[8u] Request for approval for 8172053: (ppc64) Downport of 8170153 breaks build on linux/ppc64 (big endian)

[David Holmes]

Hi Volker,

As this is a build change I have added build-dev.

The change looks fine to me.

Aside: More generally it would be nice if there was a simple way to 
record minimum compiler versions necessary for a given flag/option.

Thanks,
David

On 28/12/2016 3:17 AM, Volker Simonis wrote:
> Hi,
>
> can I please have a review and approval for pushing the following
> small, ppc64-only fix to jdk8u-dev:
>
> http://cr.openjdk.java.net/~simonis/webrevs/2016/8172053/
> https://bugs.openjdk.java.net/browse/JDK-8172053
>
> The problem is that the recent downport of  "8170153:
> PPC64/s390x/aarch64: Poor StrictMath performance due to non-optimized
> compilation" breaks the build of jdk8u with the original gcc 4.3 on
> linux/ppc64. We should however ensure, that an update-release will not
> break the original tool chain used for a released version of Java.
> Notice, that this change won't go to jdk9 because it only fixes an
> issue caused by the downport of another fix to jdk8u which doesn't
> exist in jdk9.
>
> JDK-8170153 increased the optimization level for the compilation of
> fdlibm on both linux/ppc64 and linux/ppc64le. This only worked by
> using the option '-ffp-contract=off' which guaranteed correct IEEE
> floating point behavior.
>
> Unfortunately, '-ffp-contract' is only available since gcc 4.6. For
> ppc64le that's no problem since ppc64le support only appeared in gcc
> 4.8.3. But on ppc64 (big endian) we traditionally compiled with gcc
> 4.3 which only knows '-mno-fused-madd'. However, that's still not
> enough to get the float computations right - we additionally have to
> supply '-fno-strict-aliasing'.
>
> I've tested the new configuration (i.e. '-mno-fused-madd
> -fno-strict-aliasing') with the corresponding jtreg and JCK tests and
> couldn't find any issue.
>
> Thank you and best regards,
> Volker
>