cmd/compile,runtime: redo how map assignments work

To compile:
  m[k] = v
instead of:
  mapassign(maptype, m, &k, &v), do
do:
  *mapassign(maptype, m, &k) = v

mapassign returns a pointer to the value slot in the map.  It is just
like mapaccess except that it will allocate a new slot if k is not
already present in the map.

This makes map accesses faster but potentially larger (codewise).

It is faster because the write into the map is done when the compiler
knows the concrete type, so it can be done with a few store
instructions instead of calling typedmemmove.  We also potentially
avoid stack temporaries to hold v.

The code can be larger when the map has pointers in its value type,
since there is a write barrier call in addition to the mapassign call.
That makes the code at the callsite a bit bigger (go binary is 0.3%
bigger).

This CL is in preparation for doing operations like m[k] += v with
only a single runtime call.  That will roughly double the speed of
such operations.

Update #17133
Update #5147

Change-Id: Ia435f032090a2ed905dac9234e693972fe8c2dc5
Reviewed-on: https://go-review.googlesource.com/30815
Run-TryBot: Keith Randall <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: randall77

src/cmd/compile/internal/gc/builtin.go

@@ -57,52 +57,52 @@ const runtimeimport = "" +
 	"ast64\x00\x06\x17\"\xc2\x01\x00\x00\x1d::\xc4\x01\x00\x00:\xc6\x01\x00\x00\x04\x17:\xb4\x01\x00\x00\x00\xc8\x01\x00\x00\t#m" +
 	"apaccess2_faststr\x00\x06\x17\"\xc2\x01\x00\x00\x1d::\xc4\x01\x00\x00:\xc6\x01\x00\x00\x04\x17:" +
 	"\xb4\x01\x00\x00\x00\xc8\x01\x00\x00\t\x1bmapaccess2_fat\x00\b\x17\"\xc2\x01\x00\x00\x1d::\xc4\x01\x00\x00" +
-	"\x17:\xc6\x01\x00\x00\x17\"\rzero·6\x00\x00\x04\x17:\xb4\x01\x00\x00\x00\xc8\x01\x00\x00\t\x13mapassig" +
-	"n1\x00\b\x17\"\x13mapType·1\x00\x00\x1d::\rhmap·2\x00\x00\x17:\vkey·" +
-	"3\x00\x00\x17:\vval·4\x00\x00\x00\t\x15mapiterinit\x00\x06\x17\"\xd6\x01\x00\x00\x1d::\xd8" +
-	"\x01\x00\x00\x17:\x0fhiter·3\x00\x00\x00\t\x11mapdelete\x00\x06\x17\"\xd6\x01\x00\x00\x1d::\xd8" +
-	"\x01\x00\x00\x17:\xda\x01\x00\x00\x00\t\x15mapiternext\x00\x02\x17:\x0fhiter·1\x00\x00\x00\t" +
-	"\x0fmakechan\x00\x04\x17\"\x15chanType·2\x00\x00\n\xa6\x01\x00\x00\x02\x1f\x06:\x0fhch" +
-	"an·1\x00\x00\t\x11chanrecv1\x00\x06\x17\"\x15chanType·1\x00\x00\x1f\x02:\x0f" +
-	"hchan·2\x00\x00\x17:j\x00\x00\x00\t\x11chanrecv2\x00\x06\x17\"\xea\x01\x00\x00\x1f\x02:\x0fh" +
-	"chan·3\x00\x00\x17:\relem·4\x00\x00\x01\x00\x00\t\x11chansend1\x00\x06\x17\"\xf0" +
-	"\x01\x00\x00\x1f\x04:\xf2\x01\x00\x00\x17:j\x00\x00\x00\t\x11closechan\x00\x02:\xec\x01\x00\x00\x00\a\x17wri" +
