cmd/link: don't mark a symbol's Gotype reachable

A symbol being reachable doesn't imply its type descriptor is
needed. Don't mark it.

If the type is converted to interface somewhere in the program,
there will be an explicit use of the type descriptor, which
will make it marked.

A println("hello") program before and after

-rwxr-xr-x  1 cherryyz  primarygroup  1259824 Apr 30 23:00 hello
-rwxr-xr-x  1 cherryyz  primarygroup  1169680 Apr 30 23:10 hello

Updates #38782.
Updates #6853.

Change-Id: I88884c126ce75ba073f1ba059c4b892c87d2ac96
Reviewed-on: https://go-review.googlesource.com/c/go/+/231397
Run-TryBot: Cherry Zhang <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Alessandro Arzilli <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

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

@@ -6,6 +6,7 @@ package ld
 
 import (
 	"bytes"
+	"cmd/internal/goobj2"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
 	"cmd/link/internal/loader"
@@ -154,7 +155,15 @@ func (d *deadcodePass) flood() {
 		}
 		naux := d.ldr.NAux(symIdx)
 		for i := 0; i < naux; i++ {
-			d.mark(d.ldr.Aux2(symIdx, i).Sym(), symIdx)
+			a := d.ldr.Aux2(symIdx, i)
+			if a.Type() == goobj2.AuxGotype && !d.ctxt.linkShared {
+				// A symbol being reachable doesn't imply we need its
+				// type descriptor. Don't mark it.
+				// XXX we need that for GCProg generation when linking
+				// shared library. why?
+				continue
+			}
+			d.mark(a.Sym(), symIdx)
 		}
 		// Some host object symbols have an outer object, which acts like a
 		// "carrier" symbol, or it holds all the symbols for a particular

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

@@ -14,48 +14,36 @@ import (
 	"testing"
 )
 
-// This example uses reflect.Value.Call, but not
-// reflect.{Value,Type}.Method. This should not
-// need to bring all methods live.
-const deadcodeTestSrc = `
-package main
-import "reflect"
-
-func f() { println("call") }
-
-type T int
-func (T) M() {}
-
-func main() {
-	v := reflect.ValueOf(f)
-	v.Call(nil)
-	i := interface{}(T(1))
-	println(i)
-}
-`
-
 func TestDeadcode(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
+	t.Parallel()
 
 	tmpdir, err := ioutil.TempDir("", "TestDeadcode")
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(tmpdir)
 
-	src := filepath.Join(tmpdir, "main.go")
-	err = ioutil.WriteFile(src, []byte(deadcodeTestSrc), 0666)
-	if err != nil {
-		t.Fatal(err)
-	}
-	exe := filepath.Join(tmpdir, "main.exe")
-
-	cmd := exec.Command(testenv.GoToolPath(t), "build", "-ldflags=-dumpdep", "-o", exe, src)
-	out, err := cmd.CombinedOutput()
-	if err != nil {
-		t.Fatalf("%v: %v:\n%s", cmd.Args, err, out)
+	tests := []struct {
+		src     string
+		pattern string
+	}{
+		{"reflectcall", "main.T.M"},
+		{"typedesc", "type.main.T"},
 	}
-	if bytes.Contains(out, []byte("main.T.M")) {
-		t.Errorf("main.T.M should not be reachable. Output:\n%s", out)
+	for _, test := range tests {
+		t.Run(test.src, func(t *testing.T) {
+			t.Parallel()
+			src := filepath.Join("testdata", "deadcode", test.src+".go")
+			exe := filepath.Join(tmpdir, test.src+".exe")
+			cmd := exec.Command(testenv.GoToolPath(t), "build", "-ldflags=-dumpdep", "-o", exe, src)
+			out, err := cmd.CombinedOutput()
+			if err != nil {
+				t.Fatalf("%v: %v:\n%s", cmd.Args, err, out)
+			}
+			if bytes.Contains(out, []byte(test.pattern)) {
+				t.Errorf("%s should not be reachable. Output:\n%s", test.pattern, out)
+			}
+		})
 	}
 }

src/cmd/link/internal/ld/testdata/deadcode/reflectcall.go

@@ -0,0 +1,24 @@
+// Copyright 2020 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.
+
+// This example uses reflect.Value.Call, but not
+// reflect.{Value,Type}.Method. This should not
+// need to bring all methods live.
+
+package main
+
+import "reflect"
+
+func f() { println("call") }
+
+type T int
+
+func (T) M() {}
+
+func main() {
+	v := reflect.ValueOf(f)
+	v.Call(nil)
+	i := interface{}(T(1))
+	println(i)
+}

src/cmd/link/internal/ld/testdata/deadcode/typedesc.go

@@ -0,0 +1,16 @@
+// Copyright 2020 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 a live variable doesn't bring its type
+// descriptor live.
+
+package main
+
+type T [10]string
+
+var t T
+
+func main() {
+	println(t[8])
+}