Avoid loop bounds check.

Author: klauspost

flate/matchlen_generic.go

@@ -7,27 +7,32 @@
 package flate
 
 import (
-	"encoding/binary"
 	"math/bits"
+
+	"github.com/klauspost/compress/internal/le"
 )
 
 // matchLen returns the maximum common prefix length of a and b.
 // a must be the shortest of the two.
 func matchLen(a, b []byte) (n int) {
-	for ; len(a) >= 8 && len(b) >= 8; a, b = a[8:], b[8:] {
-		diff := binary.LittleEndian.Uint64(a) ^ binary.LittleEndian.Uint64(b)
-		if diff != 0 {
-			return n + bits.TrailingZeros64(diff)>>3
+	if len(a) >= 8 && len(b) >= 8 {
+		left := len(a) - 8
+		for left >= 0 {
+			diff := le.Load64(a, n) ^ le.Load64(b, n)
+			if diff != 0 {
+				return n + bits.TrailingZeros64(diff)>>3
+			}
+			n += 8
+			left -= 8
 		}
-		n += 8
 	}
-
+	a = a[n:]
+	b = b[n:]
 	for i := range a {
 		if a[i] != b[i] {
 			break
 		}
 		n++
 	}
 	return n
-
 }

internal/le/unsafe_enabled.go

@@ -8,24 +8,32 @@ import (
 	"unsafe"
 )
 
+// Load16 will load from b at index i.
+// If the compiler can prove that b is at least 1 byte this will be without bounds check.
 func Load16[I Indexer](b []byte, i I) uint16 {
 	//return binary.LittleEndian.Uint16(b[i:])
 	//return *(*uint16)(unsafe.Pointer(&b[i]))
 	return *(*uint16)(unsafe.Pointer(uintptr(unsafe.Pointer(&b[0])) + uintptr(i)*unsafe.Sizeof(b[0])))
 }
 
+// Load32 will load from b at index i.
+// If the compiler can prove that b is at least 1 byte this will be without bounds check.
 func Load32[I Indexer](b []byte, i I) uint32 {
 	//return binary.LittleEndian.Uint32(b[i:])
 	//return *(*uint32)(unsafe.Pointer(&b[i]))
 	return *(*uint32)(unsafe.Pointer(uintptr(unsafe.Pointer(&b[0])) + uintptr(i)*unsafe.Sizeof(b[0])))
 }
 
+// Load64 will load from b at index i.
+// If the compiler can prove that b is at least 1 byte this will be without bounds check.
 func Load64[I Indexer](b []byte, i I) uint64 {
 	//return binary.LittleEndian.Uint64(b[i:])
 	//return *(*uint64)(unsafe.Pointer(&b[i]))
 	return *(*uint64)(unsafe.Pointer(uintptr(unsafe.Pointer(&b[0])) + uintptr(i)*unsafe.Sizeof(b[0])))
 }
 
+// Store16 will store v at b.
+// If the compiler can prove
 func Store16(b []byte, v uint16) {
 	//binary.LittleEndian.PutUint16(b, v)
 	*(*uint16)(unsafe.Pointer(&b[0])) = v

zstd/matchlen_generic.go

@@ -15,13 +15,19 @@ import (
 // matchLen returns the maximum common prefix length of a and b.
 // a must be the shortest of the two.
 func matchLen(a, b []byte) (n int) {
-	for ; len(a) >= 8 && len(b) >= 8; a, b = a[8:], b[8:] {
-		diff := le.Load64(a, 0) ^ le.Load64(b, 0)
-		if diff != 0 {
-			return n + bits.TrailingZeros64(diff)>>3
+	if len(a) >= 8 && len(b) >= 8 {
+		left := len(a) - 8
+		for left >= 0 {
+			diff := le.Load64(a, n) ^ le.Load64(b, n)
+			if diff != 0 {
+				return n + bits.TrailingZeros64(diff)>>3
+			}
+			n += 8
+			left -= 8
 		}
-		n += 8
 	}
+	a = a[n:]
+	b = b[n:]
 
 	for i := range a {
 		if a[i] != b[i] {