src/pkg/runtime/malloc.goc - go - Git at Google

 // Copyright 2009 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.

 // See malloc.h for overview.
 //
 // TODO(rsc): double-check stats.

 package runtime
 #include "runtime.h"
 #include "stack.h"
 #include "malloc.h"
 #include "defs.h"
 #include "type.h"

 MHeap runtime·mheap;
 extern MStats mstats;	// defined in extern.go

 extern volatile int32 runtime·MemProfileRate;

 // Same algorithm from chan.c, but a different
 // instance of the static uint32 x.
 // Not protected by a lock - let the threads use
 // the same random number if they like.
 static uint32
 fastrand1(void)
 {
 	static uint32 x = 0x49f6428aUL;

 	x += x;
 	if(x & 0x80000000L)
 		x ^= 0x88888eefUL;
 	return x;
 }

 // Allocate an object of at least size bytes.
 // Small objects are allocated from the per-thread cache's free lists.
 // Large objects (> 32 kB) are allocated straight from the heap.
 void*
 runtime·mallocgc(uintptr size, uint32 flag, int32 dogc, int32 zeroed)
 {
 	int32 sizeclass, rate;
 	MCache *c;
 	uintptr npages;
 	MSpan *s;
 	void *v;

 	if(runtime·gcwaiting && g != m->g0 && m->locks == 0)
 		runtime·gosched();
 	if(m->mallocing)
 		runtime·throw("malloc/free - deadlock");
 	m->mallocing = 1;
 	if(size == 0)
 		size = 1;

 	mstats.nmalloc++;
 	if(size <= MaxSmallSize) {
 		// Allocate from mcache free lists.
 		sizeclass = runtime·SizeToClass(size);
 		size = runtime·class_to_size[sizeclass];
 		c = m->mcache;
 		v = runtime·MCache_Alloc(c, sizeclass, size, zeroed);
 		if(v == nil)
 			runtime·throw("out of memory");
 		mstats.alloc += size;
 		mstats.total_alloc += size;
 		mstats.by_size[sizeclass].nmalloc++;
 	} else {
 		// TODO(rsc): Report tracebacks for very large allocations.

 		// Allocate directly from heap.
 		npages = size >> PageShift;
 		if((size & PageMask) != 0)
 			npages++;
 		s = runtime·MHeap_Alloc(&runtime·mheap, npages, 0, 1);
 		if(s == nil)
 			runtime·throw("out of memory");
 		size = npages<<PageShift;
 		mstats.alloc += size;
 		mstats.total_alloc += size;
 		v = (void*)(s->start << PageShift);

 		// setup for mark sweep
 		runtime·markspan(v, 0, 0, true);
 	}
 	if(!(flag & FlagNoGC))
 		runtime·markallocated(v, size, (flag&FlagNoPointers) != 0);

 	m->mallocing = 0;

 	if(!(flag & FlagNoProfiling) && (rate = runtime·MemProfileRate) > 0) {
 		if(size >= rate)
 			goto profile;
 		if(m->mcache->next_sample > size)
 			m->mcache->next_sample -= size;
 		else {
 			// pick next profile time
 			if(rate > 0x3fffffff)	// make 2*rate not overflow
 				rate = 0x3fffffff;
 			m->mcache->next_sample = fastrand1() % (2*rate);
 		profile:
 			runtime·setblockspecial(v);
 			runtime·MProf_Malloc(v, size);
 		}
 	}

 	if(dogc && mstats.heap_alloc >= mstats.next_gc)
 		runtime·gc(0);
 	return v;
 }

 void*
 runtime·malloc(uintptr size)
 {
 	return runtime·mallocgc(size, 0, 0, 1);
 }

 // Free the object whose base pointer is v.
 void
 runtime·free(void *v)
 {
 	int32 sizeclass;
 	MSpan *s;
 	MCache *c;
 	uint32 prof;
 	uintptr size;

 	if(v == nil)
 		return;

 	// If you change this also change mgc0.c:/^sweepspan,
 	// which has a copy of the guts of free.

 	if(m->mallocing)
 		runtime·throw("malloc/free - deadlock");
 	m->mallocing = 1;

 	if(!runtime·mlookup(v, nil, nil, &s)) {
 		runtime·printf("free %p: not an allocated block\n", v);
 		runtime·throw("free runtime·mlookup");
 	}
 	prof = runtime·blockspecial(v);

