runtime: support windows/arm

Updates #26148

Change-Id: I8f68b2c926c7b11dc86c9664ed7ff2d2f78b64b4
Reviewed-on: https://go-review.googlesource.com/128715
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 605 insertions, 0 deletions

diff --git a/src/runtime/sys_windows_arm.s b/src/runtime/sys_windows_arm.s
new file mode 100644
index 0000000000..409c72c554
--- /dev/null
+++ b/src/runtime/sys_windows_arm.s
@@ -0,0 +1,605 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "go_asm.h"
+#include "go_tls.h"
+#include "textflag.h"
+
+// void runtime·asmstdcall(void *c);
+TEXT runtime·asmstdcall(SB),NOSPLIT|NOFRAME,$0
+	MOVM.DB.W [R4, R5, R14], (R13)	// push {r4, r5, lr}
+	MOVW	R0, R4			// put libcall * in r4
+	MOVW	R13, R5			// save stack pointer in r5
+
+	// SetLastError(0)
+	MOVW	$0, R0
+	MRC	15, 0, R1, C13, C0, 2
+	MOVW	R0, 0x34(R1)
+
+	MOVW	8(R4), R12	// libcall->args
+
+	// Do we have more than 4 arguments?
+	MOVW	4(R4), R0	// libcall->n
+	SUB.S	$4, R0, R2
+	BLE	loadregs
+
+	// Reserve stack space for remaining args
+	SUB	R2<<2, R13
+	BIC	$0x7, R13	// alignment for ABI
+
+	// R0: count of arguments
+	// R1:
+	// R2: loop counter, from 0 to (n-4)
+	// R3: scratch
+	// R4: pointer to libcall struct
+	// R12: libcall->args
+	MOVW	$0, R2
+stackargs:
+	ADD	$4, R2, R3		// r3 = args[4 + i]
+	MOVW	R3<<2(R12), R3
+	MOVW	R3, R2<<2(R13)		// stack[i] = r3
+
+	ADD	$1, R2			// i++
+	SUB	$4, R0, R3		// while (i < (n - 4))
+	CMP	R3, R2
+	BLT	stackargs
+
+loadregs:
+	CMP	$3, R0
+	MOVW.GT 12(R12), R3
+
+	CMP	$2, R0
+	MOVW.GT 8(R12), R2
+
+	CMP	$1, R0
+	MOVW.GT 4(R12), R1
+
+	CMP	$0, R0
+	MOVW.GT 0(R12), R0
+
+	BIC	$0x7, R13		// alignment for ABI
+	MOVW	0(R4), R12		// branch to libcall->fn
+	BL	(R12)
+
+	MOVW	R5, R13			// free stack space
+	MOVW	R0, 12(R4)		// save return value to libcall->r1
+	MOVW	R1, 16(R4)
+
+	// GetLastError
+	MRC	15, 0, R1, C13, C0, 2
+	MOVW	0x34(R1), R0
+	MOVW	R0, 20(R4)		// store in libcall->err
+
+	MOVM.IA.W (R13), [R4, R5, R15]
+
+TEXT runtime·badsignal2(SB),NOSPLIT|NOFRAME,$0
+	MOVM.DB.W [R4, R14], (R13)	// push {r4, lr}
+	MOVW	R13, R4			// save original stack pointer
+	SUB	$8, R13			// space for 2 variables
+	BIC	$0x7, R13		// alignment for ABI
+
+	// stderr
+	MOVW	runtime·_GetStdHandle(SB), R1
+	MOVW	$-12, R0
+	BL	(R1)
+
+	MOVW	$runtime·badsignalmsg(SB), R1	// lpBuffer
+	MOVW	$runtime·badsignallen(SB), R2	// lpNumberOfBytesToWrite
+	MOVW	(R2), R2
+	ADD	$0x4, R13, R3		// lpNumberOfBytesWritten
+	MOVW	$0, R12			// lpOverlapped
+	MOVW	R12, (R13)
+
+	MOVW	runtime·_WriteFile(SB), R12
+	BL	(R12)
+
+	MOVW	R4, R13			// restore SP
+	MOVM.IA.W (R13), [R4, R15]	// pop {r4, pc}
+
+TEXT runtime·getlasterror(SB),NOSPLIT,$0
+	MRC	15, 0, R0, C13, C0, 2
+	MOVW	0x34(R0), R0
+	MOVW	R0, ret+0(FP)
+	RET
+
+TEXT runtime·setlasterror(SB),NOSPLIT|NOFRAME,$0
+	MRC	15, 0, R1, C13, C0, 2
+	MOVW	R0, 0x34(R1)
+	RET
+
+// Called by Windows as a Vectored Exception Handler (VEH).
