reflect: make unsafe use of SliceHeader gc-friendly

Revert workaround in compiler and
revert test for compiler workaround.

Tested that the 386 build continues to fail if
the gc change is made without the reflect change.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5312041

src/cmd/gc/reflect.c

@@ -516,6 +516,7 @@ haspointers(Type *t)
 	case TUINT32:
 	case TINT64:
 	case TUINT64:
+	case TUINTPTR:
 	case TFLOAT32:
 	case TFLOAT64:
 	case TBOOL:
@@ -533,7 +534,6 @@ haspointers(Type *t)
 	case TPTR32:
 	case TPTR64:
 	case TUNSAFEPTR:
-	case TUINTPTR:
 	case TINTER:
 	case TCHAN:
 	case TMAP:

src/pkg/reflect/value.go

@@ -1424,11 +1424,17 @@ func (v Value) Slice(beg, end int) Value {
 		typ = iv.typ.toType()
 		base = (*SliceHeader)(iv.addr).Data
 	}
-	s := new(SliceHeader)
+
+	// Declare slice so that gc can see the base pointer in it.
+	var x []byte
+
+	// Reinterpret as *SliceHeader to edit.
+	s := (*SliceHeader)(unsafe.Pointer(&x))
 	s.Data = base + uintptr(beg)*typ.Elem().Size()
 	s.Len = end - beg
-	s.Cap = cap - beg
-	return valueFromAddr(iv.flag&flagRO, typ, unsafe.Pointer(s))
+	s.Cap = end - beg
+
+	return valueFromAddr(iv.flag&flagRO, typ, unsafe.Pointer(&x))
 }
 
 // String returns the string v's underlying value, as a string.
@@ -1654,12 +1660,17 @@ func MakeSlice(typ Type, len, cap int) Value {
 	if typ.Kind() != Slice {
 		panic("reflect: MakeSlice of non-slice type")
 	}
-	s := &SliceHeader{
-		Data: uintptr(unsafe.NewArray(typ.Elem(), cap)),
-		Len:  len,
-		Cap:  cap,
-	}
-	return valueFromAddr(0, typ, unsafe.Pointer(s))
+
+	// Declare slice so that gc can see the base pointer in it.
+	var x []byte
+
+	// Reinterpret as *SliceHeader to edit.
+	s := (*SliceHeader)(unsafe.Pointer(&x))
+	s.Data = uintptr(unsafe.NewArray(typ.Elem(), cap))
+	s.Len = len
+	s.Cap = cap
+
+	return valueFromAddr(0, typ, unsafe.Pointer(&x))
 }
 
 // MakeChan creates a new channel with the specified type and buffer size.

src/pkg/runtime/gc_test.go

@@ -3,7 +3,6 @@ package runtime_test
 import (
 	"runtime"
 	"testing"
-	"unsafe"
 )
 
 func TestGcSys(t *testing.T) {
@@ -23,73 +22,3 @@ func TestGcSys(t *testing.T) {
 func workthegc() []byte {
 	return make([]byte, 1029)
 }
-
-func TestGcUintptr(t *testing.T) {
-	p1 := unsafe.Pointer(new(int))
-	*(*int)(unsafe.Pointer(p1)) = 42
-	p2 := uintptr(unsafe.Pointer(new(int)))
-	*(*int)(unsafe.Pointer(p2)) = 42
-	var a1 [1]unsafe.Pointer
-	a1[0] = unsafe.Pointer(new(int))
-	*(*int)(unsafe.Pointer(a1[0])) = 42
-	var a2 [1]uintptr
-	a2[0] = uintptr(unsafe.Pointer(new(int)))
-	*(*int)(unsafe.Pointer(a2[0])) = 42
-	s1 := make([]unsafe.Pointer, 1)
-	s1[0] = unsafe.Pointer(new(int))
-	*(*int)(unsafe.Pointer(s1[0])) = 42
-	s2 := make([]uintptr, 1)
-	s2[0] = uintptr(unsafe.Pointer(new(int)))
-	*(*int)(unsafe.Pointer(s2[0])) = 42
-	m1 := make(map[int]unsafe.Pointer)
-	m1[0] = unsafe.Pointer(new(int))
-	*(*int)(unsafe.Pointer(m1[0])) = 42
-	m2 := make(map[int]uintptr)
-	m2[0] = uintptr(unsafe.Pointer(new(int)))
-	*(*int)(unsafe.Pointer(m2[0])) = 42
-	c1 := make(chan unsafe.Pointer, 1)
-	func() {
-		p := new(int)
-		*p = 42
-		c1 <- unsafe.Pointer(p)
-	}()
-	c2 := make(chan uintptr, 1)
-	func() {
-		p := new(int)
-		*p = 42
-		c2 <- uintptr(unsafe.Pointer(p))
-	}()
-
-	runtime.GC()
-
-	if p, _ := runtime.Lookup((*byte)(unsafe.Pointer(p1))); p == nil || *(*int)(unsafe.Pointer(p)) != 42 {
-		t.Fatalf("p1 is freed")
-	}
-	if p, _ := runtime.Lookup((*byte)(unsafe.Pointer(p2))); p == nil || *(*int)(unsafe.Pointer(p)) != 42 {
-		t.Fatalf("p2 is freed")
-	}
-	if p, _ := runtime.Lookup((*byte)(unsafe.Pointer(a1[0]))); p == nil || *(*int)(unsafe.Pointer(p)) != 42 {
-		t.Fatalf("a1[0] is freed")
-	}
-	if p, _ := runtime.Lookup((*byte)(unsafe.Pointer(a2[0]))); p == nil || *(*int)(unsafe.Pointer(p)) != 42 {
-		t.Fatalf("a2[0] is freed")
-	}
-	if p, _ := runtime.Lookup((*byte)(unsafe.Pointer(s1[0]))); p == nil || *(*int)(unsafe.Pointer(p)) != 42 {
-		t.Fatalf("s1[0] is freed")
-	}
-	if p, _ := runtime.Lookup((*byte)(unsafe.Pointer(s2[0]))); p == nil || *(*int)(unsafe.Pointer(p)) != 42 {
-		t.Fatalf("s2[0] is freed")
-	}
-	if p, _ := runtime.Lookup((*byte)(unsafe.Pointer(m1[0]))); p == nil || *(*int)(unsafe.Pointer(p)) != 42 {
-		t.Fatalf("m1[0] is freed")
-	}
-	if p, _ := runtime.Lookup((*byte)(unsafe.Pointer(m2[0]))); p == nil || *(*int)(unsafe.Pointer(p)) != 42 {
-		t.Fatalf("m2[0] is freed")
-	}
-	if p, _ := runtime.Lookup((*byte)(unsafe.Pointer(<-c1))); p == nil || *(*int)(unsafe.Pointer(p)) != 42 {
-		t.Fatalf("<-c1 is freed")
-	}
-	if p, _ := runtime.Lookup((*byte)(unsafe.Pointer(<-c2))); p == nil || *(*int)(unsafe.Pointer(p)) != 42 {
-		t.Fatalf("<-c2 is freed")
-	}
-}