RFR(M): 8204965: Fix '--disable-cds' and disable CDS on AIX by default

[Volker Simonis]

On Thu, Jun 14, 2018 at 9:04 PM, Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
> Hi Volker,
>
> http://cr.openjdk.java.net/~simonis/webrevs/2018/8204965/make/autoconf/hotspot.m4.udiff.html
>
> Seems like a roundabout way to have a platform specific default value.
>
> Why not determine a default value beforehand:
>
> if test "x$OPENJDK_TARGET_OS" = "xaix"; then
>   ENABLE_CDS_DEFAULT="false"
> else
>   ENABLE_CDS_DEFAULT=true"
> fi
>
> AC_ARG_ENABLE([cds], [AS_HELP_STRING([--enable-cds@<:@=yes/no/auto@:>@],
>  [enable class data sharing feature in non-minimal VM. Default is
> ${ENABLE_CDS_DEFAULT}.])])
>
> and so on?
>

I've just followed the pattern used for '--enable-aot' right above the
code I changed.

Moreover, I don't think that we would save any code because we would
still have to check for AIX in the '--enable-cds=yes' case. Also, the
new reporting added later in the file (see "AC_MSG_CHECKING([if cds
should be enabled])" seems easier to me without the extra default
value. So if you don't mind I'd prefer to leave it as is.

> See also what we did for "8202325: [aix] disable warnings-as-errors by default".
>
> --
>
> http://cr.openjdk.java.net/~simonis/webrevs/2018/8204965/src/hotspot/share/classfile/javaClasses.cpp.udiff.html
>
> Here, do we really need to exclude this from compiling,
> DumpSharedSpaces = false is not enough?
>

Yes, we need it because the excluded code references methods (e.g.
'StringTable::create_archived_string()') which are not compiled into
libjvm.so if we disable CDS.

>
> Best Regards, Thomas
>
> On Thu, Jun 14, 2018 at 4:26 PM, Volker Simonis
> <volker.simonis at gmail.com> wrote:
>> Hi,
>>
>> can I please have a review for the following fix:
>>
>> http://cr.openjdk.java.net/~simonis/webrevs/2018/8204965/
>> https://bugs.openjdk.java.net/browse/JDK-8204965
>>
>> CDS does currently not work on AIX because of the way how we
>> reserve/commit memory on AIX. The problem is that we're using a
>> combination of shmat/mmap depending on the page size and the size of
>> the memory chunk to reserve. This makes it impossible to reliably
>> reserve the memory for the CDS archive and later on map the various
>> parts of the archive into these regions.
>>
>> In order to fix this we would have to completely rework the memory
>> reserve/commit/uncommit logic on AIX which is currently out of our
>> scope because of resource limitations.
>>
>> Unfortunately, I could not simply disable CDS in the configure step
>> because some of the shared code apparently relies on parts of the CDS
>> code which gets excluded from the build when CDS is disabled. So I
>> also fixed the offending parts in hotspot and cleaned up the configure
>> logic for CDS.
>>
>> Thank you and best regards,
>> Volker
>>
>> PS: I did run the job through the submit forest
>> (mach5-one-simonis-JDK-8204965-20180613-1946-26349) but the results
>> weren't really useful because they mention build failures on linux-x64
>> which I can't reproduce locally.