Dougall Johnson f74e9f86a3 [Aarch64] Materialize immediates with 64-bit ORR + EOR if shorter (#68287) ...

A number of useful constants can be encoded with a 64-bit ORR followed
by a 64-bit EOR, including all remaining repeated byte patterns, some
useful repeated 16-bit patterns, and some irregular masks. This patch
prioritizes that encoding over three or four instruction encodings.
Encoding with MOV + MOVK or ORR + MOVK is still preferred for fast
literal generation and readability respectively.

The method devises three candidate values, and checks if both Candidate
and (Imm ^ Candidate) are valid logical immediates. If so, Imm is
materialized with:

```
ORR Xd, XZR, #(Imm ^ Candidate)
EOR Xd, Xd, #(Candidate)
```

The method has been exhaustively tested to ensure it can solve all
possible values (excluding 0, ~0, and plain logical immediates, which
are handled earlier).

2023-10-10 08:05:51 +01:00

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