-	"teBarrier\x00\x15\x06\renabled\x00\x00\x00\vneeded\x00\x00\x00\x05cgo\x00\x00\x00" +
-	"\t\x1dwritebarrierptr\x00\x04\x17:\vdst·1\x00\x00:\vsrc·2\x00\x00" +
-	"\x00\t\x17typedmemmove\x00\x06\x17\"t\x00\x00\x17:\vdst·2\x00\x00\x17:\vsrc\xc2" +
-	"\xb73\x00\x00\x00\t\x1btypedslicecopy\x00\x06\x17\"\x06\x00\x00:\vdst·3\x00\x00:\v" +
-	"src·4\x00\x00\x01\x02\x00\t\x17selectnbsend\x00\x06\x17\"\xea\x01\x00\x00\x1f\x04:\xf6\x01\x00\x00" +
-	"\x17:\xf8\x01\x00\x00\x01\x00\x00\t\x17selectnbrecv\x00\x06\x17\"\xea\x01\x00\x00\x17:j\x00\x00\x1f\x02:\x0f" +
-	"hchan·4\x00\x00\x01\x00\x00\t\x19selectnbrecv2\x00\b\x17\"\xea\x01\x00\x00\x17:j\x00" +
-	"\x00\x17\x00\x15received·4\x00\x00\x1f\x02:\x0fhchan·5\x00\x00\x01\x00\x00\t\x11news" +
-	"elect\x00\x06\x17\"\vsel·1\x00\x00\n\x13selsize·2\x00\x00\b\rsize·" +
-	"3\x00\x00\x00\t\x13selectsend\x00\x06\x17\"\vsel·2\x00\x00\x1f\x04:\xf6\x01\x00\x00\x17:\xf8\x01" +
-	"\x00\x00\x02\x00\x15selected·1\x00\x00\t\x13selectrecv\x00\x06\x17\"\xae\x02\x00\x00\x1f\x02" +
-	":\xf6\x01\x00\x00\x17:\xf8\x01\x00\x00\x02\x00\xb0\x02\x00\x00\t\x15selectrecv2\x00\b\x17\"\xae\x02\x00\x00\x1f\x02" +
-	":\xf6\x01\x00\x00\x17:\xf8\x01\x00\x00\x17\x00\x15received·5\x00\x00\x02\x00\xb0\x02\x00\x00\t\x19selec" +
-	"tdefault\x00\x02\x17\"\xae\x02\x00\x00\x02\x00\xb0\x02\x00\x00\t\x0fselectgo\x00\x02\x17\"\xa6\x02\x00\x00" +
-	"\x00\t\tblock\x00\x00\x00\t\x11makeslice\x00\x06\x17\"\x06\x00\x00\x02\vlen·3\x00\x00\x02" +
-	"\vcap·4\x00\x00\x02\x11:\vary·1\x00\x00\t\x15makeslice64\x00\x06\x17\"\x06\x00" +
-	"\x00\n\xc0\x02\x00\x00\n\xc2\x02\x00\x00\x02\x11:\xc4\x02\x00\x00\t\x11growslice\x00\x06\x17\"\x06\x00\x00\x11:\vo" +
-	"ld·3\x00\x00\x02\xc2\x02\x00\x00\x02\x11:\xc4\x02\x00\x00\t\rmemmove\x00\x06\x17:\tto·1\x00\x00" +
-	"\x17:\vfrm·2\x00\x00\x16\x11length·3\x00^\x00\t\vmemclr\x00\x04\x17\"\vpt" +
-	"r·1\x00\x00\x16\x11length·2\x00^\x00\t\x0fmemequal\x00\x06\x17:\ax·2\x00" +
-	"\x00\x17:\ay·3\x00\x00\x16\rsize·4\x00^\x01\x00\x00\t\x11memequal8\x00\x04\x17:\xdc" +
-	"\x02\x00\x00\x17:\xde\x02\x00\x00\x01\x00\x00\t\x13memequal16\x00\x04\x17:\xdc\x02\x00\x00\x17:\xde\x02\x00\x00\x01\x00" +
-	"\x00\t\x13memequal32\x00\x04\x17:\xdc\x02\x00\x00\x17:\xde\x02\x00\x00\x01\x00\x00\t\x13memequal" +
-	"64\x00\x04\x17:\xdc\x02\x00\x00\x17:\xde\x02\x00\x00\x01\x00\x00\t\x15memequal128\x00\x04\x17:\xdc\x02\x00\x00" +
-	"\x17:\xde\x02\x00\x00\x01\x00\x00\t\x0fint64div\x00\x03\n\x00\n\x00\x01\n\x00\t\x11uint64div\x00" +
-	"\x03\x14\x00\x14\x00\x01\x14\x00\t\x0fint64mod\x00\x03\n\x00\n\x00\x01\n\x00\t\x11uint64mod\x00\x03" +
-	"\x14\x00\x14\x00\x01\x14\x00\t\x1bfloat64toint64\x00\x01\x1a\x00\x01\n\x00\t\x1dfloat64t" +
