internal/runtime/atomic: add Xchg8 for 386

For #69735

Change-Id: I5b9f57315d693d613dc88dc02c10bee39aeeef76
GitHub-Last-Rev: 690337e5b8
GitHub-Pull-Request: golang/go#69923
Reviewed-on: https://go-review.googlesource.com/c/go/+/620756
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/internal/runtime/atomic/atomic_386.go

@@ -53,6 +53,9 @@ func Xchg64(ptr *uint64, new uint64) uint64
 //go:noescape
 func Xchg(ptr *uint32, new uint32) uint32
 
+//go:noescape
+func Xchg8(ptr *uint8, new uint8) uint8
+
 //go:noescape
 func Xchguintptr(ptr *uintptr, new uintptr) uintptr
 

src/internal/runtime/atomic/atomic_386.s

@@ -153,6 +153,14 @@ addloop:
 	MOVL	CX, ret_hi+16(FP)
 	RET
 
+// uint8 Xchg8(uint8 *ptr, uint8 new)
+TEXT ·Xchg8(SB), NOSPLIT, $0-9
+	MOVL	ptr+0(FP), BX
+	MOVB	new+4(FP), AX
+	XCHGB	AX, 0(BX)
+	MOVB	AX, ret+8(FP)
+	RET
+
 TEXT ·Xchg(SB), NOSPLIT, $0-12
 	MOVL	ptr+0(FP), BX
 	MOVL	new+4(FP), AX

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 amd64 || arm64 || ppc64 || ppc64le
+//go:build 386 || amd64 || arm64 || ppc64 || ppc64le
 
 package atomic_test