cmd/compile: define func value symbols at declaration

This is mostly Russ's https://golang.org/cl/12145 but with some extra fixes to
account for the fact that function declarations without implementations now
break shared libraries, and including my test case.

Fixes #11480.

Change-Id: Iabdc2934a0378e5025e4e7affadb535eaef2c8f1
Reviewed-on: https://go-review.googlesource.com/12340Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testshared/src/dep/asm.s

+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//+build !gccgo
+
+#include "textflag.h"
+
+TEXT ·ImplementedInAsm(SB),NOSPLIT,$0-0
+       RET

misc/cgo/testshared/src/dep/gccgo.go

+//+build gccgo
+
+package dep
+
+func ImplementedInAsm() {}

misc/cgo/testshared/src/dep/stubs.go

+//+build !gccgo
+
+package dep
+
+func ImplementedInAsm()

misc/cgo/testshared/src/exe/exe.go

 package main
 
-import "dep"
+import (
+	"dep"
+	"runtime"
+)
 
 func main() {
+	defer dep.ImplementedInAsm()
+	runtime.GC()
 	dep.V = dep.F() + 1
 }

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

@@ -199,6 +199,7 @@ func makeclosure(func_ *Node) *Node {
 	xfunc.Func.Nname.Name.Funcdepth = func_.Func.Depth
 	xfunc.Func.Depth = func_.Func.Depth
 	xfunc.Func.Endlineno = func_.Func.Endlineno
+	makefuncsym(xfunc.Func.Nname.Sym)
 
 	xfunc.Nbody = func_.Nbody
 	xfunc.Func.Dcl = concat(func_.Func.Dcl, xfunc.Func.Dcl)

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

@@ -585,6 +585,10 @@ func funchdr(n *Node) {
 		Fatal("funchdr: dclcontext")
 	}
 
+	if importpkg == nil && n.Func.Nname != nil {
+		makefuncsym(n.Func.Nname.Sym)
+	}
+
 	dclcontext = PAUTO
 	markdcl()
 	Funcdepth++
@@ -1489,12 +1493,21 @@ func funcsym(s *Sym) *Sym {
 	}
 
 	s1 := Pkglookup(s.Name+"·f", s.Pkg)
-	if s1.Def == nil {
+	s.Fsym = s1
+	return s1
+}
+
+func makefuncsym(s *Sym) {
+	if isblanksym(s) {
+		return
+	}
+	if compiling_runtime != 0 && s.Name == "getg" {
+		// runtime.getg() is not a real function and so does
+		// not get a funcsym.
+		return
+	}
+	s1 := funcsym(s)
 	s1.Def = newfuncname(s1)
 	s1.Def.Func.Shortname = newname(s)
 	funcsyms = list(funcsyms, s1.Def)
-	}
-	s.Fsym = s1
-
-	return s1
 }

src/runtime/debug/stubs.go

@@ -16,5 +16,4 @@ func setMaxThreads(int) int
 
 // Implemented in package runtime.
 func readGCStats(*[]time.Duration)
-func enableGC(bool) bool
 func freeOSMemory()

src/runtime/stubs.go

@@ -249,7 +249,6 @@ func time_now() (sec int64, nsec int32)
 
 // in asm_*.s
 // not called directly; definitions here supply type information for traceback.
-func call16(fn, arg unsafe.Pointer, n, retoffset uint32)
 func call32(fn, arg unsafe.Pointer, n, retoffset uint32)
 func call64(fn, arg unsafe.Pointer, n, retoffset uint32)
 func call128(fn, arg unsafe.Pointer, n, retoffset uint32)

src/runtime/stubs32.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build 386 arm amd64p32
+
+package runtime
+
+import "unsafe"
+
+// Declarations for runtime services implemented in C or assembly that
+// are only present on 32 bit systems.
+
+func call16(fn, arg unsafe.Pointer, n, retoffset uint32)