On Fri, 2 May 2025 16:06:42 GMT, fabioromano1 <[email protected]> wrote:
>> Raffaello Giulietti has updated the pull request incrementally with one
>> additional commit since the last revision:
>>
>> Clearer and more complete spec of the *pow* methods.
>
> src/java.base/share/classes/java/lang/Math.java line 3613:
>
>> 3611: return x;
>> 3612: }
>> 3613:
>
> Suggestion:
>
>
> if (x == 2) {
> if (n >= Long.SIZE)
> throw new ArithmeticException("unsigned long overflow");
> return 1L << n;
> }
>
> if ((java.math.BigInteger.bitLengthForLong(x) - 1L) * n + 1L >
> Long.SIZE) {
> throw new ArithmeticException("unsigned long overflow");
> }
>
> With also a check for the condition `java.math.BigInteger.bitLengthForLong(x)
> * n <= Long.SIZE`, when it is true the path could be led to a loop that skips
> the checks.
I don't think there's a quick, _precise_ pre-check that would ensure that the
loop can just use simple, unchecked `*` multiplications.
Consider `unsignedPowExact(3L, 40)`, which does not overflow, versus
`unsignedPowExact(3L, 41)`, which does.
How would you pre-check these two cases using integer arithmetic?
IMO, you still need checked multiplications in the loop.
(Besides, the product in your checks can overflow, so you would have to add a
guard.)
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/25003#discussion_r2071884269
