logic in Math.nextAfter for handling -0.0 input
Tom Rodriguez
tom.rodriguez at oracle.com
Mon Jun 23 21:56:10 UTC 2014
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
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