src/pkg/runtime/panic.c - go - Git at Google

 // Copyright 2012 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 "runtime.h"
 #include "arch_GOARCH.h"
 #include "stack.h"
 #include "malloc.h"
 #include "../../cmd/ld/textflag.h"

 // Code related to defer, panic and recover.

 uint32 runtime·panicking;
 static Lock paniclk;

 // Each P holds pool for defers with arg sizes 8, 24, 40, 56 and 72 bytes.
 // Memory block is 40 (24 for 32 bits) bytes larger due to Defer header.
 // This maps exactly to malloc size classes.

 // defer size class for arg size sz
 #define DEFERCLASS(sz) (((sz)+7)>>4)
 // total size of memory block for defer with arg size sz
 #define TOTALSIZE(sz) (sizeof(Defer) - sizeof(((Defer*)nil)->args) + ROUND(sz, sizeof(uintptr)))

 // Allocate a Defer, usually using per-P pool.
 // Each defer must be released with freedefer.
 static Defer*
 newdefer(int32 siz)
 {
 	int32 total, sc;
 	Defer *d;
 	P *p;

 	d = nil;
 	sc = DEFERCLASS(siz);
 	if(sc < nelem(p->deferpool)) {
 		p = m->p;
 		d = p->deferpool[sc];
 		if(d)
 			p->deferpool[sc] = d->link;
 	}
 	if(d == nil) {
 		// deferpool is empty or just a big defer
 		total = TOTALSIZE(siz);
 		d = runtime·malloc(total);
 	}
 	d->siz = siz;
 	d->special = 0;
 	d->link = g->defer;
 	g->defer = d;
 	return d;
 }

 // Free the given defer.
 // The defer cannot be used after this call.
 static void
 freedefer(Defer *d)
 {
 	int32 sc;
 	P *p;

 	if(d->special)
 		return;
 	sc = DEFERCLASS(d->siz);
 	if(sc < nelem(p->deferpool)) {
 		p = m->p;
 		d->link = p->deferpool[sc];
 		p->deferpool[sc] = d;
 		// No need to wipe out pointers in argp/pc/fn/args,
 		// because we empty the pool before GC.
 	} else
 		runtime·free(d);
 }

 // Create a new deferred function fn with siz bytes of arguments.
 // The compiler turns a defer statement into a call to this.
 // Cannot split the stack because it assumes that the arguments
 // are available sequentially after &fn; they would not be
 // copied if a stack split occurred.  It's OK for this to call
 // functions that split the stack.
 #pragma textflag NOSPLIT
 uintptr
 runtime·deferproc(int32 siz, FuncVal *fn, ...)
 {
 	Defer *d;

 	d = newdefer(siz);
 	d->fn = fn;
 	d->pc = runtime·getcallerpc(&siz);
 	if(thechar == '5')
 		d->argp = (byte*)(&fn+2);  // skip caller's saved link register
 	else
 		d->argp = (byte*)(&fn+1);
 	runtime·memmove(d->args, d->argp, d->siz);

 	// deferproc returns 0 normally.
 	// a deferred func that stops a panic
 	// makes the deferproc return 1.
 	// the code the compiler generates always
 	// checks the return value and jumps to the
 	// end of the function if deferproc returns != 0.
 	return 0;
 }

 // Run a deferred function if there is one.
 // The compiler inserts a call to this at the end of any
 // function which calls defer.
 // If there is a deferred function, this will call runtime·jmpdefer,
 // which will jump to the deferred function such that it appears
 // to have been called by the caller of deferreturn at the point
 // just before deferreturn was called.  The effect is that deferreturn
 // is called again and again until there are no more deferred functions.
 // Cannot split the stack because we reuse the caller's frame to
 // call the deferred function.

 // The single argument isn't actually used - it just has its address
 // taken so it can be matched against pending defers.
 #pragma textflag NOSPLIT
 void
 runtime·deferreturn(uintptr arg0)
 {
 	Defer *d;
 	byte *argp;
 	FuncVal *fn;

 	d = g->defer;
 	if(d == nil)
 		return;
 	argp = (byte*)&arg0;
 	if(d->argp != argp)
 		return;

 	// Moving arguments around.
 	// Do not allow preemption here, because the garbage collector
 	// won't know the form of the arguments until the jmpdefer can
 	// flip the PC over to fn.
 	m->locks++;
 	runtime·memmove(argp, d->args, d->siz);
 	fn = d->fn;
 	g->defer = d->link;
 	freedefer(d);
 	m->locks--;
 	if(m->locks == 0 && g->preempt)
 		g->stackguard0 = StackPreempt;
 	runtime·jmpdefer(fn, argp);
 }

