internal/runtime/atomic: add Xchg8 for riscv64

For #69735

Change-Id: I34ca2b027494525ab64f94beee89ca373a5031ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/631615
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/runtime/atomic/atomic_riscv64.go

@@ -18,6 +18,9 @@ func Xadduintptr(ptr *uintptr, delta uintptr) uintptr
 //go:noescape
 func Xchg(ptr *uint32, new uint32) uint32
 
+//go:noescape
+func Xchg8(ptr *uint8, new uint8) uint8
+
 //go:noescape
 func Xchg64(ptr *uint64, new uint64) uint64
 

src/internal/runtime/atomic/atomic_riscv64.s

@@ -199,6 +199,27 @@ TEXT ·Xchg(SB), NOSPLIT, $0-20
 	MOVW	A1, ret+16(FP)
 	RET
 
+// func Xchg8(ptr *uint8, new uint8) uint8
+TEXT ·Xchg8(SB), NOSPLIT, $0-17
+	MOV	ptr+0(FP), A0
+	MOVBU	new+8(FP), A1
+	AND	$3, A0, A2
+	SLL	$3, A2
+	MOV	$255, A4
+	SLL	A2, A4
+	NOT	A4
+	AND	$~3, A0
+	SLL	A2, A1
+xchg8_again:
+	LRW	(A0), A5
+	AND	A4, A5, A3
+	OR	A1, A3
+	SCW	A3, (A0), A6
+	BNEZ	A6, xchg8_again
+	SRL	A2, A5
+	MOVB	A5, ret+16(FP)
+	RET
+
 // func Xchg64(ptr *uint64, new uint64) uint64
 TEXT ·Xchg64(SB), NOSPLIT, $0-24
 	MOV	ptr+0(FP), A0

src/internal/runtime/atomic/xchg8_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build 386 || amd64 || arm || arm64 || loong64 || ppc64 || ppc64le
+//go:build 386 || amd64 || arm || arm64 || loong64 || ppc64 || ppc64le || riscv64
 
 package atomic_test