cmd/link: only add dummy XCOFF reference if the symbol exists

On AIX when external linking, for some symbols we need to add
dummy references to prevent the external linker from discarding
them. Currently we add the reference unconditionally. But if the
symbol doesn't exist, the linking fails in a later stage for
generating external relocation of a nonexistent symbol. The
symbols are special symbols that almost always exist, except that
go:buildid may not exist if the linker is invoked without the
-buildid flag. The go command invokes the linker with the flag, so
this can only happen with manual linker invocation. Specifically,
test/run.go does this in some cases.

Fix this by checking the symbol existence before adding the
reference. Re-enable tests on AIX.

Perhaps the linker should always emit a dummy buildid even if the
flag is not set...

Fixes #54814.

Change-Id: I43d81587151595309e189e38960cbda9a1c5ca32
Reviewed-on: https://go-review.googlesource.com/c/go/+/427620
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

Author: cherrymui

src/cmd/link/internal/ld/symtab.go

@@ -684,8 +684,12 @@ func (ctxt *Link) symtab(pcln *pclntab) []sym.SymKind {
 		// Add R_XCOFFREF relocation to prevent ld's garbage collection of
 		// the following symbols. They might not be referenced in the program.
 		addRef := func(name string) {
+			s := ldr.Lookup(name, 0)
+			if s == 0 {
+				return
+			}
 			r, _ := moduledata.AddRel(objabi.R_XCOFFREF)
-			r.SetSym(ldr.Lookup(name, 0))
+			r.SetSym(s)
 			r.SetSiz(uint8(ctxt.Arch.PtrSize))
 		}
 		addRef("runtime.rodata")

test/fixedbugs/bug514.go

@@ -4,7 +4,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build cgo && !aix
+//go:build cgo
 
 package main
 

test/fixedbugs/issue40954.go

@@ -4,7 +4,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build cgo && !aix
+//go:build cgo
 
 package main
 

test/fixedbugs/issue42032.go

@@ -4,7 +4,7 @@
 // source code is governed by a BSD-style license that can be found in
 // the LICENSE file.
 
-//go:build cgo && !aix
+//go:build cgo
 
 package main
 

test/fixedbugs/issue42076.go

@@ -4,7 +4,7 @@
 // source code is governed by a BSD-style license that can be found in
 // the LICENSE file.
 
-//go:build cgo && !aix
+//go:build cgo
 
 package main
 

test/fixedbugs/issue46903.go

@@ -1,5 +1,5 @@
 // run
-//go:build goexperiment.unified && cgo && !aix
+//go:build goexperiment.unified && cgo
 
 // TODO(mdempsky): Enable test unconditionally. This test should pass
 // for non-unified mode too.

test/fixedbugs/issue51733.go

@@ -4,7 +4,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build cgo && !aix
+//go:build cgo
 
 package main