cmd/compile/internal/gc: separate builtin and real runtime packages

The builtin runtime package definitions intentionally diverge from the
actual runtime package's, but this only works as long as they never
overlap.

To make it easier to expand the builtin runtime package, this CL now
loads their definitions into a logically separate "go.runtime"
package.  By resetting the package's Prefix field to "runtime", any
references to builtin definitions will still resolve against the real
package runtime.

Fixes #14482.

Passes toolstash -cmp.

Change-Id: I539c0994deaed4506a331f38c5b4d6bc8c95433f
Reviewed-on: https://go-review.googlesource.com/37538
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -27,6 +27,14 @@ type Pkg struct {
 	Syms     map[string]*Sym
 }
 
+// isRuntime reports whether p is package runtime.
+func (p *Pkg) isRuntime() bool {
+	if compiling_runtime && p == localpkg {
+		return true
+	}
+	return p.Path == "runtime"
+}
+
 // Sym represents an object name. Most commonly, this is a Go identifier naming
 // an object declared within a package, but Syms are also used to name internal
 // synthesized objects.
@@ -153,7 +161,7 @@ var itabpkg *Pkg // fake pkg for itab entries
 
 var itablinkpkg *Pkg // fake package for runtime itab entries
 
-var Runtimepkg *Pkg // package runtime
+var Runtimepkg *Pkg // fake package runtime
 
 var racepkg *Pkg // package runtime/race
 

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

@@ -124,9 +124,14 @@ func Main() {
 	unsafepkg = mkpkg("unsafe")
 	unsafepkg.Name = "unsafe"
 
-	// real package, referred to by generated runtime calls
-	Runtimepkg = mkpkg("runtime")
+	// Pseudo-package that contains the compiler's builtin
+	// declarations for package runtime. These are declared in a
+	// separate package to avoid conflicts with package runtime's
+	// actual declarations, which may differ intentionally but
+	// insignificantly.
+	Runtimepkg = mkpkg("go.runtime")
 	Runtimepkg.Name = "runtime"
+	Runtimepkg.Prefix = "runtime"
 
 	// pseudo-packages used in symbol tables
 	itabpkg = mkpkg("go.itab")

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

@@ -1289,7 +1289,7 @@ OpSwitch:
 		if t.Results().NumFields() == 1 {
 			n.Type = l.Type.Results().Field(0).Type
 
-			if n.Op == OCALLFUNC && n.Left.Op == ONAME && (compiling_runtime || n.Left.Sym.Pkg == Runtimepkg) && n.Left.Sym.Name == "getg" {
+			if n.Op == OCALLFUNC && n.Left.Op == ONAME && n.Left.Sym.Pkg.isRuntime() && n.Left.Sym.Name == "getg" {
 				// Emit code for runtime.getg() directly instead of calling function.
 				// Most such rewrites (for example the similar one for math.Sqrt) should be done in walk,
 				// so that the ordering pass can make sure to preserve the semantics of the original code

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

@@ -1914,7 +1914,7 @@ func walkprint(nn *Node, init *Nodes) *Node {
 			on = substArgTypes(on, n.Type) // any-1
 		} else if isInt[et] {
 			if et == TUINT64 {
-				if (t.Sym.Pkg == Runtimepkg || compiling_runtime) && t.Sym.Name == "hex" {
+				if t.Sym.Pkg.isRuntime() && t.Sym.Name == "hex" {
 					on = syslook("printhex")
 				} else {
 					on = syslook("printuint")

test/runtime.go

@@ -17,5 +17,5 @@ package main
 import "runtime"
 
 func main() {
-	runtime.printbool(true)	// ERROR "unexported"
+	runtime.printbool(true)	// ERROR "unexported" "undefined"
 }