cmd/cgo: recognize untyped constants defined in different files

An untyped constant can be defined in any input file, we shouldn't
segregate them by file.

Updates #28772

Change-Id: I0347f15236833bb511eb49f86c449ee9241b0a25
Reviewed-on: https://go-review.googlesource.com/c/151600
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

misc/cgo/test/issue28545.go

@@ -22,5 +22,5 @@ const issue28772Constant = C.issue28772Constant
 func issue28545G(p **C.char) {
 	C.issue28545F(p, -1, (0))
 	C.issue28545F(p, 2+3, complex(1, 1))
-	C.issue28545F(p, issue28772Constant, (0))
+	C.issue28545F(p, issue28772Constant, issue28772Constant2)
 }

misc/cgo/test/issue28772.go

+// Copyright 2018 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.
+
+package cgotest
+
+// Constants didn't work if defined in different source file.
+
+// #define issue28772Constant2 2
+import "C"
+
+const issue28772Constant2 = C.issue28772Constant2

src/cmd/cgo/ast.go

@@ -66,7 +66,6 @@ func (f *File) ParseGo(name string, src []byte) {
 	f.Package = ast1.Name.Name
 	f.Name = make(map[string]*Name)
 	f.NamePos = make(map[*Name]token.Pos)
-	f.Consts = make(map[string]bool)
 
 	// In ast1, find the import "C" line and get any extra C preamble.
 	sawC := false
@@ -198,7 +197,7 @@ func (f *File) saveExprs(x interface{}, context astContext) {
 				vs := spec.(*ast.ValueSpec)
 				if vs.Type == nil {
 					for _, name := range spec.(*ast.ValueSpec).Names {
-						f.Consts[name.Name] = true
+						consts[name.Name] = true
 					}
 				}
 			}

src/cmd/cgo/gcc.go

@@ -1233,7 +1233,7 @@ func (p *Package) isConst(f *File, x ast.Expr) bool {
 			strings.HasPrefix(x.Name, "_Ciconst_") ||
 			strings.HasPrefix(x.Name, "_Cfconst_") ||
 			strings.HasPrefix(x.Name, "_Csconst_") ||
-			f.Consts[x.Name]
+			consts[x.Name]
 	case *ast.UnaryExpr:
 		return p.isConst(f, x.X)
 	case *ast.BinaryExpr:

src/cmd/cgo/main.go

@@ -62,9 +62,11 @@ type File struct {
 	Name     map[string]*Name    // map from Go name to Name
 	NamePos  map[*Name]token.Pos // map from Name to position of the first reference
 	Edit     *edit.Buffer
-	Consts   map[string]bool // untyped constants
 }
 
+// Untyped constants in the current package.
+var consts = make(map[string]bool)
+
 func (f *File) offset(p token.Pos) int {
 	return fset.Position(p).Offset
 }