crypto/sha256: Use bits.RotateLeft32 instead of ad-hoc implementation

Improves readability of the generic implementation.

Updates #31456.

Benchmarks (i7-4980HQ CPU)

name          old time/op    new time/op    delta
Hash8Bytes-8     339ns ± 3%     337ns ± 2%   ~     (p=0.595 n=5+5)
Hash1K-8        5.12µs ± 6%    4.97µs ± 6%   ~     (p=0.310 n=5+5)
Hash8K-8        37.6µs ± 5%    38.1µs ± 6%   ~     (p=0.841 n=5+5)

name          old speed      new speed      delta
Hash8Bytes-8  23.6MB/s ± 3%  23.8MB/s ± 3%   ~     (p=0.690 n=5+5)
Hash1K-8       200MB/s ± 6%   206MB/s ± 5%   ~     (p=0.310 n=5+5)
Hash8K-8       218MB/s ± 5%   215MB/s ± 6%   ~     (p=0.841 n=5+5)

Change-Id: Ic488841699138efde76e900bce1dd38fdbc88ec6
Reviewed-on: https://go-review.googlesource.com/c/go/+/171731Reviewed-by: Ilya Tokar <tocarip@gmail.com>
Run-TryBot: Ilya Tokar <tocarip@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/sha256/sha256block.go

@@ -8,6 +8,8 @@
 
 package sha256
 
+import "math/bits"
+
 var _K = []uint32{
 	0x428a2f98,
 	0x71374491,
@@ -87,18 +89,18 @@ func blockGeneric(dig *digest, p []byte) {
 		}
 		for i := 16; i < 64; i++ {
 			v1 := w[i-2]
-			t1 := (v1>>17 | v1<<(32-17)) ^ (v1>>19 | v1<<(32-19)) ^ (v1 >> 10)
+			t1 := (bits.RotateLeft32(v1, -17)) ^ (bits.RotateLeft32(v1, -19)) ^ (v1 >> 10)
 			v2 := w[i-15]
-			t2 := (v2>>7 | v2<<(32-7)) ^ (v2>>18 | v2<<(32-18)) ^ (v2 >> 3)
+			t2 := (bits.RotateLeft32(v2, -7)) ^ (bits.RotateLeft32(v2, -18)) ^ (v2 >> 3)
 			w[i] = t1 + w[i-7] + t2 + w[i-16]
 		}
 
 		a, b, c, d, e, f, g, h := h0, h1, h2, h3, h4, h5, h6, h7
 
 		for i := 0; i < 64; i++ {
-			t1 := h + ((e>>6 | e<<(32-6)) ^ (e>>11 | e<<(32-11)) ^ (e>>25 | e<<(32-25))) + ((e & f) ^ (^e & g)) + _K[i] + w[i]
+			t1 := h + ((bits.RotateLeft32(e, -6)) ^ (bits.RotateLeft32(e, -11)) ^ (bits.RotateLeft32(e, -25))) + ((e & f) ^ (^e & g)) + _K[i] + w[i]
 
-			t2 := ((a>>2 | a<<(32-2)) ^ (a>>13 | a<<(32-13)) ^ (a>>22 | a<<(32-22))) + ((a & b) ^ (a & c) ^ (b & c))
+			t2 := ((bits.RotateLeft32(a, -2)) ^ (bits.RotateLeft32(a, -13)) ^ (bits.RotateLeft32(a, -22))) + ((a & b) ^ (a & c) ^ (b & c))
 
 			h = g
 			g = f