+// First argument is pointer to struct containing
+// exception record and context pointers.
+// Handler function is stored in R1
+// Return 0 for 'not handled', -1 for handled.
+// int32_t sigtramp(
+//     PEXCEPTION_POINTERS ExceptionInfo,
+//     func *GoExceptionHandler);
+TEXT runtime·sigtramp(SB),NOSPLIT|NOFRAME,$0
+	MOVM.DB.W [R0, R4-R11, R14], (R13)	// push {r0, r4-r11, lr} (SP-=40)
+	SUB	$(8+20), R13		// reserve space for g, sp, and
+					// parameters/retval to go call
+
+	MOVW	R0, R6			// Save param0
+	MOVW	R1, R7			// Save param1
+
+	BL      runtime·load_g(SB)
+	CMP	$0, g			// is there a current g?
+	BL.EQ	runtime·badsignal2(SB)
+
+	// save g and SP in case of stack switch
+	MOVW	R13, 24(R13)
+	MOVW	g, 20(R13)
+
+	// do we need to switch to the g0 stack?
+	MOVW	g, R5			// R5 = g
+	MOVW	g_m(R5), R2		// R2 = m
+	MOVW	m_g0(R2), R4		// R4 = g0
+	CMP	R5, R4			// if curg == g0
+	BEQ	g0
+
+	// switch to g0 stack
+	MOVW	R4, g				// g = g0
+	MOVW	(g_sched+gobuf_sp)(g), R3	// R3 = g->gobuf.sp
+	BL      runtime·save_g(SB)
+
+	// traceback will think that we've done PUSH and SUB
+	// on this stack, so subtract them here to match.
+	// (we need room for sighandler arguments anyway).
+	// and re-save old SP for restoring later.
+	SUB	$(40+8+20), R3
+	MOVW	R13, 24(R3)		// save old stack pointer
+	MOVW	R3, R13			// switch stack
+
+g0:
+	MOVW	0(R6), R2	// R2 = ExceptionPointers->ExceptionRecord
+	MOVW	4(R6), R3	// R3 = ExceptionPointers->ContextRecord
+
+	// make it look like mstart called us on g0, to stop traceback
+	MOVW    $runtime·mstart(SB), R4
+
+	MOVW	R4, 0(R13)	// Save link register for traceback
+	MOVW	R2, 4(R13)	// Move arg0 (ExceptionRecord) into position
+	MOVW	R3, 8(R13)	// Move arg1 (ContextRecord) into position
+	MOVW	R5, 12(R13)	// Move arg2 (original g) into position
+	BL	(R7)		// Call the go routine
+	MOVW	16(R13), R4	// Fetch return value from stack
+
+	// Compute the value of the g0 stack pointer after deallocating
+	// this frame, then allocating 8 bytes. We may need to store
+	// the resume SP and PC on the g0 stack to work around
+	// control flow guard when we resume from the exception.
+	ADD	$(40+20), R13, R12
+
+	// switch back to original stack and g
+	MOVW	24(R13), R13
+	MOVW	20(R13), g
+	BL      runtime·save_g(SB)
+
+done:
+	MOVW	R4, R0				// move retval into position
+	ADD	$(8 + 20), R13			// free locals
+	MOVM.IA.W (R13), [R3, R4-R11, R14]	// pop {r3, r4-r11, lr}
+
+	// if return value is CONTINUE_SEARCH, do not set up control
+	// flow guard workaround
+	CMP	$0, R0
+	BEQ	return
+
+	// Check if we need to set up the control flow guard workaround.
+	// On Windows/ARM, the stack pointer must lie within system
+	// stack limits when we resume from exception.
+	// Store the resume SP and PC on the g0 stack,
+	// and return to returntramp on the g0 stack. returntramp
+	// pops the saved PC and SP from the g0 stack, resuming execution
+	// at the desired location.
+	// If returntramp has already been set up by a previous exception
+	// handler, don't clobber the stored SP and PC on the stack.
