cmd/compile: set itab function pointers at compile time

I noticed that we don't set an itab's function pointers at compile
time. Instead, we currently do it at executable startup.

Set the function pointers at compile time instead. This shortens
startup time. It has no effect on normal binary size. Object files
will have more relocations, but that isn't a big deal.

For PIE there are additional pointers that will need to be adjusted at
load time. There are already other pointers in an itab that need to be
adjusted, so the cache line will already be paged in. There might be
some binary size overhead to mark these pointers. The "go test -c
-buildmode=pie net/http" binary is 0.18% bigger.

Update #20505

Change-Id: I267c82489915b509ff66e512fc7319b2dd79b8f7
Reviewed-on: https://go-review.googlesource.com/44341
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>

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

@@ -1463,14 +1463,13 @@ func dumptabs() {
 		// }
 		o := dsymptr(i.lsym, 0, dtypesym(i.itype).Linksym(), 0)
 		o = dsymptr(i.lsym, o, dtypesym(i.t).Linksym(), 0)
-		o = duint32(i.lsym, o, typehash(i.t))  // copy of type hash
-		o += 4                                 // skip unused field
-		o += len(imethods(i.itype)) * Widthptr // skip fun method pointers
-		// at runtime the itab will contain pointers to types, other itabs and
-		// method functions. None are allocated on heap, so we can use obj.NOPTR.
-		ggloblsym(i.lsym, int32(o), int16(obj.DUPOK|obj.NOPTR))
-		// TODO: mark readonly after we pre-add the function pointers
-
+		o = duint32(i.lsym, o, typehash(i.t)) // copy of type hash
+		o += 4                                // skip unused field
+		for _, fn := range genfun(i.t, i.itype) {
+			o = dsymptr(i.lsym, o, fn, 0) // method pointer for each method
+		}
+		// Nothing writes static itabs, so they are read only.
+		ggloblsym(i.lsym, int32(o), int16(obj.DUPOK|obj.RODATA))
 		ilink := itablinkpkg.Lookup(i.t.ShortString() + "," + i.itype.ShortString()).Linksym()
 		dsymptr(ilink, 0, i.lsym, 0)
 		ggloblsym(ilink, int32(Widthptr), int16(obj.DUPOK|obj.RODATA))

src/runtime/iface.go

@@ -167,13 +167,6 @@ func itabAdd(m *itab) {
 	}
 }
 
-// Adds m to the set of initial itabs.
-// itabLock must be held.
-func itabAddStartup(m *itab) {
-	m.init() // TODO: remove after CL 44341
-	itabAdd(m)
-}
-
 // init fills in the m.fun array with all the code pointers for
 // the m.inter/m._type pair. If the type does not implement the interface,
 // it sets m.fun[0] to 0 and returns the name of an interface function that is missing.
@@ -230,7 +223,7 @@ func itabsinit() {
 	lock(&itabLock)
 	for _, md := range activeModules() {
 		for _, i := range md.itablinks {
-			itabAddStartup(i)
+			itabAdd(i)
 		}
 	}
 	unlock(&itabLock)

src/runtime/plugin.go

@@ -56,7 +56,7 @@ func plugin_lastmoduleinit() (path string, syms map[string]interface{}, mismatch
 
 	lock(&itabLock)
 	for _, i := range md.itablinks {
-		itabAddStartup(i)
+		itabAdd(i)
 	}
 	unlock(&itabLock)