Speedup log2 code (#7106)

* Speedup log2 code

* Update changelog

(cherry picked from commit fc3ccf2)

CHANGELOG.md

@@ -45,6 +45,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Misc Improvements
 
 * [#7093](https://github.com/osmosis-labs/osmosis/pull/7093),[#7100](https://github.com/osmosis-labs/osmosis/pull/7100) Lower CPU overheads of the Osmosis epoch.
+* [#7106](https://github.com/osmosis-labs/osmosis/pull/7106) Halve the time of log2 calculation (speeds up TWAP code)
+
 
 ## v20.5.1
 

osmomath/decimal.go

@@ -58,6 +58,11 @@ var (
 
 	// precisionFactors are used to adjust the scale of big.Int values to match the desired precision
 	precisionFactors = make(map[uint64]*big.Int)
+
+	zeroBigDec    BigDec = ZeroBigDec()
+	oneBigDec     BigDec = OneBigDec()
+	oneHalfBigDec BigDec = oneBigDec.Quo(twoBigDec)
+	negOneBigDec  BigDec = oneBigDec.Neg()
 )
 
 // Decimal errors
@@ -1028,9 +1033,9 @@ func DecApproxEq(t *testing.T, d1 BigDec, d2 BigDec, tol BigDec) (*testing.T, bo
 func (x BigDec) LogBase2() BigDec {
 	// create a new decimal to avoid mutating
 	// the receiver's int buffer.
-	xCopy := ZeroBigDec()
+	xCopy := BigDec{}
 	xCopy.i = new(big.Int).Set(x.i)
-	if xCopy.LTE(ZeroBigDec()) {
+	if xCopy.LTE(zeroBigDec) {
 		panic(fmt.Sprintf("log is not defined at <= 0, given (%s)", xCopy))
 	}
 
@@ -1040,18 +1045,18 @@ func (x BigDec) LogBase2() BigDec {
 	y := ZeroBigDec()
 
 	// repeat until: x >= 1.
-	for xCopy.LT(OneBigDec()) {
+	for xCopy.LT(oneBigDec) {
 		xCopy.i.Lsh(xCopy.i, 1)
-		y = y.Sub(OneBigDec())
+		y.AddMut(negOneBigDec)
 	}
 
 	// repeat until: x < 2.
 	for xCopy.GTE(twoBigDec) {
 		xCopy.i.Rsh(xCopy.i, 1)
-		y = y.Add(OneBigDec())
+		y.AddMut(oneBigDec)
 	}
 
-	b := OneBigDec().Quo(twoBigDec)
+	b := oneHalfBigDec.Clone()
 
 	// N.B. At this point x is a positive real number representing
 	// mantissa of the log. We estimate it using the following
@@ -1060,10 +1065,10 @@ func (x BigDec) LogBase2() BigDec {
 	// This has shown precision of 32 digits relative
 	// to Wolfram Alpha in tests.
 	for i := 0; i < maxLog2Iterations; i++ {
-		xCopy = xCopy.Mul(xCopy)
+		xCopy.MulMut(xCopy)
 		if xCopy.GTE(twoBigDec) {
 			xCopy.i.Rsh(xCopy.i, 1)
-			y = y.Add(b)
+			y.AddMut(b)
 		}
 		b.i.Rsh(b.i, 1)
 	}