RFR: 8341594: Use Unsafe to coalesce reads in java.util.zip.ZipUtils [v2]

[Chen Liang]

On Sun, 6 Oct 2024 15:07:02 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> src/java.base/share/classes/java/util/zip/ZipUtils.java line 175:
>> 
>>> 173:     public static final int get16(byte[] b, int off) {
>>> 174:         Preconditions.checkIndex(off, b.length, Preconditions.AIOOBE_FORMATTER);
>>> 175:         Preconditions.checkIndex(off + 1, b.length, Preconditions.AIOOBE_FORMATTER);
>> 
>> Please use `Preconditions.checkFromIndexSize`, which should be less overhead:
>> Suggestion:
>> 
>>         Preconditions.checkFromIndexSize(off, 2, b.length, Preconditions.AIOOBE_FORMATTER);
>> 
>> Similarly for other methods.
>
> It's actually not less overhead in my tests, since `checkIndex` is intrinsic and mostly disappears, while with `checkFromIndexSize` performance gets significantly worse (on par with baseline). It's on my todo to investigate this in-depth but I think `checkFromIndexSize` needs to be given similar intrinsification treatment as `checkIndex`.

Actually if we trust the input index to be nonnegative, we can just check our end index for out of bounds too.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/21377#discussion_r1789127887