On Tue, 17 May 2022 12:38:55 GMT, Yasumasa Suenaga <ysuen...@openjdk.org> wrote:

>> src/hotspot/share/classfile/classFileParser.cpp line 5970:
>>> 5969:   _cp->symbol_at_put(hidden_index, _class_name);
>> I don't understand these warning suppressions for symbol_at_put (here and 
>> elsewhere).  I don't see any stringops here.  What is the compiler 
>> complaining about?  (There's no mention of classfile stuff in the review 
>> cover message.)
> Like the others, it is caused by `Array::at_put()`.
> In file included from 
> /home/ysuenaga/github-forked/jdk/src/hotspot/share/oops/annotations.hpp:28,
>                  from 
> /home/ysuenaga/github-forked/jdk/src/hotspot/share/oops/instanceKlass.hpp:29,
>                  from 
> /home/ysuenaga/github-forked/jdk/src/hotspot/share/classfile/javaClasses.hpp:30,
>                  from 
> /home/ysuenaga/github-forked/jdk/src/hotspot/share/precompiled/precompiled.hpp:35:
> In member function 'void Array<T>::at_put(int, const T&) [with T = unsigned 
> char]',
>     inlined from 'void ConstantPool::tag_at_put(int, jbyte)' at 
> /home/ysuenaga/github-forked/jdk/src/hotspot/share/oops/constantPool.hpp:126:64,
>     inlined from 'void ConstantPool::symbol_at_put(int, Symbol*)' at 
> /home/ysuenaga/github-forked/jdk/src/hotspot/share/oops/constantPool.hpp:362:15,
>     inlined from 'void 
> ClassFileParser::mangle_hidden_class_name(InstanceKlass*)' at 
> /home/ysuenaga/github-forked/jdk/src/hotspot/share/classfile/classFileParser.cpp:5966:21:

`Array<T>::_data` is a pseudo flexible array member. "Pseudo" because C++
doesn't have flexible array members. The compiler is completely justified in
complaining about the apparently out-of-bounds accesses.

There is a "well-known" (though moderately ugly) approach to doing flexible
array members in C++. Something like this:

T* data() {
  return reinterpret_cast<T*>(
    reinterpret_cast<char*>(this) + data_offset());

where `data_offset()` is new and private:

static size_t data_offset() {
  return offset_of(Array, _data);

Use `data()` everywhere instead of using `_data` directly.

There are other places in HotSpot that use this kind of approach.


PR: https://git.openjdk.java.net/jdk/pull/8646

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