binary: reduces allocation during code section decoding (#1352)

Signed-off-by: Takeshi Yoneda <takeshi@tetrate.io>

internal/wasm/binary/code.go

@@ -17,7 +17,7 @@ func decodeCode(r *bytes.Reader, codeSectionStart uint64, ret *wasm.Code) (err e
 	}
 	remaining := int64(ss)
 
-	// parse locals
+	// Parse #locals.
 	ls, bytesRead, err := leb128.DecodeUint32(r)
 	remaining -= int64(bytesRead)
 	if err != nil {
@@ -26,30 +26,28 @@ func decodeCode(r *bytes.Reader, codeSectionStart uint64, ret *wasm.Code) (err e
 		return io.EOF
 	}
 
-	var nums []uint64
-	var types []wasm.ValueType
+	// Validate the locals.
+	bytesRead = 0
 	var sum uint64
-	var n uint32
 	for i := uint32(0); i < ls; i++ {
-		n, bytesRead, err = leb128.DecodeUint32(r)
-		remaining -= int64(bytesRead) + 1 // +1 for the subsequent ReadByte
+		num, n, err := leb128.DecodeUint32(r)
 		if err != nil {
 			return fmt.Errorf("read n of locals: %v", err)
 		} else if remaining < 0 {
 			return io.EOF
 		}
 
-		sum += uint64(n)
-		nums = append(nums, uint64(n))
+		sum += uint64(num)
 
 		b, err := r.ReadByte()
 		if err != nil {
 			return fmt.Errorf("read type of local: %v", err)
 		}
+
+		bytesRead += n + 1
 		switch vt := b; vt {
 		case wasm.ValueTypeI32, wasm.ValueTypeF32, wasm.ValueTypeI64, wasm.ValueTypeF64,
 			wasm.ValueTypeFuncref, wasm.ValueTypeExternref, wasm.ValueTypeV128:
-			types = append(types, vt)
 		default:
 			return fmt.Errorf("invalid local type: 0x%x", vt)
 		}
@@ -59,11 +57,29 @@ func decodeCode(r *bytes.Reader, codeSectionStart uint64, ret *wasm.Code) (err e
 		return fmt.Errorf("too many locals: %d", sum)
 	}
 
-	var localTypes []wasm.ValueType
-	for i, num := range nums {
-		t := types[i]
-		for j := uint64(0); j < num; j++ {
-			localTypes = append(localTypes, t)
+	// Rewind the buffer.
+	_, err = r.Seek(-int64(bytesRead), io.SeekCurrent)
+	if err != nil {
+		return err
+	}
+
+	localTypes := make([]wasm.ValueType, 0, sum)
+	for i := uint32(0); i < ls; i++ {
+		num, bytesRead, err := leb128.DecodeUint32(r)
+		remaining -= int64(bytesRead) + 1 // +1 for the subsequent ReadByte
+		if err != nil {
+			return fmt.Errorf("read n of locals: %v", err)
+		} else if remaining < 0 {
+			return io.EOF
+		}
+
+		b, err := r.ReadByte()
+		if err != nil {
+			return fmt.Errorf("read type of local: %v", err)
+		}
+
+		for j := uint32(0); j < num; j++ {
+			localTypes = append(localTypes, b)
 		}
 	}