RFR: 8286660: codestrings gtest fails on AArch64: "udf" in padding

[Stuart Monteith]

On Fri, 13 May 2022 09:10:09 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> On hsdis-enabled AArch64 machine this test fails even with [JDK-8274039](https://bugs.openjdk.java.net/browse/JDK-8274039):
> 
> 
> $ CONF=linux-aarch64-server-fastdebug make run-test TEST=jtreg:gtest/GTestWrapper.java
> 
> [----------] 1 test from codestrings
> [ RUN      ] codestrings.validate_vm

I didn't consider that, I assumed the code would be all zeros and nopped for a specific purpose. I'd be happy with `udf #0` as anything else would be likely to be problematic garbage. The immediate field is the least significant bottom 16 bits, so two ASCII bytes followed by two zero bytes would be disassembled as a udf instruction.

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PR: https://git.openjdk.java.net/jdk/pull/8695