runtime: warn that cputicks() might not be monotonic.

Get rid of gocputicks(), it is no longer used.

LGTM=bradfitz, dave
R=golang-codereviews, bradfitz, dave, minux
CC=golang-codereviews
https://golang.org/cl/161110044

src/runtime/asm_386.s

@@ -871,12 +871,6 @@ TEXT runtime·cputicks(SB),NOSPLIT,$0-8
 	MOVL	DX, ret_hi+4(FP)
 	RET
 
-TEXT runtime·gocputicks(SB),NOSPLIT,$0-8
-	RDTSC
-	MOVL    AX, ret_lo+0(FP)
-	MOVL    DX, ret_hi+4(FP)
-	RET
-
 TEXT runtime·ldt0setup(SB),NOSPLIT,$16-0
 	// set up ldt 7 to point at tls0
 	// ldt 1 would be fine on Linux, but on OS X, 7 is as low as we can go.

src/runtime/asm_amd64.s

@@ -855,13 +855,6 @@ TEXT runtime·cputicks(SB),NOSPLIT,$0-0
 	MOVQ	AX, ret+0(FP)
 	RET
 
-TEXT runtime·gocputicks(SB),NOSPLIT,$0-8
-	RDTSC
-	SHLQ    $32, DX
-	ADDQ    DX, AX
-	MOVQ    AX, ret+0(FP)
-	RET
-
 // hash function using AES hardware instructions
 TEXT runtime·aeshash(SB),NOSPLIT,$0-32
 	MOVQ	p+0(FP), AX	// ptr to data

src/runtime/asm_amd64p32.s

@@ -657,13 +657,6 @@ TEXT runtime·cputicks(SB),NOSPLIT,$0-0
 	MOVQ	AX, ret+0(FP)
 	RET
 
-TEXT runtime·gocputicks(SB),NOSPLIT,$0-8
-	RDTSC
-	SHLQ    $32, DX
-	ADDQ    DX, AX
-	MOVQ    AX, ret+0(FP)
-	RET
-
 // hash function using AES hardware instructions
 // For now, our one amd64p32 system (NaCl) does not
 // support using AES instructions, so have not bothered to

src/runtime/asm_arm.s

@@ -1275,9 +1275,6 @@ TEXT runtime·fastrand1(SB),NOSPLIT,$-4-4
 	MOVW	R0, ret+0(FP)
 	RET
 
-TEXT runtime·gocputicks(SB),NOSPLIT,$0
-	B runtime·cputicks(SB)
-
 TEXT runtime·return0(SB),NOSPLIT,$0
 	MOVW	$0, R0
 	RET

src/runtime/stubs.go

@@ -180,7 +180,11 @@ func exit(code int32)
 func breakpoint()
 func nanotime() int64
 func usleep(usec uint32)
+
+// careful: cputicks is not guaranteed to be monotonic!  In particular, we have
+// noticed drift between cpus on certain os/arch combinations.  See issue 8976.
 func cputicks() int64
+
 func mmap(addr unsafe.Pointer, n uintptr, prot, flags, fd int32, off uint32) unsafe.Pointer
 func munmap(addr unsafe.Pointer, n uintptr)
 func madvise(addr unsafe.Pointer, n uintptr, flags int32)