Build failure "/mnt/data/home/software/openjdk/src/hotspot/share/gc/parallel/objectStartArray.cpp:106:56: error: ‘void* memset(void*, int, size_t)’ writing to an object of type ‘class HeapWord’ with ‘private’ member ‘HeapWord::i’ [-Werror=class-memaccess]"

[Karl-Philipp Richter]

Hi,
Building jdk/jdk on Ubuntu 18.10 fails due to

```
/builds/krichter/openjdk/src/hotspot/share/gc/parallel/objectStartArray.cpp:
In member function 'void ObjectStartArray::set_covered_region(MemRegion)':
/builds/krichter/openjdk/src/hotspot/share/gc/parallel/objectStartArray.cpp:106:56:
error: 'void* memset(void*, int, size_t)' writing to an object of type
'class HeapWord' with 'private' member 'HeapWord::i'
[-Werror=class-memaccess]
     memset(_blocks_region.end(), clean_block, expand_by);
                                                        ^
In file included from
/builds/krichter/openjdk/src/hotspot/share/utilities/align.hpp:28,
                 from
/builds/krichter/openjdk/src/hotspot/share/runtime/globals.hpp:29,
                 from
/builds/krichter/openjdk/src/hotspot/share/memory/allocation.hpp:28,
                 from
/builds/krichter/openjdk/src/hotspot/share/classfile/classLoaderData.hpp:28,
                 from
/builds/krichter/openjdk/src/hotspot/share/precompiled/precompiled.hpp:34:
/builds/krichter/openjdk/src/hotspot/share/utilities/globalDefinitions.hpp:175:7:
note: 'class HeapWord' declared here
 class HeapWord {
       ^~~~~~~~
/builds/krichter/openjdk/src/hotspot/share/gc/parallel/objectStartArray.cpp:
In member function 'void ObjectStartArray::reset()':
/builds/krichter/openjdk/src/hotspot/share/gc/parallel/objectStartArray.cpp:124:73:
error: 'void* memset(void*, int, size_t)' writing to an object of type
'class HeapWord' with 'private' member 'HeapWord::i'
[-Werror=class-memaccess]
   memset(_blocks_region.start(), clean_block, _blocks_region.byte_size());
                                                                         ^
In file included from
/builds/krichter/openjdk/src/hotspot/share/utilities/align.hpp:28,
                 from
/builds/krichter/openjdk/src/hotspot/share/runtime/globals.hpp:29,
                 from
/builds/krichter/openjdk/src/hotspot/share/memory/allocation.hpp:28,
                 from
/builds/krichter/openjdk/src/hotspot/share/classfile/classLoaderData.hpp:28,
                 from
/builds/krichter/openjdk/src/hotspot/share/precompiled/precompiled.hpp:34:
/builds/krichter/openjdk/src/hotspot/share/utilities/globalDefinitions.hpp:175:7:
note: 'class HeapWord' declared here
 class HeapWord {
       ^~~~~~~~
cc1plus: all warnings being treated as errors
make[3]: *** [lib/CompileJvm.gmk:174:
/builds/krichter/openjdk/build/linux-x86_64-server-release/hotspot/variant-server/libjvm/objs/objectStartArray.o]
Error 1
make[2]: *** [make/Main.gmk:257: hotspot-server-libs] Error 2

ERROR: Build failed for target 'default (exploded-image)' in
configuration 'linux-x86_64-server-release' (exit code 2)
```

A detailed build log can be found at
https://gitlab.com/krichter/openjdk/-/jobs/140219987. The setup shows
the failure for Ubuntu 19.04-daily as well. I can produce the failure on
Ubuntu 18.10 locally as well.

experienced with jdk-13+1-59-g2472bccc3c7

-Kalle