runtime/internal/atomic: use //go:noinline to prevent inlining, not assembly nop

A bit cleanuppy for 1.6 maybe, but something I happened to notice.

Change-Id: I70f3b48445f4f527d67f7b202b6171195440b09f
Reviewed-on: https://go-review.googlesource.com/18550Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/internal/atomic/atomic_386.go

@@ -8,19 +8,15 @@ package atomic
 
 import "unsafe"
 
-// The calls to nop are to keep these functions from being inlined.
-// If they are inlined we have no guarantee that later rewrites of the
-// code by optimizers will preserve the relative order of memory accesses.
-
 //go:nosplit
+//go:noinline
 func Load(ptr *uint32) uint32 {
-	nop()
 	return *ptr
 }
 
 //go:nosplit
+//go:noinline
 func Loadp(ptr unsafe.Pointer) unsafe.Pointer {
-	nop()
 	return *(*unsafe.Pointer)(ptr)
 }
 

src/runtime/internal/atomic/atomic_amd64x.go

@@ -8,25 +8,21 @@ package atomic
 
 import "unsafe"
 
-// The calls to nop are to keep these functions from being inlined.
-// If they are inlined we have no guarantee that later rewrites of the
-// code by optimizers will preserve the relative order of memory accesses.
-
 //go:nosplit
+//go:noinline
 func Load(ptr *uint32) uint32 {
-	nop()
 	return *ptr
 }
 
 //go:nosplit
+//go:noinline
 func Loadp(ptr unsafe.Pointer) unsafe.Pointer {
-	nop()
 	return *(*unsafe.Pointer)(ptr)
 }
 
 //go:nosplit
+//go:noinline
 func Load64(ptr *uint64) uint64 {
-	nop()
 	return *ptr
 }
 

src/runtime/internal/atomic/stubs.go

@@ -12,8 +12,6 @@ func Cas(ptr *uint32, old, new uint32) bool
 // NO go:noescape annotation; see atomic_pointer.go.
 func Casp1(ptr *unsafe.Pointer, old, new unsafe.Pointer) bool
 
-func nop() // call to prevent inlining of function body
-
 //go:noescape
 func Casuintptr(ptr *uintptr, old, new uintptr) bool