// Copyright 2016 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 "funcdata.h"
#include "textflag.h"

// _rt0_s390x_lib is common startup code for s390x systems when
// using -buildmode=c-archive or -buildmode=c-shared. The linker will
// arrange to invoke this function as a global constructor (for
// c-archive) or when the shared library is loaded (for c-shared).
// We expect argc and argv to be passed in the usual C ABI registers
// R2 and R3.
TEXT _rt0_s390x_lib(SB), NOSPLIT|NOFRAME, $0
	STMG	R6, R15, 48(R15)
	MOVD	R2, _rt0_s390x_lib_argc<>(SB)
	MOVD	R3, _rt0_s390x_lib_argv<>(SB)

	// Save R6-R15 in the register save area of the calling function.
	STMG	R6, R15, 48(R15)

	// Allocate 80 bytes on the stack.
	MOVD	$-80(R15), R15

	// Save F8-F15 in our stack frame.
	FMOVD	F8, 16(R15)
	FMOVD	F9, 24(R15)
	FMOVD	F10, 32(R15)
	FMOVD	F11, 40(R15)
	FMOVD	F12, 48(R15)
	FMOVD	F13, 56(R15)
	FMOVD	F14, 64(R15)
	FMOVD	F15, 72(R15)

	// Synchronous initialization.
	MOVD	$runtime·libpreinit(SB), R1
	BL	R1

	// Create a new thread to finish Go runtime initialization.
	MOVD	_cgo_sys_thread_create(SB), R1
	CMP	R1, $0
	BEQ	nocgo
	MOVD	$_rt0_s390x_lib_go(SB), R2
	MOVD	$0, R3
	BL	R1
	BR	restore

nocgo:
	MOVD	$0x800000, R1              // stacksize
	MOVD	R1, 0(R15)
	MOVD	$_rt0_s390x_lib_go(SB), R1
	MOVD	R1, 8(R15)                 // fn
	MOVD	$runtime·newosproc(SB), R1
	BL	R1

restore:
	// Restore F8-F15 from our stack frame.
	FMOVD	16(R15), F8
	FMOVD	24(R15), F9
	FMOVD	32(R15), F10
	FMOVD	40(R15), F11
	FMOVD	48(R15), F12
	FMOVD	56(R15), F13
	FMOVD	64(R15), F14
	FMOVD	72(R15), F15
	MOVD	$80(R15), R15

	// Restore R6-R15.
	LMG	48(R15), R6, R15
	RET

// _rt0_s390x_lib_go initializes the Go runtime.
// This is started in a separate thread by _rt0_s390x_lib.
TEXT _rt0_s390x_lib_go(SB), NOSPLIT|NOFRAME, $0
	MOVD	_rt0_s390x_lib_argc<>(SB), R2
	MOVD	_rt0_s390x_lib_argv<>(SB), R3
	MOVD	$runtime·rt0_go(SB), R1
	BR	R1

DATA _rt0_s390x_lib_argc<>(SB)/8, $0
GLOBL _rt0_s390x_lib_argc<>(SB), NOPTR, $8
DATA _rt0_s90x_lib_argv<>(SB)/8, $0
GLOBL _rt0_s390x_lib_argv<>(SB), NOPTR, $8

TEXT runtime·rt0_go(SB),NOSPLIT|TOPFRAME,$0
	// R2 = argc; R3 = argv; R11 = temp; R13 = g; R15 = stack pointer
	// C TLS base pointer in AR0:AR1

	// initialize essential registers
	XOR	R0, R0

	SUB	$24, R15
	MOVW	R2, 8(R15) // argc
	MOVD	R3, 16(R15) // argv

	// create istack out of the given (operating system) stack.
	// _cgo_init may update stackguard.
	MOVD	$runtime·g0(SB), g
	MOVD	R15, R11
	SUB	$(64*1024), R11
	MOVD	R11, g_stackguard0(g)
	MOVD	R11, g_stackguard1(g)
	MOVD	R11, (g_stack+stack_lo)(g)
	MOVD	R15, (g_stack+stack_hi)(g)

	// if there is a _cgo_init, call it using the gcc ABI.
	MOVD	_cgo_init(SB), R11
	CMPBEQ	R11, $0, nocgo
	MOVW	AR0, R4			// (AR0 << 32 | AR1) is the TLS base pointer; MOVD is translated to EAR
	SLD	$32, R4, R4
	MOVW	AR1, R4			// arg 2: TLS base pointer
	MOVD	$setg_gcc<>(SB), R3 	// arg 1: setg
	MOVD	g, R2			// arg 0: G
	// C functions expect 160 bytes of space on caller stack frame
	// and an 8-byte aligned stack pointer
	MOVD	R15, R9			// save current stack (R9 is preserved in the Linux ABI)
	SUB	$160, R15		// reserve 160 bytes
	MOVD    $~7, R6
	AND 	R6, R15			// 8-byte align
	BL	R11			// this call clobbers volatile registers according to Linux ABI (R0-R5, R14)
	MOVD	R9, R15			// restore stack
	XOR	R0, R0			// zero R0

nocgo:
	// update stackguard after _cgo_init
	MOVD	(g_stack+stack_lo)(g), R2
	ADD	$const__StackGuard, R2
	MOVD	R2, g_stackguard0(g)
	MOVD	R2, g_stackguard1(g)

