RFR: 8355992: Add unsignedMultiplyExact and *powExact methods to Math and StrictMath [v2]

[Raffaello Giulietti]

On Fri, 2 May 2025 17:03:30 GMT, fabioromano1 <duke at openjdk.org> wrote:

>> 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.)
>
>> 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.)
> 
> @rgiulietti
> 
> 1. `BigInteger.bitLengthForLong(x) * n <= Long.SIZE` is a sufficient condition to ensure no overflow;
> 
> 2. `(BigInteger.bitLengthForLong(x) - 1L) * n + 1L > Long.SIZE` is a sufficient condition to ensure the overflow.
> 
> 
> Thus, there remain only the cases when ` Long.SIZE < BigInteger.bitLengthForLong(x) * n && (BigInteger.bitLengthForLong(x) - 1L) * n + 1L <= Long.SIZE`, in this cases checked multiplications in the loop are needed.
>  
> Moreover, `BigInteger.bitLengthForLong(x) - 1L` is a `long`, so the product does not overflow, so the condition at point 1 never overflows if it is evaluated only if the condition at point 2 is false.

@fabioromano1 Well, there are two checks. In one the product can overflow, you'd need to convert one of the operands to `long`.

Anyway, since the pre-checks are not precise, that would lead to an implementation with a loop with checked, and another one with unchecked multiplications. I don't think this buys you anything.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25003#discussion_r2071920476