runtime: add gc layout info for some basic types

loader/goroot.go

@@ -245,6 +245,7 @@ func pathsToOverride(goMinor int, needsSyscallPackage bool) map[string]bool {
 		"internal/cm/":                false,
 		"internal/fuzz/":              false,
 		"internal/reflectlite/":       false,
+		"internal/gclayout":           false,
 		"internal/task/":              false,
 		"internal/wasi/":              false,
 		"machine/":                    false,

src/internal/gclayout/gclayout.go

@@ -0,0 +1,33 @@
+package gclayout
+
+import "unsafe"
+
+// Internal constants for gc layout
+// See runtime/gc_precise.go
+
+var (
+	NoPtrs  unsafe.Pointer
+	Pointer unsafe.Pointer
+	String  unsafe.Pointer
+	Slice   unsafe.Pointer
+)
+
+func init() {
+	var sizeBits uintptr
+
+	switch unsafe.Sizeof(uintptr(0)) {
+	case 8:
+		sizeBits = 6
+	case 4:
+		sizeBits = 5
+	case 2:
+		sizeBits = 4
+	}
+
+	var sizeShift = sizeBits + 1
+
+	NoPtrs = unsafe.Pointer(uintptr(0b0<<sizeShift) | uintptr(0b1<<1) | uintptr(1))
+	Pointer = unsafe.Pointer(uintptr(0b1<<sizeShift) | uintptr(0b1<<1) | uintptr(1))
+	String = unsafe.Pointer(uintptr(0b01<<sizeShift) | uintptr(0b10<<1) | uintptr(1))
+	Slice = unsafe.Pointer(uintptr(0b001<<sizeShift) | uintptr(0b11<<1) | uintptr(1))
+}

src/reflect/type.go

@@ -64,6 +64,7 @@
 package reflect
 
 import (
+	"internal/gclayout"
 	"internal/itoa"
 	"unsafe"
 )
@@ -961,6 +962,26 @@ func (t *rawType) Align() int {
 	}
 }
 
+func (r *rawType) gcLayout() unsafe.Pointer {
+	kind := r.Kind()
+
+	if kind < String {
+		return gclayout.NoPtrs
+	}
+
+	switch kind {
+	case Pointer, UnsafePointer, Chan, Map:
+		return gclayout.Pointer
+	case String:
+		return gclayout.String
+	case Slice:
+		return gclayout.Slice
+	}
+
+	// Unknown (for now); let the conservative pointer scanning handle it
+	return nil
+}
+
 // FieldAlign returns the alignment if this type is used in a struct field. It
 // is currently an alias for Align() but this might change in the future.
 func (t *rawType) FieldAlign() int {

src/reflect/value.go

@@ -1479,7 +1479,8 @@ func MakeSlice(typ Type, len, cap int) Value {
 	ulen := uint(len)
 	ucap := uint(cap)
 	maxSize := (^uintptr(0)) / 2
-	elementSize := rtype.elem().Size()
+	elem := rtype.elem()
+	elementSize := elem.Size()
 	if elementSize > 1 {
 		maxSize /= uintptr(elementSize)
 	}
@@ -1493,7 +1494,9 @@ 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 := elem.gcLayout()
+
+	slice.data = alloc(size, layout)
 
 	return Value{
 		typecode: rtype,

src/runtime/slice.go

@@ -3,6 +3,7 @@ package runtime
 // This file implements compiler builtins for slices: append() and copy().
 
 import (
+	"internal/gclayout"
 	"math/bits"
 	"unsafe"
 )
@@ -47,7 +48,13 @@ 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
+	// less type info here; can only go off element size
+	if elemSize < unsafe.Sizeof(uintptr(0)) {
+		layout = gclayout.NoPtrs
+	}
+
+	buf := alloc(newCap*elemSize, layout)
 	if oldLen > 0 {
 		// copy any data to new slice
 		memmove(buf, oldBuf, oldLen*elemSize)

src/runtime/string.go

@@ -3,6 +3,7 @@ package runtime
 // This file implements functions related to Go strings.
 
 import (
+	"internal/gclayout"
 	"unsafe"
 )
 
@@ -59,7 +60,7 @@ func stringConcat(x, y _string) _string {
 		return x
 	} else {
 		length := x.length + y.length
-		buf := alloc(length, nil)
+		buf := alloc(length, gclayout.NoPtrs)
 		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 +73,7 @@ func stringFromBytes(x struct {
 	len uintptr
 	cap uintptr
 }) _string {
-	buf := alloc(x.len, nil)
+	buf := alloc(x.len, gclayout.NoPtrs)
 	memcpy(buf, unsafe.Pointer(x.ptr), x.len)
 	return _string{ptr: (*byte)(buf), length: x.len}
 }
@@ -83,7 +84,7 @@ func stringToBytes(x _string) (slice struct {
 	len uintptr
 	cap uintptr
 }) {
-	buf := alloc(x.length, nil)
+	buf := alloc(x.length, gclayout.NoPtrs)
 	memcpy(buf, unsafe.Pointer(x.ptr), x.length)
 	slice.ptr = (*byte)(buf)
 	slice.len = x.length
@@ -100,7 +101,7 @@ func stringFromRunes(runeSlice []rune) (s _string) {
 	}
 
 	// Allocate memory for the string.
-	s.ptr = (*byte)(alloc(s.length, nil))
+	s.ptr = (*byte)(alloc(s.length, gclayout.NoPtrs))
 
 	// Encode runes to UTF-8 and store the resulting bytes in the string.
 	index := uintptr(0)