Modify the function of SignCanonical in btcec, do makeCompact after i…

…sCanonicalSig.

Since function recoverKeyFromSignature is called inside function makeCompact, the performance loss calculated by function recoverKeyFromSignature is relatively large.
So do makeCompact after isCanonicalSig can reduce the number of times it is executed, when isCanonicalSig is false then continue the next loop.
The average performance improvement after modification is nearly 40%－50%.

btcsuite/btcd/btcec/privkey.go

@@ -72,14 +72,16 @@ func (p *PrivateKey) SignCanonical(curve *KoblitzCurve, hash []byte) ([]byte, er
 			return nil, err
 		}
 
+		if !isCanonicalSig(sig.R.Bytes(), sig.S.Bytes()) {
+			continue
+		}
+
 		compactSig, err := makeCompact(curve, sig, p, hash, true)
 		if err != nil {
 			continue
 		}
 
-		if isCanonical(compactSig) {
-			return compactSig, nil
-		}
+		return compactSig, nil
 	}
 	return nil, errors.New("couldn't find a canonical signature")
 }
@@ -110,3 +112,11 @@ func isCanonical(compactSig []byte) bool {
 	t4 := !(d[33] == 0 && ((d[34] & 0x80) == 0))
 	return t1 && t2 && t3 && t4
 }
+
+func isCanonicalSig(r []byte, s []byte) bool {
+	t1 := (r[0] & 0x80) == 0
+	t2 := !(r[0] == 0 && ((r[1] & 0x80) == 0))
+	t3 := (s[0] & 0x80) == 0
+	t4 := !(s[0] == 0 && ((s[1] & 0x80) == 0))
+	return t1 && t2 && t3 && t4
+}