Add 32-bit "exact int" optimization

name                          old time/op  new time/op  delta
AppendFloat32/0e+00-12        2.97ns ± 1%  2.96ns ± 1%     ~     (p=0.738 n=5+5)
AppendFloat32/1e+00-12        36.5ns ± 1%  13.1ns ± 0%  -64.04%  (p=0.008 n=5+5)
AppendFloat32/3e-01-12        32.3ns ± 2%  32.6ns ± 2%     ~     (p=0.214 n=5+5)
AppendFloat32/1e+06-12        36.3ns ± 1%  17.7ns ± 1%  -51.40%  (p=0.008 n=5+5)
AppendFloat32/-1.2345e+02-12  33.6ns ± 1%  34.3ns ± 1%   +2.08%  (p=0.024 n=5+5)

ryu.go

@@ -67,9 +67,10 @@ func AppendFloat32(b []byte, f float32) []byte {
 		return appendSpecial(b, neg, exp == 0, mant == 0)
 	}
 
-	// FIXME: add "exact int" optimization.
-
-	d := float32ToDecimal(mant, exp)
+	d, ok := float32ToDecimalExactInt(mant, exp)
+	if !ok {
+		d = float32ToDecimal(mant, exp)
+	}
 	return d.append(b, neg)
 }
 

ryu32.go

@@ -74,6 +74,24 @@ func (d dec32) append(b []byte, neg bool) []byte {
 	return b
 }
 
+func float32ToDecimalExactInt(mant, exp uint32) (d dec32, ok bool) {
+	e := exp - bias32
+	if e > mantBits32 {
+		return d, false
+	}
+	shift := mantBits32 - e
+	mant |= 1 << mantBits32 // implicit 1
+	d.m = mant >> shift
+	if d.m<<shift != mant {
+		return d, false
+	}
+	for d.m%10 == 0 {
+		d.m /= 10
+		d.e++
+	}
+	return d, true
+}
+
 func float32ToDecimal(mant, exp uint32) dec32 {
 	var e2 int32
 	var m2 uint32