math/rand: use fastrand64 if possible

Now that the top-level math/rand functions are auto-seeded by default
(issue #54880), use the runtime fastrand64 function when 1) Seed
has not been called; 2) the GODEBUG randautoseed=0 is not used.

The benchmarks run quickly and are relatively noisy, but they show
significant improvements for parallel calls to the top-level functions.

goos: linux
goarch: amd64
pkg: math/rand
cpu: 11th Gen Intel(R) Core(TM) i7-11850H @ 2.50GHz
                           │  /tmp/foo.1   │              /tmp/foo.2               │
                           │    sec/op     │    sec/op      vs base                │
Int63Threadsafe-16            11.605n ± 1%    3.094n ±  1%  -73.34% (p=0.000 n=10)
Int63ThreadsafeParallel-16   67.8350n ± 2%   0.4000n ±  1%  -99.41% (p=0.000 n=10)
Int63Unthreadsafe-16           1.947n ± 3%    1.924n ±  2%        ~ (p=0.189 n=10)
Intn1000-16                    4.295n ± 2%    4.287n ±  3%        ~ (p=0.517 n=10)
Int63n1000-16                  4.379n ± 0%    4.192n ±  2%   -4.27% (p=0.000 n=10)
Int31n1000-16                  3.641n ± 3%    3.506n ±  0%   -3.69% (p=0.000 n=10)
Float32-16                     3.330n ± 7%    3.250n ±  2%   -2.40% (p=0.017 n=10)
Float64-16                     2.194n ± 6%    2.056n ±  4%   -6.31% (p=0.004 n=10)
Perm3-16                       43.39n ± 9%    38.28n ± 12%  -11.77% (p=0.015 n=10)
Perm30-16                      324.4n ± 6%    315.9n ± 19%        ~ (p=0.315 n=10)
Perm30ViaShuffle-16            175.4n ± 1%    143.6n ±  2%  -18.15% (p=0.000 n=10)
ShuffleOverhead-16             223.4n ± 2%    215.8n ±  1%   -3.38% (p=0.000 n=10)
Read3-16                       5.428n ± 3%    5.406n ±  2%        ~ (p=0.780 n=10)
Read64-16                      41.55n ± 5%    40.14n ±  3%   -3.38% (p=0.000 n=10)
Read1000-16                    622.9n ± 4%    594.9n ±  2%   -4.50% (p=0.000 n=10)
Concurrent-16                136.300n ± 2%    4.647n ± 26%  -96.59% (p=0.000 n=10)
geomean                        23.40n         12.15n        -48.08%

Fixes #49892

Change-Id: Iba75b326145512ab0b7ece233b98ac3d4e1fb504
Reviewed-on: https://go-review.googlesource.com/c/go/+/465037
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/math/rand/default_test.go

@@ -0,0 +1,148 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package rand_test
+
+import (
+	"fmt"
+	"internal/race"
+	"internal/testenv"
+	. "math/rand"
+	"os"
+	"runtime"
+	"strconv"
+	"sync"
+	"testing"
+)
+
+// Test that racy access to the default functions behaves reasonably.
+func TestDefaultRace(t *testing.T) {
+	// Skip the test in short mode, but even in short mode run
+	// the test if we are using the race detector, because part
+	// of this is to see whether the race detector reports any problems.
+	if testing.Short() && !race.Enabled {
+		t.Skip("skipping starting another executable in short mode")
+	}
+
+	const env = "GO_RAND_TEST_HELPER_CODE"
+	if v := os.Getenv(env); v != "" {
+		doDefaultTest(t, v)
+		return
+	}
+
+	t.Parallel()
+
+	for i := 0; i < 6; i++ {
+		i := i
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			t.Parallel()
+			exe, err := os.Executable()
+			if err != nil {
+				exe = os.Args[0]
+			}
+			cmd := testenv.Command(t, exe, "-test.run=TestDefaultRace")
+			cmd = testenv.CleanCmdEnv(cmd)
+			cmd.Env = append(cmd.Env, fmt.Sprintf("GO_RAND_TEST_HELPER_CODE=%d", i/2))
+			if i%2 != 0 {
+				cmd.Env = append(cmd.Env, "GODEBUG=randautoseed=0")
+			}
+			out, err := cmd.CombinedOutput()
+			if len(out) > 0 {
+				t.Logf("%s", out)
+			}
+			if err != nil {
+				t.Error(err)
+			}
+		})
+	}
+}
+
+// doDefaultTest should be run before there have been any calls to the
+// top-level math/rand functions. Make sure that we can make concurrent
+// calls to top-level functions and to Seed without any duplicate values.
+// This will also give the race detector a change to report any problems.
+func doDefaultTest(t *testing.T, v string) {
+	code, err := strconv.Atoi(v)
+	if err != nil {
+		t.Fatalf("internal error: unrecognized code %q", v)
+	}
+
+	goroutines := runtime.GOMAXPROCS(0)
+	if goroutines < 4 {
+		goroutines = 4
+	}
+
+	ch := make(chan uint64, goroutines*3)
+	var wg sync.WaitGroup
+
+	// The various tests below should not cause race detector reports
+	// and should not produce duplicate results.
+	//
+	// Note: these tests can theoretically fail when using fastrand64
+	// in that it is possible to coincidentally get the same random
+	// number twice. That could happen something like 1 / 2**64 times,
+	// which is rare enough that it may never happen. We don't worry
+	// about that case.
+
+	switch code {
+	case 0:
+		// Call Seed and Uint64 concurrently.
+		wg.Add(goroutines)
+		for i := 0; i < goroutines; i++ {
+			go func(s int64) {
+				defer wg.Done()
+				Seed(s)
+			}(int64(i) + 100)
+		}
+		wg.Add(goroutines)
+		for i := 0; i < goroutines; i++ {
+			go func() {
+				defer wg.Done()
+				ch <- Uint64()
+			}()
+		}
+	case 1:
+		// Call Uint64 concurrently with no Seed.
+		wg.Add(goroutines)
+		for i := 0; i < goroutines; i++ {
+			go func() {
+				defer wg.Done()
+				ch <- Uint64()
+			}()
+		}
+	case 2:
+		// Start with Uint64 to pick the fast source, then call
+		// Seed and Uint64 concurrently.
+		ch <- Uint64()
+		wg.Add(goroutines)
+		for i := 0; i < goroutines; i++ {
+			go func(s int64) {
+				defer wg.Done()
+				Seed(s)
+			}(int64(i) + 100)
+		}
+		wg.Add(goroutines)
+		for i := 0; i < goroutines; i++ {
+			go func() {
+				defer wg.Done()
+				ch <- Uint64()
+			}()
+		}
+	default:
+		t.Fatalf("internal error: unrecognized code %d", code)
+	}
+
+	go func() {
+		wg.Wait()
+		close(ch)
+	}()
+
+	m := make(map[uint64]bool)
+	for i := range ch {
+		if m[i] {
+			t.Errorf("saw %d twice", i)
+		}
+		m[i] = true
+	}
+}