RFR: 8355719: Reduce memory consumption of BigInteger.pow() [v51]

[fabioromano1]

On Wed, 7 May 2025 15:38:41 GMT, Raffaello Giulietti <rgiulietti at openjdk.org> wrote:

>> fabioromano1 has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Removed needless brackets
>
> src/java.base/share/classes/java/math/BigInteger.java line 2643:
> 
>> 2641:                     : new BigInteger(result, newSign).shiftLeft(bitsToShift);
>> 2642:         } else {
>> 2643:             if ((bitLength() - 1L) * exponent >= (long) MAX_MAG_LENGTH << 5) {
> 
> Suggestion:
> 
>             if ((bitLength() - 1L) * exponent >= 32L * MAX_MAG_LENGTH) {
> 
> or
> Suggestion:
> 
>             if ((bitLength() - 1L) * exponent >= (long) Integer.SIZE * MAX_MAG_LENGTH) {
> 
> 
> Both variant are easier to read, more honest, and exactly as efficient as with the shift. The right-hand sides are compile-time constants, so they have no impact on runtime performance.
> 
> More generally, the runtime compilers are perfectly capable to optimize multiplications by constant powers of 2 and replace them with shifts, even if the other operand is not a constant.

@rgiulietti What about `(bitLength() - 1L) * exponent >= Integer.MAX_VALUE`?

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PR Review Comment: https://git.openjdk.org/jdk/pull/24690#discussion_r2078107104