 // Ensure that defer arg sizes that map to the same defer size class
 // also map to the same malloc size class.
 void
 runtime·testdefersizes(void)
 {
 	P *p;
 	int32 i, siz, defersc, mallocsc;
 	int32 map[nelem(p->deferpool)];

 	for(i=0; i<nelem(p->deferpool); i++)
 		map[i] = -1;
 	for(i=0;; i++) {
 		defersc = DEFERCLASS(i);
 		if(defersc >= nelem(p->deferpool))
 			break;
 		siz = TOTALSIZE(i);
 		mallocsc = runtime·SizeToClass(siz);
 		siz = runtime·class_to_size[mallocsc];
 		// runtime·printf("defer class %d: arg size %d, block size %d(%d)\n", defersc, i, siz, mallocsc);
 		if(map[defersc] < 0) {
 			map[defersc] = mallocsc;
 			continue;
 		}
 		if(map[defersc] != mallocsc) {
 			runtime·printf("bad defer size class: i=%d siz=%d mallocsc=%d/%d\n",
 				i, siz, map[defersc], mallocsc);
 			runtime·throw("bad defer size class");
 		}
 	}
 }

 // Run all deferred functions for the current goroutine.
 static void
 rundefer(void)
 {
 	Defer *d;

 	while((d = g->defer) != nil) {
 		g->defer = d->link;
 		reflect·call(d->fn, (byte*)d->args, d->siz, d->siz);
 		freedefer(d);
 	}
 }

 // Print all currently active panics.  Used when crashing.
 static void
 printpanics(Panic *p)
 {
 	if(p->link) {
 		printpanics(p->link);
 		runtime·printf("\t");
 	}
 	runtime·printf("panic: ");
 	runtime·printany(p->arg);
 	if(p->recovered)
 		runtime·printf(" [recovered]");
 	runtime·printf("\n");
 }

 static void recovery(G*);
 static void abortpanic(Panic*);
 static FuncVal abortpanicV = { (void(*)(void))abortpanic };

 // The implementation of the predeclared function panic.
 void
 runtime·panic(Eface e)
 {
 	Defer *d, dabort;
 	Panic p;
 	void *pc, *argp;

 	runtime·memclr((byte*)&p, sizeof p);
 	p.arg = e;
 	p.link = g->panic;
 	p.stackbase = g->stackbase;
 	g->panic = &p;

 	dabort.fn = &abortpanicV;
 	dabort.siz = sizeof(&p);
 	dabort.args[0] = &p;
 	dabort.argp = (void*)-1;  // unused because abortpanic never recovers
 	dabort.special = true;

 	for(;;) {
 		d = g->defer;
 		if(d == nil)
 			break;
 		// take defer off list in case of recursive panic
 		g->defer = d->link;
 		g->ispanic = true;	// rock for runtime·newstack, where runtime·newstackcall ends up
 		argp = d->argp;
 		pc = d->pc;

 		// The deferred function may cause another panic,
 		// so newstackcall may not return. Set up a defer
 		// to mark this panic aborted if that happens.
 		dabort.link = g->defer;
 		g->defer = &dabort;
 		p.defer = d;

 		runtime·newstackcall(d->fn, (byte*)d->args, d->siz);

 		// Newstackcall did not panic. Remove dabort.
 		if(g->defer != &dabort)
 			runtime·throw("bad defer entry in panic");
 		g->defer = dabort.link;

 		freedefer(d);
 		if(p.recovered) {
 			g->panic = p.link;
 			// Aborted panics are marked but remain on the g->panic list.
 			// Recovery will unwind the stack frames containing their Panic structs.
 			// Remove them from the list and free the associated defers.
 			while(g->panic && g->panic->aborted) {
 				freedefer(g->panic->defer);
 				g->panic = g->panic->link;
 			}
 			if(g->panic == nil)	// must be done with signal
 				g->sig = 0;
 			// Pass information about recovering frame to recovery.
 			g->sigcode0 = (uintptr)argp;
 			g->sigcode1 = (uintptr)pc;
 			runtime·mcall(recovery);
 			runtime·throw("recovery failed"); // mcall should not return
 		}
 	}

