xor_generic.go

     1  // Copyright 2013 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:build (!amd64 && !arm64 && !ppc64 && !ppc64le) || purego
     6  
     7  package subtle
     8  
     9  import (
    10  	"runtime"
    11  	"unsafe"
    12  )
    13  
    14  const wordSize = unsafe.Sizeof(uintptr(0))
    15  
    16  const supportsUnaligned = runtime.GOARCH == "386" ||
    17  	runtime.GOARCH == "amd64" ||
    18  	runtime.GOARCH == "ppc64" ||
    19  	runtime.GOARCH == "ppc64le" ||
    20  	runtime.GOARCH == "s390x"
    21  
    22  func xorBytes(dstb, xb, yb *byte, n int) {
    23  	// xorBytes assembly is written using pointers and n. Back to slices.
    24  	dst := unsafe.Slice(dstb, n)
    25  	x := unsafe.Slice(xb, n)
    26  	y := unsafe.Slice(yb, n)
    27  
    28  	if supportsUnaligned || aligned(dstb, xb, yb) {
    29  		xorLoop(words(dst), words(x), words(y))
    30  		if uintptr(n)%wordSize == 0 {
    31  			return
    32  		}
    33  		done := n &^ int(wordSize-1)
    34  		dst = dst[done:]
    35  		x = x[done:]
    36  		y = y[done:]
    37  	}
    38  	xorLoop(dst, x, y)
    39  }
    40  
    41  // aligned reports whether dst, x, and y are all word-aligned pointers.
    42  func aligned(dst, x, y *byte) bool {
    43  	return (uintptr(unsafe.Pointer(dst))|uintptr(unsafe.Pointer(x))|uintptr(unsafe.Pointer(y)))&(wordSize-1) == 0
    44  }
    45  
    46  // words returns a []uintptr pointing at the same data as x,
    47  // with any trailing partial word removed.
    48  func words(x []byte) []uintptr {
    49  	n := uintptr(len(x)) / wordSize
    50  	if n == 0 {
    51  		// Avoid creating a *uintptr that refers to data smaller than a uintptr;
    52  		// see issue 59334.
    53  		return nil
    54  	}
    55  	return unsafe.Slice((*uintptr)(unsafe.Pointer(&x[0])), n)
    56  }
    57  
    58  func xorLoop[T byte | uintptr](dst, x, y []T) {
    59  	x = x[:len(dst)] // remove bounds check in loop
    60  	y = y[:len(dst)] // remove bounds check in loop
    61  	for i := range dst {
    62  		dst[i] = x[i] ^ y[i]
    63  	}
    64  }
    65
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