[lilliput-jdk21u:lilliput] RFR: 8317954: [Lilliput/JDK21] Make C2 LoadNKlassCompactHeader more robust

[Aleksey Shipilev]

On Wed, 11 Oct 2023 13:15:38 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> Clean backport of https://github.com/openjdk/lilliput/pull/111.
> Lilliput's C2 code for generating LoadNKlass currently assumes that the disp of the incoming address is klass_offset_in_bytes. It then extracts the base register and loads from the mark_offset_in_bytes instead.
> Sometimes (apparently very rarely) it happens that C2 emits code that pre-adds obj+klass_offset into a register, and uses that as base, but with offset 0. In this case we would trip the assert or crash in release builds.
> 
> - [x] specjvm derby (which triggered the bug)
> - [x] tier1 x86_64 +UCOH
> - [x] tier1 aarch64 +UCOH

Marked as reviewed by shade (Reviewer).

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PR Review: https://git.openjdk.org/lilliput-jdk21u/pull/10#pullrequestreview-1671758302