	// set the per-goroutine and per-mach "registers"
	MOVD	$runtime·m0(SB), R2

	// save m->g0 = g0
	MOVD	g, m_g0(R2)
	// save m0 to g0->m
	MOVD	R2, g_m(g)

	BL	runtime·check(SB)

	// argc/argv are already prepared on stack
	BL	runtime·args(SB)
	BL	runtime·osinit(SB)
	BL	runtime·schedinit(SB)

	// create a new goroutine to start program
	MOVD	$runtime·mainPC(SB), R2		// entry
	SUB     $16, R15
	MOVD 	R2, 8(R15)
	MOVD 	$0, 0(R15)
	BL	runtime·newproc(SB)
	ADD	$16, R15

	// start this M
	BL	runtime·mstart(SB)

	MOVD	$0, 1(R0)
	RET

DATA	runtime·mainPC+0(SB)/8,$runtime·main(SB)
GLOBL	runtime·mainPC(SB),RODATA,$8

TEXT runtime·breakpoint(SB),NOSPLIT|NOFRAME,$0-0
	MOVD	$0, 2(R0)
	RET

TEXT runtime·asminit(SB),NOSPLIT|NOFRAME,$0-0
	RET

TEXT runtime·mstart(SB),NOSPLIT|TOPFRAME,$0
	CALL	runtime·mstart0(SB)
	RET // not reached

/*
 *  go-routine
 */

// void gogo(Gobuf*)
// restore state from Gobuf; longjmp
TEXT runtime·gogo(SB), NOSPLIT|NOFRAME, $0-8
	MOVD	buf+0(FP), R5
	MOVD	gobuf_g(R5), R6
	MOVD	0(R6), R7	// make sure g != nil
	BR	gogo<>(SB)

TEXT gogo<>(SB), NOSPLIT|NOFRAME, $0
	MOVD	R6, g
	BL	runtime·save_g(SB)

	MOVD	0(g), R4
	MOVD	gobuf_sp(R5), R15
	MOVD	gobuf_lr(R5), LR
	MOVD	gobuf_ret(R5), R3
	MOVD	gobuf_ctxt(R5), R12
	MOVD	$0, gobuf_sp(R5)
	MOVD	$0, gobuf_ret(R5)
	MOVD	$0, gobuf_lr(R5)
	MOVD	$0, gobuf_ctxt(R5)
	CMP	R0, R0 // set condition codes for == test, needed by stack split
	MOVD	gobuf_pc(R5), R6
	BR	(R6)

// void mcall(fn func(*g))
// Switch to m->g0's stack, call fn(g).
// Fn must never return.  It should gogo(&g->sched)
// to keep running g.
TEXT runtime·mcall(SB), NOSPLIT, $-8-8
	// Save caller state in g->sched
	MOVD	R15, (g_sched+gobuf_sp)(g)
	MOVD	LR, (g_sched+gobuf_pc)(g)
	MOVD	$0, (g_sched+gobuf_lr)(g)

	// Switch to m->g0 & its stack, call fn.
	MOVD	g, R3
	MOVD	g_m(g), R8
	MOVD	m_g0(R8), g
	BL	runtime·save_g(SB)
	CMP	g, R3
	BNE	2(PC)
	BR	runtime·badmcall(SB)
	MOVD	fn+0(FP), R12			// context
	MOVD	0(R12), R4			// code pointer
	MOVD	(g_sched+gobuf_sp)(g), R15	// sp = m->g0->sched.sp
	SUB	$16, R15
	MOVD	R3, 8(R15)
	MOVD	$0, 0(R15)
	BL	(R4)
	BR	runtime·badmcall2(SB)

// systemstack_switch is a dummy routine that systemstack leaves at the bottom
// of the G stack.  We need to distinguish the routine that
// lives at the bottom of the G stack from the one that lives
// at the top of the system stack because the one at the top of
// the system stack terminates the stack walk (see topofstack()).
TEXT runtime·systemstack_switch(SB), NOSPLIT, $0-0
	UNDEF
	BL	(LR)	// make sure this function is not leaf
	RET

// func systemstack(fn func())
TEXT runtime·systemstack(SB), NOSPLIT, $0-8
	MOVD	fn+0(FP), R3	// R3 = fn
	MOVD	R3, R12		// context
	MOVD	g_m(g), R4	// R4 = m