 	// ran out of deferred calls - old-school panic now
 	runtime·startpanic();
 	printpanics(g->panic);
 	runtime·dopanic(0);	// should not return
 	runtime·exit(1);	// not reached
 }

 static void
 abortpanic(Panic *p)
 {
 	p->aborted = true;
 }

 // Unwind the stack after a deferred function calls recover
 // after a panic.  Then arrange to continue running as though
 // the caller of the deferred function returned normally.
 static void
 recovery(G *gp)
 {
 	void *argp;
 	uintptr pc;

 	// Info about defer passed in G struct.
 	argp = (void*)gp->sigcode0;
 	pc = (uintptr)gp->sigcode1;

 	// Unwind to the stack frame with d's arguments in it.
 	runtime·unwindstack(gp, argp);

 	// Make the deferproc for this d return again,
 	// this time returning 1.  The calling function will
 	// jump to the standard return epilogue.
 	// The -2*sizeof(uintptr) makes up for the
 	// two extra words that are on the stack at
 	// each call to deferproc.
 	// (The pc we're returning to does pop pop
 	// before it tests the return value.)
 	// On the arm there are 2 saved LRs mixed in too.
 	if(thechar == '5')
 		gp->sched.sp = (uintptr)argp - 4*sizeof(uintptr);
 	else
 		gp->sched.sp = (uintptr)argp - 2*sizeof(uintptr);
 	gp->sched.pc = pc;
 	gp->sched.lr = 0;
 	gp->sched.ret = 1;
 	runtime·gogo(&gp->sched);
 }

 // Free stack frames until we hit the last one
 // or until we find the one that contains the sp.
 void
 runtime·unwindstack(G *gp, byte *sp)
 {
 	Stktop *top;
 	byte *stk;

 	// Must be called from a different goroutine, usually m->g0.
 	if(g == gp)
 		runtime·throw("unwindstack on self");

 	while((top = (Stktop*)gp->stackbase) != 0 && top->stackbase != 0) {
 		stk = (byte*)gp->stackguard - StackGuard;
 		if(stk <= sp && sp < (byte*)gp->stackbase)
 			break;
 		gp->stackbase = top->stackbase;
 		gp->stackguard = top->stackguard;
 		gp->stackguard0 = gp->stackguard;
 		runtime·stackfree(gp, stk, top);
 	}

 	if(sp != nil && (sp < (byte*)gp->stackguard - StackGuard || (byte*)gp->stackbase < sp)) {
 		runtime·printf("recover: %p not in [%p, %p]\n", sp, gp->stackguard - StackGuard, gp->stackbase);
 		runtime·throw("bad unwindstack");
 	}
 }

 // The implementation of the predeclared function recover.
 // Cannot split the stack because it needs to reliably
 // find the stack segment of its caller.
 #pragma textflag NOSPLIT
 void
 runtime·recover(byte *argp, GoOutput retbase, ...)
 {
 	Panic *p;
 	Stktop *top;
 	Eface *ret;

 	// Must be an unrecovered panic in progress.
 	// Must be on a stack segment created for a deferred call during a panic.
 	// Must be at the top of that segment, meaning the deferred call itself
 	// and not something it called. The top frame in the segment will have
 	// argument pointer argp == top - top->argsize.
 	// The subtraction of g->panicwrap allows wrapper functions that
 	// do not count as official calls to adjust what we consider the top frame
 	// while they are active on the stack. The linker emits adjustments of
 	// g->panicwrap in the prologue and epilogue of functions marked as wrappers.
 	ret = (Eface*)&retbase;
 	top = (Stktop*)g->stackbase;
 	p = g->panic;
 	if(p != nil && !p->recovered && top->panic && argp == (byte*)top - top->argsize - g->panicwrap) {
 		p->recovered = 1;
 		*ret = p->arg;
 	} else {
 		ret->type = nil;
 		ret->data = nil;
 	}
 }

