RFR(XXS) 8249087 Symbol constructor unnecessarily initializes _body[0]

[David Holmes]

Hi Ioi,

On 21/07/2020 12:50 pm, Ioi Lam wrote:
> 
> 
> On 7/20/20 7:36 PM, David Holmes wrote:
>> Hi Ioi,
>>
>> On 21/07/2020 10:12 am, Ioi Lam wrote:
>>> Hi please review this very simple fix:
>>>
>>> diff -r 19f26d72a8d0 src/hotspot/share/oops/symbol.cpp
>>> --- a/src/hotspot/share/oops/symbol.cpp    Mon Jul 20 14:24:19 2020 
>>> -0700
>>> +++ b/src/hotspot/share/oops/symbol.cpp    Mon Jul 20 17:11:57 2020 
>>> -0700
>>> @@ -51,8 +51,11 @@
>>>   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
>>>     memcpy(_body, name, length);
>>> +  // For symbols of length 0 and 1: _body[0] (and _body[1]) are 
>>> uninitialized and may
>>
>> Can we ever have a Symbol of length zero?
>>
>> If the Symbol name is length 1 then surely _body[0] is initialized to 
>> the single character of that name?
>>
>> The change seems harmless given a zero length symbol is meaningless, 
>> but the commentary just confuses things to me.
>>
> 
> Hi David,
> 
> We can have a valid Symbol of length 0. All UTF8 constants in 
> classfiles, including the literal string "", are represented as Symbols.

Ah I see.
> How about
> 
> // Random, uninitialized values may appear in _body[0] and _body[1]
> // for Symbols of length 0 and 1. These random values never change during
> // the lifetime of the Symbol, and are read only by 
> Symbol::identity_hash(),

What if as_quoted_ascii() were called on the zero-length symbol? That 
reads _body[0] unconditionally. The same for use of base(). ??

Thanks,
David
-----

> // which would tolerate such randomness.
> 
> 
> Thanks
> - Ioi
>> Thanks,
>> David
>> -----
>>
>>> +  // contain random values, which will only be read by 
>>> Symbol::identity_hash(),
>>> +  // which would tolerate such randomness. These values never change 
>>> during the lifetime
>>> +  // of the Symbol.
>>>   }
>>>
>>>
>>> Passed hs tiers 1/2. Running tiers 3/4 now.
>>>
>>> Thanks
>>> - Ioi
>