math/rand/v2: simplify Perm

The compiler says Perm is being inlined into BenchmarkPerm,
and yet BenchmarkPerm30ViaShuffle, which you'd think is the
same code, still runs significantly faster.

The benchmarks are mystifying but this is clearly still a step in
the right direction, since BenchmarkPerm30ViaShuffle is still
the fastest and we avoid having two copies of that logic.

goos: linux
goarch: amd64
pkg: math/rand/v2
cpu: AMD Ryzen 9 7950X 16-Core Processor
                        │ e1bbe739fb.amd64 │           8993506f2f.amd64           │
                        │      sec/op      │    sec/op     vs base                │
SourceUint64-32                1.316n ± 2%    1.325n ± 1%        ~ (p=0.208 n=20)
GlobalInt64-32                 2.048n ± 1%    2.240n ± 1%   +9.38% (p=0.000 n=20)
GlobalInt64Parallel-32        0.1037n ± 1%   0.1041n ± 1%        ~ (p=0.774 n=20)
GlobalUint64-32                2.039n ± 2%    2.072n ± 3%        ~ (p=0.115 n=20)
GlobalUint64Parallel-32       0.1013n ± 1%   0.1008n ± 1%        ~ (p=0.417 n=20)
Int64-32                       1.692n ± 2%    1.716n ± 1%        ~ (p=0.122 n=20)
Uint64-32                      1.643n ± 2%    1.665n ± 1%        ~ (p=0.062 n=20)
GlobalIntN1000-32              3.287n ± 1%    3.335n ± 1%        ~ (p=0.147 n=20)
IntN1000-32                    2.678n ± 2%    2.484n ± 1%   -7.24% (p=0.000 n=20)
Int64N1000-32                  2.684n ± 2%    2.502n ± 2%   -6.80% (p=0.000 n=20)
Int64N1e8-32                   2.663n ± 2%    2.484n ± 2%   -6.76% (p=0.000 n=20)
Int64N1e9-32                   2.633n ± 1%    2.502n ± 0%   -4.98% (p=0.000 n=20)
Int64N2e9-32                   2.657n ± 1%    2.502n ± 0%   -5.87% (p=0.000 n=20)
Int64N1e18-32                  3.125n ± 2%    3.201n ± 1%   +2.43% (p=0.000 n=20)
Int64N2e18-32                  3.476n ± 1%    3.504n ± 1%   +0.83% (p=0.009 n=20)
Int64N4e18-32                  4.795n ± 1%    4.873n ± 1%        ~ (p=0.106 n=20)
Int32N1000-32                  2.485n ± 2%    2.639n ± 1%   +6.20% (p=0.000 n=20)
Int32N1e8-32                   2.457n ± 1%    2.686n ± 2%   +9.34% (p=0.000 n=20)
Int32N1e9-32                   2.452n ± 1%    2.636n ± 1%   +7.52% (p=0.000 n=20)
Int32N2e9-32                   2.453n ± 1%    2.660n ± 1%   +8.44% (p=0.000 n=20)
Float32-32                     2.254n ± 1%    2.261n ± 1%        ~ (p=0.888 n=20)
Float64-32                     2.262n ± 1%    2.280n ± 1%        ~ (p=0.040 n=20)
ExpFloat64-32                  3.777n ± 2%    3.891n ± 1%   +3.03% (p=0.000 n=20)
NormFloat64-32                 3.606n ± 1%    3.711n ± 1%   +2.91% (p=0.000 n=20)
Perm3-32                       33.12n ± 2%    32.60n ± 2%        ~ (p=0.045 n=20)
Perm30-32                      176.1n ± 1%    204.2n ± 0%  +15.96% (p=0.000 n=20)
Perm30ViaShuffle-32            109.3n ± 1%    121.7n ± 2%  +11.30% (p=0.000 n=20)
ShuffleOverhead-32             112.5n ± 1%    106.2n ± 2%   -5.56% (p=0.000 n=20)
Concurrent-32                  2.099n ± 0%    2.190n ± 5%   +4.36% (p=0.001 n=20)

