[pr/110] RFR: 8317812: [Lilliput] Make C2 LoadNKlassCompactHeader more robust

[Roman Kennke]

On Wed, 11 Oct 2023 08:27:57 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> On the other hand, this suggests that we can pass `$mem$$index$Register` right away?
>> 
>> https://github.com/openjdk/lilliput/pull/111/files#diff-018aa61d1a7aafcf70a535fcd40a318a4bd6511fd40ac39ce4be90cc52216749L7350
>
> In fact, I don't quite see why `x86_64.ad` cannot do what `aarch64.ad` does:
> 
> 
> __ load_nklass_compact($dst$$Register, $mem$$base$$Register, $mem$$index$$Register, $mem$$scale, $mem$$disp);

Problem is the index=4 does not decode to noreg (it decodes to rsp) and scale=0 does not decode to no_scale (it's invalid, I think). Hence the extra hoops there.

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PR Review Comment: https://git.openjdk.org/lilliput/pull/111#discussion_r1354551486