runtime, cmd/link/internal/ld: disable memory profiling when data unreachable

If runtime.MemProfile is unreachable, default to not collecting any
memory profiling samples, to save memory on the hash table.

Fixes #42347

Change-Id: I9a4894a5fc77035fe59b1842e1ec77a1182e70c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/299671
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Keith Randall <khr@golang.org>

Author: bradfitz

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

@@ -224,3 +224,103 @@ func testWindowsBuildmodeCSharedASLR(t *testing.T, useASLR bool) {
 		t.Error("IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE flag should not be set")
 	}
 }
+
+// TestMemProfileCheck tests that cmd/link sets
+// runtime.disableMemoryProfiling if the runtime.MemProfile
+// symbol is unreachable after deadcode (and not dynlinking).
+// The runtime then uses that to set the default value of
+// runtime.MemProfileRate, which this test checks.
+func TestMemProfileCheck(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		prog    string
+		wantOut string
+	}{
+		{
+			"no_memprofile",
+			`
+package main
+import "runtime"
+func main() {
+	println(runtime.MemProfileRate)
+}
+`,
+			"0",
+		},
+		{
+			"with_memprofile",
+			`
+package main
+import "runtime"
+func main() {
+	runtime.MemProfile(nil, false)
+	println(runtime.MemProfileRate)
+}
+`,
+			"524288",
+		},
+		{
+			"with_memprofile_indirect",
+			`
+package main
+import "runtime"
+var f = runtime.MemProfile
+func main() {
+	if f == nil {
+		panic("no f")
+	}
+	println(runtime.MemProfileRate)
+}
+`,
+			"524288",
+		},
+		{
+			"with_memprofile_runtime_pprof",
+			`
+package main
+import "runtime"
+import "runtime/pprof"
+func main() {
+        _ = pprof.Profiles()
+	println(runtime.MemProfileRate)
+}
+`,
+			"524288",
+		},
+		{
+			"with_memprofile_http_pprof",
+			`
+package main
+import "runtime"
+import _ "net/http/pprof"
+func main() {
+	println(runtime.MemProfileRate)
+}
+`,
+			"524288",
+		},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			tempDir := t.TempDir()
+			src := filepath.Join(tempDir, "x.go")
+			if err := ioutil.WriteFile(src, []byte(tt.prog), 0644); err != nil {
+				t.Fatal(err)
+			}
+			cmd := exec.Command(testenv.GoToolPath(t), "run", src)
+			out, err := cmd.CombinedOutput()
+			if err != nil {
+				t.Fatal(err)
+			}
+			got := strings.TrimSpace(string(out))
+			if got != tt.wantOut {
+				t.Errorf("got %q; want %q", got, tt.wantOut)
+			}
+		})
+	}
+}

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

@@ -790,6 +790,18 @@ func (ctxt *Link) linksetup() {
 			sb.SetSize(0)
 			sb.AddUint8(uint8(objabi.GOARM))
 		}
+
+		// Set runtime.disableMemoryProfiling bool if
+		// runtime.MemProfile is not retained in the binary after
+		// deadcode (and we're not dynamically linking).
+		memProfile := ctxt.loader.Lookup("runtime.MemProfile", sym.SymVerABIInternal)
+		if memProfile != 0 && !ctxt.loader.AttrReachable(memProfile) && !ctxt.DynlinkingGo() {
+			memProfSym := ctxt.loader.LookupOrCreateSym("runtime.disableMemoryProfiling", 0)
+			sb := ctxt.loader.MakeSymbolUpdater(memProfSym)
+			sb.SetType(sym.SDATA)
+			sb.SetSize(0)
+			sb.AddUint8(1) // true bool
+		}
 	} else {
 		// If OTOH the module does not contain the runtime package,
 		// create a local symbol for the moduledata.

src/runtime/mprof.go

@@ -490,7 +490,22 @@ func (r *StackRecord) Stack() []uintptr {
 // memory profiling rate should do so just once, as early as
 // possible in the execution of the program (for example,
 // at the beginning of main).
-var MemProfileRate int = 512 * 1024
+var MemProfileRate int = defaultMemProfileRate(512 * 1024)
+
+// defaultMemProfileRate returns 0 if disableMemoryProfiling is set.
+// It exists primarily for the godoc rendering of MemProfileRate
+// above.
+func defaultMemProfileRate(v int) int {
+	if disableMemoryProfiling {
+		return 0
+	}
+	return v
+}
+
+// disableMemoryProfiling is set by the linker if runtime.MemProfile
+// is not used and the link type guarantees nobody else could use it
+// elsewhere.
+var disableMemoryProfiling bool
 
 // A MemProfileRecord describes the live objects allocated
 // by a particular call sequence (stack trace).