Avoid some GCC 10.X warnings in HotSpot

[David Holmes]

On 18/06/2020 3:04 pm, Ioi Lam wrote:
> 
> 
> On 6/17/20 6:56 PM, 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.
>>
> 
> Actually I don't know why we are setting _body[0] to zero. The content 
> of _body is not supposed to be 0-terminated so we don't need a zero in 
> there.

It may not be required but it is cleaner/safer to make it NUL. Though 
arguably we should just be rejecting a length of zero to begin with.

> E.g., if the length is 1, _body[1] will have garbage anyway.

If the length is 1 then _body[1] is "out of bounds", so I'm not sure 
what your point is. If the actual length is one then _body[0] has the 
true symbol content (non NUL-terminated) and _body[1] is junk we 
shouldn't touch.

But the definition and allocation of Symbols confuses me as I mentioned 
because I can't see why we define _body with size 2 rather than just 1. 
Nor can I see how the length we pass to placement new accounts for the 
additional fields that a Symbol has. ??

Cheers,
David

> "java" hit Breakpoint 2, Symbol::Symbol (this=0x7fffa1621e38, 
> name=0x7ffff6dceb9c "*", length=1, refcount=65535)
> (step until the loop finishes)
> 
> (gdb) p *this
> $1 = {<MetaspaceObj> = {}, _hash_and_refcount = 2027159551, _length = 1, 
> _body = <incomplete sequence \361>}
> (gdb) p _body[0]
> $2 = 42 '*'
> (gdb) p _body[1]
> $3 = 241 '\361'
> 
> Thanks
> - Ioi
> 
>> 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();
>>
>>
>