[9] RFR(S): 8074869: C2 code generator can replace -0.0f with +0.0f on Linux

[Vladimir Kozlov]

On 3/12/15 8:28 AM, Vitaly Davidovich wrote:
> Hi Zoltan,
>
> Is -0.0{f,d} not replaced with an immediate because the immediate then increases instruction size? Just curious ...

I did not get the question. If you look on usage of immD0/immF0 it is replaced with 0 (or G0) value which we can easy 
embed into instructions. It is not easy to use 0x80000000 such way. It will need to be loaded into register in separate 
instruction.

But I think -0. is rare so it should not be a problem.

Regards,
Vladimir

>
> Thanks
>
> On Thu, Mar 12, 2015 at 8:43 AM, Zoltán Majó <zoltan.majo at oracle.com <mailto:zoltan.majo at oracle.com>> wrote:
>
>     Hi,
>
>
>     please review the following small patch.
>
>     Bug: https://bugs.openjdk.java.net/__browse/JDK-8074869 <https://bugs.openjdk.java.net/browse/JDK-8074869>
>
>
>     Problem: On Linux, the C2 code generator can replace the value -0.0f with +0.0f (and also the value -0.0d with
>     +0.0d). The reason is that in some *.ad files both the value -0.0f and +0.0f is treated as being +0.0f and can
>     therefore be replaced with an immediate +0.0f embedded into an instruction.
>
>     For example, in the sparc.ad <http://sparc.ad> file, the 'fpclass' function is used to decide if a float node's
>     content is +0.0:
>
>     predicate((n->getf() == 0) && (fpclass(n->getf()) == FP_PZERO));
>
>     On Solaris, 'fpclass' returns FP_PZERO if the parameter is +0.0f and FP_NZERO if the parameter is -0.0f. As a
>     result, +0.0f and -0.0f are distinguished by the compiler.
>
>     On Linux, however, 'fpclass' is not available and therefore 'fpclassify' is used. 'fpclassify' does not distinguish
>     between ±0.0f, it returns FP_ZERO for both +0.0f and -0.0f.
>
>
>     Solution: Instead of 'fpclass', use cast float->int and double->long to check if value is +0.0f and +0.0d,
>     respectively. This logic is already use on some architectures, for example on x86_32 and on x86_64.
>
>     As 'fpclass' is not used any more, remove its declarations from globalDefintions_*. The include of ieeefp.h must be
>     kept as we rely on some other functionality from this header on solaris.
>
>
>     Webrev: http://cr.openjdk.java.net/~__zmajo/8074869/webrev.00/ <http://cr.openjdk.java.net/~zmajo/8074869/webrev.00/>
>
>
>     Testing:
>     - JPRT testing on all supported platforms (does *not* include aarch64 and ppc64)
>
>     - manual testing on aarch64:
>
>     All DaCapo benchmarks with the small input size. I used the default JVM flags and tested the VM w/ and w/o the fix.
>     All benchmarks pass except eclipse. For eclipse, the same Java-level failure appears both w/ and w/o the fix, so I
>     think the problem with eclipse is not due to these changes.
>
>     I also tested with the "-Xcomp -XX:-TieredCompilation -server" flags. Eclipse fails in this case as well.
>     Additionally, tradebeans and tradesoap fail with a Java-level failure. As the failure happens also with both builds
>     (w/ and w/o the fix), I don't think the problem is caused by these changes either.
>
>     - no testing on ppc64: I don't have access to a ppc64 machine. Could somebody with access to a ppc64 machine please
>     build and test the VM with this patch (and then maybe confirm that it works)?
>
>
>     Thank you and best regards,
>
>
>     Zoltan
>
>