RFR: 8276220: Reduce excessive allocations in DateTimeFormatter [v3]

[Claes Redestad]

On Wed, 3 Nov 2021 11:53:52 GMT, Stephen Colebourne <scolebourne at openjdk.org> wrote:

>> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add fallback for values outside the allowable range
>
> src/java.base/share/classes/java/time/format/DateTimeFormatterBuilder.java line 3158:
> 
>> 3156: 
>> 3157:         // only instantiated and used if there's ever a value outside the allowed range
>> 3158:         private FractionPrinterParser fallback;
> 
> This class has to be safe in a multi-threaded environment. I'm not convinced it is safe right now, as the usage doesn't follow the standard racy single check idiom. At a minimum, there needs to be a comment explaining the thread-safety issues.

Yes, I'll make sure to read into a local variable.

> src/java.base/share/classes/java/time/format/DateTimeFormatterBuilder.java line 3266:
> 
>> 3264:             if (!field.range().isValidIntValue(value)) {
>> 3265:                 if (fallback == null) {
>> 3266:                     fallback = new FractionPrinterParser(field, minWidth, maxWidth, decimalPoint, subsequentWidth);
> 
> It would be nice to see a test case cover this.

I'll see to it.

> src/java.base/share/classes/java/time/format/DateTimeFormatterBuilder.java line 3290:
> 
>> 3288:             range.checkValidValue(value, field);
>> 3289:             BigDecimal minBD = BigDecimal.valueOf(range.getMinimum());
>> 3290:             BigDecimal rangeBD = BigDecimal.valueOf(range.getMaximum()).subtract(minBD).add(BigDecimal.ONE);
> 
> I wouldn't be surprised if there is a way to replace the use of `BigDecimal` with calculations using `long`.  Fundamentally, calculations like 15/60 -> 0.25 are not hard, but it depends on whether the exact results can be matched across a wide range of possible inputs.

I think the main engineering challenge is writing tests that ensure that we don't lose precision on an arbitrary fractional field.

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PR: https://git.openjdk.java.net/jdk/pull/6188