau, Tom <tom.deneau at amd.com> wrote:
> 
> 
> The JDK method Math.nextAfter contains the logic shown below to handle an input of -0.0
> 
> When the graal compiler compiles this for the hsail backend, it makes the reasonable
> assumption that "start + 0.0d" can be reduced to "start" which is a problem for this algorithm.
> 
> From what I could tell, c2 or graal for amd64 backend do not do this optimization
> and so get the right answer for -0.0 input.  How do they know not to do this optimization?
> 
> 
>        } else {        // start > direction or start < direction
>            // Add +0.0 to get rid of a -0.0 (+0.0 + -0.0 => +0.0)
>            // then bitwise convert start to integer.
>            long transducer = Double.doubleToRawLongBits(start + 0.0d);         <==============
> 
>            if (direction > start) { // Calculate next greater value
>                transducer = transducer + (transducer >= 0L ? 1L:-1L);
>            } else  { // Calculate next lesser value
>                  ....
> 
> -- Tom