runtime: find g in Windows profiler using SP

The architecture-specific interpretation of m->tls[0]
is unnecessary and fragile. Delete it.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: I927345e52fa2f1741d4914478a29d1fb8acb0dc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/288806
Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/os_windows.go

@@ -1132,21 +1132,21 @@ func profilem(mp *m, thread uintptr) {
 	c.contextflags = _CONTEXT_CONTROL
 	stdcall2(_GetThreadContext, thread, uintptr(unsafe.Pointer(c)))
 
-	gp := gFromTLS(mp)
+	gp := gFromSP(mp, c.sp())
 
 	sigprof(c.ip(), c.sp(), c.lr(), gp, mp)
 }
 
-func gFromTLS(mp *m) *g {
+func gFromSP(mp *m, sp uintptr) *g {
-	switch GOARCH {
+	if gp := mp.g0; gp != nil && gp.stack.lo < sp && sp < gp.stack.hi {
-	case "arm":
+		return gp
-		tls := &mp.tls[0]
+	}
-		return **((***g)(unsafe.Pointer(tls)))
+	if gp := mp.gsignal; gp != nil && gp.stack.lo < sp && sp < gp.stack.hi {
-	case "386", "amd64":
+		return gp
-		tls := &mp.tls[0]
+	}
-		return *((**g)(unsafe.Pointer(tls)))
+	if gp := mp.curg; gp != nil && gp.stack.lo < sp && sp < gp.stack.hi {
+		return gp
 	}
-	throw("unsupported architecture")
 	return nil
 }
 
@@ -1295,7 +1295,7 @@ func preemptM(mp *m) {
 	unlock(&suspendLock)
 
 	// Does it want a preemption and is it safe to preempt?
-	gp := gFromTLS(mp)
+	gp := gFromSP(mp, c.sp())
 	if wantAsyncPreempt(gp) {
 		if ok, newpc := isAsyncSafePoint(gp, c.ip(), c.sp(), c.lr()); ok {
 			// Inject call to asyncPreempt

src/runtime/sys_windows_arm.s

@@ -350,9 +350,6 @@ TEXT runtime·tstart_stdcall(SB),NOSPLIT|NOFRAME,$0
 	MOVW	R0, g_m(g)
 	BL	runtime·save_g(SB)
 
-	// do per-thread TLS initialization
-	BL	init_thread_tls<>(SB)
-
 	// Layout new m scheduler stack on os stack.
 	MOVW	R13, R0
 	MOVW	R0, g_stack+stack_hi(g)
@@ -581,39 +578,8 @@ TEXT runtime·_initcgo(SB),NOSPLIT|NOFRAME,$0
 	MOVW 	$runtime·tls_g(SB), R1
 	MOVW	R0, (R1)
 
-	BL	init_thread_tls<>(SB)
-
 	MOVW	R4, R13
 	MOVM.IA.W (R13), [R4, R15]	// pop {r4, pc}
 
-// void init_thread_tls()
-//
-// Does per-thread TLS initialization. Saves a pointer to the TLS slot
-// holding G, in the current m.
-//
-//     g->m->tls[0] = &_TEB->TlsSlots[tls_g]
-//
-// The purpose of this is to enable the profiling handler to get the
-// current g associated with the thread. We cannot use m->curg because curg
-// only holds the current user g. If the thread is executing system code or
-// external code, m->curg will be NULL. The thread's TLS slot always holds
-// the current g, so save a reference to this location so the profiling
-// handler can get the real g from the thread's m.
-//
-// Clobbers R0-R3
-TEXT init_thread_tls<>(SB),NOSPLIT|NOFRAME,$0
-	// compute &_TEB->TlsSlots[tls_g]
-	MRC	15, 0, R0, C13, C0, 2
-	ADD	$0xe10, R0
-	MOVW 	$runtime·tls_g(SB), R1
-	MOVW	(R1), R1
-	MOVW	R1<<2, R1
-	ADD	R1, R0
-
-	// save in g->m->tls[0]
-	MOVW	g_m(g), R1
-	MOVW	R0, m_tls(R1)
-	RET
-
 // Holds the TLS Slot, which was allocated by TlsAlloc()
 GLOBL runtime·tls_g+0(SB), NOPTR, $4