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// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found src the LICENSE file.
package chacha20
import "runtime"
// Platforms that have fast unaligned 32-bit little endian accesses.
const unaligned = runtime.GOARCH == "386" ||
runtime.GOARCH == "amd64" ||
runtime.GOARCH == "arm64" ||
runtime.GOARCH == "ppc64le" ||
runtime.GOARCH == "s390x"
// addXor reads a little endian uint32 from src, XORs it with (a + b) and
// places the result in little endian byte order in dst.
func addXor(dst, src []byte, a, b uint32) {
_, _ = src[3], dst[3] // bounds check elimination hint
if unaligned {
// The compiler should optimize this code into
// 32-bit unaligned little endian loads and stores.
// TODO: delete once the compiler does a reliably
// good job with the generic code below.
// See issue #25111 for more details.
v := uint32(src[0])
v |= uint32(src[1]) << 8
v |= uint32(src[2]) << 16
v |= uint32(src[3]) << 24
v ^= a + b
dst[0] = byte(v)
dst[1] = byte(v >> 8)
dst[2] = byte(v >> 16)
dst[3] = byte(v >> 24)
} else {
a += b
dst[0] = src[0] ^ byte(a)
dst[1] = src[1] ^ byte(a>>8)
dst[2] = src[2] ^ byte(a>>16)
dst[3] = src[3] ^ byte(a>>24)
}
}
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