-	"ouint64\x00\x01\x1a\x00\x01\x14\x00\t\x1dfloat64touint32\x00\x01\x1a\x00\x01\x12\x00\t\x1b" +
-	"int64tofloat64\x00\x01\n\x00\x01\x1a\x00\t\x1duint64tofloat64\x00\x01" +
-	"\x14\x00\x01\x1a\x00\t\x1duint32tofloat64\x00\x01\x12\x00\x01\x1a\x00\t\x19complex12" +
-	"8div\x00\x04\x1e\vnum·2\x00\x00\x1e\vden·3\x00\x00\x02\x1e\vquo·1\x00\x00\t\x19r" +
-	"acefuncenter\x00\x01\x16^\x00\t\x17racefuncexit\x00\x00\x00\t\x0frace" +
-	"read\x00\x01\x16^\x00\t\x11racewrite\x00\x01\x16^\x00\t\x19racereadrange" +
-	"\x00\x04\x16\raddr·1\x00^\x16\rsize·2\x00^\x00\t\x1bracewriterang" +
-	"e\x00\x04\x16\x92\x03\x00^\x16\x94\x03\x00^\x00\t\x0fmsanread\x00\x04\x16\x92\x03\x00^\x16\x94\x03\x00^\x00\t\x11m" +
-	"sanwrite\x00\x04\x16\x92\x03\x00^\x16\x94\x03\x00^\x00\v\xf6\x01\v\x00\x01\x00\n$$\n"
+	"\x17:\xc6\x01\x00\x00\x17\"\rzero·6\x00\x00\x04\x17:\xb4\x01\x00\x00\x00\xc8\x01\x00\x00\t\x11mapassig" +
+	"n\x00\x06\x17\"\xa4\x01\x00\x00\x1d::\xb0\x01\x00\x00\x17:\xb2\x01\x00\x00\x02\x17:\xb4\x01\x00\x00\t\x15mapiterin" +
+	"it\x00\x06\x17\"\x13mapType·1\x00\x00\x1d::\rhmap·2\x00\x00\x17:\x0fhiter" +
+	"·3\x00\x00\x00\t\x11mapdelete\x00\x06\x17\"\xd8\x01\x00\x00\x1d::\xda\x01\x00\x00\x17:\vkey·" +
+	"3\x00\x00\x00\t\x15mapiternext\x00\x02\x17:\x0fhiter·1\x00\x00\x00\t\x0fmakec" +
+	"han\x00\x04\x17\"\x15chanType·2\x00\x00\n\xa6\x01\x00\x00\x02\x1f\x06:\x0fhchan·1\x00" +
+	"\x00\t\x11chanrecv1\x00\x06\x17\"\x15chanType·1\x00\x00\x1f\x02:\x0fhchan\xc2" +
+	"\xb72\x00\x00\x17:j\x00\x00\x00\t\x11chanrecv2\x00\x06\x17\"\xe8\x01\x00\x00\x1f\x02:\x0fhchan·" +
+	"3\x00\x00\x17:\relem·4\x00\x00\x01\x00\x00\t\x11chansend1\x00\x06\x17\"\xee\x01\x00\x00\x1f\x04:" +
+	"\xf0\x01\x00\x00\x17:j\x00\x00\x00\t\x11closechan\x00\x02:\xea\x01\x00\x00\x00\a\x17writeBarr" +
+	"ier\x00\x15\x06\renabled\x00\x00\x00\vneeded\x00\x00\x00\x05cgo\x00\x00\x00\t\x1dwrit" +
+	"ebarrierptr\x00\x04\x17:\vdst·1\x00\x00:\vsrc·2\x00\x00\x00\t\x17typ" +
+	"edmemmove\x00\x06\x17\"t\x00\x00\x17:\vdst·2\x00\x00\x17:\vsrc·3\x00\x00\x00\t" +
+	"\x1btypedslicecopy\x00\x06\x17\"\x06\x00\x00:\vdst·3\x00\x00:\vsrc·4" +
+	"\x00\x00\x01\x02\x00\t\x17selectnbsend\x00\x06\x17\"\xe8\x01\x00\x00\x1f\x04:\xf4\x01\x00\x00\x17:\xf6\x01\x00\x00" +
+	"\x01\x00\x00\t\x17selectnbrecv\x00\x06\x17\"\xe8\x01\x00\x00\x17:j\x00\x00\x1f\x02:\x0fhchan\xc2" +
+	"\xb74\x00\x00\x01\x00\x00\t\x19selectnbrecv2\x00\b\x17\"\xe8\x01\x00\x00\x17:j\x00\x00\x17\x00\x15re" +
+	"ceived·4\x00\x00\x1f\x02:\x0fhchan·5\x00\x00\x01\x00\x00\t\x11newselect\x00" +
+	"\x06\x17\"\vsel·1\x00\x00\n\x13selsize·2\x00\x00\b\rsize·3\x00\x00\x00\t\x13" +
