RFR: 8357959: (bf) ByteBuffer.allocateDirect initialization can result in large TTSP spikes [v7]

[Aleksey Shipilev]

On Tue, 3 Jun 2025 09:47:01 GMT, Rohitash Kumar <duke at openjdk.org> wrote:

>> src/java.base/share/classes/java/nio/Bits.java line 260:
>> 
>>> 258:             offset += len;
>>> 259:         }
>>> 260:     }
>> 
>> So this becomes:
>> 
>> Suggestion:
>> 
>>     static void setMemory(long srcAddr, long count, byte value) {
>>         long offset = 0;
>>         while (offset < count) {
>>             long len = Math.min(UNSAFE_SET_THRESHOLD, count - offset);
>>             UNSAFE.setMemory(srcAddr + offset, len, value);
>>             offset += len;
>>         }
>>     }
>
> It might be trival but isn't it better to keep `len` declaration outside of while loop?

I don't think so, reliability argument wins: put the variables in their closest scope. Here, for example, it shows directly that `len` is (re-)defined for every iteration. There are hardly any performance implications of doing this, barring compiler nitpicks.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25487#discussion_r2123349452