RFR: 8276453: Undefined behavior in C1 LIR_OprDesc causes SEGV in fastdebug build [v5]

[Dean Long]

On Wed, 10 Nov 2021 04:04:59 GMT, Man Cao <manc at openjdk.org> wrote:

> That could work. Do you think it is better to split this into two or three RFEs:

I would be in favor of that, if it's ok with @vnkozlov 

> Do you think it is feasible to replace all the NULL and 0 with LIR_Opr::illegalOpr()?

I don't think so, if we want to preserve existing behavior.  NULL and illegalOpr() were two different values before, 
and now LIR_Opr() introduces a new value that is different from both NULL and illegalOpr(), and with the value 0 I believe it becomes a valid pointer LIR_Opt.  To preserve exisiting behavior, we should crash if any attempt is made to use a NULL LIR_Opr.  illegalOpr() doesn't do that.  We could change existing behavior rather than preserve it, but then each use of NULL would need to be examined on a case-by-case basis.

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PR: https://git.openjdk.java.net/jdk/pull/6221