 void
 runtime·startpanic(void)
 {
 	if(runtime·mheap.cachealloc.size == 0) { // very early
 		runtime·printf("runtime: panic before malloc heap initialized\n");
 		m->mallocing = 1; // tell rest of panic not to try to malloc
 	} else if(m->mcache == nil) // can happen if called from signal handler or throw
 		m->mcache = runtime·allocmcache();
 	switch(m->dying) {
 	case 0:
 		m->dying = 1;
 		if(g != nil)
 			g->writebuf = nil;
 		runtime·xadd(&runtime·panicking, 1);
 		runtime·lock(&paniclk);
 		if(runtime·debug.schedtrace > 0 || runtime·debug.scheddetail > 0)
 			runtime·schedtrace(true);
 		runtime·freezetheworld();
 		return;
 	case 1:
 		// Something failed while panicing, probably the print of the
 		// argument to panic().  Just print a stack trace and exit.
 		m->dying = 2;
 		runtime·printf("panic during panic\n");
 		runtime·dopanic(0);
 		runtime·exit(3);
 	case 2:
 		// This is a genuine bug in the runtime, we couldn't even
 		// print the stack trace successfully.
 		m->dying = 3;
 		runtime·printf("stack trace unavailable\n");
 		runtime·exit(4);
 	default:
 		// Can't even print!  Just exit.
 		runtime·exit(5);
 	}
 }

 void
 runtime·dopanic(int32 unused)
 {
 	static bool didothers;
 	bool crash;
 	int32 t;

 	if(g->sig != 0)
 		runtime·printf("[signal %x code=%p addr=%p pc=%p]\n",
 			g->sig, g->sigcode0, g->sigcode1, g->sigpc);

 	if((t = runtime·gotraceback(&crash)) > 0){
 		if(g != m->g0) {
 			runtime·printf("\n");
 			runtime·goroutineheader(g);
 			runtime·traceback((uintptr)runtime·getcallerpc(&unused), (uintptr)runtime·getcallersp(&unused), 0, g);
 		} else if(t >= 2 || m->throwing > 0) {
 			runtime·printf("\nruntime stack:\n");
 			runtime·traceback((uintptr)runtime·getcallerpc(&unused), (uintptr)runtime·getcallersp(&unused), 0, g);
 		}
 		if(!didothers) {
 			didothers = true;
 			runtime·tracebackothers(g);
 		}
 	}
 	runtime·unlock(&paniclk);
 	if(runtime·xadd(&runtime·panicking, -1) != 0) {
 		// Some other m is panicking too.
 		// Let it print what it needs to print.
 		// Wait forever without chewing up cpu.
 		// It will exit when it's done.
 		static Lock deadlock;
 		runtime·lock(&deadlock);
 		runtime·lock(&deadlock);
 	}

 	if(crash)
 		runtime·crash();

 	runtime·exit(2);
 }

 void
 runtime·panicindex(void)
 {
 	runtime·panicstring("index out of range");
 }

 void
 runtime·panicslice(void)
 {
 	runtime·panicstring("slice bounds out of range");
 }

 void
 runtime·throwreturn(void)
 {
 	// can only happen if compiler is broken
 	runtime·throw("no return at end of a typed function - compiler is broken");
 }

 void
 runtime·throwinit(void)
 {
 	// can only happen with linker skew
 	runtime·throw("recursive call during initialization - linker skew");
 }

 bool
 runtime·canpanic(G *gp)
 {
 	byte g;

 	USED(&g);  // don't use global g, it points to gsignal

 	// Is it okay for gp to panic instead of crashing the program?
 	// Yes, as long as it is running Go code, not runtime code,
 	// and not stuck in a system call.
 	if(gp == nil || gp != m->curg)
 		return false;
 	if(m->locks-m->softfloat != 0 || m->mallocing != 0 || m->throwing != 0 || m->gcing != 0 || m->dying != 0)
 		return false;
 	if(gp->status != Grunning || gp->syscallsp != 0)
 		return false;
 #ifdef GOOS_windows
 	if(m->libcallsp != 0)
 		return false;
 #endif
 	return true;
 }

 void
 runtime·throw(int8 *s)
 {
 	if(m->throwing == 0)
 		m->throwing = 1;
 	runtime·startpanic();
 	runtime·printf("fatal error: %s\n", s);
 	runtime·dopanic(0);
 	*(int32*)0 = 0;	// not reached
 	runtime·exit(1);	// even more not reached
 }

 void
 runtime·panicstring(int8 *s)
 {
 	Eface err;