+	"selectsend\x00\x06\x17\"\vsel·2\x00\x00\x1f\x04:\xf4\x01\x00\x00\x17:\xf6\x01\x00\x00\x02\x00\x15s" +
+	"elected·1\x00\x00\t\x13selectrecv\x00\x06\x17\"\xac\x02\x00\x00\x1f\x02:\xf4\x01\x00\x00\x17" +
+	":\xf6\x01\x00\x00\x02\x00\xae\x02\x00\x00\t\x15selectrecv2\x00\b\x17\"\xac\x02\x00\x00\x1f\x02:\xf4\x01\x00\x00\x17" +
+	":\xf6\x01\x00\x00\x17\x00\x15received·5\x00\x00\x02\x00\xae\x02\x00\x00\t\x19selectdefau" +
+	"lt\x00\x02\x17\"\xac\x02\x00\x00\x02\x00\xae\x02\x00\x00\t\x0fselectgo\x00\x02\x17\"\xa4\x02\x00\x00\x00\t\tblo" +
+	"ck\x00\x00\x00\t\x11makeslice\x00\x06\x17\"\x06\x00\x00\x02\vlen·3\x00\x00\x02\vcap·" +
+	"4\x00\x00\x02\x11:\vary·1\x00\x00\t\x15makeslice64\x00\x06\x17\"\x06\x00\x00\n\xbe\x02\x00\x00" +
+	"\n\xc0\x02\x00\x00\x02\x11:\xc2\x02\x00\x00\t\x11growslice\x00\x06\x17\"\x06\x00\x00\x11:\vold·3\x00" +
+	"\x00\x02\xc0\x02\x00\x00\x02\x11:\xc2\x02\x00\x00\t\rmemmove\x00\x06\x17:\tto·1\x00\x00\x17:\vfrm" +
+	"·2\x00\x00\x16\x11length·3\x00^\x00\t\vmemclr\x00\x04\x17\"\vptr·1\x00\x00" +
+	"\x16\x11length·2\x00^\x00\t\x0fmemequal\x00\x06\x17:\ax·2\x00\x00\x17:\ay\xc2" +
+	"\xb73\x00\x00\x16\rsize·4\x00^\x01\x00\x00\t\x11memequal8\x00\x04\x17:\xda\x02\x00\x00\x17:\xdc" +
+	"\x02\x00\x00\x01\x00\x00\t\x13memequal16\x00\x04\x17:\xda\x02\x00\x00\x17:\xdc\x02\x00\x00\x01\x00\x00\t\x13mem" +
+	"equal32\x00\x04\x17:\xda\x02\x00\x00\x17:\xdc\x02\x00\x00\x01\x00\x00\t\x13memequal64\x00\x04\x17:" +
+	"\xda\x02\x00\x00\x17:\xdc\x02\x00\x00\x01\x00\x00\t\x15memequal128\x00\x04\x17:\xda\x02\x00\x00\x17:\xdc\x02\x00\x00" +
+	"\x01\x00\x00\t\x0fint64div\x00\x03\n\x00\n\x00\x01\n\x00\t\x11uint64div\x00\x03\x14\x00\x14\x00\x01" +
+	"\x14\x00\t\x0fint64mod\x00\x03\n\x00\n\x00\x01\n\x00\t\x11uint64mod\x00\x03\x14\x00\x14\x00\x01\x14" +
+	"\x00\t\x1bfloat64toint64\x00\x01\x1a\x00\x01\n\x00\t\x1dfloat64touint6" +
+	"4\x00\x01\x1a\x00\x01\x14\x00\t\x1dfloat64touint32\x00\x01\x1a\x00\x01\x12\x00\t\x1bint64t" +
+	"ofloat64\x00\x01\n\x00\x01\x1a\x00\t\x1duint64tofloat64\x00\x01\x14\x00\x01\x1a\x00\t" +
+	"\x1duint32tofloat64\x00\x01\x12\x00\x01\x1a\x00\t\x19complex128div\x00\x04" +
+	"\x1e\vnum·2\x00\x00\x1e\vden·3\x00\x00\x02\x1e\vquo·1\x00\x00\t\x19racefun" +
+	"center\x00\x01\x16^\x00\t\x17racefuncexit\x00\x00\x00\t\x0fraceread\x00\x01" +
+	"\x16^\x00\t\x11racewrite\x00\x01\x16^\x00\t\x19racereadrange\x00\x04\x16\rad" +
+	"dr·1\x00^\x16\rsize·2\x00^\x00\t\x1bracewriterange\x00\x04\x16\x90\x03" +
+	"\x00^\x16\x92\x03\x00^\x00\t\x0fmsanread\x00\x04\x16\x90\x03\x00^\x16\x92\x03\x00^\x00\t\x11msanwri" +
+	"te\x00\x04\x16\x90\x03\x00^\x16\x92\x03\x00^\x00\v\xf6\x01\v\x00\x01\x00\n$$\n"
 
