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 <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/hashmap.go

@@ -481,20 +481,19 @@ func mapaccess2_fat(t *maptype, h *hmap, key, zero unsafe.Pointer) (unsafe.Point
 	return v, true
 }
 
-func mapassign1(t *maptype, h *hmap, key unsafe.Pointer, val unsafe.Pointer) {
+// Like mapaccess, but allocates a slot for the key if it is not present in the map.
+func mapassign(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
 	if h == nil {
 		panic(plainError("assignment to entry in nil map"))
 	}
 	if raceenabled {
 		callerpc := getcallerpc(unsafe.Pointer(&t))
-		pc := funcPC(mapassign1)
+		pc := funcPC(mapassign)
 		racewritepc(unsafe.Pointer(h), callerpc, pc)
 		raceReadObjectPC(t.key, key, callerpc, pc)
-		raceReadObjectPC(t.elem, val, callerpc, pc)
 	}
 	if msanenabled {
 		msanread(key, t.key.size)
-		msanread(val, t.elem.size)
 	}
 	if h.flags&hashWriting != 0 {
 		throw("concurrent map writes")
@@ -521,35 +520,29 @@ again:
 
 	var inserti *uint8
 	var insertk unsafe.Pointer
-	var insertv unsafe.Pointer
+	var val unsafe.Pointer
 	for {
 		for i := uintptr(0); i < bucketCnt; i++ {
 			if b.tophash[i] != top {
 				if b.tophash[i] == empty && inserti == nil {
 					inserti = &b.tophash[i]
 					insertk = add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
-					insertv = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
+					val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
 				}
 				continue
 			}
 			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
-			k2 := k
 			if t.indirectkey {
-				k2 = *((*unsafe.Pointer)(k2))
+				k = *((*unsafe.Pointer)(k))
 			}
-			if !alg.equal(key, k2) {
+			if !alg.equal(key, k) {
 				continue
 			}
 			// already have a mapping for key. Update it.
 			if t.needkeyupdate {
-				typedmemmove(t.key, k2, key)
+				typedmemmove(t.key, k, key)
 			}
-			v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
-			v2 := v
-			if t.indirectvalue {
-				v2 = *((*unsafe.Pointer)(v2))
-			}
-			typedmemmove(t.elem, v2, val)
+			val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
 			goto done
 		}
 		ovf := b.overflow(t)
@@ -574,7 +567,7 @@ again:
 		h.setoverflow(t, b, newb)
 		inserti = &newb.tophash[0]
 		insertk = add(unsafe.Pointer(newb), dataOffset)
-		insertv = add(insertk, bucketCnt*uintptr(t.keysize))
+		val = add(insertk, bucketCnt*uintptr(t.keysize))
 	}
 
 	// store new key/value at insert position
@@ -585,11 +578,9 @@ again:
 	}
 	if t.indirectvalue {
 		vmem := newobject(t.elem)
-		*(*unsafe.Pointer)(insertv) = vmem
-		insertv = vmem
+		*(*unsafe.Pointer)(val) = vmem
 	}
 	typedmemmove(t.key, insertk, key)
-	typedmemmove(t.elem, insertv, val)
 	*inserti = top
 	h.count++
 
@@ -598,6 +589,10 @@ done:
 		throw("concurrent map writes")
 	}
 	h.flags &^= hashWriting
+	if t.indirectvalue {
+		val = *((*unsafe.Pointer)(val))
+	}
+	return val
 }
 
 func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
@@ -1128,7 +1123,8 @@ func reflect_mapaccess(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
 
 //go:linkname reflect_mapassign reflect.mapassign
 func reflect_mapassign(t *maptype, h *hmap, key unsafe.Pointer, val unsafe.Pointer) {
-	mapassign1(t, h, key, val)
+	p := mapassign(t, h, key)
+	typedmemmove(t.elem, p, val)
 }
 
 //go:linkname reflect_mapdelete reflect.mapdelete