internal/runtime/maps: return after fatal to help register allocator

Seems simple, but putting the return after fatal ensures that at the
point of the small group loop, no call has happened so the key is
still in a register. This ensures that we don't have to restore the
key from the stack before the comparison on each iteration. That gets
rid of a load from the inner loop.

name                                       old time/op  new time/op  delta
MapAccessHit/Key=int64/Elem=int64/len=6-8  4.01ns ± 6%  3.85ns ± 3%  -3.92%  (p=0.001 n=10+10)

Change-Id: Ia23ac48e6c5522be88f7d9be0ff3489b2dfc52fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/624255
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

src/internal/runtime/maps/map.go

@@ -560,6 +560,7 @@ func (m *Map) putSlotSmall(typ *abi.SwissMapType, hash uintptr, key unsafe.Point
 	match = g.ctrls().matchEmptyOrDeleted()
 	if match == 0 {
 		fatal("small map with no empty slot (concurrent map writes?)")
+		return nil
 	}
 
 	i := match.first()

src/internal/runtime/maps/runtime_fast32_swiss.go

@@ -27,6 +27,7 @@ func runtime_mapaccess1_fast32(typ *abi.SwissMapType, m *Map, key uint32) unsafe
 
 	if m.writing != 0 {
 		fatal("concurrent map read and map write")
+		return nil
 	}
 
 	if m.dirLen == 0 {
@@ -91,6 +92,7 @@ func runtime_mapaccess2_fast32(typ *abi.SwissMapType, m *Map, key uint32) (unsaf
 
 	if m.writing != 0 {
 		fatal("concurrent map read and map write")
+		return nil, false
 	}
 
 	if m.dirLen == 0 {

src/internal/runtime/maps/runtime_fast64_swiss.go

@@ -27,6 +27,7 @@ func runtime_mapaccess1_fast64(typ *abi.SwissMapType, m *Map, key uint64) unsafe
 
 	if m.writing != 0 {
 		fatal("concurrent map read and map write")
+		return nil
 	}
 
 	if m.dirLen == 0 {
@@ -91,6 +92,7 @@ func runtime_mapaccess2_fast64(typ *abi.SwissMapType, m *Map, key uint64) (unsaf
 
 	if m.writing != 0 {
 		fatal("concurrent map read and map write")
+		return nil, false
 	}
 
 	if m.dirLen == 0 {

src/internal/runtime/maps/runtime_faststr_swiss.go

@@ -55,6 +55,7 @@ func runtime_mapaccess1_faststr(typ *abi.SwissMapType, m *Map, key string) unsaf
 
 	if m.writing != 0 {
 		fatal("concurrent map read and map write")
+		return nil
 	}
 
 	hash := typ.Hasher(abi.NoEscape(unsafe.Pointer(&key)), m.seed)
@@ -112,6 +113,7 @@ func runtime_mapaccess2_faststr(typ *abi.SwissMapType, m *Map, key string) (unsa
 
 	if m.writing != 0 {
 		fatal("concurrent map read and map write")
+		return nil, false
 	}
 
 	hash := typ.Hasher(abi.NoEscape(unsafe.Pointer(&key)), m.seed)

src/internal/runtime/maps/table.go

@@ -608,6 +608,7 @@ func (it *Iter) Next() {
 
 	if it.m.writing != 0 {
 		fatal("concurrent map iteration and map write")
+		return
 	}
 
 	if it.dirIdx < 0 {