 const unsafeimport = "" +
 	"version 2\n\n\x00\x00\x01\vunsafe\x00\t\x0fOffsetof\x00\x01:\x00\x01\x16\x00\t" +

src/cmd/compile/internal/gc/builtin/runtime.go

@@ -93,7 +93,7 @@ func mapaccess2_fast32(mapType *byte, hmap map[any]any, key any) (val *any, pres
 func mapaccess2_fast64(mapType *byte, hmap map[any]any, key any) (val *any, pres bool)
 func mapaccess2_faststr(mapType *byte, hmap map[any]any, key any) (val *any, pres bool)
 func mapaccess2_fat(mapType *byte, hmap map[any]any, key *any, zero *byte) (val *any, pres bool)
-func mapassign1(mapType *byte, hmap map[any]any, key *any, val *any)
+func mapassign(mapType *byte, hmap map[any]any, key *any) (val *any)
 func mapiterinit(mapType *byte, hmap map[any]any, hiter *any)
 func mapdelete(mapType *byte, hmap map[any]any, key *any)
 func mapiternext(hiter *any)

src/cmd/compile/internal/gc/order.go

@@ -188,9 +188,9 @@ func isaddrokay(n *Node) bool {
 	return islvalue(n) && (n.Op != ONAME || n.Class == PEXTERN || istemp(n))
 }
 
-// Orderaddrtemp ensures that *np is okay to pass by address to runtime routines.
-// If the original argument *np is not okay, orderaddrtemp creates a tmp, emits
-// tmp = *np, and then sets *np to the tmp variable.
+// Orderaddrtemp ensures that n is okay to pass by address to runtime routines.
+// If the original argument n is not okay, orderaddrtemp creates a tmp, emits
+// tmp = n, and then returns tmp.
 func orderaddrtemp(n *Node, order *Order) *Node {
 	if isaddrokay(n) {
 		return n
@@ -395,13 +395,8 @@ func ordercall(n *Node, order *Order) {
 }
 