	MOVD	m_gsignal(R4), R5	// R5 = gsignal
	CMPBEQ	g, R5, noswitch

	MOVD	m_g0(R4), R5	// R5 = g0
	CMPBEQ	g, R5, noswitch

	MOVD	m_curg(R4), R6
	CMPBEQ	g, R6, switch

	// Bad: g is not gsignal, not g0, not curg. What is it?
	// Hide call from linker nosplit analysis.
	MOVD	$runtime·badsystemstack(SB), R3
	BL	(R3)
	BL	runtime·abort(SB)

switch:
	// save our state in g->sched.  Pretend to
	// be systemstack_switch if the G stack is scanned.
	BL	gosave_systemstack_switch<>(SB)

	// switch to g0
	MOVD	R5, g
	BL	runtime·save_g(SB)
	MOVD	(g_sched+gobuf_sp)(g), R15

	// call target function
	MOVD	0(R12), R3	// code pointer
	BL	(R3)

	// switch back to g
	MOVD	g_m(g), R3
	MOVD	m_curg(R3), g
	BL	runtime·save_g(SB)
	MOVD	(g_sched+gobuf_sp)(g), R15
	MOVD	$0, (g_sched+gobuf_sp)(g)
	RET

noswitch:
	// already on m stack, just call directly
	// Using a tail call here cleans up tracebacks since we won't stop
	// at an intermediate systemstack.
	MOVD	0(R12), R3	// code pointer
	MOVD	0(R15), LR	// restore LR
	ADD	$8, R15
	BR	(R3)

/*
 * support for morestack
 */

// Called during function prolog when more stack is needed.
// Caller has already loaded:
// R3: framesize, R4: argsize, R5: LR
//
// The traceback routines see morestack on a g0 as being
// the top of a stack (for example, morestack calling newstack
// calling the scheduler calling newm calling gc), so we must
// record an argument size. For that purpose, it has no arguments.
TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
	// Cannot grow scheduler stack (m->g0).
	MOVD	g_m(g), R7
	MOVD	m_g0(R7), R8
	CMPBNE	g, R8, 3(PC)
	BL	runtime·badmorestackg0(SB)
	BL	runtime·abort(SB)

	// Cannot grow signal stack (m->gsignal).
	MOVD	m_gsignal(R7), R8
	CMP	g, R8
	BNE	3(PC)
	BL	runtime·badmorestackgsignal(SB)
	BL	runtime·abort(SB)

	// Called from f.
	// Set g->sched to context in f.
	MOVD	R15, (g_sched+gobuf_sp)(g)
	MOVD	LR, R8
	MOVD	R8, (g_sched+gobuf_pc)(g)
	MOVD	R5, (g_sched+gobuf_lr)(g)
	MOVD	R12, (g_sched+gobuf_ctxt)(g)

	// Called from f.
	// Set m->morebuf to f's caller.
	MOVD	R5, (m_morebuf+gobuf_pc)(R7)	// f's caller's PC
	MOVD	R15, (m_morebuf+gobuf_sp)(R7)	// f's caller's SP
	MOVD	g, (m_morebuf+gobuf_g)(R7)

	// Call newstack on m->g0's stack.
	MOVD	m_g0(R7), g
	BL	runtime·save_g(SB)
	MOVD	(g_sched+gobuf_sp)(g), R15
	// Create a stack frame on g0 to call newstack.
	MOVD	$0, -8(R15)	// Zero saved LR in frame
	SUB	$8, R15
	BL	runtime·newstack(SB)

	// Not reached, but make sure the return PC from the call to newstack
	// is still in this function, and not the beginning of the next.
	UNDEF

TEXT runtime·morestack_noctxt(SB),NOSPLIT|NOFRAME,$0-0
	// Force SPWRITE. This function doesn't actually write SP,
	// but it is called with a special calling convention where
	// the caller doesn't save LR on stack but passes it as a
	// register (R5), and the unwinder currently doesn't understand.
	// Make it SPWRITE to stop unwinding. (See issue 54332)
	MOVD	R15, R15

	MOVD	$0, R12
	BR	runtime·morestack(SB)

// reflectcall: call a function with the given argument list
// func call(stackArgsType *_type, f *FuncVal, stackArgs *byte, stackArgsSize, stackRetOffset, frameSize uint32, regArgs *abi.RegArgs).
// we don't have variable-sized frames, so we use a small number
// of constant-sized-frame functions to encode a few bits of size in the pc.
// Caution: ugly multiline assembly macros in your future!

#define DISPATCH(NAME,MAXSIZE)		\
	MOVD	$MAXSIZE, R4;		\
	CMP	R3, R4;		\
	BGT	3(PC);			\
	MOVD	$NAME(SB), R5;	\
	BR	(R5)
// Note: can't just "BR NAME(SB)" - bad inlining results.

