RFR: 8352422: [ubsan] Out-of-range reported in ciMethod.cpp:917:20: runtime error: 2.68435e+09 is outside the range of representable values of type 'int'

[Emanuel Peter]

On Wed, 23 Apr 2025 10:58:54 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

> The double `(double)count * prof_factor * method_life / counter_life + 0.5`
> can overflow a 32-bit int, causing UB on casting, but in practice computing
> a wrong scale, probably.
> 
> We just need to compare that the cast is not going to overflow. This is possible
> because `INT_MAX` is exactly representable in a `double`. It is also good to
> notice that the expression `(double)count * prof_factor * method_life / counter_life + 0.5`
> cannot overflow a `double`:
> - `count` is a int, max value = 2^31-1 < 2.2e9
> - `method_lie` is a int, max value < 2.2e9
> - `prof_factor` is a float, max value < 3.5e38
> - `counter_life` is a int, positive at this point, so min value = 1
> So, the whole expression is bounded by 16.94e56 + 0.5, which is much smaller than the
> max value of a double (about 1.8e308). We probably would have precision issues, but
> it probably doesn't matter a lot.
> 
> The semantic I picked here is basically `min(INT_MAX, count_d)`, so it'd always fit.
> 
> Thanks,
> Marc

Looks reasonable, I would just add a little comment to the code.

src/hotspot/share/ci/ciMethod.cpp line 919:

> 917:       double count_d = (double)count * prof_factor * method_life / counter_life + 0.5;
> 918:       if (count_d >= static_cast<double>(INT_MAX)) {
> 919:         count = INT_MAX;

Suggestion:

        // Clamp in case of overflowing int range.
        count = INT_MAX;

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

Marked as reviewed by epeter (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24824#pullrequestreview-2804298750
PR Review Comment: https://git.openjdk.org/jdk/pull/24824#discussion_r2066928575