RFR: 8320971: Use BufferedInputStream.buf directly when param of implTransferTo() is trusted [v17]

[Vladimir Sitnikov]

On Fri, 22 Dec 2023 14:27:16 GMT, Markus KARG <duke at openjdk.org> wrote:

>> Can anybody give a hint how one can create assertions in OpenJDK test code that would check the amount of allocated heap for the tested method?
>> 
>> Since the change here is "removal of an allocation", the assert in the code should probably allow only a very small allocation in `.transferTo`.
>> 
>> Does `com.sun.management.ThreadMXBean#getCurrentThreadAllocatedBytes()` sound right for the test?
>
> I think there is a misundertanding. This PR is not intendend to reduce the *amount* of allocated heap, it is about sparing time by not creating *temporary  copies*. The latter should be rather easy to check: Invoke `transferTo(out)` two times in a row and compare the *identity* of the two byte arrays passed to `out.write()`. If they stay the same, then apparently no *temporary copy* was created. Two achieve this, the BIS must be wrapper around an extendable input stream (like `FileInputStream`) so between calls the stream could get extended (e. g. by writing into the file).

Markus, could you please double-check? A temporary allocation *is* an allocation, and it is the point of the change to avoid allocation and copying the temp data when the output stream is known to behave well. I am sure the allocation "before the change" would be non-trivial (more than several KiB), and after the change the allocation within .transferTo would be minimal (few bytes) assuming the output stream has already allocated its buffer

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PR Review Comment: https://git.openjdk.org/jdk/pull/16879#discussion_r1435120325