62 lines
2.3 KiB
Markdown
62 lines
2.3 KiB
Markdown
Gir has five optimization passes.
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joinAdjacentOps
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---------------
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* *consumes*: commands without offsets
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* *produces*: commands without offsets
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* *unknown commands*: passed through unmodified
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* *acts on*: `moveHead`, `add`
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* *other known commands*: `loop` (recurses)
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`joinAdjacentOps` joins adjacent `moveHead`s and `add`s together. Normally
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`parse` joins runs of `<>` or `+-` into one command, but if the runs are
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interrupted by another character two commands get generated. `moveHead`s are
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joined first, before the pass goes over the generated commands again and
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joins `add`s.
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transformClearLoops
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-------------------
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* *consumes*: commands without offsets
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* *produces*: commands without offsets
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* *unknown commands*: passed through unmodified
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* *acts on*: `loop` with one command, which is `add 1` or `add -1`
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* *other known commands*: `loop` (recurses)
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`transformClearLoops` changes loops of the form `[-]` or `[+]` into a single
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`clear` command.
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addOffsetProperties
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-------------------
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* *consumes*: commands without offsets
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* *produces*: commands with offsets
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* *unknown commands*: raise UnknownIRError
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* *acts on*: `moveHead`, `add`, `clear`, `writeByte`, `readByte`, `loop`
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`addOffsetProperties` adds an `offset` property to `add`, `clear`,
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`writeByte`, and `readByte`. The `offset` tells the offset at which from the
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current tape head location the operations are performed. It also adds an
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`isBalanced` property to loops. This property tells if execution of loop
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body ends at the same cell where it began, which is useful for performing
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further optimizations.
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transformMultiplyLoops
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----------------------
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* *consumed*: commands with offsets
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* *unknown commands*: passed through unmodified
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* *acts on*: balanced `loop`s that only have adds and where adds at offset 0 are together 1 or -1
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* *other known commands*: `loop` (recurses)
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`transformMultiplyLoops` changes loops of the form `[>+>++<<-]` into a
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single `multiply` command.
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flattenLoops
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------------
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* *consumes*: commands with offsets
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* *produces*: flattened array commands with offsets
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* *unknown commands*: passed through unmodified
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* *acts on*: `loop`
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`flattenLoops` changes the nested structure where `loop` commands have a
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`content` property with the loop body inside to a flat structure with
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`jumpIfZero` and `jumpIfNonZero`.
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