xchg8.go

     1  // Copyright 2025 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  package atomic
     6  
     7  import (
     8  	"internal/goarch"
     9  	"unsafe"
    10  )
    11  
    12  //go:nosplit
    13  func goXchg8(addr *uint8, v uint8) uint8 {
    14  	// Align down to 4 bytes and use 32-bit CAS.
    15  	addr32 := (*uint32)(unsafe.Pointer(uintptr(unsafe.Pointer(addr)) &^ 3))
    16  	shift := (uintptr(unsafe.Pointer(addr)) & 3)
    17  	if goarch.BigEndian {
    18  		shift = shift ^ 3
    19  	}
    20  	shift = shift * 8
    21  	word := uint32(v) << shift
    22  	mask := uint32(0xFF) << shift
    23  
    24  	for {
    25  		old := *addr32 // Read the old 32-bit value
    26  		// Clear the old 8 bits then insert the new value
    27  		if Cas(addr32, old, (old&^mask)|word) {
    28  			// Return the old 8-bit value
    29  			return uint8((old & mask) >> shift)
    30  		}
    31  	}
    32  }
    33
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