[aarch64-port-dev ] [9] RFR(S): 8074869: C2 code generator can replace -0.0f with +0.0f on Linux

[Vladimir Kozlov]

Good.

Thanks,
Vladimir

On 3/13/15 11:29 AM, Zoltán Majó wrote:
> Thank you, Vladimir, Volker, and Dean, for the review! (Dean gave his feedback in a private discussion.)
>
> Here is the patch I intend to push:
>
> http://cr.openjdk.java.net/~zmajo/8074869/webrev.01/
>
> The patch includes Volker's test.
>
> Thank you and best regards,
>
>
> Zoltan
>
>
> On 03/13/2015 04:54 PM, Volker Simonis wrote:
>> Hi Zoltan,
>>
>> I've tested the change on Linux/ppc64 and AIX and it works fine.
>>
>> However I don't really see the bug which you pretend to fix with this change.
>> All the Linux architectures which are in the OpenJDK use the following
>> predicate to check for +0.0:
>>
>> predicate((n->getf() == 0) &&
>>            (fpclassify(n->getf()) == FP_ZERO) && (signbit(n->getf()) == 0));
>>
>> In particular they explicitly check the sign bit of the float/double
>> argument which should ensure correct operation.
>>
>> I wrote a small regression test (added to the new webrev for your
>> convenience [1]) which proves the correct operation with the current
>> code. If I remove the "signbit" check the regression test will fail.
>>
>> Nevertheless I think your change is good because it is a nice cleanup
>> and simplification. So please go ahead and push it.
>>
>> Regards,
>> Volker
>>
>> PS: I've also removed some trailing whitespace from your original change
>>
>> [1] http://cr.openjdk.java.net/~simonis/webrevs/2015/8074869/
>>
>>
>> On Thu, Mar 12, 2015 at 1:43 PM, Zoltán Majó <zoltan.majo at oracle.com> wrote:
>>> Hi,
>>>
>>>
>>> please review the following small patch.
>>>
>>> Bug: 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 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/
>>>
>>>
>>> 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
>>>
>