Avoid some GCC 10.X warnings in HotSpot

[David Holmes]

On 19/06/2020 11:59 am, Koichi Sakata wrote:
> Hi David,
> 
>  > This is in relation to the hotspot part as these issues need to be
>  > handled separately. I have filed:
>  >
>  > https://bugs.openjdk.java.net/browse/JDK-8247818
> 
> Thank you, David.I have something to ask you.
> Should I send only the other part of the patch (i.e. NetworkInterface.c 
> and k_standard.c) to core-lib ML again? I've sent the whole one to 
> core-lib before.

Probably best to re-send as the mention of "Hotspot" in subject might 
put off core-libs folk from looking at it. :)

Cheers,
David

>  > I'm not really clear on the warning here but this is an area where we
>  > trick the compiler somewhat. The _body[] is declared with a size of 2,
>  > but when we allocate Symbols we allocate sufficient memory for _body to
>  > contain the entire symbol.
>  >
>  > That said I'm struggling to see how we allocate the additional space
>  > needed for the _hash_and_refcount and _length fields ???
> 
> I was thinking exactly the same thing. I've learned a lot from Ioi's 
> explanation.
> 
>  > The check for length==0 introduces more overhead than just always
>  > setting _body[0]=0, so there is no need to add it.
> 
> I understood that clearly. Thank you for teaching me.
> 
> Thanks,
> Koichi
> 
> On 2020/06/18 10:56, David Holmes wrote:
>> Hi Koichi,
>>
>> This is in relation to the hotspot part as these issues need to be 
>> handled separately. I have filed:
>>
>> https://bugs.openjdk.java.net/browse/JDK-8247818
>>
>> On 18/06/2020 8:46 am, Koichi Sakata wrote:
>>> Hi all,
>>>
>>> I tried to build OpenJDK fastdebug with GCC 10.1 on Ubuntu 18.04, but 
>>> I saw some compiler warnings as follows:
>>>
>>> In file included from 
>>> /home/jyukutyo/code/jdk/src/hotspot/share/classfile/systemDictionary.hpp:31, 
>>>
>>>                   from 
>>> /home/jyukutyo/code/jdk/src/hotspot/share/classfile/javaClasses.hpp:28,
>>>                   from 
>>> /home/jyukutyo/code/jdk/src/hotspot/share/precompiled/precompiled.hpp:35: 
>>>
>>> In member function 'void Symbol::byte_at_put(int, u1)',
>>>      inlined from 'Symbol::Symbol(const u1*, int, int)' at 
>>> /home/jyukutyo/code/jdk/src/hotspot/share/oops/symbol.cpp:55:16:
>>> /home/jyukutyo/code/jdk/src/hotspot/share/oops/symbol.hpp:130:18: 
>>> error: writing 1 byte into a region of size 0 
>>> [-Werror=stringop-overflow=]
>>>    130 |     _body[index] = value;
>>>        |     ~~~~~~~~~~~~~^~~~~~~
>>
>> I'm not really clear on the warning here but this is an area where we 
>> trick the compiler somewhat. The _body[] is declared with a size of 2, 
>> but when we allocate Symbols we allocate sufficient memory for _body 
>> to contain the entire symbol.
>>
>> That said I'm struggling to see how we allocate the additional space 
>> needed for the _hash_and_refcount and _length fields ???
>>
>>> I can resolve them with the following patch. I believe it fixes those 
>>> potential bugs, so I'd like to contribute it.
>>> (Our company has signed OCA.)
>>>
>>> Thanks,
>>> Koichi
>>>
>>> ===== PATCH =====
>>> diff -r 20d92fe3ac52 src/hotspot/share/oops/symbol.cpp
>>> --- a/src/hotspot/share/oops/symbol.cpp    Tue Jun 16 03:16:41 2020 
>>> +0000
>>> +++ b/src/hotspot/share/oops/symbol.cpp    Thu Jun 18 07:08:50 2020 
>>> +0900
>>> @@ -50,9 +50,10 @@
>>>   Symbol::Symbol(const u1* name, int length, int refcount) {
>>>     _hash_and_refcount =  pack_hash_and_refcount((short)os::random(), 
>>> refcount);
>>>     _length = length;
>>> -  _body[0] = 0;  // in case length == 0
>>> -  for (int i = 0; i < length; i++) {
>>> -    byte_at_put(i, name[i]);
>>> +  if (length == 0) {
>>> +    _body[0] = 0;
>>
>> The check for length==0 introduces more overhead than just always 
>> setting _body[0]=0, so there is no need to add it.
>>
>>> +  } else {
>>> +    memcpy(_body, name, length);
>>>     }
>>>   }
>>
>> So you are replacing byte_at_put with a memcpy call. On the surface 
>> that seems reasonable, but I have to wonder why we were using the loop 
>> in the first place. It may just be historical or it may relate to an 
>> alignment issue, or something else. Hopefully someone else (e.g. 
>> Coleen :) ) can shed more light here.
>>
>> Thanks,
>> David
>> -----
>>
>>> diff -r 20d92fe3ac52 src/hotspot/share/oops/symbol.hpp
>>> --- a/src/hotspot/share/oops/symbol.hpp    Tue Jun 16 03:16:41 2020 
>>> +0000
>>> +++ b/src/hotspot/share/oops/symbol.hpp    Thu Jun 18 07:08:50 2020 
>>> +0900
>>> @@ -125,11 +125,6 @@
>>>       return (int)heap_word_size(byte_size(length));
>>>     }
>>>
>>> -  void byte_at_put(int index, u1 value) {
>>> -    assert(index >=0 && index < length(), "symbol index overflow");
>>> -    _body[index] = value;
>>> -  }
>>> -
>>>     Symbol(const u1* name, int length, int refcount);
>>>     void* operator new(size_t size, int len) throw();
>>>     void* operator new(size_t size, int len, Arena* arena) throw();
>>
>>