perf: optimize math.Int.Size for small values

This is a simpler version of cosmos#16263 that only optimizes values that fit
in 53 bits. It is possible to optimize values that fit in 64 bits by
using big.Int.BitLen and math.Log2(10), but it doesn't seem worth the
complexity, especially given the revert of cosmos#16263.

I failed to beat big.Int.Marshal for values that don't fit in 64 bits.

math/fuzz_test.go

@@ -22,3 +22,15 @@ func FuzzLegacyNewDecFromStr(f *testing.F) {
 		}
 	})
 }
+
+func FuzzSmallIntSize(f *testing.F) {
+	f.Add(int64(2<<53 - 1))
+	f.Add(-int64(2<<53 - 1))
+	f.Fuzz(func(t *testing.T, input int64) {
+		i := NewInt(input)
+		exp, _ := i.Marshal()
+		if i.Size() != len(exp) {
+			t.Fatalf("input %d: i.Size()=%d, len(input)=%d", input, i.Size(), len(exp))
+		}
+	})
+}

math/int.go

@@ -4,6 +4,7 @@ import (
 	"encoding"
 	"encoding/json"
 	"fmt"
+	stdmath "math"
 	"math/big"
 	"strings"
 	"sync"
@@ -429,6 +430,24 @@ func (i *Int) Unmarshal(data []byte) error {
 
 // Size implements the gogo proto custom type interface.
 func (i *Int) Size() int {
+	if i.i == nil {
+		return 1
+	}
+	// A float64 can store 52 bits exactly, which allows us to use
+	// math.Log10 to compute the size fast and garbage free.
+	if i.i.BitLen() <= 52 {
+		i64 := i.i.Int64()
+		if i64 == 0 {
+			return 1
+		}
+		size := 0
+		if i64 < 0 {
+			i64 = -i64
+			size++
+		}
+		return size + 1 + int(stdmath.Log10(float64(i64)))
+	}
+	// Slow path.
 	bz, _ := i.Marshal()
 	return len(bz)
 }

math/int_test.go

@@ -592,14 +592,16 @@ func TestNewIntFromString(t *testing.T) {
 }
 
 func BenchmarkIntSize(b *testing.B) {
+	var tests []math.Int
+	for _, st := range sizeTests {
+		ii, _ := math.NewIntFromString(st.s)
+		tests = append(tests, ii)
+	}
+	b.ResetTimer()
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {
-		for _, st := range sizeTests {
-			ii, _ := math.NewIntFromString(st.s)
+		for _, ii := range tests {
 			got := ii.Size()
-			if got != st.want {
-				b.Errorf("%q:: got=%d, want=%d", st.s, got, st.want)
-			}
 			sink = got
 		}
 	}