cmd/cgo: support floating point #define macros

Current code doesn't support floating point #define macros.
This CL compiles floats to a object file and retrive values from it.
That approach is the same work as we've already done for integers.

Updates #18720

Change-Id: I88b7ab174d0f73bda975cf90c5aeb797961fe034
Reviewed-on: https://go-review.googlesource.com/35511Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/test/issue6612.go

@@ -74,18 +74,15 @@ func testNaming(t *testing.T) {
 		}
 	}
 
-	// This would be nice, but it has never worked.
-	/*
-		if c := C.myfloat_def; c != 1.5 {
-			t.Errorf("C.myint_def = %v, want 1.5", c)
-		}
-		{
-			const c = C.myfloat_def
-			if c != 1.5 {
+	if c := C.myfloat_def; c != 1.5 {
+		t.Errorf("C.myint_def = %v, want 1.5", c)
+	}
+	{
+		const c = C.myfloat_def
+		if c != 1.5 {
 			t.Errorf("C.myint as const = %v, want 1.5", c)
-			}
 		}
-	*/
+	}
 
 	if s := C.mystring_def; s != "hello" {
 		t.Errorf("C.mystring_def = %q, want %q", s, "hello")

src/cmd/cgo/main.go

@@ -88,7 +88,7 @@ type Name struct {
 	Mangle   string // name used in generated Go
 	C        string // name used in C
 	Define   string // #define expansion
-	Kind     string // "const", "type", "var", "fpvar", "func", "not-type"
+	Kind     string // "iconst", "fconst", "sconst", "type", "var", "fpvar", "func", "not-type"
 	Type     *Type  // the type of xxx
 	FuncType *FuncType
 	AddError bool
@@ -100,6 +100,11 @@ func (n *Name) IsVar() bool {
 	return n.Kind == "var" || n.Kind == "fpvar"
 }
 
+// IsConst reports whether Kind is either "iconst", "fconst" or "sconst"
+func (n *Name) IsConst() bool {
+	return n.Kind == "iconst" || n.Kind == "fconst" || n.Kind == "sconst"
+}
+
 // A ExpFunc is an exported function, callable from C.
 // Such functions are identified in the Go input file
 // by doc comments containing the line //export ExpName

src/cmd/cgo/out.go

@@ -185,7 +185,7 @@ func (p *Package) writeDefs() {
 	for _, key := range nameKeys(p.Name) {
 		n := p.Name[key]
 		if n.Const != "" {
-			fmt.Fprintf(fgo2, "const _Cconst_%s = %s\n", n.Go, n.Const)
+			fmt.Fprintf(fgo2, "const _C%s_%s = %s\n", n.Kind, n.Go, n.Const)
 		}
 	}
 	fmt.Fprintf(fgo2, "\n")