RFR: 8356865: C2: Unreasonable values for debug flag FastAllocateSizeLimit can lead to left-shift-overflow, which is UB [v3]

[Benoît Maillard]

On Wed, 18 Jun 2025 19:42:07 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

>> Thanks for the comments!
>> 
>> I added the assert because the issue in the JBS mentioned a specific case where we ended up with negative values.
>> 
>> Should I leave it like this, or rather convert it to a more specific check (ie. making sure that the `LogBytesPerLong - log2_esize` most significant bits are not used **before** shifting)?
>
> IMO your assert is obfuscating the overflow problem. 
> I think the assert should be before doing the shift.
> It can be like:
> 
> assert((fast_size_limit == 0) || (count_leading_zeros(fast_size_limit) > (LogBytesPerLong - log2_esize), "fast_size_limit (%d) overflow when shifted left by %d", fast_size_limit, (LogBytesPerLong - log2_esize));

Thanks for the tip, I made the requested changes!

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PR Review Comment: https://git.openjdk.org/jdk/pull/25834#discussion_r2159359020