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

[Raffaello Giulietti]

On Fri, 2 May 2025 17:30:55 GMT, fabioromano1 <duke at openjdk.org> wrote:

>> Again, I don't think that `n == 1` is a frequent case which would make any practical difference.
>> 
>> As for the `bitLength` check, the product might overflow.
>> Further, `bitLength` might not be that cheap.
>> Finally, the test would just help to _fail_ faster at the expense of making the successful runs slightly slower.
>
>> As for the `bitLength` check, the product might overflow. Further, `bitLength` might not be that cheap.
> 
> The current implementations of `bitLength()` call `numberOfLeadingZeros()`, which are:
> 
> public static int numberOfLeadingZeros(long i) {
>         int x = (int)(i >>> 32);
>         return x == 0 ? 32 + Integer.numberOfLeadingZeros((int)i)
>                 : Integer.numberOfLeadingZeros(x);
>     }
> 
> public static int numberOfLeadingZeros(int i) {
>         // HD, Count leading 0's
>         if (i <= 0)
>             return i == 0 ? 32 : 0;
>         int n = 31;
>         if (i >= 1 << 16) { n -= 16; i >>>= 16; }
>         if (i >= 1 <<  8) { n -=  8; i >>>=  8; }
>         if (i >= 1 <<  4) { n -=  4; i >>>=  4; }
>         if (i >= 1 <<  2) { n -=  2; i >>>=  2; }
>         return n - (i >>> 1);
>     }

Yes, I'm familiar with both this Java code and the intrinsic code.

Compare this with the much simpler proposed code.
The checked multiplication `unsignedMultiplyExact` apparently performs two 64x64->64 multiplications, but on some architectures it might end up in a single 64x64->128 multiplication and one check.
So the proposed code performs 6 such multiplications if the method returns + 5 ordinary multiplications in the worst case.

As a general rule, the simpler the code, the better the outcome of the optimizing compiler.

Again, to me there's no point in failing fast at the expense of the successful case.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/25003#discussion_r2071955574