 	// m->softfloat is set during software floating point,
 	// which might cause a fault during a memory load.
 	// It increments m->locks to avoid preemption.
 	// If we're panicking, the software floating point frames
 	// will be unwound, so decrement m->locks as they would.
 	if(m->softfloat) {
 		m->locks--;
 		m->softfloat = 0;
 	}

 	if(m->mallocing) {
 		runtime·printf("panic: %s\n", s);
 		runtime·throw("panic during malloc");
 	}
 	if(m->gcing) {
 		runtime·printf("panic: %s\n", s);
 		runtime·throw("panic during gc");
 	}
 	if(m->locks) {
 		runtime·printf("panic: %s\n", s);
 		runtime·throw("panic holding locks");
 	}
 	runtime·newErrorCString(s, &err);
 	runtime·panic(err);
 }

 void
 runtime·Goexit(void)
 {
 	rundefer();
 	runtime·goexit();
 }

 void
 runtime·panicdivide(void)
 {
 	runtime·panicstring("integer divide by zero");
 }
	// Copyright 2012 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 "runtime.h"
	#include "arch_GOARCH.h"
	#include "stack.h"
	#include "malloc.h"
	#include "../../cmd/ld/textflag.h"

	// Code related to defer, panic and recover.

	uint32 runtime·panicking;
	static Lock paniclk;

	// Each P holds pool for defers with arg sizes 8, 24, 40, 56 and 72 bytes.
	// Memory block is 40 (24 for 32 bits) bytes larger due to Defer header.
	// This maps exactly to malloc size classes.

	// defer size class for arg size sz
	#define DEFERCLASS(sz) (((sz)+7)>>4)
	// total size of memory block for defer with arg size sz
	#define TOTALSIZE(sz) (sizeof(Defer) - sizeof(((Defer*)nil)->args) + ROUND(sz, sizeof(uintptr)))

	// Allocate a Defer, usually using per-P pool.
	// Each defer must be released with freedefer.
	static Defer*
	newdefer(int32 siz)
	{
	int32 total, sc;
	Defer *d;
	P *p;

	d = nil;
	sc = DEFERCLASS(siz);
	if(sc < nelem(p->deferpool)) {
	p = m->p;
	d = p->deferpool[sc];
	if(d)
	p->deferpool[sc] = d->link;
	}
	if(d == nil) {
	// deferpool is empty or just a big defer
	total = TOTALSIZE(siz);
	d = runtime·malloc(total);
	}
	d->siz = siz;
	d->special = 0;
	d->link = g->defer;
	g->defer = d;
	return d;
	}

	// Free the given defer.
	// The defer cannot be used after this call.
	static void
	freedefer(Defer *d)
	{
	int32 sc;
	P *p;

	if(d->special)
	return;
	sc = DEFERCLASS(d->siz);
	if(sc < nelem(p->deferpool)) {
	p = m->p;
	d->link = p->deferpool[sc];
	p->deferpool[sc] = d;
	// No need to wipe out pointers in argp/pc/fn/args,
	// because we empty the pool before GC.
	} else
	runtime·free(d);
	}

	// Create a new deferred function fn with siz bytes of arguments.
	// The compiler turns a defer statement into a call to this.
	// Cannot split the stack because it assumes that the arguments
	// are available sequentially after &fn; they would not be
	// copied if a stack split occurred. It's OK for this to call
	// functions that split the stack.
	#pragma textflag NOSPLIT
	uintptr
	runtime·deferproc(int32 siz, FuncVal *fn, ...)
	{
	Defer *d;

	d = newdefer(siz);
	d->fn = fn;
	d->pc = runtime·getcallerpc(&siz);
	if(thechar == '5')
	d->argp = (byte*)(&fn+2); // skip caller's saved link register
	else
	d->argp = (byte*)(&fn+1);
	runtime·memmove(d->args, d->argp, d->siz);

	// deferproc returns 0 normally.
	// a deferred func that stops a panic
	// makes the deferproc return 1.
	// the code the compiler generates always
	// checks the return value and jumps to the
	// end of the function if deferproc returns != 0.
	return 0;
	}

	// Run a deferred function if there is one.
	// The compiler inserts a call to this at the end of any
	// function which calls defer.
	// If there is a deferred function, this will call runtime·jmpdefer,
	// which will jump to the deferred function such that it appears
	// to have been called by the caller of deferreturn at the point
	// just before deferreturn was called. The effect is that deferreturn
	// is called again and again until there are no more deferred functions.
	// Cannot split the stack because we reuse the caller's frame to
	// call the deferred function.

