RFR: 8299308: Add Assembler::testw register + immediate function for x86

[Erik Österlund]

On Fri, 23 Dec 2022 10:32:53 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

> Is it really needed, `testw r, i16` has the prefix 0x66 as a length changing prefix, which will bottle the predecoder. It is better to zero-extend to a 32-bit value and use `testl r, i32`.
> 
> Thanks.

Interesting. I did indeed run into strange perf issues with testw with Address operand and ultimately changed to testl. Where this code was invoked I thought I care a bit more about the footprint of the generated code and less about getting optimal performance. But maybe I should just nuke it and use testl there as well.

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PR: https://git.openjdk.org/jdk/pull/11772