reflect: Remove imprecise techniques from channel/select operations.

Reflect used to communicate to the runtime using interface words,
which is bad for precise GC because sometimes iwords hold a pointer
and sometimes they don't.  This change rewrites channel and select
operations to always pass pointers to the runtime.

reflect.Select gets somewhat more expensive, as we now do an allocation
per receive case instead of one allocation whose size is the max of
all the received types.  This seems unavoidable to get preciseness
(unless we move the allocation into selectgo, which is a much bigger
change).

Fixes #6490

R=golang-codereviews, dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/52900043

src/pkg/reflect/value.go

@@ -212,51 +212,11 @@ func methodName() string {
 // bigger than a pointer, its word is a pointer to v's data.
 // Otherwise, its word holds the data stored
 // in its leading bytes (so is not a pointer).
-// Because the value sometimes holds a pointer, we use
-// unsafe.Pointer to represent it, so that if iword appears
-// in a struct, the garbage collector knows that might be
-// a pointer.
-// TODO: get rid of all occurrences of iword (except in the interface decls below?)
-// We want to get rid of the "feature" that an unsafe.Pointer is sometimes a pointer
-// and sometimes a uintptr.
+// This type is very dangerous for the garbage collector because
+// it must be treated conservatively.  We try to never expose it
+// to the GC here so that GC remains precise.
 type iword unsafe.Pointer
 
-// Get an iword that represents this value.
-// TODO: this function goes away at some point
-func (v Value) iword() iword {
-	t := v.typ
-	if t == nil {
-		return iword(nil)
-	}
-	if v.flag&flagIndir != 0 {
-		if v.typ.size > ptrSize {
-			return iword(v.ptr)
-		}
-		// Have indirect but want direct word.
-		if t.pointers() {
-			return iword(*(*unsafe.Pointer)(v.ptr))
-		}
-		return iword(loadScalar(v.ptr, v.typ.size))
-	}
-	if t.pointers() {
-		return iword(v.ptr)
-	}
-	return iword(v.scalar)
-}
-
-// Build a Value from a type/iword pair, plus any extra flags.
-// TODO: this function goes away at some point
-func fromIword(t *rtype, w iword, fl flag) Value {
-	fl |= flag(t.Kind()) << flagKindShift
-	if t.size > ptrSize {
-		return Value{t, unsafe.Pointer(w), 0, fl | flagIndir}
-	} else if t.pointers() {
-		return Value{t, unsafe.Pointer(w), 0, fl}
-	} else {
-		return Value{t, nil, uintptr(w), fl}
-	}
-}
-
 // loadScalar loads n bytes at p from memory into a uintptr
 // that forms the second word of an interface.  The data
 // must be non-pointer in nature.
@@ -1458,9 +1418,21 @@ func (v Value) recv(nb bool) (val Value, ok bool) {
 	if ChanDir(tt.dir)&RecvDir == 0 {
 		panic("reflect: recv on send-only channel")
 	}
-	word, selected, ok := chanrecv(v.typ, v.pointer(), nb)
-	if selected {
-		val = fromIword(tt.elem, word, 0)
+	t := tt.elem
+	val = Value{t, nil, 0, flag(t.Kind()) << flagKindShift}
+	var p unsafe.Pointer
+	if t.size > ptrSize {
+		p = unsafe_New(t)
+		val.ptr = p
+		val.flag |= flagIndir
+	} else if t.pointers() {
+		p = unsafe.Pointer(&val.ptr)
+	} else {
+		p = unsafe.Pointer(&val.scalar)
+	}
+	selected, ok := chanrecv(v.typ, v.pointer(), nb, p)
+	if !selected {
+		val = Value{}
 	}
 	return
 }
@@ -1483,7 +1455,15 @@ func (v Value) send(x Value, nb bool) (selected bool) {
 	}
 	x.mustBeExported()
 	x = x.assignTo("reflect.Value.Send", tt.elem, nil)
-	return chansend(v.typ, v.pointer(), x.iword(), nb)
+	var p unsafe.Pointer
+	if x.flag&flagIndir != 0 {
+		p = x.ptr
+	} else if x.typ.pointers() {
+		p = unsafe.Pointer(&x.ptr)
+	} else {
+		p = unsafe.Pointer(&x.scalar)
+	}
+	return chansend(v.typ, v.pointer(), p, nb)
 }
 
 // Set assigns x to the value v.
@@ -2049,17 +2029,18 @@ func Copy(dst, src Value) int {
 // A runtimeSelect is a single case passed to rselect.
 // This must match ../runtime/chan.c:/runtimeSelect
 type runtimeSelect struct {
-	dir uintptr // 0, SendDir, or RecvDir
-	typ *rtype  // channel type
-	ch  iword   // interface word for channel
-	val iword   // interface word for value (for SendDir)
+	dir uintptr        // 0, SendDir, or RecvDir
+	typ *rtype         // channel type
+	ch  unsafe.Pointer // channel
+	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
 }
 
