[Aarch64] Materialize immediates with 64-bit ORR + EOR if shorter #68287
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✅ With the latest revision this PR passed the C/C++ code formatter. |
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Contributing to LLVM asks me to select suitable reviewers – I don't think I have permission to do that on Github, but any chance of a review, @davemgreen or @resistor? |
davemgreen
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Oct 8, 2023
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).
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davemgreen
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Oct 10, 2023
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Thanks for the review! I'm not able to hit submit, but I am happy for this to be merged. |
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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:
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).