RFR(S): 8080684: PPC64: Fix little-endian build after "8077838: Recent developments for ppc"

[Volker Simonis]

Hi Sasha,

you're right - my fix was too complicated. I should have just used
function_entry() instead of emit_fd() because function_entry() already
wraps either emit_fd() or pc() depending on the platform. That's
exactly what I've done in the new patch:

http://cr.openjdk.java.net/~simonis/webrevs/2015/8080684.v2/

As a bonus I've also fixed the Power 8 detection which was broken
because we issued an illegal 'lqarx' instruction to determine if we're
on Power 8.

Regards,
Volker

PS: I was actually able to test this change on a REAL ppc64le machine
this time :)



On Wed, May 20, 2015 at 3:25 AM, Alexander Smundak <asmundak at google.com> wrote:
> It fails to compile because the code still references FileDescriptor,
> even if ABI_ELFv2 is defined.
> The revised patch which succeeds is here:
> http://cr.openjdk.java.net/~asmundak/8080684/hotspot/webrev
> (it's for jdk8u-dev branch)
> but IMHO the proposed change isn't semantically right --
> if a method is called emit_fd, it should create a FileDescriptor.
> I am not sure that rationale behind this patch is right -- a
> compile-time error  is usually easy to fix and verify that the semantics
> is correct while fixing it, whereas runtime error usually requires
> more effort.
>
>
> On Tue, May 19, 2015 at 9:41 AM, Volker Simonis
> <volker.simonis at gmail.com> wrote:
>> Hi,
>>
>> can I please get a review for the following fix of the ppc64le build.
>>
>> @Sasha, Tiago: I would be also especially interested if somebody with
>> a little-endian ppc64 system can check if the fix really works there
>> as I have currently no access to such a system.
>>
>> https://bugs.openjdk.java.net/browse/JDK-8080684
>> http://cr.openjdk.java.net/~simonis/webrevs/2015/8080684/
>>
>> Following the details:
>>
>> On big-endian ppc64 we need so called 'function descriptors' instead
>> of simple pointers in order to call functions. That's why the
>> Assembler class on ppc64 offers an 'emit_fd()' method which can be
>> used to create such a function descriptor.
>>
>> On little-endian ppc64 the ABI was changed (i.e. ABI_ELFv2) and
>> function descriptors have been removed. That's why the before
>> mentioned 'emit_fd()' is being excluded from the build with the help
>> of preprocessor macros if the HotSpot is being build in a little
>> endian environment:
>>
>> #if !defined(ABI_ELFv2)
>> inline address emit_fd(...)
>> #endif
>>
>> The drawback of this approach is that every call site which uses
>> 'emit_fd()' has to conditionally handle the case where 'emit_fd()'
>> isn't present as well. This was exactly the problem with change
>> "8077838: Recent developments for ppc" which introduced an
>> unconditional call to 'emit_fd()' in 'VM_Version::config_dscr() which
>> lead to a build failure on little endian.
>>
>> A better approach would be to make 'emit_fd()' available on both,
>> little- and big-endian platforms and handle the difference internally,
>> within the function itself. On little-endian, the function will just
>> return the current PC without emitting any code at all while the
>> big-endian variant emits the required function descriptor.
>>
>> Thank you and best regards,
>> Volker