runtime: replace TestFutexsleep with TestTimediv

TestFutexsleep was originally created in CL 7876043 as a
regression test for buggy division logic in futexsleep. Several months
later CL 11575044 moved this logic to timediv (called by futexsleep).

This test calls runtime.Futexsleep, which temporarily disables
asynchronous preemption. Unfortunately, TestFutexSleep calls this from
multiple goroutines, creating a race condition that may result in
asynchronous preemption remaining disabled for the remainder of the
process lifetime.

We could fix this by moving the async preemption disable to the main
test function, however this test has had a history of flakiness. As an
alternative, this CL replaces the test wholesale with a new test for
timediv, covering the overflow logic without the difficulty of dealing
with futex.

Fixes #50749.

Change-Id: If9e1dac63ef1535adb49f9a9ffcaff99b9135895
Reviewed-on: https://go-review.googlesource.com/c/go/+/380058
Trust: Michael Pratt <[email protected]>
Run-TryBot: Michael Pratt <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: prattmic

src/runtime/export_futex_test.go

@@ -1328,3 +1328,5 @@ func Acquirem() {
 func Releasem() {
 	releasem(getg().m)
 }
+
+var Timediv = timediv

src/runtime/export_test.go

@@ -6,6 +6,7 @@ package runtime_test
 
 import (
 	"flag"
+	"fmt"
 	"io"
 	. "runtime"
 	"runtime/debug"
@@ -362,3 +363,78 @@ func TestVersion(t *testing.T) {
 		t.Fatalf("cr/nl in version: %q", vers)
 	}
 }
+
+func TestTimediv(t *testing.T) {
+	for _, tc := range []struct {
+		num int64
+		div int32
+		ret int32
+		rem int32
+	}{
+		{
+			num: 8,
+			div: 2,
+			ret: 4,
+			rem: 0,
+		},
+		{
+			num: 9,
+			div: 2,
+			ret: 4,
+			rem: 1,
+		},
+		{
+			// Used by runtime.check.
+			num: 12345*1000000000 + 54321,
+			div: 1000000000,
+			ret: 12345,
+			rem: 54321,
+		},
+		{
+			num: 1<<32 - 1,
+			div: 2,
+			ret: 1<<31 - 1, // no overflow.
+			rem: 1,
+		},
+		{
+			num: 1 << 32,
+			div: 2,
+			ret: 1<<31 - 1, // overflow.
+			rem: 0,
+		},
+		{
+			num: 1 << 40,
+			div: 2,
+			ret: 1<<31 - 1, // overflow.
+			rem: 0,
+		},
+		{
+			num: 1<<40 + 1,
+			div: 1 << 10,
+			ret: 1 << 30,
+			rem: 1,
+		},
+	} {
+		name := fmt.Sprintf("%d div %d", tc.num, tc.div)
+		t.Run(name, func(t *testing.T) {
+			// Double check that the inputs make sense using
+			// standard 64-bit division.
+			ret64 := tc.num / int64(tc.div)
+			rem64 := tc.num % int64(tc.div)
+			if ret64 != int64(int32(ret64)) {
+				// Simulate timediv overflow value.
+				ret64 = 1<<31 - 1
+				rem64 = 0
+			}
+			if ret64 != int64(tc.ret) {
+				t.Errorf("%d / %d got ret %d rem %d want ret %d rem %d", tc.num, tc.div, ret64, rem64, tc.ret, tc.rem)
+			}
+
+			var rem int32
+			ret := Timediv(tc.num, tc.div, &rem)
+			if ret != tc.ret || rem != tc.rem {
+				t.Errorf("timediv %d / %d got ret %d rem %d want ret %d rem %d", tc.num, tc.div, ret, rem, tc.ret, tc.rem)
+			}
+		})
+	}
+}