RFR: 8247373: ArraysSupport.newLength doc, test, and exception message

[Roger Riggs]

On Fri, 4 Dec 2020 06:50:14 GMT, Stuart Marks <smarks at openjdk.org> wrote:

> This rewrites the doc of ArraysSupport.newLength, adds detail to the exception message, and adds a test. In addition to some renaming and a bit of refactoring of the actual code, I also made two changes of substance to the code:
> 
> 1. I fixed a problem with overflow checking. In the original code, if oldLength and prefGrowth were both very large (say, Integer.MAX_VALUE), this method could return a negative value. It turns out that writing tests helps find bugs!
> 
> 2. Under the old policy, if oldLength and minGrowth required a length above SOFT_MAX_ARRAY_LENGTH but not above Integer.MAX_VALUE, this method would return Integer.MAX_VALUE. That doesn't make any sense, because attempting to allocate an array of that length will almost certainly cause the Hotspot to throw OOME because its implementation limit was exceeded. Instead, if the required length is in this range, this method returns that required length.
> 
> Separately, I'll work on retrofitting various call sites around the JDK to use this method.

Nice clean description of the algorithm.

src/java.base/share/classes/jdk/internal/util/ArraysSupport.java line 654:

> 652:             return SOFT_MAX_ARRAY_LENGTH;
> 653:         } else {
> 654:             return minLength;

Isn't this last `else if... then.. else` the same as:
`return Math.max(minLength, SOFT_MAX_ARRAY_LENGTH)`

src/java.base/share/classes/jdk/internal/util/ArraysSupport.java line 640:

> 638:         int prefLength = oldLength + Math.max(minGrowth, prefGrowth); // might overflow
> 639:         if (0 < prefLength && prefLength <= SOFT_MAX_ARRAY_LENGTH) {
> 640:             return prefLength;

In spite of the assert `minGrowth > 0`, that is unchecked, I would suggest prefLength == 0 to return prefLength.
    ```if (0 <= prefLength && prefLength <= SOFT_MAX_ARRAY_LENGTH) {
            return prefLength;```
Otherwise, it falls into hughLength(...) which will return the SOFT_MAX_ARRAY_LENGTH.

It would be more robust if the algorithm was well defined if either min or pref were zero.

test/jdk/jdk/internal/util/ArraysSupport/NewLength.java line 70:

> 68:             { IMAX-2, 1,      IMAX,   IMAX-1 },
> 69:             { IMAX-1, 1,      IMAX,   IMAX   }
> 70:         };

Adding test cases for zero (0) for the min and preferred would be good to include and show any unpredictable behavior.

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PR: https://git.openjdk.java.net/jdk/pull/1617