[release-branch.go1.8] cmd/compile,runtime: fix atomic And8 for mipsle

Removing stray xori that came from big endian copy/paste.
Adding atomicand8 check to runtime.check() that would have revealed
this error.
Might fix #19396.

Change-Id: If8d6f25d3e205496163541eb112548aa66df9c2a
Reviewed-on: https://go-review.googlesource.com/39597
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/ssa/gen/MIPS.rules

@@ -404,8 +404,7 @@
 				(ANDconst  <config.fe.TypeUInt32()> [3] ptr)))
 		(NORconst [0] <config.fe.TypeUInt32()> (SLL <config.fe.TypeUInt32()>
 			(MOVWconst [0xff]) (SLLconst <config.fe.TypeUInt32()> [3]
-				(ANDconst <config.fe.TypeUInt32()> [3]
-					(XORconst <config.fe.TypeUInt32()> [3] ptr)))))) mem)
+				(ANDconst <config.fe.TypeUInt32()> [3] ptr))))) mem)
 
 // AtomicOr8(ptr,val) -> LoweredAtomicOr(ptr&^3,uint32(val) << (((ptr^3) & 3) * 8))
 (AtomicOr8 ptr val mem) && config.BigEndian ->

src/cmd/compile/internal/ssa/rewriteMIPS.go

@@ -712,7 +712,7 @@ func rewriteValueMIPS_OpAtomicAnd8(v *Value, config *Config) bool {
 	_ = b
 	// match: (AtomicAnd8  ptr val mem)
 	// cond: !config.BigEndian
-	// result: (LoweredAtomicAnd (AND <config.fe.TypeUInt32().PtrTo()> (MOVWconst [^3]) ptr) 		(OR <config.fe.TypeUInt32()> (SLL <config.fe.TypeUInt32()> (ZeroExt8to32 val) 			(SLLconst <config.fe.TypeUInt32()> [3] 				(ANDconst  <config.fe.TypeUInt32()> [3] ptr))) 		(NORconst [0] <config.fe.TypeUInt32()> (SLL <config.fe.TypeUInt32()> 			(MOVWconst [0xff]) (SLLconst <config.fe.TypeUInt32()> [3] 				(ANDconst <config.fe.TypeUInt32()> [3] 					(XORconst <config.fe.TypeUInt32()> [3] ptr)))))) mem)
+	// result: (LoweredAtomicAnd (AND <config.fe.TypeUInt32().PtrTo()> (MOVWconst [^3]) ptr) 		(OR <config.fe.TypeUInt32()> (SLL <config.fe.TypeUInt32()> (ZeroExt8to32 val) 			(SLLconst <config.fe.TypeUInt32()> [3] 				(ANDconst  <config.fe.TypeUInt32()> [3] ptr))) 		(NORconst [0] <config.fe.TypeUInt32()> (SLL <config.fe.TypeUInt32()> 			(MOVWconst [0xff]) (SLLconst <config.fe.TypeUInt32()> [3] 				(ANDconst <config.fe.TypeUInt32()> [3] ptr))))) mem)
 	for {
 		ptr := v.Args[0]
 		val := v.Args[1]
@@ -750,10 +750,7 @@ func rewriteValueMIPS_OpAtomicAnd8(v *Value, config *Config) bool {
 		v10.AuxInt = 3
 		v11 := b.NewValue0(v.Line, OpMIPSANDconst, config.fe.TypeUInt32())
 		v11.AuxInt = 3
-		v12 := b.NewValue0(v.Line, OpMIPSXORconst, config.fe.TypeUInt32())
-		v12.AuxInt = 3
-		v12.AddArg(ptr)
-		v11.AddArg(v12)
+		v11.AddArg(ptr)
 		v10.AddArg(v11)
 		v8.AddArg(v10)
 		v7.AddArg(v8)

src/runtime/runtime1.go

@@ -260,6 +260,12 @@ func check() {
 		throw("atomicor8")
 	}
 
+	m = [4]byte{0xff, 0xff, 0xff, 0xff}
+	atomic.And8(&m[1], 0x1)
+	if m[0] != 0xff || m[1] != 0x1 || m[2] != 0xff || m[3] != 0xff {
+		throw("atomicand8")
+	}
+
 	*(*uint64)(unsafe.Pointer(&j)) = ^uint64(0)
 	if j == j {
 		throw("float64nan")