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

[Zoltán Majó]

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
>>