internal/runtime/atomic: add arm native implementations of And8/Or8

With LDREXB/STREXB now available for the arm assembler we can implement these operations natively. The instructions are armv6k+ but for simplicity I only use them on armv7.

Benchmark results for a raspberry Pi 3 model B+:

	goos: linux
	goarch: arm
	pkg: internal/runtime/atomic
	cpu: ARMv7 Processor rev 4 (v7l)
			 │   old.txt    │               new.txt               │
			 │    sec/op    │   sec/op     vs base                │
	And8-4             127.65n ± 0%   68.74n ± 0%  -46.15% (p=0.000 n=10)

Change-Id: Ic87f307c35f7d7f56010980302f253056f6d54dc
GitHub-Last-Rev: a7351802fd
GitHub-Pull-Request: golang/go#70002
Cq-Include-Trybots: luci.golang.try:gotip-linux-arm
Reviewed-on: https://go-review.googlesource.com/c/go/+/622075
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/runtime/atomic/atomic_arm.go

@@ -159,8 +159,11 @@ func goStore64(addr *uint64, v uint64) {
 	addrLock(addr).unlock()
 }
 
+//go:noescape
+func Or8(addr *uint8, v uint8)
+
 //go:nosplit
-func Or8(addr *uint8, v uint8) {
+func goOr8(addr *uint8, v uint8) {
 	// Align down to 4 bytes and use 32-bit CAS.
 	uaddr := uintptr(unsafe.Pointer(addr))
 	addr32 := (*uint32)(unsafe.Pointer(uaddr &^ 3))
@@ -173,8 +176,11 @@ func Or8(addr *uint8, v uint8) {
 	}
 }
 
+//go:noescape
+func And8(addr *uint8, v uint8)
+
 //go:nosplit
-func And8(addr *uint8, v uint8) {
+func goAnd8(addr *uint8, v uint8) {
 	// Align down to 4 bytes and use 32-bit CAS.
 	uaddr := uintptr(unsafe.Pointer(addr))
 	addr32 := (*uint32)(unsafe.Pointer(uaddr &^ 3))

src/internal/runtime/atomic/atomic_arm.s

@@ -228,6 +228,42 @@ store64loop:
 	DMB	MB_ISH
 	RET
 
+TEXT armAnd8<>(SB),NOSPLIT,$0-5
+	// addr is already in R1
+	MOVB	v+4(FP), R2
+
+and8loop:
+	LDREXB	(R1), R6
+
+	DMB	MB_ISHST
+
+	AND 	R2, R6
+	STREXB	R6, (R1), R0
+	CMP	$0, R0
+	BNE	and8loop
+
+	DMB	MB_ISH
+
+	RET
+
+TEXT armOr8<>(SB),NOSPLIT,$0-5
+	// addr is already in R1
+	MOVB	v+4(FP), R2
+
+or8loop:
+	LDREXB	(R1), R6
+
+	DMB	MB_ISHST
+
+	ORR 	R2, R6
+	STREXB	R6, (R1), R0
+	CMP	$0, R0
+	BNE	or8loop
+
+	DMB	MB_ISH
+
+	RET
+
 // The following functions all panic if their address argument isn't
 // 8-byte aligned. Since we're calling back into Go code to do this,
 // we have to cooperate with stack unwinding. In the normal case, the
@@ -310,3 +346,31 @@ TEXT ·Store64(SB),NOSPLIT,$-4-12
 	JMP	·goStore64(SB)
 #endif
 	JMP	armStore64<>(SB)
+
+TEXT ·And8(SB),NOSPLIT,$-4-5
+	NO_LOCAL_POINTERS
+	MOVW	addr+0(FP), R1
+
+// Uses STREXB/LDREXB that is armv6k or later.
+// For simplicity we only enable this on armv7.
+#ifndef GOARM_7
+	MOVB	internal∕cpu·ARM+const_offsetARMHasV7Atomics(SB), R11
+	CMP	$1, R11
+	BEQ	2(PC)
+	JMP	·goAnd8(SB)
+#endif
+	JMP	armAnd8<>(SB)
+
+TEXT ·Or8(SB),NOSPLIT,$-4-5
+	NO_LOCAL_POINTERS
+	MOVW	addr+0(FP), R1
+
+// Uses STREXB/LDREXB that is armv6k or later.
+// For simplicity we only enable this on armv7.
+#ifndef GOARM_7
+	MOVB	internal∕cpu·ARM+const_offsetARMHasV7Atomics(SB), R11
+	CMP	$1, R11
+	BEQ	2(PC)
+	JMP	·goOr8(SB)
+#endif
+	JMP	armOr8<>(SB)