math/bits: support negative rotation count and remove RotateRight

For details see the discussion on the issue below.

RotateLeft functions can now be inlined because the don't panic
anymore for negative rotation counts.

name            old time/op  new time/op  delta
RotateLeft-8    6.72ns ± 2%  1.86ns ± 0%  -72.33%  (p=0.016 n=5+4)
RotateLeft8-8   4.41ns ± 2%  1.67ns ± 1%  -62.15%  (p=0.008 n=5+5)
RotateLeft16-8  4.46ns ± 6%  1.65ns ± 0%  -63.06%  (p=0.008 n=5+5)
RotateLeft32-8  4.50ns ± 5%  1.67ns ± 1%  -62.86%  (p=0.008 n=5+5)
RotateLeft64-8  4.54ns ± 1%  1.85ns ± 1%  -59.32%  (p=0.008 n=5+5)

https://perf.golang.org/search?q=upload:20170411.4

(Measured on 2.3 GHz Intel Core i7 running macOS 10.12.3.)

For #18616.

Change-Id: I0828d80d54ec24f8d44954a57b3d6aeedb69c686
Reviewed-on: https://go-review.googlesource.com/40394Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/bits/bits.go

@@ -163,7 +163,8 @@ func OnesCount64(x uint64) int {
 
 // --- RotateLeft ---
 
-// RotateLeft returns the value of x rotated left by k bits; k must not be negative.
+// RotateLeft returns the value of x rotated left by (k mod UintSize) bits.
+// To rotate x right by k bits, call RotateLeft(x, -k).
 func RotateLeft(x uint, k int) uint {
 	if UintSize == 32 {
 		return uint(RotateLeft32(uint32(x), k))
@@ -171,96 +172,38 @@ func RotateLeft(x uint, k int) uint {
 	return uint(RotateLeft64(uint64(x), k))
 }
 
-// RotateLeft8 returns the value of x rotated left by k bits; k must not be negative.
+// RotateLeft8 returns the value of x rotated left by (k mod 8) bits.
+// To rotate x right by k bits, call RotateLeft8(x, -k).
 func RotateLeft8(x uint8, k int) uint8 {
-	if k < 0 {
-		panic("negative rotation count")
-	}
 	const n = 8
 	s := uint(k) & (n - 1)
 	return x<<s | x>>(n-s)
 }
 
-// RotateLeft16 returns the value of x rotated left by k bits; k must not be negative.
+// RotateLeft16 returns the value of x rotated left by (k mod 16) bits.
+// To rotate x right by k bits, call RotateLeft16(x, -k).
 func RotateLeft16(x uint16, k int) uint16 {
-	if k < 0 {
-		panic("negative rotation count")
-	}
 	const n = 16
 	s := uint(k) & (n - 1)
 	return x<<s | x>>(n-s)
 }
 
-// RotateLeft32 returns the value of x rotated left by k bits; k must not be negative.
+// RotateLeft32 returns the value of x rotated left by (k mod 32) bits.
+// To rotate x right by k bits, call RotateLeft32(x, -k).
 func RotateLeft32(x uint32, k int) uint32 {
-	if k < 0 {
-		panic("negative rotation count")
-	}
 	const n = 32
 	s := uint(k) & (n - 1)
 	return x<<s | x>>(n-s)
 }
 
-// RotateLeft64 returns the value of x rotated left by k bits; k must not be negative.
+// RotateLeft64 returns the value of x rotated left by (k mod 64) bits.
+// To rotate x right by k bits, call RotateLeft64(x, -k).
 func RotateLeft64(x uint64, k int) uint64 {
-	if k < 0 {
-		panic("negative rotation count")
-	}
 	const n = 64
 	s := uint(k) & (n - 1)
 	return x<<s | x>>(n-s)
 }
 
