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

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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>
This commit is contained in:
Austin Clements 2019-05-31 15:45:06 -04:00
parent b402bd4499
commit dde7c770ef
2 changed files with 32 additions and 8 deletions
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32
src/cmd/compile/internal/gc/noder.go
View file

@ -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