[dev.link] cmd/link: fix hash collision check

For content-addressable symbols, we build its content hash based
on the symbol data and relocations. When the compiler builds the
symbol data, it may not always include the trailing zeros, e.g.
the data of [10]int64{1,2,3} is only the first 24 bytes.
Therefore, we may end up with symbols with the same contents
(thus same hash) but different sizes. This is not actually a hash
collision. In this case, we can deduplicate them and keep the one
with the larger size.

Change-Id: If6834542d7914cc00f917d7db151955e5aee6f30
Reviewed-on: https://go-review.googlesource.com/c/go/+/243718
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/link/link_test.go

@@ -778,3 +778,23 @@ func TestPErsrc(t *testing.T) {
 		t.Fatalf("binary does not contain expected content")
 	}
 }
+
+func TestContentAddressableSymbols(t *testing.T) {
+	// Test that the linker handles content-addressable symbols correctly.
+	testenv.MustHaveGoBuild(t)
+
+	t.Parallel()
+
+	tmpdir, err := ioutil.TempDir("", "TestContentAddressableSymbols")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	src := filepath.Join("testdata", "testHashedSyms", "p.go")
+	cmd := exec.Command(testenv.GoToolPath(t), "run", src)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Errorf("command %s failed: %v\n%s", cmd, err, out)
+	}
+}