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Merge pull request #4448 from georgerennie/shiftadd_gating

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peepopt shiftadd: Only match for sufficiently small constant widths
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Emil J 2024-11-20 13:34:09 +01:00 • committed by GitHub
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passes/pmgen/peepopt_shiftadd.pmg
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@ -53,6 +53,11 @@ match add
select port(add, constport).is_fully_const() select port(add, constport).is_fully_const()
define <IdString> varport (constport == \A ? \B : \A) define <IdString> varport (constport == \A ? \B : \A)
// only optimize for constants up to a fixed width. this prevents cases
// with a blowup in internal term size and prevents larger constants being
// casted to int incorrectly
select (GetSize(port(add, constport)) <= 24)
// if a value of var is able to wrap the output, the transformation might give wrong results // if a value of var is able to wrap the output, the transformation might give wrong results
// an addition/substraction can at most flip one more bit than the largest operand (the carry bit) // an addition/substraction can at most flip one more bit than the largest operand (the carry bit)
// as long as the output can show this bit, no wrap should occur (assuming all signed-ness make sense) // as long as the output can show this bit, no wrap should occur (assuming all signed-ness make sense)