-// rselect runs a select. It returns the index of the chosen case,
-// and if the case was a receive, the interface word of the received
-// value and the conventional OK bool to indicate whether the receive
-// corresponds to a sent value.
-func rselect([]runtimeSelect) (chosen int, recv iword, recvOK bool)
+// rselect runs a select.  It returns the index of the chosen case.
+// If the case was a receive, val is filled in with the received value.
+// The conventional OK bool indicates whether the receive corresponds
+// to a sent value.
+//go:noescape
+func rselect([]runtimeSelect) (chosen int, recvOK bool)
 
 // A SelectDir describes the communication direction of a select case.
 type SelectDir int
@@ -2139,7 +2120,7 @@ func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool) {
 			if ChanDir(tt.dir)&SendDir == 0 {
 				panic("reflect.Select: SendDir case using recv-only channel")
 			}
-			rc.ch = ch.iword()
+			rc.ch = ch.pointer()
 			rc.typ = &tt.rtype
 			v := c.Send
 			if !v.IsValid() {
@@ -2147,7 +2128,13 @@ func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool) {
 			}
 			v.mustBeExported()
 			v = v.assignTo("reflect.Select", tt.elem, nil)
-			rc.val = v.iword()
+			if v.flag&flagIndir != 0 {
+				rc.val = v.ptr
+			} else if v.typ.pointers() {
+				rc.val = unsafe.Pointer(&v.ptr)
+			} else {
+				rc.val = unsafe.Pointer(&v.scalar)
+			}
 
 		case SelectRecv:
 			if c.Send.IsValid() {
@@ -2160,18 +2147,28 @@ func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool) {
 			ch.mustBe(Chan)
 			ch.mustBeExported()
 			tt := (*chanType)(unsafe.Pointer(ch.typ))
-			rc.typ = &tt.rtype
 			if ChanDir(tt.dir)&RecvDir == 0 {
 				panic("reflect.Select: RecvDir case using send-only channel")
 			}
-			rc.ch = ch.iword()
+			rc.ch = ch.pointer()
+			rc.typ = &tt.rtype
+			rc.val = unsafe_New(tt.elem)
 		}
 	}
 
-	chosen, word, recvOK := rselect(runcases)
+	chosen, recvOK = rselect(runcases)
 	if runcases[chosen].dir == uintptr(SelectRecv) {
 		tt := (*chanType)(unsafe.Pointer(runcases[chosen].typ))
-		recv = fromIword(tt.elem, word, 0)
+		t := tt.elem
+		p := runcases[chosen].val
+		fl := flag(t.Kind()) << flagKindShift
+		if t.size > ptrSize {
+			recv = Value{t, p, 0, fl | flagIndir}
+		} else if t.pointers() {
+			recv = Value{t, *(*unsafe.Pointer)(p), 0, fl}
+		} else {
+			recv = Value{t, nil, loadScalar(p, t.size), fl}
+		}
 	}
 	return chosen, recv, recvOK
 }
@@ -2624,8 +2621,12 @@ func cvtI2I(v Value, typ Type) Value {
 func chancap(ch unsafe.Pointer) int
 func chanclose(ch unsafe.Pointer)
 func chanlen(ch unsafe.Pointer) int
-func chanrecv(t *rtype, ch unsafe.Pointer, nb bool) (val iword, selected, received bool)
-func chansend(t *rtype, ch unsafe.Pointer, val iword, nb bool) bool
+
+//go:noescape
+func chanrecv(t *rtype, ch unsafe.Pointer, nb bool, val unsafe.Pointer) (selected, received bool)
+
+//go:noescape
+func chansend(t *rtype, ch unsafe.Pointer, val unsafe.Pointer, nb bool) bool
 
 func makechan(typ *rtype, size uint64) (ch unsafe.Pointer)
 func makemap(t *rtype) (m unsafe.Pointer)

src/pkg/runtime/chan.c

@@ -541,18 +541,16 @@ runtime·selectnbrecv2(ChanType *t, byte *v, bool *received, Hchan *c, bool sele
 }
 