	// The single argument isn't actually used - it just has its address
	// taken so it can be matched against pending defers.
	#pragma textflag NOSPLIT
	void
	runtime·deferreturn(uintptr arg0)
	{
	Defer *d;
	byte *argp;
	FuncVal *fn;

	d = g->defer;
	if(d == nil)
	return;
	argp = (byte*)&arg0;
	if(d->argp != argp)
	return;

	// Moving arguments around.
	// Do not allow preemption here, because the garbage collector
	// won't know the form of the arguments until the jmpdefer can
	// flip the PC over to fn.
	m->locks++;
	runtime·memmove(argp, d->args, d->siz);
	fn = d->fn;
	g->defer = d->link;
	freedefer(d);
	m->locks--;
	if(m->locks == 0 && g->preempt)
	g->stackguard0 = StackPreempt;
	runtime·jmpdefer(fn, argp);
	}

	// Ensure that defer arg sizes that map to the same defer size class
	// also map to the same malloc size class.
	void
	runtime·testdefersizes(void)
	{
	P *p;
	int32 i, siz, defersc, mallocsc;
	int32 map[nelem(p->deferpool)];

	for(i=0; i<nelem(p->deferpool); i++)
	map[i] = -1;
	for(i=0;; i++) {
	defersc = DEFERCLASS(i);
	if(defersc >= nelem(p->deferpool))
	break;
	siz = TOTALSIZE(i);
	mallocsc = runtime·SizeToClass(siz);
	siz = runtime·class_to_size[mallocsc];
	// runtime·printf("defer class %d: arg size %d, block size %d(%d)\n", defersc, i, siz, mallocsc);
	if(map[defersc] < 0) {
	map[defersc] = mallocsc;
	continue;
	}
	if(map[defersc] != mallocsc) {
	runtime·printf("bad defer size class: i=%d siz=%d mallocsc=%d/%d\n",
	i, siz, map[defersc], mallocsc);
	runtime·throw("bad defer size class");
	}
	}
	}

	// Run all deferred functions for the current goroutine.
	static void
	rundefer(void)
	{
	Defer *d;

	while((d = g->defer) != nil) {
	g->defer = d->link;
	reflect·call(d->fn, (byte*)d->args, d->siz, d->siz);
	freedefer(d);
	}
	}

	// Print all currently active panics. Used when crashing.
	static void
	printpanics(Panic *p)
	{
	if(p->link) {
	printpanics(p->link);
	runtime·printf("\t");
	}
	runtime·printf("panic: ");
	runtime·printany(p->arg);
	if(p->recovered)
	runtime·printf(" [recovered]");
	runtime·printf("\n");
	}

	static void recovery(G*);
	static void abortpanic(Panic*);
	static FuncVal abortpanicV = { (void(*)(void))abortpanic };

	// The implementation of the predeclared function panic.
	void
	runtime·panic(Eface e)
	{
	Defer *d, dabort;
	Panic p;
	void pc, argp;

	runtime·memclr((byte*)&p, sizeof p);
	p.arg = e;
	p.link = g->panic;
	p.stackbase = g->stackbase;
	g->panic = &p;

	dabort.fn = &abortpanicV;
	dabort.siz = sizeof(&p);
	dabort.args[0] = &p;
	dabort.argp = (void*)-1; // unused because abortpanic never recovers
	dabort.special = true;

	for(;;) {
	d = g->defer;
	if(d == nil)
	break;
	// take defer off list in case of recursive panic
	g->defer = d->link;
	g->ispanic = true; // rock for runtime·newstack, where runtime·newstackcall ends up
	argp = d->argp;
	pc = d->pc;

	// The deferred function may cause another panic,
	// so newstackcall may not return. Set up a defer
	// to mark this panic aborted if that happens.
	dabort.link = g->defer;
	g->defer = &dabort;
	p.defer = d;

	runtime·newstackcall(d->fn, (byte*)d->args, d->siz);

	// Newstackcall did not panic. Remove dabort.
	if(g->defer != &dabort)
	runtime·throw("bad defer entry in panic");
	g->defer = dabort.link;

	freedefer(d);
	if(p.recovered) {
	g->panic = p.link;
	// Aborted panics are marked but remain on the g->panic list.
	// Recovery will unwind the stack frames containing their Panic structs.
	// Remove them from the list and free the associated defers.
	while(g->panic && g->panic->aborted) {
	freedefer(g->panic->defer);
	g->panic = g->panic->link;
	}
	if(g->panic == nil) // must be done with signal
	g->sig = 0;
	// Pass information about recovering frame to recovery.
	g->sigcode0 = (uintptr)argp;
	g->sigcode1 = (uintptr)pc;
	runtime·mcall(recovery);
	runtime·throw("recovery failed"); // mcall should not return
	}
	}

