RFR: 5038439: Warning message for literal shift amounts outside the canonical domain [v3]

[Jan Lahoda]

On Mon, 3 Nov 2025 19:12:43 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> Hmm, obviously this is a judgement call and I'm curious what others think.
>> 
>> My opinion is that I still think there should be a warning for shifts of -1.
>> 
>> I mean, maybe to a _real_ hacker, then of course `foo << -1` makes perfect sense because it's just stating the "obvious" which is "shift the low order bit into the high order position and set all the other bits to zero", right?? :)
>> 
>> I just don't think the average Java programmer automatically understands that.
>> 
>> Yes, it's an idiom or "handy trick", but if hasn't attained the status of being universally recognized then it doesn't deserve a special exception.
>> 
>> On the other hand, excluding -1 from the warning would be less disruptive (at least, to the real hackers out there), and that has its own merits. So I don't have a super strong opinion about it.
>
> This might be worth a corpus scan to see how common it is to shift by -1 or other constants, either int or long.
> Someone, might interpret `v << -1` as a right shift, is it more likely to misinterpret it as using more than 5/6 bits of the shift or considering it signed.

I am working on a corpus run, will take some days probably. But I am not sure if producing a warning for code that is "correct, but advanced use" is realistic. Maybe the line needs to be draw closer to "the code is probably buggy/highly suspicious".

@rgiulietti - thanks, the explanation makes sense.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27102#discussion_r2495062625