FWIW, adinn's branchless code together with
PR https://git.openjdk.java.net/jdk/pull/4660
make both methods less vulnerable to timing attacks.
Greetings
Raffaello
On 2021-07-02 15:50, Andrew Haley wrote:
On Fri, 2 Jul 2021 11:06:06 GMT, Andrew Dinn <ad...@openjdk.org> wrote:
You can also do that branchlessly which might prove better
```
long result = Math.multiplyHigh(x, y);
result += (y & (x >> 63));
result += (x & (y >> 63));
return result;
```
I doubt very much that it would be better, because these days branch prediction
is excellent, and we also have conditional select instructions. Exposing the
condition helps C2 to eliminate it if the range of args is known. The `if` code
is easier to understand.
Benchmark results, with one of the operands changing signs every iteration,
1000 iterations:
Benchmark Mode Cnt Score Error Units
MulHiTest.mulHiTest1 (aph) avgt 3 1570.587 ± 16.602 ns/op
MulHiTest.mulHiTest2 (adinn) avgt 3 2237.637 ± 4.740 ns/op
In any case, note that with this optimization the unsigned mulHi is in the
nanosecond range, so Good Enough. IMO.
-------------
PR: https://git.openjdk.java.net/jdk/pull/4644