	// ran out of deferred calls - old-school panic now
	runtime·startpanic();
	printpanics(g->panic);
	runtime·dopanic(0); // should not return
	runtime·exit(1); // not reached
	}

	static void
	abortpanic(Panic *p)
	{
	p->aborted = true;
	}

	// Unwind the stack after a deferred function calls recover
	// after a panic. Then arrange to continue running as though
	// the caller of the deferred function returned normally.
	static void
	recovery(G *gp)
	{
	void *argp;
	uintptr pc;

	// Info about defer passed in G struct.
	argp = (void*)gp->sigcode0;
	pc = (uintptr)gp->sigcode1;

	// Unwind to the stack frame with d's arguments in it.
	runtime·unwindstack(gp, argp);

	// Make the deferproc for this d return again,
	// this time returning 1. The calling function will
	// jump to the standard return epilogue.
	// The -2*sizeof(uintptr) makes up for the
	// two extra words that are on the stack at
	// each call to deferproc.
	// (The pc we're returning to does pop pop
	// before it tests the return value.)
	// On the arm there are 2 saved LRs mixed in too.
	if(thechar == '5')
	gp->sched.sp = (uintptr)argp - 4*sizeof(uintptr);
	else
	gp->sched.sp = (uintptr)argp - 2*sizeof(uintptr);
	gp->sched.pc = pc;
	gp->sched.lr = 0;
	gp->sched.ret = 1;
	runtime·gogo(&gp->sched);
	}

	// Free stack frames until we hit the last one
	// or until we find the one that contains the sp.
	void
	runtime·unwindstack(G gp, byte sp)
	{
	Stktop *top;
	byte *stk;

	// Must be called from a different goroutine, usually m->g0.
	if(g == gp)
	runtime·throw("unwindstack on self");

	while((top = (Stktop*)gp->stackbase) != 0 && top->stackbase != 0) {
	stk = (byte*)gp->stackguard - StackGuard;
	if(stk <= sp && sp < (byte*)gp->stackbase)
	break;
	gp->stackbase = top->stackbase;
	gp->stackguard = top->stackguard;
	gp->stackguard0 = gp->stackguard;
	runtime·stackfree(gp, stk, top);
	}

	if(sp != nil && (sp < (byte)gp->stackguard - StackGuard \|\| (byte)gp->stackbase < sp)) {
	runtime·printf("recover: %p not in [%p, %p]\n", sp, gp->stackguard - StackGuard, gp->stackbase);
	runtime·throw("bad unwindstack");
	}
	}

	// The implementation of the predeclared function recover.
	// Cannot split the stack because it needs to reliably
	// find the stack segment of its caller.
	#pragma textflag NOSPLIT
	void
	runtime·recover(byte *argp, GoOutput retbase, ...)
	{
	Panic *p;
	Stktop *top;
	Eface *ret;

	// Must be an unrecovered panic in progress.
	// Must be on a stack segment created for a deferred call during a panic.
	// Must be at the top of that segment, meaning the deferred call itself
	// and not something it called. The top frame in the segment will have
	// argument pointer argp == top - top->argsize.
	// The subtraction of g->panicwrap allows wrapper functions that
	// do not count as official calls to adjust what we consider the top frame
	// while they are active on the stack. The linker emits adjustments of
	// g->panicwrap in the prologue and epilogue of functions marked as wrappers.
	ret = (Eface*)&retbase;
	top = (Stktop*)g->stackbase;
	p = g->panic;
	if(p != nil && !p->recovered && top->panic && argp == (byte*)top - top->argsize - g->panicwrap) {
	p->recovered = 1;
	*ret = p->arg;
	} else {
	ret->type = nil;
	ret->data = nil;
	}
	}

	void
	runtime·startpanic(void)
	{
	if(runtime·mheap.cachealloc.size == 0) { // very early
	runtime·printf("runtime: panic before malloc heap initialized\n");
	m->mallocing = 1; // tell rest of panic not to try to malloc
	} else if(m->mcache == nil) // can happen if called from signal handler or throw
	m->mcache = runtime·allocmcache();
	switch(m->dying) {
	case 0:
	m->dying = 1;
	if(g != nil)
	g->writebuf = nil;
	runtime·xadd(&runtime·panicking, 1);
	runtime·lock(&paniclk);
	if(runtime·debug.schedtrace > 0 \|\| runtime·debug.scheddetail > 0)
	runtime·schedtrace(true);
	runtime·freezetheworld();
	return;
	case 1:
	// Something failed while panicing, probably the print of the
	// argument to panic(). Just print a stack trace and exit.
	m->dying = 2;
	runtime·printf("panic during panic\n");
	runtime·dopanic(0);
	runtime·exit(3);
	case 2:
	// This is a genuine bug in the runtime, we couldn't even
	// print the stack trace successfully.
	m->dying = 3;
	runtime·printf("stack trace unavailable\n");
	runtime·exit(4);
	default:
	// Can't even print! Just exit.
	runtime·exit(5);
	}
	}

