[dev.link] cmd: reference symbols by name when linking against Go shared library

When building a program that links against Go shared libraries,
it needs to reference symbols defined in the shared library. At
compile time, we don't know where the shared library boundary is.
If we reference a symbol in package p by index, and package p is
actually part of a shared library, we cannot resolve the index at
link time, as the linker doesn't see the object file of p.

So when linking against Go shared libraries, always use named
reference for now.

To do this, the compiler needs to know whether we will be linking
against Go shared libraries. The -dynlink flag kind of indicates
that (as the document says), but currently it is actually
overloaded: it is also used when building a plugin or a shared
library, which is self-contained (if -linkshared is not otherwise
specified) and could use index for symbol reference. So we
introduce another compiler flag, -linkshared, specifically for
linking against Go shared libraries. The go command will pass
this flag if its -linkshared flag is specified
("go build -linkshared").

There may be better way to handle this. For example, we can
put the symbol indices in a special section in the shared library
that the linker can read. Or we can generate some per-package
description file to include the indices. (Currently we generate
a .shlibname file for each package that is included in a shared
library, which contains the path of the library. We could
consider extending this.) That said, this CL is a stop-gap
solution. And it is no worse than the old object files.

If we were to redesign the build system so that the shared
library boundary is known at compile time, we could use indices
for symbol references that do not cross shared library boundary,
as well as doing other things better.

Change-Id: I9c02aad36518051cc4785dbe25c4b4cef8f3faeb
Reviewed-on: https://go-review.googlesource.com/c/go/+/201818
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

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

@@ -251,6 +251,7 @@ func Main(archInit func(*Arch)) {
 	if supportsDynlink(thearch.LinkArch.Arch) {
 		flag.BoolVar(&flag_shared, "shared", false, "generate code that can be linked into a shared library")
 		flag.BoolVar(&flag_dynlink, "dynlink", false, "support references to Go symbols defined in other shared libraries")
+		flag.BoolVar(&Ctxt.Flag_linkshared, "linkshared", false, "generate code that will be linked against Go shared libraries")
 	}
 	flag.StringVar(&cpuprofile, "cpuprofile", "", "write cpu profile to `file`")
 	flag.StringVar(&memprofile, "memprofile", "", "write memory profile to `file`")

src/cmd/go/alldocs.go

@@ -145,8 +145,8 @@
 // 	-ldflags '[pattern=]arg list'
 // 		arguments to pass on each go tool link invocation.
 // 	-linkshared
-// 		link against shared libraries previously created with
-// 		-buildmode=shared.
+// 		build code that will be linked against shared libraries previously
+// 		created with -buildmode=shared.
 // 	-mod mode
 // 		module download mode to use: readonly or vendor.
 // 		See 'go help modules' for more.

src/cmd/go/internal/work/build.go

@@ -97,8 +97,8 @@ and test commands:
 	-ldflags '[pattern=]arg list'
 		arguments to pass on each go tool link invocation.
 	-linkshared
-		link against shared libraries previously created with
-		-buildmode=shared.
+		build code that will be linked against shared libraries previously
+		created with -buildmode=shared.
 	-mod mode
 		module download mode to use: readonly or vendor.
 		See 'go help modules' for more.

src/cmd/go/internal/work/init.go

@@ -224,6 +224,7 @@ func buildModeInit() {
 				base.Fatalf("-linkshared not supported on %s\n", platform)
 			}
 			codegenArg = "-dynlink"
+			forcedGcflags = append(forcedGcflags, "-linkshared")
 			// TODO(mwhudson): remove -w when that gets fixed in linker.
 			forcedLdflags = append(forcedLdflags, "-linkshared", "-w")
 		}

src/cmd/internal/obj/link.go

@@ -648,6 +648,7 @@ type Link struct {
 	Debugpcln          string
 	Flag_shared        bool
 	Flag_dynlink       bool
+	Flag_linkshared    bool
 	Flag_optimize      bool
 	Flag_locationlists bool
 	Flag_newobj        bool // use new object file format

src/cmd/internal/obj/sym.go

@@ -184,7 +184,7 @@ func (ctxt *Link) NumberSyms(asm bool) {
 
 	var idx, nonpkgidx int32 = 0, 0
 	ctxt.traverseSyms(traverseDefs, func(s *LSym) {
-		if asm || s.Pkg == "_" || s.DuplicateOK() {
+		if asm || s.Pkg == "_" || s.DuplicateOK() || ctxt.Flag_linkshared {
 			s.PkgIdx = goobj2.PkgIdxNone
 			s.SymIdx = nonpkgidx
 			if nonpkgidx != int32(len(ctxt.nonpkgdefs)) {