 	// Find size class for v.
 	sizeclass = s->sizeclass;
 	if(sizeclass == 0) {
 		// Large object.
 		size = s->npages<<PageShift;
 		*(uintptr*)(s->start<<PageShift) = 1;	// mark as "needs to be zeroed"
 		// Must mark v freed before calling unmarkspan and MHeap_Free:
 		// they might coalesce v into other spans and change the bitmap further.
 		runtime·markfreed(v, size);
 		runtime·unmarkspan(v, 1<<PageShift);
 		runtime·MHeap_Free(&runtime·mheap, s, 1);
 	} else {
 		// Small object.
 		c = m->mcache;
 		size = runtime·class_to_size[sizeclass];
 		if(size > sizeof(uintptr))
 			((uintptr*)v)[1] = 1;	// mark as "needs to be zeroed"
 		// Must mark v freed before calling MCache_Free:
 		// it might coalesce v and other blocks into a bigger span
 		// and change the bitmap further.
 		runtime·markfreed(v, size);
 		mstats.by_size[sizeclass].nfree++;
 		runtime·MCache_Free(c, v, sizeclass, size);
 	}
 	mstats.alloc -= size;
 	if(prof)
 		runtime·MProf_Free(v, size);
 	m->mallocing = 0;
 }

 int32
 runtime·mlookup(void *v, byte **base, uintptr *size, MSpan **sp)
 {
 	uintptr n, i;
 	byte *p;
 	MSpan *s;

 	mstats.nlookup++;
 	s = runtime·MHeap_LookupMaybe(&runtime·mheap, v);
 	if(sp)
 		*sp = s;
 	if(s == nil) {
 		runtime·checkfreed(v, 1);
 		if(base)
 			*base = nil;
 		if(size)
 			*size = 0;
 		return 0;
 	}

 	p = (byte*)((uintptr)s->start<<PageShift);
 	if(s->sizeclass == 0) {
 		// Large object.
 		if(base)
 			*base = p;
 		if(size)
 			*size = s->npages<<PageShift;
 		return 1;
 	}

 	if((byte*)v >= (byte*)s->limit) {
 		// pointers past the last block do not count as pointers.
 		return 0;
 	}

 	n = runtime·class_to_size[s->sizeclass];
 	i = ((byte*)v - p)/n;
 	if(base)
 		*base = p + i*n;
 	if(size)
 		*size = n;

 	return 1;
 }

 MCache*
 runtime·allocmcache(void)
 {
 	MCache *c;

 	runtime·lock(&runtime·mheap);
 	c = runtime·FixAlloc_Alloc(&runtime·mheap.cachealloc);
 	mstats.mcache_inuse = runtime·mheap.cachealloc.inuse;
 	mstats.mcache_sys = runtime·mheap.cachealloc.sys;
 	runtime·unlock(&runtime·mheap);
 	return c;
 }

 uintptr runtime·sizeof_C_MStats = sizeof(MStats);

 #define MaxArena32 (2U<<30)

 void
 runtime·mallocinit(void)
 {
 	byte *p;
 	uintptr arena_size, bitmap_size;
 	extern byte end[];

 	runtime·InitSizes();