TEXT ·reflectcall(SB), NOSPLIT, $-8-48
	MOVWZ	frameSize+32(FP), R3
	DISPATCH(runtime·call16, 16)
	DISPATCH(runtime·call32, 32)
	DISPATCH(runtime·call64, 64)
	DISPATCH(runtime·call128, 128)
	DISPATCH(runtime·call256, 256)
	DISPATCH(runtime·call512, 512)
	DISPATCH(runtime·call1024, 1024)
	DISPATCH(runtime·call2048, 2048)
	DISPATCH(runtime·call4096, 4096)
	DISPATCH(runtime·call8192, 8192)
	DISPATCH(runtime·call16384, 16384)
	DISPATCH(runtime·call32768, 32768)
	DISPATCH(runtime·call65536, 65536)
	DISPATCH(runtime·call131072, 131072)
	DISPATCH(runtime·call262144, 262144)
	DISPATCH(runtime·call524288, 524288)
	DISPATCH(runtime·call1048576, 1048576)
	DISPATCH(runtime·call2097152, 2097152)
	DISPATCH(runtime·call4194304, 4194304)
	DISPATCH(runtime·call8388608, 8388608)
	DISPATCH(runtime·call16777216, 16777216)
	DISPATCH(runtime·call33554432, 33554432)
	DISPATCH(runtime·call67108864, 67108864)
	DISPATCH(runtime·call134217728, 134217728)
	DISPATCH(runtime·call268435456, 268435456)
	DISPATCH(runtime·call536870912, 536870912)
	DISPATCH(runtime·call1073741824, 1073741824)
	MOVD	$runtime·badreflectcall(SB), R5
	BR	(R5)

#define CALLFN(NAME,MAXSIZE)			\
TEXT NAME(SB), WRAPPER, $MAXSIZE-48;		\
	NO_LOCAL_POINTERS;			\
	/* copy arguments to stack */		\
	MOVD	stackArgs+16(FP), R4;			\
	MOVWZ	stackArgsSize+24(FP), R5;		\
	MOVD	$stack-MAXSIZE(SP), R6;		\
loopArgs: /* copy 256 bytes at a time */	\
	CMP	R5, $256;			\
	BLT	tailArgs;			\
	SUB	$256, R5;			\
	MVC	$256, 0(R4), 0(R6);		\
	MOVD	$256(R4), R4;			\
	MOVD	$256(R6), R6;			\
	BR	loopArgs;			\
tailArgs: /* copy remaining bytes */		\
	CMP	R5, $0;				\
	BEQ	callFunction;			\
	SUB	$1, R5;				\
	EXRL	$callfnMVC<>(SB), R5;		\
callFunction:					\
	MOVD	f+8(FP), R12;			\
	MOVD	(R12), R8;			\
	PCDATA  $PCDATA_StackMapIndex, $0;	\
	BL	(R8);				\
	/* copy return values back */		\
	MOVD	stackArgsType+0(FP), R7;		\
	MOVD	stackArgs+16(FP), R6;			\
	MOVWZ	stackArgsSize+24(FP), R5;			\
	MOVD	$stack-MAXSIZE(SP), R4;		\
	MOVWZ	stackRetOffset+28(FP), R1;		\
	ADD	R1, R4;				\
	ADD	R1, R6;				\
	SUB	R1, R5;				\
	BL	callRet<>(SB);			\
	RET

// callRet copies return values back at the end of call*. This is a
// separate function so it can allocate stack space for the arguments
// to reflectcallmove. It does not follow the Go ABI; it expects its
// arguments in registers.
TEXT callRet<>(SB), NOSPLIT, $40-0
	MOVD	R7, 8(R15)
	MOVD	R6, 16(R15)
	MOVD	R4, 24(R15)
	MOVD	R5, 32(R15)
	MOVD	$0, 40(R15)
	BL	runtime·reflectcallmove(SB)
	RET

CALLFN(·call16, 16)
CALLFN(·call32, 32)
CALLFN(·call64, 64)
CALLFN(·call128, 128)
CALLFN(·call256, 256)
CALLFN(·call512, 512)
CALLFN(·call1024, 1024)
CALLFN(·call2048, 2048)
CALLFN(·call4096, 4096)
CALLFN(·call8192, 8192)
CALLFN(·call16384, 16384)
CALLFN(·call32768, 32768)
CALLFN(·call65536, 65536)
CALLFN(·call131072, 131072)
CALLFN(·call262144, 262144)
CALLFN(·call524288, 524288)
CALLFN(·call1048576, 1048576)
CALLFN(·call2097152, 2097152)
CALLFN(·call4194304, 4194304)
CALLFN(·call8388608, 8388608)
CALLFN(·call16777216, 16777216)
CALLFN(·call33554432, 33554432)
CALLFN(·call67108864, 67108864)
CALLFN(·call134217728, 134217728)
CALLFN(·call268435456, 268435456)
CALLFN(·call536870912, 536870912)
CALLFN(·call1073741824, 1073741824)