goos: darwin
goarch: arm64
pkg: math/rand/v2
cpu: Apple M1
                       │ e1bbe739fb.arm64 │           8993506f2f.arm64           │
                       │      sec/op      │    sec/op     vs base                │
SourceUint64-8                2.290n ± 1%    2.271n ± 0%        ~ (p=0.015 n=20)
GlobalInt64-8                 2.180n ± 1%    2.161n ± 1%        ~ (p=0.180 n=20)
GlobalInt64Parallel-8        0.4294n ± 0%   0.4303n ± 0%   +0.19% (p=0.001 n=20)
GlobalUint64-8                2.170n ± 1%    2.164n ± 1%        ~ (p=0.673 n=20)
GlobalUint64Parallel-8       0.4283n ± 0%   0.4287n ± 0%        ~ (p=0.128 n=20)
Int64-8                       2.481n ± 1%    2.478n ± 1%        ~ (p=0.867 n=20)
Uint64-8                      2.464n ± 1%    2.460n ± 1%        ~ (p=0.763 n=20)
GlobalIntN1000-8              2.814n ± 0%    2.814n ± 2%        ~ (p=0.969 n=20)
IntN1000-8                    2.934n ± 2%    3.003n ± 2%   +2.35% (p=0.000 n=20)
Int64N1000-8                  2.957n ± 1%    2.954n ± 0%        ~ (p=0.285 n=20)
Int64N1e8-8                   2.935n ± 2%    2.956n ± 0%   +0.73% (p=0.002 n=20)
Int64N1e9-8                   2.935n ± 2%    3.325n ± 0%  +13.29% (p=0.000 n=20)
Int64N2e9-8                   2.933n ± 4%    2.956n ± 2%        ~ (p=0.163 n=20)
Int64N1e18-8                  3.781n ± 1%    3.780n ± 1%        ~ (p=0.805 n=20)
Int64N2e18-8                  4.362n ± 0%    4.385n ± 0%        ~ (p=0.077 n=20)
Int64N4e18-8                  6.576n ± 1%    6.527n ± 0%        ~ (p=0.024 n=20)
Int32N1000-8                  2.942n ± 2%    2.964n ± 1%        ~ (p=0.073 n=20)
Int32N1e8-8                   2.941n ± 1%    2.964n ± 1%        ~ (p=0.058 n=20)
Int32N1e9-8                   2.938n ± 2%    2.963n ± 2%   +0.87% (p=0.003 n=20)
Int32N2e9-8                   2.982n ± 2%    2.961n ± 2%        ~ (p=0.056 n=20)
Float32-8                     3.441n ± 0%    3.442n ± 0%        ~ (p=0.030 n=20)
Float64-8                     3.441n ± 0%    3.442n ± 0%   +0.03% (p=0.001 n=20)
ExpFloat64-8                  4.472n ± 0%    4.472n ± 0%        ~ (p=0.877 n=20)
NormFloat64-8                 4.716n ± 0%    4.734n ± 0%   +0.38% (p=0.000 n=20)
Perm3-8                       26.66n ± 0%    26.55n ± 0%   -0.39% (p=0.000 n=20)
Perm30-8                      143.3n ± 0%    181.9n ± 0%  +26.97% (p=0.000 n=20)
Perm30ViaShuffle-8            142.9n ± 0%    143.1n ± 0%        ~ (p=0.669 n=20)
ShuffleOverhead-8             121.1n ± 1%    120.6n ± 1%   -0.41% (p=0.004 n=20)
Concurrent-8                  2.379n ± 2%    2.357n ± 2%        ~ (p=0.337 n=20)