-// --- RotateRight ---
-
-// RotateRight returns the value of x rotated left by k bits; k must not be negative.
-func RotateRight(x uint, k int) uint {
-	if UintSize == 32 {
-		return uint(RotateRight32(uint32(x), k))
-	}
-	return uint(RotateRight64(uint64(x), k))
-}
-
-// RotateRight8 returns the value of x rotated left by k bits; k must not be negative.
-func RotateRight8(x uint8, k int) uint8 {
-	if k < 0 {
-		panic("negative rotation count")
-	}
-	const n = 8
-	s := uint(k) & (n - 1)
-	return x<<(n-s) | x>>s
-}
-
-// RotateRight16 returns the value of x rotated left by k bits; k must not be negative.
-func RotateRight16(x uint16, k int) uint16 {
-	if k < 0 {
-		panic("negative rotation count")
-	}
-	const n = 16
-	s := uint(k) & (n - 1)
-	return x<<(n-s) | x>>s
-}
-
-// RotateRight32 returns the value of x rotated left by k bits; k must not be negative.
-func RotateRight32(x uint32, k int) uint32 {
-	if k < 0 {
-		panic("negative rotation count")
-	}
-	const n = 32
-	s := uint(k) & (n - 1)
-	return x<<(n-s) | x>>s
-}
-
-// RotateRight64 returns the value of x rotated left by k bits; k must not be negative.
-func RotateRight64(x uint64, k int) uint64 {
-	if k < 0 {
-		panic("negative rotation count")
-	}
-	const n = 64
-	s := uint(k) & (n - 1)
-	return x<<(n-s) | x>>s
-}
-
 // --- Reverse ---
 
 // Reverse returns the value of x with its bits in reversed order.

src/math/bits/bits_test.go

@@ -342,6 +342,10 @@ func TestRotateLeft(t *testing.T) {
 		if got8 != want8 {
 			t.Fatalf("RotateLeft8(%#02x, %d) == %#02x; want %#02x", x8, k, got8, want8)
 		}
+		got8 = RotateLeft8(want8, -int(k))
+		if got8 != x8 {
+			t.Fatalf("RotateLeft8(%#02x, -%d) == %#02x; want %#02x", want8, k, got8, x8)
+		}
 
 		x16 := uint16(m)
 		got16 := RotateLeft16(x16, int(k))
@@ -349,6 +353,10 @@ func TestRotateLeft(t *testing.T) {
 		if got16 != want16 {
 			t.Fatalf("RotateLeft16(%#04x, %d) == %#04x; want %#04x", x16, k, got16, want16)
 		}
+		got16 = RotateLeft16(want16, -int(k))
+		if got16 != x16 {
+			t.Fatalf("RotateLeft16(%#04x, -%d) == %#04x; want %#04x", want16, k, got16, x16)
+		}
 
 		x32 := uint32(m)
 		got32 := RotateLeft32(x32, int(k))
@@ -356,6 +364,10 @@ func TestRotateLeft(t *testing.T) {
 		if got32 != want32 {
 			t.Fatalf("RotateLeft32(%#08x, %d) == %#08x; want %#08x", x32, k, got32, want32)
 		}
+		got32 = RotateLeft32(want32, -int(k))
+		if got32 != x32 {
+			t.Fatalf("RotateLeft32(%#08x, -%d) == %#08x; want %#08x", want32, k, got32, x32)
+		}
 		if UintSize == 32 {
 			x := uint(m)
 			got := RotateLeft(x, int(k))
@@ -363,6 +375,10 @@ func TestRotateLeft(t *testing.T) {
 			if got != want {
 				t.Fatalf("RotateLeft(%#08x, %d) == %#08x; want %#08x", x, k, got, want)
 			}
+			got = RotateLeft(want, -int(k))
+			if got != x {
+				t.Fatalf("RotateLeft(%#08x, -%d) == %#08x; want %#08x", want, k, got, x)
+			}
 		}
 