 // Ordermapassign appends n to order->out, introducing temporaries
-// to make sure that all map assignments have the form m[k] = x,
-// where x is addressable.
-// (Orderexpr has already been called on n, so we know k is addressable.)
-//
-// If n is m[k] = x where x is not addressable, the rewrite is:
-//	tmp = x
-//	m[k] = tmp
+// to make sure that all map assignments have the form m[k] = x.
+// (Note: orderexpr has already been called on n, so we know k is addressable.)
 //
 // If n is the multiple assignment form ..., m[k], ... = ..., the rewrite is
 //	t1 = m
@@ -428,7 +423,7 @@ func ordermapassign(n *Node, order *Order) {
 		// We call writebarrierfat only for values > 4 pointers long. See walk.go.
 		// TODO(mdempsky): writebarrierfat doesn't exist anymore, but removing that
 		// logic causes net/http's tests to become flaky; see CL 21242.
-		if (n.Left.Op == OINDEXMAP || (needwritebarrier(n.Left, n.Right) && n.Left.Type.Width > int64(4*Widthptr))) && !isaddrokay(n.Right) {
+		if needwritebarrier(n.Left, n.Right) && n.Left.Type.Width > int64(4*Widthptr) && !isaddrokay(n.Right) {
 			m := n.Left
 			n.Left = ordertemp(m.Type, order, false)
 			a := nod(OAS, m, n.Left)
@@ -1061,8 +1056,14 @@ func orderexpr(n *Node, order *Order, lhs *Node) *Node {
 		// key must be addressable
 	case OINDEXMAP:
 		n.Left = orderexpr(n.Left, order, nil)
-
 		n.Right = orderexpr(n.Right, order, nil)
+		needCopy := false
+
+		if n.Etype == 0 && instrumenting {
+			// Race detector needs the copy so it can
+			// call treecopy on the result.
+			needCopy = true
+		}
 
 		// For x = m[string(k)] where k is []byte, the allocation of
 		// backing bytes for the string can be avoided by reusing
@@ -1076,12 +1077,13 @@ func orderexpr(n *Node, order *Order, lhs *Node) *Node {
 		// conversion (by the ordercopyexpr a few lines below).
 		if n.Etype == 0 && n.Right.Op == OARRAYBYTESTR {
 			n.Right.Op = OARRAYBYTESTRTMP
+			needCopy = true
 		}
 
+		// Map calls need to take the address of the key.
 		n.Right = orderaddrtemp(n.Right, order)
-		if n.Etype == 0 {
-			// use of value (not being assigned);
-			// make copy in temporary.
+
+		if needCopy {
 			n = ordercopyexpr(n, n.Type, order, 0)
 		}
 

src/cmd/compile/internal/gc/racewalk.go

@@ -474,8 +474,8 @@ func isartificial(n *Node) bool {
 func callinstr(np **Node, init *Nodes, wr int, skip int) bool {
 	n := *np
 
-	//print("callinstr for %+N [ %O ] etype=%E class=%d\n",
-	//	  n, n->op, n->type ? n->type->etype : -1, n->class);
+	//fmt.Printf("callinstr for %v [ %v ] etype=%v class=%v\n",
+	//	n, n.Op, n.Type.Etype, n.Class)
 
 	if skip != 0 || n.Type == nil || n.Type.Etype >= TIDEAL {
 		return false

src/cmd/compile/internal/gc/syntax.go

@@ -52,7 +52,7 @@ type Node struct {
 	Op        Op
 	Ullman    uint8 // sethi/ullman number
 	Addable   bool  // addressable
-	Etype     EType // op for OASOP, etype for OTYPE, exclam for export, 6g saved reg, ChanDir for OTCHAN
+	Etype     EType // op for OASOP, etype for OTYPE, exclam for export, 6g saved reg, ChanDir for OTCHAN, for OINDEXMAP 1=LHS,0=RHS
 	Bounded   bool  // bounds check unnecessary
 	NonNil    bool  // guaranteed to be non-nil
 	Class     Class // PPARAM, PAUTO, PEXTERN, etc

