xts: avoid redundant bounds checks

Small changes to improve performance of xts by about 6%.

This removes all bounds checks, which can be verified by running:

  go build -gcflags="-d=ssa/check_bce/debug=1" .

Before this would show mutiple Found IsInBounds/IsSliceInBounds in the
inner loops of Encrypt and Decrypt.

Tweaked the benchmark to use a larger buffer to make the improvement
more visible. XTS is often used with disk sector sizes, so larger values
are appropriate.

  goos: linux
  goarch: amd64
  pkg: golang.org/x/crypto/xts
  cpu: Intel(R) Xeon(R) W-2135 CPU @ 3.70GHz
         │ before.txt  │             after.txt              │
         │   sec/op    │   sec/op     vs base               │
  XTS-12   1.720µ ± 2%   1.619µ ± 2%  -5.87% (p=0.000 n=10)

         │  before.txt  │              after.txt              │
         │     B/s      │     B/s       vs base               │
  XTS-12   177.4Mi ± 2%   188.5Mi ± 2%  +6.24% (p=0.000 n=10)

         │ before.txt │           after.txt            │
         │    B/op    │    B/op     vs base            │
  XTS-12   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
  ¹ all samples are equal

         │ before.txt │           after.txt            │
         │ allocs/op  │ allocs/op   vs base            │
  XTS-12   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
  ¹ all samples are equal

Author: ncruces

xts/xts.go

@@ -87,7 +87,7 @@ func (c *Cipher) Encrypt(ciphertext, plaintext []byte, sectorNum uint64) {
 
 	c.k2.Encrypt(tweak[:], tweak[:])
 
-	for len(plaintext) > 0 {
+	for len(plaintext) >= blockSize && len(ciphertext) >= blockSize {
 		for j := range tweak {
 			ciphertext[j] = plaintext[j] ^ tweak[j]
 		}
@@ -126,7 +126,7 @@ func (c *Cipher) Decrypt(plaintext, ciphertext []byte, sectorNum uint64) {
 
 	c.k2.Encrypt(tweak[:], tweak[:])
 
-	for len(ciphertext) > 0 {
+	for len(ciphertext) >= blockSize && len(plaintext) >= blockSize {
 		for j := range tweak {
 			plaintext[j] = ciphertext[j] ^ tweak[j]
 		}

xts/xts_test.go

@@ -106,13 +106,14 @@ func TestShorterCiphertext(t *testing.T) {
 
 func BenchmarkXTS(b *testing.B) {
 	b.ReportAllocs()
+	b.SetBytes(320)
 	c, err := NewCipher(aes.NewCipher, make([]byte, 32))
 	if err != nil {
 		b.Fatalf("NewCipher failed: %s", err)
 	}
-	plaintext := make([]byte, 32)
-	encrypted := make([]byte, 48)
-	decrypted := make([]byte, 48)
+	plaintext := make([]byte, 320)
+	encrypted := make([]byte, 336)
+	decrypted := make([]byte, 336)
 
 	for i := 0; i < b.N; i++ {
 		c.Encrypt(encrypted, plaintext, 0)