logic in Math.nextAfter for handling -0.0 input

[Deneau, Tom]

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<mailto: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