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| -rw-r--r-- | doc/manual.txt | 45 |
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diff --git a/doc/manual.txt b/doc/manual.txt index 5fdc5a884..d2fc7e5c9 100644 --- a/doc/manual.txt +++ b/doc/manual.txt @@ -4561,6 +4561,51 @@ tested against via linear scanning. If put into the last branch of the whole ``case`` statement, the whole ``case`` statement uses linear scanning. +computedGoto pragma +------------------- +The `computedGoto`:idx: pragma can be used to tell the compiler how to +compile a Nimrod `case`:idx: in a ``while true`` statement. +Syntactically it has to be used as a statement inside the loop: + +.. code-block:: nimrod + + type + MyEnum = enum + enumA, enumB, enumC, enumD, enumE + + proc vm() = + var instructions: array [0..100, MyEnum] + instructions[2] = enumC + instructions[3] = enumD + instructions[4] = enumA + instructions[5] = enumD + instructions[6] = enumC + instructions[7] = enumA + instructions[8] = enumB + + instructions[12] = enumE + var pc = 0 + while true: + {.computedGoto.} + let instr = instructions[pc] + case instr + of enumA: + echo "yeah A" + of enumC, enumD: + echo "yeah CD" + of enumB: + echo "yeah B" + of enumE: + break + inc(pc) + + vm() + +As the example shows ``computedGoto`` is mostly useful for interpreters. If +the underlying backend (C compiler) does not support the computed goto +extension the pragma is simply ignored. + + unroll pragma ------------- The `unroll`:idx: pragma can be used to tell the compiler that it should unroll |