cmd/link: stop checking reflect.Value.Call in deadcode pass

In the linker's deadcode pass, we need to keep a method live if
it can be reached through reflection. We do this by marking all
exported method live if reflect.Value.Method or
reflect.Type.Method is used. Currently we also check for
reflect.Value.Call, which is unnecessary because in order to call
a method through reflection, the method must be obtained through
reflect.Value.Method or reflect.Type.Method, which we already
check.

Per discussion in https://groups.google.com/d/msg/golang-dev/eG9It63-Bxg/_bnoVy-eAwAJ
Thanks Brad, Russ, and Ian for bringing this up.

Change-Id: I8e9529a224bb898dbf5752674cc9d155db386c14
Reviewed-on: https://go-review.googlesource.com/c/go/+/228792
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: cherrymui

src/cmd/link/internal/ld/deadcode2.go

@@ -219,7 +219,6 @@ func deadcode2(ctxt *Link) {
 	d.init()
 	d.flood()
 
-	callSym := ldr.Lookup("reflect.Value.Call", sym.SymVerABIInternal)
 	methSym := ldr.Lookup("reflect.Value.Method", sym.SymVerABIInternal)
 	if ctxt.DynlinkingGo() {
 		// Exported methods may satisfy interfaces we don't know
@@ -231,7 +230,7 @@ func deadcode2(ctxt *Link) {
 		// Methods might be called via reflection. Give up on
 		// static analysis, mark all exported methods of
 		// all reachable types as reachable.
-		d.reflectSeen = d.reflectSeen || (callSym != 0 && ldr.AttrReachable(callSym)) || (methSym != 0 && ldr.AttrReachable(methSym))
+		d.reflectSeen = d.reflectSeen || (methSym != 0 && ldr.AttrReachable(methSym))
 
 		// Mark all methods that could satisfy a discovered
 		// interface as reachable. We recheck old marked interfaces