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AVM: Extract divideCeilUnsafely to help document opcode costing rationale #3867

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Merged
michaeldiamant merged 2 commits into from
Apr 11, 2022
Merged

AVM: Extract divideCeilUnsafely to help document opcode costing rationale #3867

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b793a8c
Extract divideCeil to help document opcode costing rationale
michaeldiamant Apr 7, 2022
c80832d
Rename divideCeil to highlight absence of divide-by-zero check
michaeldiamant Apr 7, 2022
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9 changes: 8 additions & 1 deletion data/transactions/logic/opcodes.go
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Original file line number Diff line number Diff line change
Expand Up @@ -69,10 +69,17 @@ type linearCost struct {
chunkSize int
}

// divideCeilUnsafely provides `math.Ceil` semantics using integer division. The technique avoids slower floating point operations as suggested in https://stackoverflow.com/a/2745086.
// The method does _not_ check for divide-by-zero.
func divideCeilUnsafely(numerator int, denominator int) int {
return (numerator + denominator - 1) / denominator
}

func (lc *linearCost) compute(stack []stackValue) int {
cost := lc.baseCost
if lc.chunkCost != 0 && lc.chunkSize != 0 {
cost += lc.chunkCost * (len(stack[len(stack)-1].Bytes) + lc.chunkSize - 1) / lc.chunkSize
// Uses divideCeilUnsafely rather than (len/size) to match how Ethereum discretizes hashing costs.
cost += divideCeilUnsafely(lc.chunkCost*len(stack[len(stack)-1].Bytes), lc.chunkSize)
}
return cost
}
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