cmd/link: put zero-sized data symbols at same address as runtime.zerobase

Put zero-sized data symbols at same address as runtime.zerobase,
so zero-sized global variables have the same address as zero-sized
allocations.

Change-Id: Ib3145dc1b663a9794dfabc0e6abd2384960f2c49
Reviewed-on: https://go-review.googlesource.com/c/go/+/490435
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

2 files changed, 42 insertions, 3 deletions

diff --git a/src/cmd/link/internal/ld/data.go b/src/cmd/link/internal/ld/data.go
index 7c135ae7e6..d0efcdc052 100644
--- a/src/cmd/link/internal/ld/data.go
+++ b/src/cmd/link/internal/ld/data.go
@@ -2156,7 +2156,7 @@ type symNameSize struct {
 }
 
 func (state *dodataState) dodataSect(ctxt *Link, symn sym.SymKind, syms []loader.Sym) (result []loader.Sym, maxAlign int32) {
-	var head, tail loader.Sym
+	var head, tail, zerobase loader.Sym
 	ldr := ctxt.loader
 	sl := make([]symNameSize, len(syms))
 
@@ -2196,20 +2196,26 @@ func (state *dodataState) dodataSect(ctxt *Link, symn sym.SymKind, syms []loader
 			}
 		}
 	}
+	zerobase = ldr.Lookup("runtime.zerobase", 0)
 
 	// Perform the sort.
 	if symn != sym.SPCLNTAB {
 		sort.Slice(sl, func(i, j int) bool {
 			si, sj := sl[i].sym, sl[j].sym
+			isz, jsz := sl[i].sz, sl[j].sz
 			switch {
 			case si == head, sj == tail:
 				return true
 			case sj == head, si == tail:
 				return false
+			// put zerobase right after all the zero-sized symbols,
+			// so zero-sized symbols have the same address as zerobase.
+			case si == zerobase:
+				return jsz != 0 // zerobase < nonzero-sized
+			case sj == zerobase:
+				return isz == 0 // 0-sized < zerobase
 			}
 			if checkSize {
-				isz := sl[i].sz
-				jsz := sl[j].sz
 				if isz != jsz {
 					return isz < jsz
 				}
diff --git a/test/zerosize.go b/test/zerosize.go
new file mode 100644
index 0000000000..53a29f7927
--- /dev/null
+++ b/test/zerosize.go
@@ -0,0 +1,33 @@
+// run
+
+// Copyright 2023 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 that zero-sized variables get same address as
+// runtime.zerobase.
+
+package main
+
+var x, y [0]int
+var p, q = new([0]int), new([0]int) // should get &runtime.zerobase
+
+func main() {
+	if &x != &y {
+		// Failing for now. x and y are at same address, but compiler optimizes &x==&y to false. Skip.
+		// print("&x=", &x, " &y=", &y, " &x==&y = ", &x==&y, "\n")
+		// panic("FAIL")
+	}
+	if p != q {
+		print("p=", p, " q=", q, " p==q = ", p==q, "\n")
+		panic("FAIL")
+	}
+	if &x != p {
+		print("&x=", &x, " p=", p, " &x==p = ", &x==p, "\n")
+		panic("FAIL")
+	}
+	if &y != p {
+		print("&y=", &y, " p=", p, " &y==p = ", &y==p, "\n")
+		panic("FAIL")
+	}
+}