cmd/compile: inline convT2{I,E} when result doesn't escape

No point in calling a function when we can build the interface
using a known type (or itab) and the address of a local.

Get rid of third arg (preallocated stack space) to convT2{I,E}.

Makes go binary smaller by 0.2%

benchmark                   old ns/op     new ns/op     delta
BenchmarkEfaceInteger-8     16.7          10.1          -39.52%

Update #17118
Update #15375

Change-Id: I9724a1f802bfa1e3957bf1856b55558278e198a2
Reviewed-on: https://go-review.googlesource.com/29373
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 6 insertions, 10 deletions

diff --git a/src/runtime/iface.go b/src/runtime/iface.go
index 7f24a6e69c..476ec7e8b2 100644
--- a/src/runtime/iface.go
+++ b/src/runtime/iface.go
@@ -152,7 +152,7 @@ func itabsinit() {
 	unlock(&ifaceLock)
 }
 
-func convT2E(t *_type, elem unsafe.Pointer, x unsafe.Pointer) (e eface) {
+func convT2E(t *_type, elem unsafe.Pointer) (e eface) {
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(unsafe.Pointer(&t)), funcPC(convT2E))
 	}
@@ -162,18 +162,16 @@ func convT2E(t *_type, elem unsafe.Pointer, x unsafe.Pointer) (e eface) {
 	if isDirectIface(t) {
 		throw("direct convT2E")
 	}
-	if x == nil {
-		x = newobject(t)
-		// TODO: We allocate a zeroed object only to overwrite it with
-		// actual data. Figure out how to avoid zeroing. Also below in convT2I.
-	}
+	x := newobject(t)
+	// TODO: We allocate a zeroed object only to overwrite it with
+	// actual data. Figure out how to avoid zeroing. Also below in convT2I.
 	typedmemmove(t, x, elem)
 	e._type = t
 	e.data = x
 	return
 }
 
-func convT2I(tab *itab, elem unsafe.Pointer, x unsafe.Pointer) (i iface) {
+func convT2I(tab *itab, elem unsafe.Pointer) (i iface) {
 	t := tab._type
 	if raceenabled {
 		raceReadObjectPC(t, elem, getcallerpc(unsafe.Pointer(&tab)), funcPC(convT2I))
@@ -184,9 +182,7 @@ func convT2I(tab *itab, elem unsafe.Pointer, x unsafe.Pointer) (i iface) {
 	if isDirectIface(t) {
 		throw("direct convT2I")
 	}
-	if x == nil {
-		x = newobject(t)
-	}
+	x := newobject(t)
 	typedmemmove(t, x, elem)
 	i.tab = tab
 	i.data = x