// Not a function: target for EXRL (execute relative long) instruction.
TEXT callfnMVC<>(SB),NOSPLIT|NOFRAME,$0-0
	MVC	$1, 0(R4), 0(R6)

TEXT runtime·procyield(SB),NOSPLIT,$0-0
	RET

// Save state of caller into g->sched,
// but using fake PC from systemstack_switch.
// Must only be called from functions with no locals ($0)
// or else unwinding from systemstack_switch is incorrect.
// Smashes R1.
TEXT gosave_systemstack_switch<>(SB),NOSPLIT|NOFRAME,$0
	MOVD	$runtime·systemstack_switch(SB), R1
	ADD	$16, R1	// get past prologue
	MOVD	R1, (g_sched+gobuf_pc)(g)
	MOVD	R15, (g_sched+gobuf_sp)(g)
	MOVD	$0, (g_sched+gobuf_lr)(g)
	MOVD	$0, (g_sched+gobuf_ret)(g)
	// Assert ctxt is zero. See func save.
	MOVD	(g_sched+gobuf_ctxt)(g), R1
	CMPBEQ	R1, $0, 2(PC)
	BL	runtime·abort(SB)
	RET

// func asmcgocall(fn, arg unsafe.Pointer) int32
// Call fn(arg) on the scheduler stack,
// aligned appropriately for the gcc ABI.
// See cgocall.go for more details.
TEXT ·asmcgocall(SB),NOSPLIT,$0-20
	// R2 = argc; R3 = argv; R11 = temp; R13 = g; R15 = stack pointer
	// C TLS base pointer in AR0:AR1
	MOVD	fn+0(FP), R3
	MOVD	arg+8(FP), R4

	MOVD	R15, R2		// save original stack pointer
	MOVD	g, R5

	// Figure out if we need to switch to m->g0 stack.
	// We get called to create new OS threads too, and those
	// come in on the m->g0 stack already. Or we might already
	// be on the m->gsignal stack.
	MOVD	g_m(g), R6
	MOVD	m_gsignal(R6), R7
	CMPBEQ	R7, g, g0
	MOVD	m_g0(R6), R7
	CMPBEQ	R7, g, g0
	BL	gosave_systemstack_switch<>(SB)
	MOVD	R7, g
	BL	runtime·save_g(SB)
	MOVD	(g_sched+gobuf_sp)(g), R15

	// Now on a scheduling stack (a pthread-created stack).
g0:
	// Save room for two of our pointers, plus 160 bytes of callee
	// save area that lives on the caller stack.
	SUB	$176, R15
	MOVD	$~7, R6
	AND	R6, R15                 // 8-byte alignment for gcc ABI
	MOVD	R5, 168(R15)             // save old g on stack
	MOVD	(g_stack+stack_hi)(R5), R5
	SUB	R2, R5
	MOVD	R5, 160(R15)             // save depth in old g stack (can't just save SP, as stack might be copied during a callback)
	MOVD	$0, 0(R15)              // clear back chain pointer (TODO can we give it real back trace information?)
	MOVD	R4, R2                  // arg in R2
	BL	R3                      // can clobber: R0-R5, R14, F0-F3, F5, F7-F15

	XOR	R0, R0                  // set R0 back to 0.
	// Restore g, stack pointer.
	MOVD	168(R15), g
	BL	runtime·save_g(SB)
	MOVD	(g_stack+stack_hi)(g), R5
	MOVD	160(R15), R6
	SUB	R6, R5
	MOVD	R5, R15

	MOVW	R2, ret+16(FP)
	RET

// cgocallback(fn, frame unsafe.Pointer, ctxt uintptr)
// See cgocall.go for more details.
TEXT ·cgocallback(SB),NOSPLIT,$24-24
	NO_LOCAL_POINTERS

	// Load m and g from thread-local storage.
	MOVB	runtime·iscgo(SB), R3
	CMPBEQ	R3, $0, nocgo
	BL	runtime·load_g(SB)

nocgo:
	// If g is nil, Go did not create the current thread.
	// Call needm to obtain one for temporary use.
	// In this case, we're running on the thread stack, so there's
	// lots of space, but the linker doesn't know. Hide the call from
	// the linker analysis by using an indirect call.
	CMPBEQ	g, $0, needm

	MOVD	g_m(g), R8
	MOVD	R8, savedm-8(SP)
	BR	havem

needm:
	MOVD	g, savedm-8(SP) // g is zero, so is m.
	MOVD	$runtime·needm(SB), R3
	BL	(R3)

