RFR: 8220283: ZGC fails to build on GCC 4.4.7: ATTRIBUTE_ALIGNED compatibility issue

[Kim Barrett]

> On Mar 7, 2019, at 1:27 PM, Severin Gehwolf <sgehwolf at redhat.com> wrote:
> 
> On Thu, 2019-03-07 at 18:40 +0100, Aleksey Shipilev wrote:
>> On 3/7/19 6:23 PM, Severin Gehwolf wrote:
>>> http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8220283/01/webrev/
>> 
>> Some background. After Severin reported this failure, I found this trick by pure chance, fiddling
>> with godbolt: https://godbolt.org/z/OmVQ-B
>> 
>> We should extend the "NOTE!" comment with this new knowledge, for instance:
>> 
>> // NOTE! The "+0" below is a workaround for a known bug in older GCC versions
>> // (known to fail with 4.6.0, fixed in 4.9.0). This bug affects systems such as
>> // RedHat/Oracle Linux 7.5, which ships with GCC 4.8.5. For more details, see
>> // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55382 and
>> // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53017
>> //
>> // GCC versions older than 4.6.4 would fail even with "+0", and needs additional
>> // cast to typeof(x) to work around the similar bug.
>> //
>> #define ATTRIBUTE_ALIGNED(x) __attribute__((aligned((typeof(x))x+0)))
> 
> Sure, makes sense. Thanks! Updated webrev:
> http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8220283/02/webrev/
> 
> Could I get a second review for this, please?
> 
> Thanks,
> Severin

Looks okay, but yuck! :(

Reviewed.