cmd/compile: allow bodyless function if it is linkname'd

In assembly free packages (aka "complete" or "pure go"), allow
bodyless functions if they are linkname'd to something else.

Presumably the thing the function is linkname'd to has a definition.
If not, the linker will complain. And linkname is unsafe, so we expect
users to know what they are doing.

Note this handles only one direction, where the linkname directive
is in the local package. If the linkname directive is in the remote
package, this CL won't help. (See os/signal/sig.s for an example.)

Fixes #23311

Change-Id: I824361b4b582ee05976d94812e5b0e8b0f7a18a6
Reviewed-on: https://go-review.googlesource.com/c/151318
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -497,9 +497,20 @@ func (p *noder) funcDecl(fun *syntax.FuncDecl) *Node {
 		}
 	} else {
 		if pure_go || strings.HasPrefix(f.funcname(), "init.") {
+			// Linknamed functions are allowed to have no body. Hopefully
+			// the linkname target has a body. See issue 23311.
+			isLinknamed := false
+			for _, n := range p.linknames {
+				if f.funcname() == n.local {
+					isLinknamed = true
+					break
+				}
+			}
+			if !isLinknamed {
 				yyerrorl(f.Pos, "missing function body")
 			}
 		}
+	}
 
 	return f
 }

src/runtime/testdata/testprog/empty.s → test/fixedbugs/issue23311.dir/main.go

@@ -2,4 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// This exists solely so we can linkname in symbols from runtime.
+package main
+
+import _ "unsafe" // for linkname
+
+//go:linkname f runtime.GC
+func f()
+
+func main() {
+	f()
+}

src/internal/syscall/unix/empty.s → test/fixedbugs/issue23311.go

+// compiledir
+
 // 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.
 
-// This exists solely so we can linkname in symbols from syscall.
+package ignored