	// Set m->sched.sp = SP, so that if a panic happens
	// during the function we are about to execute, it will
	// have a valid SP to run on the g0 stack.
	// The next few lines (after the havem label)
	// will save this SP onto the stack and then write
	// the same SP back to m->sched.sp. That seems redundant,
	// but if an unrecovered panic happens, unwindm will
	// restore the g->sched.sp from the stack location
	// and then systemstack will try to use it. If we don't set it here,
	// that restored SP will be uninitialized (typically 0) and
	// will not be usable.
	MOVD	g_m(g), R8
	MOVD	m_g0(R8), R3
	MOVD	R15, (g_sched+gobuf_sp)(R3)

havem:
	// Now there's a valid m, and we're running on its m->g0.
	// Save current m->g0->sched.sp on stack and then set it to SP.
	// Save current sp in m->g0->sched.sp in preparation for
	// switch back to m->curg stack.
	// NOTE: unwindm knows that the saved g->sched.sp is at 8(R1) aka savedsp-16(SP).
	MOVD	m_g0(R8), R3
	MOVD	(g_sched+gobuf_sp)(R3), R4
	MOVD	R4, savedsp-24(SP)	// must match frame size
	MOVD	R15, (g_sched+gobuf_sp)(R3)

	// Switch to m->curg stack and call runtime.cgocallbackg.
	// Because we are taking over the execution of m->curg
	// but *not* resuming what had been running, we need to
	// save that information (m->curg->sched) so we can restore it.
	// We can restore m->curg->sched.sp easily, because calling
	// runtime.cgocallbackg leaves SP unchanged upon return.
	// To save m->curg->sched.pc, we push it onto the curg stack and
	// open a frame the same size as cgocallback's g0 frame.
	// Once we switch to the curg stack, the pushed PC will appear
	// to be the return PC of cgocallback, so that the traceback
	// will seamlessly trace back into the earlier calls.
	MOVD	m_curg(R8), g
	BL	runtime·save_g(SB)
	MOVD	(g_sched+gobuf_sp)(g), R4 // prepare stack as R4
	MOVD	(g_sched+gobuf_pc)(g), R5
	MOVD	R5, -(24+8)(R4)	// "saved LR"; must match frame size
	// Gather our arguments into registers.
	MOVD	fn+0(FP), R1
	MOVD	frame+8(FP), R2
	MOVD	ctxt+16(FP), R3
	MOVD	$-(24+8)(R4), R15	// switch stack; must match frame size
	MOVD	R1, 8(R15)
	MOVD	R2, 16(R15)
	MOVD	R3, 24(R15)
	BL	runtime·cgocallbackg(SB)

	// Restore g->sched (== m->curg->sched) from saved values.
	MOVD	0(R15), R5
	MOVD	R5, (g_sched+gobuf_pc)(g)
	MOVD	$(24+8)(R15), R4	// must match frame size
	MOVD	R4, (g_sched+gobuf_sp)(g)

	// Switch back to m->g0's stack and restore m->g0->sched.sp.
	// (Unlike m->curg, the g0 goroutine never uses sched.pc,
	// so we do not have to restore it.)
	MOVD	g_m(g), R8
	MOVD	m_g0(R8), g
	BL	runtime·save_g(SB)
	MOVD	(g_sched+gobuf_sp)(g), R15
	MOVD	savedsp-24(SP), R4	// must match frame size
	MOVD	R4, (g_sched+gobuf_sp)(g)

	// If the m on entry was nil, we called needm above to borrow an m
	// for the duration of the call. Since the call is over, return it with dropm.
	MOVD	savedm-8(SP), R6
	CMPBNE	R6, $0, droppedm
	MOVD	$runtime·dropm(SB), R3
	BL	(R3)
droppedm:

	// Done!
	RET

// void setg(G*); set g. for use by needm.
TEXT runtime·setg(SB), NOSPLIT, $0-8
	MOVD	gg+0(FP), g
	// This only happens if iscgo, so jump straight to save_g
	BL	runtime·save_g(SB)
	RET

// void setg_gcc(G*); set g in C TLS.
// Must obey the gcc calling convention.
TEXT setg_gcc<>(SB),NOSPLIT|NOFRAME,$0-0
	// The standard prologue clobbers LR (R14), which is callee-save in
	// the C ABI, so we have to use NOFRAME and save LR ourselves.
	MOVD	LR, R1
	// Also save g, R10, and R11 since they're callee-save in C ABI
	MOVD	R10, R3
	MOVD	g, R4
	MOVD	R11, R5

	MOVD	R2, g
	BL	runtime·save_g(SB)