src/cmd/compile/internal/gc/walk.go

@@ -1292,44 +1292,48 @@ opswitch:
 		}
 
 	case OINDEXMAP:
-		if n.Etype == 1 {
-			break
-		}
+		// Replace m[k] with *map{access1,assign}(maptype, m, &k)
 		n.Left = walkexpr(n.Left, init)
 		n.Right = walkexpr(n.Right, init)
-
-		t := n.Left.Type
-		p := ""
-		if t.Val().Width <= 128 { // Check ../../runtime/hashmap.go:maxValueSize before changing.
-			switch algtype(t.Key()) {
-			case AMEM32:
-				p = "mapaccess1_fast32"
-			case AMEM64:
-				p = "mapaccess1_fast64"
-			case ASTRING:
-				p = "mapaccess1_faststr"
+		map_ := n.Left
+		key := n.Right
+		t := map_.Type
+		if n.Etype == 1 {
+			// This m[k] expression is on the left-hand side of an assignment.
+			// orderexpr made sure key is addressable.
+			key = nod(OADDR, key, nil)
+			n = mkcall1(mapfn("mapassign", t), nil, init, typename(t), map_, key)
+		} else {
+			// m[k] is not the target of an assignment.
+			p := ""
+			if t.Val().Width <= 128 { // Check ../../runtime/hashmap.go:maxValueSize before changing.
+				switch algtype(t.Key()) {
+				case AMEM32:
+					p = "mapaccess1_fast32"
+				case AMEM64:
+					p = "mapaccess1_fast64"
+				case ASTRING:
+					p = "mapaccess1_faststr"
+				}
 			}
-		}
 
-		var key *Node
-		if p != "" {
-			// fast versions take key by value
-			key = n.Right
-		} else {
-			// standard version takes key by reference.
-			// orderexpr made sure key is addressable.
-			key = nod(OADDR, n.Right, nil)
-			p = "mapaccess1"
-		}
+			if p == "" {
+				// standard version takes key by reference.
+				// orderexpr made sure key is addressable.
+				key = nod(OADDR, key, nil)
+				p = "mapaccess1"
+			}
 
-		if w := t.Val().Width; w <= 1024 { // 1024 must match ../../../../runtime/hashmap.go:maxZero
-			n = mkcall1(mapfn(p, t), ptrto(t.Val()), init, typename(t), n.Left, key)
-		} else {
-			p = "mapaccess1_fat"
-			z := zeroaddr(w)
-			n = mkcall1(mapfn(p, t), ptrto(t.Val()), init, typename(t), n.Left, key, z)
+			if w := t.Val().Width; w <= 1024 { // 1024 must match ../../../../runtime/hashmap.go:maxZero
+				n = mkcall1(mapfn(p, t), ptrto(t.Val()), init, typename(t), map_, key)
+			} else {
+				p = "mapaccess1_fat"
+				z := zeroaddr(w)
+				n = mkcall1(mapfn(p, t), ptrto(t.Val()), init, typename(t), map_, key, z)
+			}
 		}
-		n.NonNil = true // mapaccess always returns a non-nil pointer
+		n.Type = ptrto(t.Val())
+		n.NonNil = true // mapaccess1* and mapassign always return non-nil pointers.
 		n = nod(OIND, n, nil)
 		n.Type = t.Val()
 		n.Typecheck = 1
@@ -2306,22 +2310,6 @@ func convas(n *Node, init *Nodes) *Node {
 		goto out
 	}
 
-	if n.Left.Op == OINDEXMAP {
-		map_ := n.Left.Left
-		key := n.Left.Right
-		val := n.Right
-		map_ = walkexpr(map_, init)
-		key = walkexpr(key, init)
-		val = walkexpr(val, init)
-
-		// orderexpr made sure key and val are addressable.
-		key = nod(OADDR, key, nil)
-
-		val = nod(OADDR, val, nil)
-		n = mkcall1(mapfn("mapassign1", map_.Type), nil, init, typename(map_.Type), map_, key, val)
-		goto out
-	}
-
 	if !eqtype(lt, rt) {
 		n.Right = assignconv(n.Right, lt, "assignment")
 		n.Right = walkexpr(n.Right, init)