runtime/pprof: harden CPU profile test against smart backend

A couple of the CPU profiling testpoints make calls to helper
functions (cpuHog1, for example) where the computed value is always
thrown away by the caller without being used. A smart compiler back
end (in this case LLVM) can detect this fact and delete the contents
of the called function, which can cause tests to fail. Harden the test
slighly by passing in a value read from a global and insuring that the
caller stores the value back to a global; this prevents any optimizer
mischief.

Change-Id: Icbd6e3e32ff299c68a6397dc1404a52b21eaeaab
Reviewed-on: https://go-review.googlesource.com/76230
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

src/runtime/pprof/pprof_test.go

@@ -26,16 +26,18 @@ import (
 	"time"
 )
 
-func cpuHogger(f func() int, dur time.Duration) {
+func cpuHogger(f func(x int) int, y *int, dur time.Duration) {
 	// We only need to get one 100 Hz clock tick, so we've got
 	// a large safety buffer.
 	// But do at least 500 iterations (which should take about 100ms),
 	// otherwise TestCPUProfileMultithreaded can fail if only one
 	// thread is scheduled during the testing period.
 	t0 := time.Now()
+	accum := *y
 	for i := 0; i < 500 || time.Since(t0) < dur; i++ {
-		f()
+		accum = f(accum)
 	}
+	*y = accum
 }
 
 var (
@@ -46,8 +48,8 @@ var (
 // The actual CPU hogging function.
 // Must not call other functions nor access heap/globals in the loop,
 // otherwise under race detector the samples will be in the race runtime.
-func cpuHog1() int {
-	foo := salt1
+func cpuHog1(x int) int {
+	foo := x
 	for i := 0; i < 1e5; i++ {
 		if foo > 0 {
 			foo *= foo
@@ -58,8 +60,8 @@ func cpuHog1() int {
 	return foo
 }
 
-func cpuHog2() int {
-	foo := salt2
+func cpuHog2(x int) int {
+	foo := x
 	for i := 0; i < 1e5; i++ {
 		if foo > 0 {
 			foo *= foo
@@ -72,7 +74,7 @@ func cpuHog2() int {
 
 func TestCPUProfile(t *testing.T) {
 	testCPUProfile(t, []string{"runtime/pprof.cpuHog1"}, func(dur time.Duration) {
-		cpuHogger(cpuHog1, dur)
+		cpuHogger(cpuHog1, &salt1, dur)
 	})
 }
 
@@ -81,29 +83,29 @@ func TestCPUProfileMultithreaded(t *testing.T) {
 	testCPUProfile(t, []string{"runtime/pprof.cpuHog1", "runtime/pprof.cpuHog2"}, func(dur time.Duration) {
 		c := make(chan int)
 		go func() {
-			cpuHogger(cpuHog1, dur)
+			cpuHogger(cpuHog1, &salt1, dur)
 			c <- 1
 		}()
-		cpuHogger(cpuHog2, dur)
+		cpuHogger(cpuHog2, &salt2, dur)
 		<-c
 	})
 }
 
 func TestCPUProfileInlining(t *testing.T) {
 	testCPUProfile(t, []string{"runtime/pprof.inlinedCallee", "runtime/pprof.inlinedCaller"}, func(dur time.Duration) {
-		cpuHogger(inlinedCaller, dur)
+		cpuHogger(inlinedCaller, &salt1, dur)
 	})
 }
 
-func inlinedCaller() int {
-	inlinedCallee()
-	return 0
+func inlinedCaller(x int) int {
+	x = inlinedCallee(x)
+	return x
 }
 
-func inlinedCallee() {
+func inlinedCallee(x int) int {
 	// We could just use cpuHog1, but for loops prevent inlining
 	// right now. :(
-	foo := salt1
+	foo := x
 	i := 0
 loop:
 	if foo > 0 {
@@ -114,7 +116,7 @@ loop:
 	if i++; i < 1e5 {
 		goto loop
 	}
-	salt1 = foo
+	return foo
 }
 
 func parseProfile(t *testing.T, valBytes []byte, f func(uintptr, []*profile.Location, map[string][]string)) {
@@ -843,7 +845,7 @@ func TestEmptyCallStack(t *testing.T) {
 func TestCPUProfileLabel(t *testing.T) {
 	testCPUProfile(t, []string{"runtime/pprof.cpuHogger;key=value"}, func(dur time.Duration) {
 		Do(context.Background(), Labels("key", "value"), func(context.Context) {
-			cpuHogger(cpuHog1, dur)
+			cpuHogger(cpuHog1, &salt1, dur)
 		})
 	})
 }
@@ -856,14 +858,15 @@ func TestLabelRace(t *testing.T) {
 		start := time.Now()
 		var wg sync.WaitGroup
 		for time.Since(start) < dur {
+			var salts [10]int
 			for i := 0; i < 10; i++ {
 				wg.Add(1)
-				go func() {
+				go func(j int) {
 					Do(context.Background(), Labels("key", "value"), func(context.Context) {
-						cpuHogger(cpuHog1, time.Millisecond)
+						cpuHogger(cpuHog1, &salts[j], time.Millisecond)
 					})
 					wg.Done()
-				}()
+				}(i)
 			}
 			wg.Wait()
 		}