	MOVD	R5, R11
	MOVD	R4, g
	MOVD	R3, R10
	MOVD	R1, LR
	RET

TEXT runtime·abort(SB),NOSPLIT|NOFRAME,$0-0
	MOVW	(R0), R0
	UNDEF

// int64 runtime·cputicks(void)
TEXT runtime·cputicks(SB),NOSPLIT,$0-8
	// The TOD clock on s390 counts from the year 1900 in ~250ps intervals.
	// This means that since about 1972 the msb has been set, making the
	// result of a call to STORE CLOCK (stck) a negative number.
	// We clear the msb to make it positive.
	STCK	ret+0(FP)      // serialises before and after call
	MOVD	ret+0(FP), R3  // R3 will wrap to 0 in the year 2043
	SLD	$1, R3
	SRD	$1, R3
	MOVD	R3, ret+0(FP)
	RET

// AES hashing not implemented for s390x
TEXT runtime·memhash(SB),NOSPLIT|NOFRAME,$0-32
	JMP	runtime·memhashFallback(SB)
TEXT runtime·strhash(SB),NOSPLIT|NOFRAME,$0-24
	JMP	runtime·strhashFallback(SB)
TEXT runtime·memhash32(SB),NOSPLIT|NOFRAME,$0-24
	JMP	runtime·memhash32Fallback(SB)
TEXT runtime·memhash64(SB),NOSPLIT|NOFRAME,$0-24
	JMP	runtime·memhash64Fallback(SB)

TEXT runtime·return0(SB), NOSPLIT, $0
	MOVW	$0, R3
	RET

// Called from cgo wrappers, this function returns g->m->curg.stack.hi.
// Must obey the gcc calling convention.
TEXT _cgo_topofstack(SB),NOSPLIT|NOFRAME,$0
	// g (R13), R10, R11 and LR (R14) are callee-save in the C ABI, so save them
	MOVD	g, R1
	MOVD	R10, R3
	MOVD	LR, R4
	MOVD	R11, R5

	BL	runtime·load_g(SB)	// clobbers g (R13), R10, R11
	MOVD	g_m(g), R2
	MOVD	m_curg(R2), R2
	MOVD	(g_stack+stack_hi)(R2), R2

	MOVD	R1, g
	MOVD	R3, R10
	MOVD	R4, LR
	MOVD	R5, R11
	RET

// The top-most function running on a goroutine
// returns to goexit+PCQuantum.
TEXT runtime·goexit(SB),NOSPLIT|NOFRAME|TOPFRAME,$0-0
	BYTE $0x07; BYTE $0x00; // 2-byte nop
	BL	runtime·goexit1(SB)	// does not return
	// traceback from goexit1 must hit code range of goexit
	BYTE $0x07; BYTE $0x00; // 2-byte nop

TEXT ·publicationBarrier(SB),NOSPLIT|NOFRAME,$0-0
	// Stores are already ordered on s390x, so this is just a
	// compile barrier.
	RET

// This is called from .init_array and follows the platform, not Go, ABI.
// We are overly conservative. We could only save the registers we use.
// However, since this function is only called once per loaded module
// performance is unimportant.
TEXT runtime·addmoduledata(SB),NOSPLIT|NOFRAME,$0-0
	// Save R6-R15 in the register save area of the calling function.
	// Don't bother saving F8-F15 as we aren't doing any calls.
	STMG	R6, R15, 48(R15)

	// append the argument (passed in R2, as per the ELF ABI) to the
	// moduledata linked list.
	MOVD	runtime·lastmoduledatap(SB), R1
	MOVD	R2, moduledata_next(R1)
	MOVD	R2, runtime·lastmoduledatap(SB)

	// Restore R6-R15.
	LMG	48(R15), R6, R15
	RET

TEXT ·checkASM(SB),NOSPLIT,$0-1
	MOVB	$1, ret+0(FP)
	RET

// gcWriteBarrier performs a heap pointer write and informs the GC.
//
// gcWriteBarrier does NOT follow the Go ABI. It takes two arguments:
// - R2 is the destination of the write
// - R3 is the value being written at R2.
// It clobbers R10 (the temp register) and R1 (used by PLT stub).
// It does not clobber any other general-purpose registers,
// but may clobber others (e.g., floating point registers).
TEXT runtime·gcWriteBarrier(SB),NOSPLIT,$96
	// Save the registers clobbered by the fast path.
	MOVD	R4, 96(R15)
	MOVD	g_m(g), R1
	MOVD	m_p(R1), R1
	// Increment wbBuf.next position.
	MOVD	$16, R4
	ADD	(p_wbBuf+wbBuf_next)(R1), R4
	MOVD	R4, (p_wbBuf+wbBuf_next)(R1)
	MOVD	(p_wbBuf+wbBuf_end)(R1), R1
	// Record the write.
	MOVD	R3, -16(R4) // Record value
	MOVD	(R2), R10   // TODO: This turns bad writes into bad reads.
	MOVD	R10, -8(R4) // Record *slot
	// Is the buffer full?
	CMPBEQ	R4, R1, flush
ret:
	MOVD	96(R15), R4
	// Do the write.
	MOVD	R3, (R2)
	RET

flush:
	// Save all general purpose registers since these could be
	// clobbered by wbBufFlush and were not saved by the caller.
	STMG	R2, R3, 8(R15)   // set R2 and R3 as arguments for wbBufFlush
	MOVD	R0, 24(R15)
	// R1 already saved.
	// R4 already saved.
	STMG	R5, R12, 32(R15) // save R5 - R12
	// R13 is g.
	// R14 is LR.
	// R15 is SP.

