RFR: 8282182: Document algorithm used to encode aarch64 logical immediate operands. [v3]

[Nick Gasson]

On Mon, 21 Feb 2022 13:15:34 GMT, Andrew Dinn <adinn at openjdk.org> wrote:

>> This *documentation only* change explains how logical immediate mask values are derived from valid logical instruction operands. The encoding function is used to populate a sparse array that maps valid masks to a unique set of input operand values and a reverse lookup array that maps inputs to the associated mask.
>
> Andrew Dinn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   include unsaved changes

src/hotspot/cpu/aarch64/immediate_aarch64.cpp line 197:

> 195: // derivation of k and p and validity checks:
> 196: //
> 197: // when immN is 1 then k == 6 and immr/imms are masked to 6 bit

There's a comment below "looks like 7 given the spec above but this cannot be!" - which spec is that referring to?

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PR: https://git.openjdk.java.net/jdk/pull/7536