8239357: Revert gcc implementation of offset_of

Revert definition of offset_of and reenable -Winvalid-offsetof.

Reviewed-by: erikj, ihse, dholmes

make/hotspot/lib/CompileJvm.gmk

@@ -86,7 +86,7 @@ DISABLED_WARNINGS_gcc := parentheses comment unknown-pragmas address \
     delete-non-virtual-dtor char-subscripts array-bounds int-in-bool-context \
     ignored-qualifiers  missing-field-initializers implicit-fallthrough \
     empty-body strict-overflow sequence-point maybe-uninitialized \
-    misleading-indentation cast-function-type invalid-offsetof
+    misleading-indentation cast-function-type
 
 ifeq ($(call check-jvm-feature, zero), true)
   DISABLED_WARNINGS_gcc += return-type switch clobbered
@@ -95,8 +95,7 @@ endif
 DISABLED_WARNINGS_clang := tautological-compare \
     undefined-var-template sometimes-uninitialized unknown-pragmas \
     delete-non-virtual-dtor missing-braces char-subscripts \
-    ignored-qualifiers missing-field-initializers mismatched-tags \
-    invalid-offsetof
+    ignored-qualifiers missing-field-initializers mismatched-tags
 
 DISABLED_WARNINGS_solstudio := labelnotused hidef w_novirtualdescr inlafteruse \
     unknownpragma doubunder w_enumnotused w_toomanyenumnotused \

src/hotspot/share/utilities/globalDefinitions_gcc.hpp

@@ -235,7 +235,11 @@ inline int wcslen(const jchar* x) { return wcslen((const wchar_t*)x); }
 #define FORMAT64_MODIFIER "ll"
 #endif // _LP64
 
-#define offset_of(klass,field) offsetof(klass,field)
+// gcc warns about applying offsetof() to non-POD object or calculating
+// offset directly when base address is NULL. Use 16 to get around the
+// warning. The -Wno-invalid-offsetof option could be used to suppress
+// this warning, but we instead just avoid the use of offsetof().
+#define offset_of(klass,field) (size_t)((intx)&(((klass*)16)->field) - 16)
 
 #if defined(_LP64) && defined(__APPLE__)
 #define JLONG_FORMAT          "%ld"