goos: linux
goarch: 386
pkg: math/rand/v2
cpu: AMD Ryzen 9 7950X 16-Core Processor
                        │ e1bbe739fb.386 │            8993506f2f.386            │
                        │     sec/op     │    sec/op     vs base                │
SourceUint64-32              2.087n ± 1%    2.102n ± 2%        ~ (p=0.507 n=20)
GlobalInt64-32               3.538n ± 2%    3.542n ± 2%        ~ (p=0.425 n=20)
GlobalInt64Parallel-32      0.3207n ± 1%   0.3202n ± 0%        ~ (p=0.963 n=20)
GlobalUint64-32              3.543n ± 1%    3.507n ± 1%        ~ (p=0.034 n=20)
GlobalUint64Parallel-32     0.3170n ± 0%   0.3170n ± 1%        ~ (p=0.920 n=20)
Int64-32                     2.548n ± 1%    2.516n ± 1%        ~ (p=0.139 n=20)
Uint64-32                    2.565n ± 2%    2.544n ± 1%        ~ (p=0.394 n=20)
GlobalIntN1000-32            6.300n ± 1%    6.237n ± 1%        ~ (p=0.029 n=20)
IntN1000-32                  4.750n ± 0%    4.670n ± 2%        ~ (p=0.034 n=20)
Int64N1000-32                5.515n ± 2%    5.412n ± 1%   -1.86% (p=0.009 n=20)
Int64N1e8-32                 5.527n ± 0%    5.414n ± 2%   -2.05% (p=0.002 n=20)
Int64N1e9-32                 5.531n ± 2%    5.473n ± 1%        ~ (p=0.047 n=20)
Int64N2e9-32                 5.514n ± 2%    5.487n ± 1%        ~ (p=0.298 n=20)
Int64N1e18-32                9.059n ± 1%    8.901n ± 2%        ~ (p=0.037 n=20)
Int64N2e18-32                9.594n ± 1%    9.521n ± 1%        ~ (p=0.051 n=20)
Int64N4e18-32                12.05n ± 2%    11.92n ± 1%        ~ (p=0.357 n=20)
Int32N1000-32                4.840n ± 2%    4.785n ± 1%        ~ (p=0.189 n=20)
Int32N1e8-32                 4.832n ± 2%    4.748n ± 1%        ~ (p=0.042 n=20)
Int32N1e9-32                 4.815n ± 2%    4.810n ± 1%        ~ (p=0.878 n=20)
Int32N2e9-32                 4.813n ± 1%    4.812n ± 1%        ~ (p=0.542 n=20)
Float32-32                   10.90n ± 2%    10.48n ± 4%   -3.85% (p=0.007 n=20)
Float64-32                   20.32n ± 4%    19.79n ± 3%        ~ (p=0.553 n=20)
ExpFloat64-32                12.95n ± 3%    12.91n ± 3%        ~ (p=0.909 n=20)
NormFloat64-32               7.570n ± 1%    7.462n ± 1%   -1.44% (p=0.004 n=20)
Perm3-32                     37.80n ± 2%    35.98n ± 2%   -4.79% (p=0.000 n=20)
Perm30-32                    214.0n ± 1%    241.5n ± 1%  +12.85% (p=0.000 n=20)
Perm30ViaShuffle-32          188.7n ± 2%    187.3n ± 2%        ~ (p=0.029 n=20)
ShuffleOverhead-32           160.8n ± 1%    160.2n ± 1%        ~ (p=0.180 n=20)
Concurrent-32                3.288n ± 0%    3.308n ± 3%        ~ (p=0.037 n=20)

For #61716.

Change-Id: I342b611456c3569520d3c91c849d29eba325d87e
Reviewed-on: https://go-review.googlesource.com/c/go/+/502504
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Rob Pike <[email protected]>

Author: rsc

src/math/rand/v2/example_test.go

@@ -93,7 +93,7 @@ func Example_rand() {
 	// IntN(10)    8                    4                   5
 	// Int32N(10)  1                    8                   5
 	// Int64N(10)  4                    2                   6
-	// Perm        [0 4 1 3 2]          [4 0 1 3 2]         [4 1 3 0 2]
+	// Perm        [0 2 4 3 1]          [0 4 2 3 1]         [2 1 3 0 4]
 }
 