	// This takes arguments R2 and R3.
	CALL	runtime·wbBufFlush(SB)

	LMG	8(R15), R2, R3   // restore R2 - R3
	MOVD	24(R15), R0      // restore R0
	LMG	32(R15), R5, R12 // restore R5 - R12
	JMP	ret

// Note: these functions use a special calling convention to save generated code space.
// Arguments are passed in registers, but the space for those arguments are allocated
// in the caller's stack frame. These stubs write the args into that stack space and
// then tail call to the corresponding runtime handler.
// The tail call makes these stubs disappear in backtraces.
TEXT runtime·panicIndex(SB),NOSPLIT,$0-16
	MOVD	R0, x+0(FP)
	MOVD	R1, y+8(FP)
	JMP	runtime·goPanicIndex(SB)
TEXT runtime·panicIndexU(SB),NOSPLIT,$0-16
	MOVD	R0, x+0(FP)
	MOVD	R1, y+8(FP)
	JMP	runtime·goPanicIndexU(SB)
TEXT runtime·panicSliceAlen(SB),NOSPLIT,$0-16
	MOVD	R1, x+0(FP)
	MOVD	R2, y+8(FP)
	JMP	runtime·goPanicSliceAlen(SB)
TEXT runtime·panicSliceAlenU(SB),NOSPLIT,$0-16
	MOVD	R1, x+0(FP)
	MOVD	R2, y+8(FP)
	JMP	runtime·goPanicSliceAlenU(SB)
TEXT runtime·panicSliceAcap(SB),NOSPLIT,$0-16
	MOVD	R1, x+0(FP)
	MOVD	R2, y+8(FP)
	JMP	runtime·goPanicSliceAcap(SB)
TEXT runtime·panicSliceAcapU(SB),NOSPLIT,$0-16
	MOVD	R1, x+0(FP)
	MOVD	R2, y+8(FP)
	JMP	runtime·goPanicSliceAcapU(SB)
TEXT runtime·panicSliceB(SB),NOSPLIT,$0-16
	MOVD	R0, x+0(FP)
	MOVD	R1, y+8(FP)
	JMP	runtime·goPanicSliceB(SB)
TEXT runtime·panicSliceBU(SB),NOSPLIT,$0-16
	MOVD	R0, x+0(FP)
	MOVD	R1, y+8(FP)
	JMP	runtime·goPanicSliceBU(SB)
TEXT runtime·panicSlice3Alen(SB),NOSPLIT,$0-16
	MOVD	R2, x+0(FP)
	MOVD	R3, y+8(FP)
	JMP	runtime·goPanicSlice3Alen(SB)
TEXT runtime·panicSlice3AlenU(SB),NOSPLIT,$0-16
	MOVD	R2, x+0(FP)
	MOVD	R3, y+8(FP)
	JMP	runtime·goPanicSlice3AlenU(SB)
TEXT runtime·panicSlice3Acap(SB),NOSPLIT,$0-16
	MOVD	R2, x+0(FP)
	MOVD	R3, y+8(FP)
	JMP	runtime·goPanicSlice3Acap(SB)
TEXT runtime·panicSlice3AcapU(SB),NOSPLIT,$0-16
	MOVD	R2, x+0(FP)
	MOVD	R3, y+8(FP)
	JMP	runtime·goPanicSlice3AcapU(SB)
TEXT runtime·panicSlice3B(SB),NOSPLIT,$0-16
	MOVD	R1, x+0(FP)
	MOVD	R2, y+8(FP)
	JMP	runtime·goPanicSlice3B(SB)
TEXT runtime·panicSlice3BU(SB),NOSPLIT,$0-16
	MOVD	R1, x+0(FP)
	MOVD	R2, y+8(FP)
	JMP	runtime·goPanicSlice3BU(SB)
TEXT runtime·panicSlice3C(SB),NOSPLIT,$0-16
	MOVD	R0, x+0(FP)
	MOVD	R1, y+8(FP)
	JMP	runtime·goPanicSlice3C(SB)
TEXT runtime·panicSlice3CU(SB),NOSPLIT,$0-16
	MOVD	R0, x+0(FP)
	MOVD	R1, y+8(FP)
	JMP	runtime·goPanicSlice3CU(SB)
TEXT runtime·panicSliceConvert(SB),NOSPLIT,$0-16
	MOVD	R2, x+0(FP)
	MOVD	R3, y+8(FP)
	JMP	runtime·goPanicSliceConvert(SB)