+	MOVW	4(R3), R3			// PEXCEPTION_POINTERS->Context
+	MOVW	0x40(R3), R2			// load PC from context record
+	MOVW	$runtime·returntramp(SB), R1
+	CMP	R1, R2
+	B.EQ	return				// do not clobber saved SP/PC
+
+	// Save resume SP and PC on g0 stack
+	MOVW	0x38(R3), R2			// load SP from context record
+	MOVW	R2, 0(R12)			// Store resume SP on g0 stack
+	MOVW	0x40(R3), R2			// load PC from context record
+	MOVW	R2, 4(R12)			// Store resume PC on g0 stack
+
+	// Set up context record to return to returntramp on g0 stack
+	MOVW	R12, 0x38(R3)			// save g0 stack pointer
+						// in context record
+	MOVW	$runtime·returntramp(SB), R2	// save resume address
+	MOVW	R2, 0x40(R3)			// in context record
+
+return:
+	B	(R14)				// return
+
+//
+// Trampoline to resume execution from exception handler.
+// This is part of the control flow guard workaround.
+// It switches stacks and jumps to the continuation address.
+//
+TEXT runtime·returntramp(SB),NOSPLIT|NOFRAME,$0
+	MOVM.IA	(R13), [R13, R15]		// ldm sp, [sp, pc]
+
+TEXT runtime·exceptiontramp(SB),NOSPLIT|NOFRAME,$0
+	MOVW	$runtime·exceptionhandler(SB), R1
+	B	runtime·sigtramp(SB)
+
+TEXT runtime·firstcontinuetramp(SB),NOSPLIT|NOFRAME,$0
+	MOVW	$runtime·firstcontinuehandler(SB), R1
+	B	runtime·sigtramp(SB)
+
+TEXT runtime·lastcontinuetramp(SB),NOSPLIT|NOFRAME,$0
+	MOVW	$runtime·lastcontinuehandler(SB), R1
+	B	runtime·sigtramp(SB)
+
+TEXT runtime·ctrlhandler(SB),NOSPLIT|NOFRAME,$0
+	MOVW	$runtime·ctrlhandler1(SB), R1
+	B	runtime·externalthreadhandler(SB)
+
+TEXT runtime·profileloop(SB),NOSPLIT|NOFRAME,$0
+	MOVW	$runtime·profileloop1(SB), R1
+	B	runtime·externalthreadhandler(SB)
+
+// int32 externalthreadhandler(uint32 arg, int (*func)(uint32))
+// stack layout:
+//   +----------------+
+//   | callee-save    |
+//   | registers      |
+//   +----------------+
+//   | m              |
+//   +----------------+
+// 20| g              |
+//   +----------------+
+// 16| func ptr (r1)  |
+//   +----------------+
+// 12| argument (r0)  |
+//---+----------------+
+// 8 | param1         |
+//   +----------------+
+// 4 | param0         |
+//   +----------------+
+// 0 | retval         |
+//   +----------------+
+//
+TEXT runtime·externalthreadhandler(SB),NOSPLIT|NOFRAME,$0
+	MOVM.DB.W [R4-R11, R14], (R13)		// push {r4-r11, lr}
+	SUB	$(m__size + g__size + 20), R13	// space for locals
+	MOVW	R0, 12(R13)
+	MOVW	R1, 16(R13)
+
+	// zero out m and g structures
+	ADD	$20, R13, R0			// compute pointer to g
+	MOVW	R0, 4(R13)
+	MOVW	$(m__size + g__size), R0
+	MOVW	R0, 8(R13)
+	BL	runtime·memclrNoHeapPointers(SB)
+
+	// initialize m and g structures
+	ADD	$20, R13, R2			// R2 = g
+	ADD	$(20 + g__size), R13, R3	// R3 = m
+	MOVW	R2, m_g0(R3)			// m->g0 = g
+	MOVW	R3, g_m(R2)			// g->m = m
+	MOVW	R2, m_curg(R3)			// m->curg = g
+
+	MOVW	R2, g
+	BL	runtime·save_g(SB)
+
+	// set up stackguard stuff
+	MOVW	R13, R0
+	MOVW	R0, g_stack+stack_hi(g)
+	SUB	$(32*1024), R0
+	MOVW	R0, (g_stack+stack_lo)(g)