	void
	runtime·dopanic(int32 unused)
	{
	static bool didothers;
	bool crash;
	int32 t;

	if(g->sig != 0)
	runtime·printf("[signal %x code=%p addr=%p pc=%p]\n",
	g->sig, g->sigcode0, g->sigcode1, g->sigpc);

	if((t = runtime·gotraceback(&crash)) > 0){
	if(g != m->g0) {
	runtime·printf("\n");
	runtime·goroutineheader(g);
	runtime·traceback((uintptr)runtime·getcallerpc(&unused), (uintptr)runtime·getcallersp(&unused), 0, g);
	} else if(t >= 2 \|\| m->throwing > 0) {
	runtime·printf("\nruntime stack:\n");
	runtime·traceback((uintptr)runtime·getcallerpc(&unused), (uintptr)runtime·getcallersp(&unused), 0, g);
	}
	if(!didothers) {
	didothers = true;
	runtime·tracebackothers(g);
	}
	}
	runtime·unlock(&paniclk);
	if(runtime·xadd(&runtime·panicking, -1) != 0) {
	// Some other m is panicking too.
	// Let it print what it needs to print.
	// Wait forever without chewing up cpu.
	// It will exit when it's done.
	static Lock deadlock;
	runtime·lock(&deadlock);
	runtime·lock(&deadlock);
	}

	if(crash)
	runtime·crash();

	runtime·exit(2);
	}

	void
	runtime·panicindex(void)
	{
	runtime·panicstring("index out of range");
	}

	void
	runtime·panicslice(void)
	{
	runtime·panicstring("slice bounds out of range");
	}

	void
	runtime·throwreturn(void)
	{
	// can only happen if compiler is broken
	runtime·throw("no return at end of a typed function - compiler is broken");
	}

	void
	runtime·throwinit(void)
	{
	// can only happen with linker skew
	runtime·throw("recursive call during initialization - linker skew");
	}

	bool
	runtime·canpanic(G *gp)
	{
	byte g;

	USED(&g); // don't use global g, it points to gsignal

	// Is it okay for gp to panic instead of crashing the program?
	// Yes, as long as it is running Go code, not runtime code,
	// and not stuck in a system call.
	if(gp == nil \|\| gp != m->curg)
	return false;
	if(m->locks-m->softfloat != 0 \|\| m->mallocing != 0 \|\| m->throwing != 0 \|\| m->gcing != 0 \|\| m->dying != 0)
	return false;
	if(gp->status != Grunning \|\| gp->syscallsp != 0)
	return false;
	#ifdef GOOS_windows
	if(m->libcallsp != 0)
	return false;
	#endif
	return true;
	}

	void
	runtime·throw(int8 *s)
	{
	if(m->throwing == 0)
	m->throwing = 1;
	runtime·startpanic();
	runtime·printf("fatal error: %s\n", s);
	runtime·dopanic(0);
	(int32)0 = 0; // not reached
	runtime·exit(1); // even more not reached
	}

	void
	runtime·panicstring(int8 *s)
	{
	Eface err;

	// m->softfloat is set during software floating point,
	// which might cause a fault during a memory load.
	// It increments m->locks to avoid preemption.
	// If we're panicking, the software floating point frames
	// will be unwound, so decrement m->locks as they would.
	if(m->softfloat) {
	m->locks--;
	m->softfloat = 0;
	}

	if(m->mallocing) {
	runtime·printf("panic: %s\n", s);
	runtime·throw("panic during malloc");
	}
	if(m->gcing) {
	runtime·printf("panic: %s\n", s);
	runtime·throw("panic during gc");
	}
	if(m->locks) {
	runtime·printf("panic: %s\n", s);
	runtime·throw("panic holding locks");
	}
	runtime·newErrorCString(s, &err);
	runtime·panic(err);
	}

	void
	runtime·Goexit(void)
	{
	rundefer();
	runtime·goexit();
	}

	void
	runtime·panicdivide(void)
	{
	runtime·panicstring("integer divide by zero");
	}