RFR: 8346916: [REDO] align_up has potential overflow [v7]

[Casper Norrbin]

On Wed, 5 Mar 2025 13:42:32 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> That solution still works. With `can_align_up` and setting `aligned_bytes` to 0, we can use the logic that is already there a few lines down:
>> 
>> https://github.com/openjdk/jdk/blob/caaf4098452476d981183ad4302b76b9c883a72b/src/hotspot/share/gc/parallel/psOldGen.cpp#L201-L207
>> 
>> To me it felt better to continue to try to expand instead of aborting. If you prefer the other version I can swap back to it.
>
>> try to expand instead of aborting.
> 
> Did the previous version abort? I thought the logic is essentially:
> 
> 
> if (remaining == 0) {
>   return;
> }
> 
> aligned_bytes = align_up(min2(bytes, remaining), alignment).
> 
> 
> The below `if (aligned_bytes == 0) { ` should not be reachable any more, since align-up-overflow can't occur.

I meant aborting in the sense that we abort the expand operation by returning early instead of continuing.

If we don't that, the `if (aligned_bytes == 0) {` should be reachable with `can_align_up` if the align were to fail and we set aligned_bytes to 0. The `remaining == 0` would then be caught later in the function inside `expand_by(aligned_bytes)`.

I'll go ahead and revert to the other version, and remove the `if (aligned_bytes == 0) {` block as well.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/23711#discussion_r1981475286