 // For reflect:
-//	func chansend(c chan, val iword, nb bool) (selected bool)
-// where an iword is the same word an interface value would use:
-// the actual data if it fits, or else a pointer to the data.
+//	func chansend(c chan, val *any, nb bool) (selected bool)
+// where val points to the data to be sent.
 //
 // The "uintptr selected" is really "bool selected" but saying
 // uintptr gets us the right alignment for the output parameter block.
 #pragma textflag NOSPLIT
 void
-reflect·chansend(ChanType *t, Hchan *c, uintptr val, bool nb, uintptr selected)
+reflect·chansend(ChanType *t, Hchan *c, byte *val, bool nb, uintptr selected)
 {
 	bool *sp;
-	byte *vp;
 
 	if(nb) {
 		selected = false;
@@ -562,21 +560,16 @@ reflect·chansend(ChanType *t, Hchan *c, uintptr val, bool nb, uintptr selected)
 		FLUSH(&selected);
 		sp = nil;
 	}
-	if(t->elem->size <= sizeof(val))
-		vp = (byte*)&val;
-	else
-		vp = (byte*)val;
-	runtime·chansend(t, c, vp, sp, runtime·getcallerpc(&t));
+	runtime·chansend(t, c, val, sp, runtime·getcallerpc(&t));
 }
 
 // For reflect:
-//	func chanrecv(c chan, nb bool) (val iword, selected, received bool)
-// where an iword is the same word an interface value would use:
-// the actual data if it fits, or else a pointer to the data.
+//	func chanrecv(c chan, nb bool, val *any) (selected, received bool)
+// where val points to a data area that will be filled in with the
+// received value.  val must have the size and type of the channel element type.
 void
-reflect·chanrecv(ChanType *t, Hchan *c, bool nb, uintptr val, bool selected, bool received)
+reflect·chanrecv(ChanType *t, Hchan *c, bool nb, byte *val, bool selected, bool received)
 {
-	byte *vp;
 	bool *sp;
 
 	if(nb) {
@@ -589,15 +582,7 @@ reflect·chanrecv(ChanType *t, Hchan *c, bool nb, uintptr val, bool selected, bo
 	}
 	received = false;
 	FLUSH(&received);
-	if(t->elem->size <= sizeof(val)) {
-		val = 0;
-		vp = (byte*)&val;
-	} else {
-		vp = runtime·mal(t->elem->size);
-		val = (uintptr)vp;
-		FLUSH(&val);
-	}
-	runtime·chanrecv(t, c, vp, sp, &received);
+	runtime·chanrecv(t, c, val, sp, &received);
 }
 
 static void newselect(int32, Select**);
@@ -1150,7 +1135,7 @@ struct runtimeSelect
 	uintptr dir;
 	ChanType *typ;
 	Hchan *ch;
-	uintptr val;
+	byte *val;
 };
 
 // This enum must match ../reflect/value.go:/SelectDir.
@@ -1160,32 +1145,18 @@ enum SelectDir {
 	SelectDefault,
 };
 
-// func rselect(cases []runtimeSelect) (chosen int, word uintptr, recvOK bool)
+// func rselect(cases []runtimeSelect) (chosen int, recvOK bool)
 void
-reflect·rselect(Slice cases, intgo chosen, uintptr word, bool recvOK)
+reflect·rselect(Slice cases, intgo chosen, bool recvOK)
 {
 	int32 i;
 	Select *sel;
 	runtimeSelect* rcase, *rc;
-	void *elem;
-	void *recvptr;
-	uintptr maxsize;
 
 	chosen = -1;
-	word = 0;
 	recvOK = false;
 
-	maxsize = 0;
 	rcase = (runtimeSelect*)cases.array;
-	for(i=0; i<cases.len; i++) {
-		rc = &rcase[i];
-		if(rc->dir == SelectRecv && rc->ch != nil && maxsize < rc->typ->elem->size)
-			maxsize = rc->typ->elem->size;
-	}
-
-	recvptr = nil;
-	if(maxsize > sizeof(void*))
-		recvptr = runtime·mal(maxsize);
 
 	newselect(cases.len, &sel);
 	for(i=0; i<cases.len; i++) {
@@ -1197,30 +1168,19 @@ reflect·rselect(Slice cases, intgo chosen, uintptr word, bool recvOK)
 		case SelectSend:
 			if(rc->ch == nil)
 				break;
-			if(rc->typ->elem->size > sizeof(void*))
-				elem = (void*)rc->val;
-			else
-				elem = (void*)&rc->val;
-			selectsend(sel, rc->ch, (void*)i, elem, 0);
+			selectsend(sel, rc->ch, (void*)i, rc->val, 0);
 			break;
 		case SelectRecv:
 			if(rc->ch == nil)
 				break;
-			if(rc->typ->elem->size > sizeof(void*))
-				elem = recvptr;
-			else
-				elem = &word;
-			selectrecv(sel, rc->ch, (void*)i, elem, &recvOK, 0);
+			selectrecv(sel, rc->ch, (void*)i, rc->val, &recvOK, 0);
 			break;
 		}
 	}
 
 	chosen = (intgo)(uintptr)selectgo(&sel);
-	if(rcase[chosen].dir == SelectRecv && rcase[chosen].typ->elem->size > sizeof(void*))
-		word = (uintptr)recvptr;
 
 	FLUSH(&chosen);
-	FLUSH(&word);
 	FLUSH(&recvOK);
 }