perf(mcache): no alloc

lang/mcache/mcache.go

@@ -16,6 +16,7 @@ package mcache
 
 import (
 	"sync"
+	"unsafe"
 
 	"github.com/bytedance/gopkg/lang/dirtmake"
 )
@@ -25,12 +26,19 @@ const maxSize = 46
 // index contains []byte which cap is 1<<index
 var caches [maxSize]sync.Pool
 
+type bytesHeader struct {
+	Data *byte
+	Len  int
+	Cap  int
+}
+
 func init() {
 	for i := 0; i < maxSize; i++ {
 		size := 1 << i
 		caches[i].New = func() interface{} {
 			buf := dirtmake.Bytes(0, size)
-			return buf
+			h := (*bytesHeader)(unsafe.Pointer(&buf))
+			return h.Data
 		}
 	}
 }
@@ -57,8 +65,14 @@ func Malloc(size int, capacity ...int) []byte {
 	if len(capacity) > 0 && capacity[0] > size {
 		c = capacity[0]
 	}
-	var ret = caches[calcIndex(c)].Get().([]byte)
-	ret = ret[:size]
+
+	i := calcIndex(c)
+
+	ret := []byte{}
+	h := (*bytesHeader)(unsafe.Pointer(&ret))
+	h.Len = size
+	h.Cap = 1 << i
+	h.Data = caches[i].Get().(*byte)
 	return ret
 }
 
@@ -68,6 +82,6 @@ func Free(buf []byte) {
 	if !isPowerOfTwo(size) {
 		return
 	}
-	buf = buf[:0]
-	caches[bsr(size)].Put(buf)
+	h := (*bytesHeader)(unsafe.Pointer(&buf))
+	caches[bsr(size)].Put(h.Data)
 }

lang/mcache/utils.go

@@ -21,5 +21,5 @@ func bsr(x int) int {
 }
 
 func isPowerOfTwo(x int) bool {
-	return (x & (-x)) == x
+	return (x != 0) && ((x & (-x)) == x)
 }