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

[Albert Mingkun Yang]

On Tue, 4 Mar 2025 15:48:17 GMT, Casper Norrbin <cnorrbin at openjdk.org> wrote:

>> Hi everyone,
>> 
>> The `align_up` function can potentially overflow, resulting in undefined behavior. Most use cases rely on the assumption that aligned_result >= original. To address this, I've added an assertion to verify this condition.
>> 
>> The original PR (#20808) missed cases where overflow checks already existed, so I've now went through usages of `align_up` and found the places with explicit checks. Most notably, #23168 added `align_up_or_null` to metaspace, but this function is also useful elsewhere. Given this, I relocated it to `align.hpp`, alongside the rest of the alignment functions.
>
> Casper Norrbin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   align comments

src/hotspot/share/gc/parallel/psOldGen.cpp line 193:

> 191: #endif
> 192:   const size_t alignment = virtual_space()->alignment();
> 193:   size_t aligned_bytes = can_align_up(bytes, alignment) ? align_up(bytes, alignment) : 0;

Why doesn't the previous revision using early-return + min2 work?

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PR Review Comment: https://git.openjdk.org/jdk/pull/23711#discussion_r1981369341