cmd/link: don't mark shared library symbols reachable unconditionally

During the transitioning period, we mark symbols from Go shared
libraries reachable unconditionally. That might be useful when
there was still a large portion of the linker using sym.Symbols,
and only reachable symbols were converted to sym.Symbols. Marking
them reachable brings them to the dynamic symbol table, even if
they are not needed, increased the binary size unexpectedly.

That time has passed. Now we largely operate on loader symbols,
and it is not needed to mark them reachable anymore.

Fixes #40416.

Change-Id: I1e2bdb93a960ba7dc96575fabe15af93d8e95329
Reviewed-on: https://go-review.googlesource.com/c/go/+/244839
Run-TryBot: Cherry Zhang <[email protected]>
Reviewed-by: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: cherrymui

misc/cgo/testshared/shared_test.go

@@ -462,6 +462,7 @@ func TestTrivialExecutable(t *testing.T) {
 	run(t, "trivial executable", "../../bin/trivial")
 	AssertIsLinkedTo(t, "../../bin/trivial", soname)
 	AssertHasRPath(t, "../../bin/trivial", gorootInstallDir)
+	checkSize(t, "../../bin/trivial", 100000) // it is 19K on linux/amd64, 100K should be enough
 }
 
 // Build a trivial program in PIE mode that links against the shared runtime and check it runs.
@@ -470,6 +471,18 @@ func TestTrivialExecutablePIE(t *testing.T) {
 	run(t, "trivial executable", "./trivial.pie")
 	AssertIsLinkedTo(t, "./trivial.pie", soname)
 	AssertHasRPath(t, "./trivial.pie", gorootInstallDir)
+	checkSize(t, "./trivial.pie", 100000) // it is 19K on linux/amd64, 100K should be enough
+}
+
+// Check that the file size does not exceed a limit.
+func checkSize(t *testing.T, f string, limit int64) {
+	fi, err := os.Stat(f)
+	if err != nil {
+		t.Fatalf("stat failed: %v", err)
+	}
+	if sz := fi.Size(); sz > limit {
+		t.Errorf("file too large: got %d, want <= %d", sz, limit)
+	}
 }
 
 // Build a division test program and check it runs.

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

@@ -2191,17 +2191,6 @@ func ldshlibsyms(ctxt *Link, shlib string) {
 		l.SetSymElfType(s, elf.ST_TYPE(elfsym.Info))
 		su.SetSize(int64(elfsym.Size))
 		if elfsym.Section != elf.SHN_UNDEF {
-			// If it's not undefined, mark the symbol as reachable
-			// so as to protect it from dead code elimination,
-			// even if there aren't any explicit references to it.
-			// Under the previous sym.Symbol based regime this
-			// wasn't necessary, but for the loader-based deadcode
-			// it is definitely needed.
-			//
-			// FIXME: have a more general/flexible mechanism for this?
-			//
-			l.SetAttrReachable(s, true)
-
 			// Set .File for the library that actually defines the symbol.
 			l.SetSymPkg(s, libpath)