runtime: fix windows cpu profiler

Currently it periodically fails with the following message.
The immediate cause is the wrong base register when obtaining g
in sys_windows_amd64/386.s.
But there are several secondary problems as well.

runtime: unknown pc 0x0 after stack split
panic: invalid memory address or nil pointer dereference
fatal error: panic during malloc
[signal 0xc0000005 code=0x0 addr=0x60 pc=0x42267a]

runtime stack:
runtime.panic(0x7914c0, 0xc862af)
        c:/src/perfer/work/windows-amd64-a15f344a9efa/go/src/pkg/runtime/panic.c:217 +0x2c
runtime: unexpected return pc for runtime.externalthreadhandler called from 0x0

R=rsc, alex.brainman
CC=golang-codereviews
https://golang.org/cl/63310043

src/pkg/runtime/os_windows.c

@@ -71,6 +71,9 @@ extern void *runtime·WriteFile;
 
 void *runtime·GetQueuedCompletionStatusEx;
 
+extern uintptr runtime·externalthreadhandlerp;
+void runtime·externalthreadhandler(void);
+
 static int32
 getproccount(void)
 {
@@ -86,6 +89,8 @@ runtime·osinit(void)
 	void *kernel32;
 	void *SetProcessPriorityBoost;
 
+	runtime·externalthreadhandlerp = (uintptr)runtime·externalthreadhandler;
+
 	runtime·stdcall(runtime·SetConsoleCtrlHandler, 2, runtime·ctrlhandler, (uintptr)1);
 	runtime·stdcall(runtime·timeBeginPeriod, 1, (uintptr)1);
 	runtime·ncpu = getproccount();
@@ -293,9 +298,11 @@ runtime·stdcall(void *fn, int32 count, ...)
 	m->libcall.args = (uintptr*)&count + 1;
 	if(m->profilehz != 0) {
 		// leave pc/sp for cpu profiler
+		m->libcallg = g;
 		m->libcallpc = (uintptr)runtime·getcallerpc(&fn);
+		// sp must be the last, because once async cpu profiler finds
+		// all three values to be non-zero, it will use them
 		m->libcallsp = (uintptr)runtime·getcallersp(&fn);
-		m->libcallg = g;
 	}
 	runtime·asmcgocall(runtime·asmstdcall, &m->libcall);
 	m->libcallsp = 0;

src/pkg/runtime/proc.c

@@ -3029,6 +3029,7 @@ runtime·testSchedLocalQueueSteal(void)
 }
 
 extern void runtime·morestack(void);
+uintptr runtime·externalthreadhandlerp;
 
 // Does f mark the top of a goroutine stack?
 bool
@@ -3039,7 +3040,8 @@ runtime·topofstack(Func *f)
 		f->entry == (uintptr)runtime·mcall ||
 		f->entry == (uintptr)runtime·morestack ||
 		f->entry == (uintptr)runtime·lessstack ||
-		f->entry == (uintptr)_rt0_go;
+		f->entry == (uintptr)_rt0_go ||
+		(runtime·externalthreadhandlerp != 0 && f->entry == runtime·externalthreadhandlerp);
 }
 
 void

src/pkg/runtime/sys_windows_386.s

@@ -347,10 +347,12 @@ TEXT runtime·usleep1(SB),NOSPLIT,$0
 	// leave pc/sp for cpu profiler
 	MOVL	(SP), SI
 	MOVL	SI, m_libcallpc(BP)
+	MOVL	g(CX), SI
+	MOVL	SI, m_libcallg(BP)
+	// sp must be the last, because once async cpu profiler finds
+	// all three values to be non-zero, it will use them
 	LEAL	4(SP), SI
 	MOVL	SI, m_libcallsp(BP)
-	MOVL	g(BP), SI
-	MOVL	SI, m_libcallg(BP)
 
 	MOVL	m_g0(BP), SI
 	CMPL	g(CX), SI

src/pkg/runtime/sys_windows_amd64.s

@@ -342,10 +342,12 @@ TEXT runtime·usleep1(SB),NOSPLIT,$0
 	// leave pc/sp for cpu profiler
 	MOVQ	(SP), R12
 	MOVQ	R12, m_libcallpc(R13)
+	MOVQ	g(R15), R12
+	MOVQ	R12, m_libcallg(R13)
+	// sp must be the last, because once async cpu profiler finds
+	// all three values to be non-zero, it will use them
 	LEAQ	8(SP), R12
 	MOVQ	R12, m_libcallsp(R13)
-	MOVQ	g(R13), R12
-	MOVQ	R12, m_libcallg(R13)
 
 	MOVQ	m_g0(R13), R14
 	CMPQ	g(R15), R14