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peepopt shiftadd: Only match for sufficiently small constant widths

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This addresses issue #4445
This commit is contained in:
George Rennie 2024-06-12 14:38:12 +01:00
parent a55e8594b7
commit 41aaaa153e
1 changed files with 5 additions and 0 deletions
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5
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)