+	MOVW	R0, g_stackguard0(g)
+	MOVW	R0, g_stackguard1(g)
+
+	// move argument into position and call function
+	MOVW	12(R13), R0
+	MOVW	R0, 4(R13)
+	MOVW	16(R13), R1
+	BL	(R1)
+
+	// clear g
+	MOVW	$0, g
+	BL	runtime·save_g(SB)
+
+	MOVW	0(R13), R0			// load return value
+	ADD	$(m__size + g__size + 20), R13	// free locals
+	MOVM.IA.W (R13), [R4-R11, R15]		// pop {r4-r11, pc}
+
+GLOBL runtime·cbctxts(SB), NOPTR, $4
+
+TEXT runtime·callbackasm1(SB),NOSPLIT|NOFRAME,$0
+	MOVM.DB.W [R4-R11, R14], (R13)	// push {r4-r11, lr}
+	SUB	$36, R13		// space for locals
+
+	// save callback arguments to stack. We currently support up to 4 arguments
+	ADD	$16, R13, R4
+	MOVM.IA	[R0-R3], (R4)
+
+	// load cbctxts[i]. The trampoline in zcallback_windows.s puts the callback
+	// index in R12
+	MOVW	runtime·cbctxts(SB), R4
+	MOVW	R12<<2(R4), R4		// R4 holds pointer to wincallbackcontext structure
+
+	// extract callback context
+	MOVW	wincallbackcontext_argsize(R4), R5
+	MOVW	wincallbackcontext_gobody(R4), R4
+
+	// we currently support up to 4 arguments
+	CMP	$(4 * 4), R5
+	BL.GT	runtime·abort(SB)
+
+	// extend argsize by size of return value
+	ADD	$4, R5
+
+	// Build 'type args struct'
+	MOVW	R4, 4(R13)		// fn
+	ADD	$16, R13, R0		// arg (points to r0-r3, ret on stack)
+	MOVW	R0, 8(R13)
+	MOVW	R5, 12(R13)		// argsize
+
+	BL	runtime·load_g(SB)
+	BL	runtime·cgocallback_gofunc(SB)
+
+	ADD	$16, R13, R0		// load arg
+	MOVW	12(R13), R1		// load argsize
+	SUB	$4, R1			// offset to return value
+	MOVW	R1<<0(R0), R0		// load return value
+
+	ADD	$36, R13		// free locals
+	MOVM.IA.W (R13), [R4-R11, R15]	// pop {r4-r11, pc}
+
+// uint32 tstart_stdcall(M *newm);
+TEXT runtime·tstart_stdcall(SB),NOSPLIT|NOFRAME,$0
+	MOVM.DB.W [R4-R11, R14], (R13)		// push {r4-r11, lr}
+
+	MOVW	m_g0(R0), g
+	MOVW	R0, g_m(g)
+	BL	runtime·save_g(SB)
+
+	// Layout new m scheduler stack on os stack.
+	MOVW	R13, R0
+	MOVW	R0, g_stack+stack_hi(g)
+	SUB	$(64*1024), R0
+	MOVW	R0, (g_stack+stack_lo)(g)
+	MOVW	R0, g_stackguard0(g)
+	MOVW	R0, g_stackguard1(g)
+
+	BL	runtime·emptyfunc(SB)	// fault if stack check is wrong
+	BL	runtime·mstart(SB)
+
+	// Exit the thread.
+	MOVW	$0, R0
+	MOVM.IA.W (R13), [R4-R11, R15]		// pop {r4-r11, pc}
+
+// onosstack calls fn on OS stack.
+// adapted from asm_arm.s : systemstack
+// func onosstack(fn unsafe.Pointer, arg uint32)
+TEXT runtime·onosstack(SB),NOSPLIT,$0
+	MOVW	fn+0(FP), R5		// R5 = fn
+	MOVW	arg+4(FP), R6		// R6 = arg
+
+	// This function can be called when there is no g,
+	// for example, when we are handling a callback on a non-go thread.
+	// In this case we're already on the system stack.
+	CMP	$0, g
+	BEQ	noswitch
+
+	MOVW	g_m(g), R1		// R1 = m
+
+	MOVW	m_gsignal(R1), R2	// R2 = gsignal
+	CMP	g, R2
+	B.EQ	noswitch
+
+	MOVW	m_g0(R1), R2		// R2 = g0
+	CMP	g, R2
+	B.EQ	noswitch
+
+	MOVW	m_curg(R1), R3
+	CMP	g, R3
+	B.EQ	switch
+
+	// Bad: g is not gsignal, not g0, not curg. What is it?
+	// Hide call from linker nosplit analysis.
