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

[David Holmes]

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.

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