cmd/compile: make map.zero symbol content-addressable

The compiler machinery that generates "map.zero" symbols marks them as
RODATA and DUPOK, which is problematic when a given application has
multiple map zero symbols (from different packages) with varying
sizes: the dupok path in the loader assumes that if two symbols have
the same name, it is safe to pick any of the versions. In the case of
map.zero, the link needs to select the largest symbol, not an
arbitrary sym.

To fix this problem, mark map.zero symbols as content-addressable,
since the loader's content addressability processing path already
supports selection of the larger symbol in cases where there are dups.

Fixes #46653.

Change-Id: Iabd2feef01d448670ba795c7eaddc48c191ea276
Reviewed-on: https://go-review.googlesource.com/c/go/+/326211
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

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

@@ -148,6 +148,7 @@ func dumpdata() {
 	if reflectdata.ZeroSize > 0 {
 		zero := base.PkgLinksym("go.map", "zero", obj.ABI0)
 		objw.Global(zero, int32(reflectdata.ZeroSize), obj.DUPOK|obj.RODATA)
+		zero.Set(obj.AttrContentAddressable, true)
 	}
 
 	staticdata.WriteFuncSyms()

test/fixedbugs/issue46653.dir/bad/bad.go

@@ -0,0 +1,64 @@
+// Copyright 2021 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.
+
+package a
+
+func Bad() {
+	m := make(map[int64]A)
+	a := m[0]
+	if len(a.B.C1.D2.E2.F1) != 0 ||
+		len(a.B.C1.D2.E2.F2) != 0 ||
+		len(a.B.C1.D2.E2.F3) != 0 ||
+		len(a.B.C1.D2.E2.F4) != 0 ||
+		len(a.B.C1.D2.E2.F5) != 0 ||
+		len(a.B.C1.D2.E2.F6) != 0 ||
+		len(a.B.C1.D2.E2.F7) != 0 ||
+		len(a.B.C1.D2.E2.F8) != 0 ||
+		len(a.B.C1.D2.E2.F9) != 0 ||
+		len(a.B.C1.D2.E2.F10) != 0 ||
+		len(a.B.C1.D2.E2.F11) != 0 ||
+		len(a.B.C1.D2.E2.F16) != 0 {
+		panic("bad")
+	}
+}
+
+type A struct {
+	B
+}
+
+type B struct {
+	C1 C
+	C2 C
+}
+
+type C struct {
+	D1 D
+	D2 D
+}
+
+type D struct {
+	E1 E
+	E2 E
+	E3 E
+	E4 E
+}
+
+type E struct {
+	F1  string
+	F2  string
+	F3  string
+	F4  string
+	F5  string
+	F6  string
+	F7  string
+	F8  string
+	F9  string
+	F10 string
+	F11 string
+	F12 string
+	F13 string
+	F14 string
+	F15 string
+	F16 string
+}

test/fixedbugs/issue46653.dir/main.go

@@ -0,0 +1,27 @@
+// Copyright 2021 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.
+
+package main
+
+import (
+	bad "issue46653.dir/bad"
+)
+
+func main() {
+	bad.Bad()
+}
+
+func neverCalled() L {
+	m := make(map[string]L)
+	return m[""]
+}
+
+type L struct {
+	A Data
+	B Data
+}
+
+type Data struct {
+	F1 [22][]string
+}

test/fixedbugs/issue46653.go

@@ -0,0 +1,10 @@
+// runindir
+
+// Copyright 2021 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.
+
+// Test to verify compiler and linker handling of multiple
+// competing map.zero symbol definitions.
+
+package ignored