RFR: 8343506: [s390x] multiple test failures with ubsan

[Dean Long]

On Tue, 5 Nov 2024 10:05:02 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

>> src/hotspot/cpu/s390/s390.ad line 2550:
>> 
>>> 2548: // Unsigned Integer Immediate: 9-bit
>>> 2549: operand SSlenDW() %{
>>> 2550:   predicate(Immediate::is_uimm8((julong)n->get_long()-1));
>> 
>> Suggestion:
>> 
>>   predicate(n->get_long() >= 1 && Immediate::is_uimm8((julong)n->get_long()-1));
>
> I don't think this is necessary. Unsigned subtraction with wrap-around is not undefined behavior.

Right, it's not UB, but sometimes it is a bug, and would be flagged by things like -fsanitize=unsigned-integer-overflow, so my preference would be to avoid it if possible.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21864#discussion_r1830259148