runtime: use the nsec system call instead of /dev/bintime on Plan 9

LGTM=0intro
R=0intro
CC=ality, dave, golang-codereviews, jas, mischief, rsc
https://golang.org/cl/104570043

src/pkg/runtime/os_plan9.c

@@ -150,29 +150,13 @@ runtime·usleep(uint32 µs)
 int64
 runtime·nanotime(void)
 {
-	static int32 fd = -1;
-	byte b[8];
-	uint32 hi, lo;
-
-	// As long as all goroutines share the same file
-	// descriptor table we can get away with using
-	// just a static fd.  Without a lock the file can
-	// be opened twice but that's okay.
-	//
-	// Using /dev/bintime gives us a latency on the
-	// order of ten microseconds between two calls.
-	//
-	// The naïve implementation (without the cached
-	// file descriptor) is roughly four times slower
-	// in 9vx on a 2.16 GHz Intel Core 2 Duo.
-
-	if(fd < 0 && (fd = runtime·open("/dev/bintime", OREAD|OCEXEC, 0)) < 0)
-		return 0;
-	if(runtime·pread(fd, b, sizeof b, 0) != sizeof b)
-		return 0;
-	hi = b[0]<<24 | b[1]<<16 | b[2]<<8 | b[3];
-	lo = b[4]<<24 | b[5]<<16 | b[6]<<8 | b[7];
-	return (int64)hi<<32 | (int64)lo;
+	int64 ns, scratch;
+
+	ns = runtime·nsec(&scratch);
+	// TODO(aram): remove hack after I fix _nsec in the pc64 kernel.
+	if(ns == 0)
+		return scratch;
+	return ns;
 }
 
 void

src/pkg/runtime/os_plan9.h

@@ -15,6 +15,7 @@ int32	runtime·plan9_tsemacquire(uint32 *addr, int32 ms);
 int32 	runtime·plan9_semrelease(uint32 *addr, int32 count);
 int32	runtime·notify(void (*fn)(void*, int8*));
 int32	runtime·noted(int32);
+int64	runtime·nsec(int64*);
 void	runtime·sigtramp(void*, int8*);
 void	runtime·sigpanic(void);
 void	runtime·goexitsall(int8*);

src/pkg/runtime/sys_plan9_386.s

@@ -64,6 +64,16 @@ TEXT runtime·plan9_tsemacquire(SB),NOSPLIT,$0
 	INT	$64
 	RET
 
+TEXT runtime·nsec(SB),NOSPLIT,$0
+	MOVL	$53, AX
+	INT	$64
+	CMPL	AX, $-1
+	JNE	4(PC)
+	MOVL	a+0(FP), CX
+	MOVL	AX, 0(CX)
+	MOVL	AX, 4(CX)
+	RET
+
 TEXT runtime·notify(SB),NOSPLIT,$0
 	MOVL	$28, AX
 	INT	$64

src/pkg/runtime/sys_plan9_amd64.s

@@ -77,6 +77,11 @@ TEXT runtime·plan9_tsemacquire(SB),NOSPLIT,$0
 	SYSCALL
 	RET
 
+TEXT runtime·nsec(SB),NOSPLIT,$0
+	MOVQ	$53, BP
+	SYSCALL
+	RET
+
 TEXT runtime·notify(SB),NOSPLIT,$0
 	MOVQ	$28, BP
 	SYSCALL