internal/runtime/maps: use simpler calculation for slot element

This reduces the adds required at the return point from 3 to 1.
(The multiply inside g.elem() does get CSE'd with the one inside
g.key(), but the rest of the adds don't.)

Instead, compute the element as just a fixed offset from the key.

Change-Id: Ia4d7664efafcdca5e9daeb77d270651bb186232c
Reviewed-on: https://go-review.googlesource.com/c/go/+/629535
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

4 files changed, 14 insertions, 10 deletions

diff --git a/src/internal/runtime/maps/runtime_fast32_swiss.go b/src/internal/runtime/maps/runtime_fast32_swiss.go
index 2ab30bce6c..ff5815abdd 100644
--- a/src/internal/runtime/maps/runtime_fast32_swiss.go
+++ b/src/internal/runtime/maps/runtime_fast32_swiss.go
@@ -66,7 +66,7 @@ func runtime_mapaccess1_fast32(typ *abi.SwissMapType, m *Map, key uint32) unsafe
 
 			slotKey := g.key(typ, i)
 			if key == *(*uint32)(slotKey) {
-				slotElem := g.elem(typ, i)
+				slotElem := unsafe.Pointer(uintptr(slotKey) + typ.ElemOff)
 				return slotElem
 			}
 			match = match.removeFirst()
@@ -134,7 +134,7 @@ func runtime_mapaccess2_fast32(typ *abi.SwissMapType, m *Map, key uint32) (unsaf
 
 			slotKey := g.key(typ, i)
 			if key == *(*uint32)(slotKey) {
-				slotElem := g.elem(typ, i)
+				slotElem := unsafe.Pointer(uintptr(slotKey) + typ.ElemOff)
 				return slotElem, true
 			}
 			match = match.removeFirst()
diff --git a/src/internal/runtime/maps/runtime_fast64_swiss.go b/src/internal/runtime/maps/runtime_fast64_swiss.go
index 396c63c236..f4716dffda 100644
--- a/src/internal/runtime/maps/runtime_fast64_swiss.go
+++ b/src/internal/runtime/maps/runtime_fast64_swiss.go
@@ -66,7 +66,7 @@ func runtime_mapaccess1_fast64(typ *abi.SwissMapType, m *Map, key uint64) unsafe
 
 			slotKey := g.key(typ, i)
 			if key == *(*uint64)(slotKey) {
-				slotElem := g.elem(typ, i)
+				slotElem := unsafe.Pointer(uintptr(slotKey) + typ.ElemOff)
 				return slotElem
 			}
 			match = match.removeFirst()
@@ -134,7 +134,7 @@ func runtime_mapaccess2_fast64(typ *abi.SwissMapType, m *Map, key uint64) (unsaf
 
 			slotKey := g.key(typ, i)
 			if key == *(*uint64)(slotKey) {
-				slotElem := g.elem(typ, i)
+				slotElem := unsafe.Pointer(uintptr(slotKey) + typ.ElemOff)
 				return slotElem, true
 			}
 			match = match.removeFirst()
diff --git a/src/internal/runtime/maps/runtime_faststr_swiss.go b/src/internal/runtime/maps/runtime_faststr_swiss.go
index 38170a1821..eed8d8666d 100644
--- a/src/internal/runtime/maps/runtime_faststr_swiss.go
+++ b/src/internal/runtime/maps/runtime_faststr_swiss.go
@@ -141,7 +141,7 @@ func runtime_mapaccess1_faststr(typ *abi.SwissMapType, m *Map, key string) unsaf
 
 			slotKey := g.key(typ, i)
 			if key == *(*string)(slotKey) {
-				slotElem := g.elem(typ, i)
+				slotElem := unsafe.Pointer(uintptr(slotKey) + typ.ElemOff)
 				return slotElem
 			}
 			match = match.removeFirst()
@@ -199,7 +199,7 @@ func runtime_mapaccess2_faststr(typ *abi.SwissMapType, m *Map, key string) (unsa
 
 			slotKey := g.key(typ, i)
 			if key == *(*string)(slotKey) {
-				slotElem := g.elem(typ, i)
+				slotElem := unsafe.Pointer(uintptr(slotKey) + typ.ElemOff)
 				return slotElem, true
 			}
 			match = match.removeFirst()
diff --git a/src/internal/runtime/maps/runtime_swiss.go b/src/internal/runtime/maps/runtime_swiss.go
index 58ac893486..3f4f970fb7 100644
--- a/src/internal/runtime/maps/runtime_swiss.go
+++ b/src/internal/runtime/maps/runtime_swiss.go
@@ -90,11 +90,12 @@ func runtime_mapaccess1(typ *abi.SwissMapType, m *Map, key unsafe.Pointer) unsaf
 			i := match.first()
 
 			slotKey := g.key(typ, i)
+			slotKeyOrig := slotKey
 			if typ.IndirectKey() {
 				slotKey = *((*unsafe.Pointer)(slotKey))
 			}
 			if typ.Key.Equal(key, slotKey) {
-				slotElem := g.elem(typ, i)
+				slotElem := unsafe.Pointer(uintptr(slotKeyOrig) + typ.ElemOff)
 				if typ.IndirectElem() {
 					slotElem = *((*unsafe.Pointer)(slotElem))
 				}
@@ -163,11 +164,12 @@ func runtime_mapaccess2(typ *abi.SwissMapType, m *Map, key unsafe.Pointer) (unsa
 			i := match.first()
 
 			slotKey := g.key(typ, i)
+			slotKeyOrig := slotKey
 			if typ.IndirectKey() {
 				slotKey = *((*unsafe.Pointer)(slotKey))
 			}
 			if typ.Key.Equal(key, slotKey) {
-				slotElem := g.elem(typ, i)
+				slotElem := unsafe.Pointer(uintptr(slotKeyOrig) + typ.ElemOff)
 				if typ.IndirectElem() {
 					slotElem = *((*unsafe.Pointer)(slotElem))
 				}
@@ -256,6 +258,7 @@ outer:
 				i := match.first()
 
 				slotKey := g.key(typ, i)
+				slotKeyOrig := slotKey
 				if typ.IndirectKey() {
 					slotKey = *((*unsafe.Pointer)(slotKey))
 				}
@@ -264,7 +267,7 @@ outer:
 						typedmemmove(typ.Key, slotKey, key)
 					}
 
-					slotElem = g.elem(typ, i)
+					slotElem = unsafe.Pointer(uintptr(slotKeyOrig) + typ.ElemOff)
 					if typ.IndirectElem() {
 						slotElem = *((*unsafe.Pointer)(slotElem))
 					}
@@ -298,6 +301,7 @@ outer:
 				// If there is room left to grow, just insert the new entry.
 				if t.growthLeft > 0 {
 					slotKey := g.key(typ, i)
+					slotKeyOrig := slotKey
 					if typ.IndirectKey() {
 						kmem := newobject(typ.Key)
 						*(*unsafe.Pointer)(slotKey) = kmem
@@ -305,7 +309,7 @@ outer:
 					}
 					typedmemmove(typ.Key, slotKey, key)
 
-					slotElem = g.elem(typ, i)
+					slotElem = unsafe.Pointer(uintptr(slotKeyOrig) + typ.ElemOff)
 					if typ.IndirectElem() {
 						emem := newobject(typ.Elem)
 						*(*unsafe.Pointer)(slotElem) = emem