Avoid some GCC 10.X warnings in HotSpot

[Koichi Sakata]

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

I will do so.Thank you.

Thanks,
Koichi

On 2020/06/19 14:56, David Holmes wrote:
> 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();
>>>
>>>