+	MOVW	$runtime·badsystemstack(SB), R0
+	BL	(R0)
+	B	runtime·abort(SB)
+
+switch:
+	// save our state in g->sched. Pretend to
+	// be systemstack_switch if the G stack is scanned.
+	MOVW	$runtime·systemstack_switch(SB), R3
+	ADD	$4, R3, R3 // get past push {lr}
+	MOVW	R3, (g_sched+gobuf_pc)(g)
+	MOVW	R13, (g_sched+gobuf_sp)(g)
+	MOVW	LR, (g_sched+gobuf_lr)(g)
+	MOVW	g, (g_sched+gobuf_g)(g)
+
+	// switch to g0
+	MOVW	R2, g
+	MOVW	(g_sched+gobuf_sp)(R2), R3
+	// make it look like mstart called systemstack on g0, to stop traceback
+	SUB	$4, R3, R3
+	MOVW	$runtime·mstart(SB), R4
+	MOVW	R4, 0(R3)
+	MOVW	R3, R13
+
+	// call target function
+	MOVW	R6, R0		// arg
+	BL	(R5)
+
+	// switch back to g
+	MOVW	g_m(g), R1
+	MOVW	m_curg(R1), g
+	MOVW	(g_sched+gobuf_sp)(g), R13
+	MOVW	$0, R3
+	MOVW	R3, (g_sched+gobuf_sp)(g)
+	RET
+
+noswitch:
+	// Using a tail call here cleans up tracebacks since we won't stop
+	// at an intermediate systemstack.
+	MOVW.P	4(R13), R14	// restore LR
+	MOVW	R6, R0		// arg
+	B	(R5)
+
+// Runs on OS stack. Duration (in 100ns units) is in R0.
+TEXT runtime·usleep2(SB),NOSPLIT|NOFRAME,$0
+	MOVM.DB.W [R4, R14], (R13)	// push {r4, lr}
+	MOVW	R13, R4			// Save SP
+	SUB	$8, R13			// R13 = R13 - 8
+	BIC	$0x7, R13		// Align SP for ABI
+	RSB	$0, R0, R3		// R3 = -R0
+	MOVW	$0, R1			// R1 = FALSE (alertable)
+	MOVW	$-1, R0			// R0 = handle
+	MOVW	R13, R2			// R2 = pTime
+	MOVW	R3, 0(R2)		// time_lo
+	MOVW	R0, 4(R2)		// time_hi
+	MOVW	runtime·_NtWaitForSingleObject(SB), R3
+	BL	(R3)
+	MOVW	R4, R13			// Restore SP
+	MOVM.IA.W (R13), [R4, R15]	// pop {R4, pc}
+
+// Runs on OS stack.
+TEXT runtime·switchtothread(SB),NOSPLIT|NOFRAME,$0
+	MOVM.DB.W [R4, R14], (R13)  	// push {R4, lr}
+	MOVW    R13, R4
+	BIC	$0x7, R13		// alignment for ABI
+	MOVW	runtime·_SwitchToThread(SB), R0
+	BL	(R0)
+	MOVW 	R4, R13			// restore stack pointer 
+	MOVM.IA.W (R13), [R4, R15]	// pop {R4, pc}
+
+TEXT ·publicationBarrier(SB),NOSPLIT|NOFRAME,$0-0
+	B	runtime·armPublicationBarrier(SB)
+
+// never called (cgo not supported)
+TEXT runtime·read_tls_fallback(SB),NOSPLIT|NOFRAME,$0
+	MOVW	$0xabcd, R0
+	MOVW	R0, (R0)
+	RET
+
+// See http://www.dcl.hpi.uni-potsdam.de/research/WRK/2007/08/getting-os-information-the-kuser_shared_data-structure/
+// Must read hi1, then lo, then hi2. The snapshot is valid if hi1 == hi2.
