RFR: 8346914: UB issue in scalbnA

[David Holmes]

On Wed, 18 Jun 2025 08:16:36 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>>> `+  // We know that k+n <= (int)0x7fe, and might be negative if n is negative.`
>> 
>> It can be negative if `n` is positive too.
>
>> But that is not correct - we should only take this "overflow" path for
>> `((k+n) > 0x7fe && (k+n) <= INT_MAX)`. Your suggestion makes us take this
>> path if `(k+n)` overflows to negative. ??
> 
> It is intentional that the new test is true for the `(k+n)` => overflow case.
> It fully handles the overflow case, eliminating the need for the later fixup
> of the case where `((k <= -54) && (n > 5000))` (though `(n > 0)` would work
> just as well; I don't know why that `5000` was inserted). That fixup returned
> the same `hugeX`-based result as here.
> 
>> It can be negative if n is positive too.
> 
> `(k+n)` cannot be negative with `n` positive here, even under wrapping
> semantics, because we can't get here in that case due to the prior overflow
> detection.

Okay @kimbarrett  I suggest that you take over this issue and PR. I do not have the knowledge of the code that you have and I cannot affirm that your statements are correct.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25656#discussion_r2155825085