 func ExamplePerm() {

src/math/rand/v2/rand.go

@@ -230,18 +230,12 @@ func (r *Rand) Float32() float32 {
 // Perm returns, as a slice of n ints, a pseudo-random permutation of the integers
 // in the half-open interval [0,n).
 func (r *Rand) Perm(n int) []int {
-	m := make([]int, n)
-	// In the following loop, the iteration when i=0 always swaps m[0] with m[0].
-	// A change to remove this useless iteration is to assign 1 to i in the init
-	// statement. But Perm also effects r. Making this change will affect
-	// the final state of r. So this change can't be made for compatibility
-	// reasons for Go 1.
-	for i := 0; i < n; i++ {
-		j := r.IntN(i + 1)
-		m[i] = m[j]
-		m[j] = i
+	p := make([]int, n)
+	for i := range p {
+		p[i] = i
 	}
-	return m
+	r.Shuffle(len(p), func(i, j int) { p[i], p[j] = p[j], p[i] })
+	return p
 }
 
 // Shuffle pseudo-randomizes the order of elements.

src/math/rand/v2/regress_test.go

@@ -437,24 +437,24 @@ var regressGolden = []any{
 
 	[]int{},                             // Perm(0)
 	[]int{0},                            // Perm(1)
-	[]int{0, 4, 2, 1, 3},                // Perm(5)
-	[]int{2, 4, 5, 0, 7, 1, 3, 6},       // Perm(8)
-	[]int{6, 4, 1, 5, 7, 3, 0, 8, 2},    // Perm(9)
-	[]int{8, 0, 1, 2, 3, 9, 5, 4, 7, 6}, // Perm(10)
-	[]int{0, 13, 14, 7, 1, 4, 15, 10, 11, 12, 9, 5, 3, 6, 8, 2}, // Perm(16)
+	[]int{0, 4, 2, 3, 1},                // Perm(5)
+	[]int{4, 5, 7, 0, 6, 3, 2, 1},       // Perm(8)
+	[]int{2, 5, 4, 0, 7, 8, 1, 6, 3},    // Perm(9)
+	[]int{9, 8, 7, 1, 3, 2, 5, 4, 0, 6}, // Perm(10)
+	[]int{1, 5, 8, 11, 14, 2, 7, 10, 15, 9, 13, 6, 0, 3, 12, 4}, // Perm(16)
 	[]int{},                             // Perm(0)
 	[]int{0},                            // Perm(1)
-	[]int{3, 2, 4, 0, 1},                // Perm(5)
-	[]int{7, 1, 6, 4, 2, 3, 5, 0},       // Perm(8)
-	[]int{1, 7, 2, 6, 3, 5, 8, 4, 0},    // Perm(9)
-	[]int{1, 5, 7, 0, 3, 6, 4, 9, 2, 8}, // Perm(10)
-	[]int{6, 13, 2, 11, 14, 7, 10, 12, 4, 5, 3, 0, 15, 9, 1, 8}, // Perm(16)
+	[]int{4, 1, 2, 0, 3},                // Perm(5)
+	[]int{7, 0, 3, 5, 4, 1, 2, 6},       // Perm(8)
+	[]int{6, 7, 1, 2, 0, 5, 8, 3, 4},    // Perm(9)
+	[]int{7, 2, 8, 6, 1, 5, 9, 0, 3, 4}, // Perm(10)
+	[]int{11, 0, 5, 1, 12, 4, 13, 9, 7, 2, 15, 10, 8, 14, 6, 3}, // Perm(16)
 	[]int{},                             // Perm(0)
 	[]int{0},                            // Perm(1)
-	[]int{0, 4, 2, 1, 3},                // Perm(5)
-	[]int{0, 7, 1, 4, 3, 6, 2, 5},       // Perm(8)
-	[]int{1, 3, 0, 4, 5, 2, 8, 7, 6},    // Perm(9)
-	[]int{5, 4, 7, 9, 6, 1, 0, 3, 8, 2}, // Perm(10)
+	[]int{2, 4, 0, 3, 1},                // Perm(5)
+	[]int{4, 2, 5, 0, 6, 3, 1, 7},       // Perm(8)
+	[]int{3, 2, 8, 6, 5, 7, 1, 4, 0},    // Perm(9)
+	[]int{2, 0, 7, 5, 6, 1, 8, 3, 4, 9}, // Perm(10)
 
 	uint32(1298498081), // Uint32()
 	uint32(2019727887), // Uint32()