cmd/link: fix -buildmode=pie / -linkshared combination

main.main and main.init were not being marked as reachable.

Fixes #17076

Change-Id: Ib3e29bd35ba6252962e6ba89173ca321ed6849b9
Reviewed-on: https://go-review.googlesource.com/28996
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testshared/shared_test.go

@@ -394,6 +394,14 @@ func TestTrivialExecutable(t *testing.T) {
 	AssertHasRPath(t, "./bin/trivial", gorootInstallDir)
 }
 
+// Build a trivial program in PIE mode that links against the shared runtime and check it runs.
+func TestTrivialExecutablePIE(t *testing.T) {
+	goCmd(t, "build", "-buildmode=pie", "-o", "trivial.pie", "-linkshared", "trivial")
+	run(t, "trivial executable", "./trivial.pie")
+	AssertIsLinkedTo(t, "./trivial.pie", soname)
+	AssertHasRPath(t, "./trivial.pie", gorootInstallDir)
+}
+
 // Build an executable that uses cgo linked against the shared runtime and check it
 // runs.
 func TestCgoExecutable(t *testing.T) {

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

@@ -241,7 +241,7 @@ func (d *deadcodepass) init() {
 		// In a normal binary, start at main.main and the init
 		// functions and mark what is reachable from there.
 		names = append(names, *flagEntrySymbol)
-		if *FlagLinkshared && Buildmode == BuildmodeExe {
+		if *FlagLinkshared && (Buildmode == BuildmodeExe || Buildmode == BuildmodePIE) {
 			names = append(names, "main.main", "main.init")
 		}
 		for _, name := range markextra {