 		x64 := uint64(m)
@@ -371,6 +387,10 @@ func TestRotateLeft(t *testing.T) {
 		if got64 != want64 {
 			t.Fatalf("RotateLeft64(%#016x, %d) == %#016x; want %#016x", x64, k, got64, want64)
 		}
+		got64 = RotateLeft64(want64, -int(k))
+		if got64 != x64 {
+			t.Fatalf("RotateLeft64(%#016x, -%d) == %#016x; want %#016x", want64, k, got64, x64)
+		}
 		if UintSize == 64 {
 			x := uint(m)
 			got := RotateLeft(x, int(k))
@@ -378,6 +398,10 @@ func TestRotateLeft(t *testing.T) {
 			if got != want {
 				t.Fatalf("RotateLeft(%#016x, %d) == %#016x; want %#016x", x, k, got, want)
 			}
+			got = RotateLeft(want, -int(k))
+			if got != x {
+				t.Fatalf("RotateLeft(%#08x, -%d) == %#08x; want %#08x", want, k, got, x)
+			}
 		}
 	}
 }
@@ -422,96 +446,6 @@ func BenchmarkRotateLeft64(b *testing.B) {
 	Output = int(s)
 }
 
-func TestRotateRight(t *testing.T) {
-	var m uint64 = deBruijn64
-
-	for k := uint(0); k < 128; k++ {
-		x8 := uint8(m)
-		got8 := RotateRight8(x8, int(k))
-		want8 := x8>>(k&0x7) | x8<<(8-k&0x7)
-		if got8 != want8 {
-			t.Fatalf("RotateRight8(%#02x, %d) == %#02x; want %#02x", x8, k, got8, want8)
-		}
-
-		x16 := uint16(m)
-		got16 := RotateRight16(x16, int(k))
-		want16 := x16>>(k&0xf) | x16<<(16-k&0xf)
-		if got16 != want16 {
-			t.Fatalf("RotateRight16(%#04x, %d) == %#04x; want %#04x", x16, k, got16, want16)
-		}
-
-		x32 := uint32(m)
-		got32 := RotateRight32(x32, int(k))
-		want32 := x32>>(k&0x1f) | x32<<(32-k&0x1f)
-		if got32 != want32 {
-			t.Fatalf("RotateRight32(%#08x, %d) == %#08x; want %#08x", x32, k, got32, want32)
-		}
-		if UintSize == 32 {
-			x := uint(m)
-			got := RotateRight(x, int(k))
-			want := x>>(k&0x1f) | x<<(32-k&0x1f)
-			if got != want {
-				t.Fatalf("RotateRight(%#08x, %d) == %#08x; want %#08x", x, k, got, want)
-			}
-		}
-
-		x64 := uint64(m)
-		got64 := RotateRight64(x64, int(k))
-		want64 := x64>>(k&0x3f) | x64<<(64-k&0x3f)
-		if got64 != want64 {
-			t.Fatalf("RotateRight64(%#016x, %d) == %#016x; want %#016x", x64, k, got64, want64)
-		}
-		if UintSize == 64 {
-			x := uint(m)
-			got := RotateRight(x, int(k))
-			want := x>>(k&0x3f) | x<<(64-k&0x3f)
-			if got != want {
-				t.Fatalf("RotateRight(%#016x, %d) == %#016x; want %#016x", x, k, got, want)
-			}
-		}
-	}
-}
-
-func BenchmarkRotateRight(b *testing.B) {
-	var s uint
-	for i := 0; i < b.N; i++ {
-		s += RotateRight(uint(Input), i)
-	}
-	Output = int(s)
-}
-
-func BenchmarkRotateRight8(b *testing.B) {
-	var s uint8
-	for i := 0; i < b.N; i++ {
-		s += RotateRight8(uint8(Input), i)
-	}
-	Output = int(s)
-}
-
-func BenchmarkRotateRight16(b *testing.B) {
-	var s uint16
-	for i := 0; i < b.N; i++ {
-		s += RotateRight16(uint16(Input), i)
-	}
-	Output = int(s)
-}
-
-func BenchmarkRotateRight32(b *testing.B) {
-	var s uint32
-	for i := 0; i < b.N; i++ {
-		s += RotateRight32(uint32(Input), i)
-	}
-	Output = int(s)
-}
-
-func BenchmarkRotateRight64(b *testing.B) {
-	var s uint64
-	for i := 0; i < b.N; i++ {
-		s += RotateRight64(uint64(Input), i)
-	}
-	Output = int(s)
-}
-
 func TestReverse(t *testing.T) {
 	// test each bit
 	for i := uint(0); i < 64; i++ {