+#define _INTERRUPT_TIME 0x7ffe0008
+#define _SYSTEM_TIME 0x7ffe0014
+#define time_lo 0
+#define time_hi1 4
+#define time_hi2 8
+
+TEXT runtime·nanotime(SB),NOSPLIT,$0-8
+	MOVW	$0, R0
+	MOVB	runtime·useQPCTime(SB), R0
+	CMP	$0, R0
+	BNE	useQPC
+	MOVW	$_INTERRUPT_TIME, R3
+loop:
+	MOVW	time_hi1(R3), R1
+	MOVW	time_lo(R3), R0
+	MOVW	time_hi2(R3), R2
+	CMP	R1, R2
+	BNE	loop
+
+	// wintime = R1:R0, multiply by 100
+	MOVW	$100, R2
+	MULLU	R0, R2, (R4, R3)    // R4:R3 = R1:R0 * R2
+	MULA	R1, R2, R4, R4
+
+	// wintime*100 = R4:R3, subtract startNano and return
+	MOVW    runtime·startNano+0(SB), R0
+	MOVW    runtime·startNano+4(SB), R1
+	SUB.S   R0, R3
+	SBC	R1, R4
+	MOVW	R3, ret_lo+0(FP)
+	MOVW	R4, ret_hi+4(FP)
+	RET
+useQPC:
+	B	runtime·nanotimeQPC(SB)		// tail call
+	RET
+
+TEXT time·now(SB),NOSPLIT,$0-20
+	MOVW    $0, R0
+	MOVB    runtime·useQPCTime(SB), R0
+	CMP	$0, R0
+	BNE	useQPC
+	MOVW	$_INTERRUPT_TIME, R3
+loop:
+	MOVW	time_hi1(R3), R1
+	MOVW	time_lo(R3), R0
+	MOVW	time_hi2(R3), R2
+	CMP	R1, R2
+	BNE	loop
+
+	// wintime = R1:R0, multiply by 100
+	MOVW	$100, R2
+	MULLU	R0, R2, (R4, R3)    // R4:R3 = R1:R0 * R2
+	MULA	R1, R2, R4, R4
+
+	// wintime*100 = R4:R3, subtract startNano and return
+	MOVW    runtime·startNano+0(SB), R0
+	MOVW    runtime·startNano+4(SB), R1
+	SUB.S   R0, R3
+	SBC	R1, R4
+	MOVW	R3, mono+12(FP)
+	MOVW	R4, mono+16(FP)
+
+	MOVW	$_SYSTEM_TIME, R3
+wall:
+	MOVW	time_hi1(R3), R1
+	MOVW	time_lo(R3), R0
+	MOVW	time_hi2(R3), R2
+	CMP	R1, R2
+	BNE	wall
+
+	// w = R1:R0 in 100ns untis
+	// convert to Unix epoch (but still 100ns units)
+	#define delta 116444736000000000
+	SUB.S   $(delta & 0xFFFFFFFF), R0
+	SBC     $(delta >> 32), R1
+
+	// Convert to nSec
+	MOVW    $100, R2
+	MULLU   R0, R2, (R4, R3)    // R4:R3 = R1:R0 * R2
+	MULA    R1, R2, R4, R4
+	// w = R2:R1 in nSec
+	MOVW    R3, R1	      // R4:R3 -> R2:R1
+	MOVW    R4, R2
+
+	// multiply nanoseconds by reciprocal of 10**9 (scaled by 2**61)
+	// to get seconds (96 bit scaled result)
+	MOVW	$0x89705f41, R3		// 2**61 * 10**-9
+	MULLU	R1,R3,(R6,R5)		// R7:R6:R5 = R2:R1 * R3
+	MOVW	$0,R7
+	MULALU	R2,R3,(R7,R6)
+
+	// unscale by discarding low 32 bits, shifting the rest by 29
+	MOVW	R6>>29,R6		// R7:R6 = (R7:R6:R5 >> 61)
+	ORR	R7<<3,R6
+	MOVW	R7>>29,R7
+
+	// subtract (10**9 * sec) from nsec to get nanosecond remainder
+	MOVW	$1000000000, R5	// 10**9
+	MULLU	R6,R5,(R9,R8)   // R9:R8 = R7:R6 * R5
+	MULA	R7,R5,R9,R9
+	SUB.S	R8,R1		// R2:R1 -= R9:R8
+	SBC	R9,R2
+
+	// because reciprocal was a truncated repeating fraction, quotient
+	// may be slightly too small -- adjust to make remainder < 10**9
+	CMP	R5,R1	// if remainder > 10**9
+	SUB.HS	R5,R1   //    remainder -= 10**9
+	ADD.HS	$1,R6	//    sec += 1
+
+	MOVW	R6,sec_lo+0(FP)
+	MOVW	R7,sec_hi+4(FP)
+	MOVW	R1,nsec+8(FP)
+	RET
+useQPC:
+	B	runtime·nanotimeQPC(SB)		// tail call
+	RET
+