cmd/compile: generate table of main symbol types

For each exported symbol in package main, add its name and type to
go.plugin.tabs symbol. This is used by the runtime when loading a
plugin to return a typed interface{} value.

Change-Id: I23c39583e57180acb8f7a74d218dae4368614f46
Reviewed-on: https://go-review.googlesource.com/27818
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -130,6 +130,7 @@ func dumpobj1(outfile string, mode int) {
 	externs := len(externdcl)
 
 	dumpglobls()
+	dumpptabs()
 	dumptypestructs()
 
 	// Dump extra globals.
@@ -163,6 +164,35 @@ func dumpobj1(outfile string, mode int) {
 	bout.Close()
 }
 
+func dumpptabs() {
+	if !Ctxt.Flag_dynlink || localpkg.Name != "main" {
+		return
+	}
+	for _, exportn := range exportlist {
+		s := exportn.Sym
+		n := s.Def
+		if n == nil {
+			continue
+		}
+		if n.Op != ONAME {
+			continue
+		}
+		if !exportname(s.Name) {
+			continue
+		}
+		if s.Pkg.Name != "main" {
+			continue
+		}
+		if n.Type.Etype == TFUNC && n.Class == PFUNC {
+			// function
+			ptabs = append(ptabs, ptabEntry{s: s, t: s.Def.Type})
+		} else {
+			// variable
+			ptabs = append(ptabs, ptabEntry{s: s, t: typPtr(s.Def.Type)})
+		}
+	}
+}
+
 func dumpglobls() {
 	// add globals
 	for _, n := range externdcl {

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

@@ -18,9 +18,15 @@ type itabEntry struct {
 	sym      *Sym
 }
 
+type ptabEntry struct {
+	s *Sym
+	t *Type
+}
+
 // runtime interface and reflection data structures
 var signatlist []*Node
 var itabs []itabEntry
+var ptabs []ptabEntry
 
 type Sig struct {
 	name   string
@@ -1405,6 +1411,31 @@ func dumptypestructs() {
 		ggloblsym(ilink, int32(Widthptr), int16(obj.DUPOK|obj.RODATA))
 	}
 
+	// process ptabs
+	if localpkg.Name == "main" && len(ptabs) > 0 {
+		ot := 0
+		s := obj.Linklookup(Ctxt, "go.plugin.tabs", 0)
+		for _, p := range ptabs {
+			// Dump ptab symbol into go.pluginsym package.
+			//
+			// type ptab struct {
+			//	name nameOff
+			//	typ  typeOff // pointer to symbol
+			// }
+			nsym := dname(p.s.Name, "", nil, true)
+			ot = dsymptrOffLSym(s, ot, nsym, 0)
+			ot = dsymptrOffLSym(s, ot, Linksym(typesym(p.t)), 0)
+		}
+		ggloblLSym(s, int32(ot), int16(obj.RODATA))
+
+		ot = 0
+		s = obj.Linklookup(Ctxt, "go.plugin.exports", 0)
+		for _, p := range ptabs {
+			ot = dsymptrLSym(s, ot, Linksym(p.s), 0)
+		}
+		ggloblLSym(s, int32(ot), int16(obj.RODATA))
+	}
+
 	// generate import strings for imported packages
 	if forceObjFileStability {
 		// Sorting the packages is not necessary but to compare binaries created