cmd/compile: make the second argument to go:linkname optional

The //go:linkname directive can be used to make a symbol accessible to
another package (when it wouldn't normally be). Sometimes you want to
do this without actually changing the symbol's object file symbol
name; for example, in gccgo this makes unexported symbols non-static,
and in gc this provides ABI0 wrappers for Go symbols so they can be
called from assembly in other packages. Currently, this results in
stutter like

   //go:linkname entersyscall runtime.entersyscall

This CL makes the second argument to go:linkname optional for the case
where the intent is simply to expose the symbol rather than to rename
it in the object file.

Updates #31230.

Change-Id: Id06d9c4b2ec3d8e27f9b8a0d65212ab8048d734f
Reviewed-on: https://go-review.googlesource.com/c/go/+/179861
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

diff --git a/src/cmd/compile/doc.go b/src/cmd/compile/doc.go
index 5291a8b0ebacf1d8961e19212cd1fc394937dce4..5b437d6804ce121a4a5e88633d238ad37b783bf5 100644 (file)
--- a/src/cmd/compile/doc.go
+++ b/src/cmd/compile/doc.go
@@ -216,11 +216,15 @@ not include a stack overflow check. This is most commonly used by low-level
 runtime sources invoked at times when it is unsafe for the calling goroutine to be
 preempted.
 
-       //go:linkname localname importpath.name
+       //go:linkname localname [importpath.name]
 
 The //go:linkname directive instructs the compiler to use ``importpath.name'' as the
 object file symbol name for the variable or function declared as ``localname'' in the
-source code. Because this directive can subvert the type system and package
+source code.
+If the ``importpath.name'' argument is omitted, the directive uses the
+symbol's default object file symbol name and only has the effect of making
+the symbol accessible to other packages.
+Because this directive can subvert the type system and package
 modularity, it is only enabled in files that have imported "unsafe".
 */
 package main

diff --git a/src/cmd/compile/internal/gc/noder.go b/src/cmd/compile/internal/gc/noder.go
index e83ae7c5eba0e2335463e33efd410f6e609df689..93d355278e1a03a1a75f93a881e29bd6922b0d1d 100644 (file)
--- a/src/cmd/compile/internal/gc/noder.go
+++ b/src/cmd/compile/internal/gc/noder.go
@@ -244,10 +244,21 @@ func (p *noder) node() {
        xtop = append(xtop, p.decls(p.file.DeclList)...)
 
        for _, n := range p.linknames {
-               if imported_unsafe {
-                       lookup(n.local).Linkname = n.remote
-               } else {
+               if !imported_unsafe {
                        p.yyerrorpos(n.pos, "//go:linkname only allowed in Go files that import \"unsafe\"")
+                       continue
+               }
+               s := lookup(n.local)
+               if n.remote != "" {
+                       s.Linkname = n.remote
+               } else {
+                       // Use the default object symbol name if the
+                       // user didn't provide one.
+                       if myimportpath == "" {
+                               p.yyerrorpos(n.pos, "//go:linkname requires linkname argument or -p compiler flag")
+                       } else {
+                               s.Linkname = objabi.PathToPrefix(myimportpath) + "." + n.local
+                       }
                }
        }
 
@@ -1476,11 +1487,20 @@ func (p *noder) pragma(pos syntax.Pos, text string) syntax.Pragma {
 
        case strings.HasPrefix(text, "go:linkname "):
                f := strings.Fields(text)
-               if len(f) != 3 {
-                       p.error(syntax.Error{Pos: pos, Msg: "usage: //go:linkname localname linkname"})
+               if !(2 <= len(f) && len(f) <= 3) {
+                       p.error(syntax.Error{Pos: pos, Msg: "usage: //go:linkname localname [linkname]"})
                        break
                }
-               p.linknames = append(p.linknames, linkname{pos, f[1], f[2]})
+               // The second argument is optional. If omitted, we use
+               // the default object symbol name for this and
+               // linkname only serves to mark this symbol as
+               // something that may be referenced via the object
+               // symbol name from another package.
+               var target string
+               if len(f) == 3 {
+                       target = f[2]
+               }
+               p.linknames = append(p.linknames, linkname{pos, f[1], target})
 
        case strings.HasPrefix(text, "go:cgo_import_dynamic "):
                // This is permitted for general use because Solaris