runtime: add gc layout info for some basic types

Author: dgryski

src/reflect/gclayout_none.go

@@ -0,0 +1,7 @@
+//go:build !gc.precise
+
+package reflect
+
+func (r *rawType) gcLayout() unsafe.Pointer {
+    return nil
+}

src/reflect/gclayout_precise.go

@@ -0,0 +1,37 @@
+//go:build gc.precise
+
+package reflect
+
+import "unsafe"
+
+var (
+	// Constants for use with alloc()
+	// See runtime/gc_precise.go
+	// pppppppp_pppppppp_pppppppp_ppsssss1
+	gcLayoutNoPtrs  = unsafe.Pointer(uintptr(0b0000_00001_1))
+	gcLayoutPointer = unsafe.Pointer(uintptr(0b0001_00001_1))
+	gcLayoutString  = unsafe.Pointer(uintptr(0b0001_00010_1))
+	gcLayoutSlice   = unsafe.Pointer(uintptr(0b0001_00011_1))
+)
+
+func (r *rawType) gcLayout() unsafe.Pointer {
+
+	if r.Kind() < String {
+		return gcLayoutNoPtrs
+	}
+
+	if r.Kind() == Pointer || r.Kind() == UnsafePointer {
+		return gcLayoutPointer
+	}
+
+	if r.Kind() == String {
+		return gcLayoutString
+	}
+
+	if r.Kind() == Slice {
+		return gcLayoutSlice
+	}
+
+	// Unknown (for now); less the conservative pointer scanning handle it
+	return nil
+}

src/reflect/value.go

@@ -1493,7 +1493,9 @@ func MakeSlice(typ Type, len, cap int) Value {
 	var slice sliceHeader
 	slice.cap = uintptr(ucap)
 	slice.len = uintptr(ulen)
-	slice.data = alloc(size, nil)
+	layout := rtype.elem.gcLayout()
+
+	slice.data = alloc(size, layout)
 
 	return Value{
 		typecode: rtype,

src/runtime/slice.go

@@ -47,7 +47,13 @@ func sliceGrow(oldBuf unsafe.Pointer, oldLen, oldCap, newCap, elemSize uintptr)
 	// memory allocators, this causes some difficult to debug issues.
 	newCap = 1 << bits.Len(uint(newCap))
 
-	buf := alloc(newCap*elemSize, nil)
+	var layout unsafe.Pointer
+	if elemSize == 1 {
+		// []byte
+		layout = gcLayoutNoPtrs
+	}
+
+	buf := alloc(newCap*elemSize, layout)
 	if oldLen > 0 {
 		// copy any data to new slice
 		memmove(buf, oldBuf, oldLen*elemSize)

src/runtime/string.go

@@ -32,6 +32,11 @@ func stringEqual(x, y string) bool {
 	return true
 }
 
+var (
+	// Constants for use with alloc()
+	gcLayoutNoPtrs = unsafe.Pointer(uintptr(0x3))
+)
+
 // Return true iff x < y.
 //
 //go:nobounds
@@ -59,7 +64,7 @@ func stringConcat(x, y _string) _string {
 		return x
 	} else {
 		length := x.length + y.length
-		buf := alloc(length, nil)
+		buf := alloc(length, gcLayoutNoPtrs)
 		memcpy(buf, unsafe.Pointer(x.ptr), x.length)
 		memcpy(unsafe.Add(buf, x.length), unsafe.Pointer(y.ptr), y.length)
 		return _string{ptr: (*byte)(buf), length: length}
@@ -72,7 +77,7 @@ func stringFromBytes(x struct {
 	len uintptr
 	cap uintptr
 }) _string {
-	buf := alloc(x.len, nil)
+	buf := alloc(x.len, gcLayoutNoPtrs)
 	memcpy(buf, unsafe.Pointer(x.ptr), x.len)
 	return _string{ptr: (*byte)(buf), length: x.len}
 }
@@ -83,7 +88,7 @@ func stringToBytes(x _string) (slice struct {
 	len uintptr
 	cap uintptr
 }) {
-	buf := alloc(x.length, nil)
+	buf := alloc(x.length, gcLayoutNoPtrs)
 	memcpy(buf, unsafe.Pointer(x.ptr), x.length)
 	slice.ptr = (*byte)(buf)
 	slice.len = x.length
@@ -100,7 +105,7 @@ func stringFromRunes(runeSlice []rune) (s _string) {
 	}
 
 	// Allocate memory for the string.
-	s.ptr = (*byte)(alloc(s.length, nil))
+	s.ptr = (*byte)(alloc(s.length, gcLayoutNoPtrs))
 
 	// Encode runes to UTF-8 and store the resulting bytes in the string.
 	index := uintptr(0)