 	// Set up the allocation arena, a contiguous area of memory where
 	// allocated data will be found.  The arena begins with a bitmap large
 	// enough to hold 4 bits per allocated word.
 	if(sizeof(void*) == 8) {
 		// On a 64-bit machine, allocate from a single contiguous reservation.
 		// 16 GB should be big enough for now.
 		//
 		// The code will work with the reservation at any address, but ask
 		// SysReserve to use 0x000000f800000000 if possible.
 		// Allocating a 16 GB region takes away 36 bits, and the amd64
 		// doesn't let us choose the top 17 bits, so that leaves the 11 bits
 		// in the middle of 0x00f8 for us to choose.  Choosing 0x00f8 means
 		// that the valid memory addresses will begin 0x00f8, 0x00f9, 0x00fa, 0x00fb.
 		// None of the bytes f8 f9 fa fb can appear in valid UTF-8, and
 		// they are otherwise as far from ff (likely a common byte) as possible.
 		// Choosing 0x00 for the leading 6 bits was more arbitrary, but it
 		// is not a common ASCII code point either.  Using 0x11f8 instead
 		// caused out of memory errors on OS X during thread allocations.
 		// These choices are both for debuggability and to reduce the
 		// odds of the conservative garbage collector not collecting memory
 		// because some non-pointer block of memory had a bit pattern
 		// that matched a memory address.
 		//
 		// Actually we reserve 17 GB (because the bitmap ends up being 1 GB)
 		// but it hardly matters: fc is not valid UTF-8 either, and we have to
 		// allocate 15 GB before we get that far.
 		arena_size = 16LL<<30;
 		bitmap_size = arena_size / (sizeof(void*)*8/4);
 		p = runtime·SysReserve((void*)(0x00f8ULL<<32), bitmap_size + arena_size);
 		if(p == nil)
 			runtime·throw("runtime: cannot reserve arena virtual address space");
 	} else {
 		// On a 32-bit machine, we can't typically get away
 		// with a giant virtual address space reservation.
 		// Instead we map the memory information bitmap
 		// immediately after the data segment, large enough
 		// to handle another 2GB of mappings (256 MB),
 		// along with a reservation for another 512 MB of memory.
 		// When that gets used up, we'll start asking the kernel
 		// for any memory anywhere and hope it's in the 2GB
 		// following the bitmap (presumably the executable begins
 		// near the bottom of memory, so we'll have to use up
 		// most of memory before the kernel resorts to giving out
 		// memory before the beginning of the text segment).
 		//
 		// Alternatively we could reserve 512 MB bitmap, enough
 		// for 4GB of mappings, and then accept any memory the
 		// kernel threw at us, but normally that's a waste of 512 MB
 		// of address space, which is probably too much in a 32-bit world.
 		bitmap_size = MaxArena32 / (sizeof(void*)*8/4);
 		arena_size = 512<<20;

 		// SysReserve treats the address we ask for, end, as a hint,
 		// not as an absolute requirement.  If we ask for the end
 		// of the data segment but the operating system requires
 		// a little more space before we can start allocating, it will
 		// give out a slightly higher pointer.  That's fine.
 		// Run with what we get back.
 		p = runtime·SysReserve(end, bitmap_size + arena_size);
 		if(p == nil)
 			runtime·throw("runtime: cannot reserve arena virtual address space");
 	}
 	if((uintptr)p & (((uintptr)1<<PageShift)-1))
 		runtime·throw("runtime: SysReserve returned unaligned address");

 	runtime·mheap.bitmap = p;
 	runtime·mheap.arena_start = p + bitmap_size;
 	runtime·mheap.arena_used = runtime·mheap.arena_start;
 	runtime·mheap.arena_end = runtime·mheap.arena_start + arena_size;

 	// Initialize the rest of the allocator.
 	runtime·MHeap_Init(&runtime·mheap, runtime·SysAlloc);
 	m->mcache = runtime·allocmcache();

 	// See if it works.
 	runtime·free(runtime·malloc(1));
 }

 void*
 runtime·MHeap_SysAlloc(MHeap *h, uintptr n)
 {
 	byte *p;

 	if(n <= h->arena_end - h->arena_used) {
 		// Keep taking from our reservation.
 		p = h->arena_used;
 		runtime·SysMap(p, n);
 		h->arena_used += n;
 		runtime·MHeap_MapBits(h);
 		return p;
 	}

 	// On 64-bit, our reservation is all we have.
 	if(sizeof(void*) == 8)
 		return nil;

 	// On 32-bit, once the reservation is gone we can
 	// try to get memory at a location chosen by the OS
 	// and hope that it is in the range we allocated bitmap for.
 	p = runtime·SysAlloc(n);
 	if(p == nil)
 		return nil;

 	if(p < h->arena_start || p+n - h->arena_start >= MaxArena32) {
 		runtime·printf("runtime: memory allocated by OS not in usable range\n");
 		runtime·SysFree(p, n);
 		return nil;
 	}

 	if(p+n > h->arena_used) {
 		h->arena_used = p+n;
 		if(h->arena_used > h->arena_end)
 			h->arena_end = h->arena_used;
 		runtime·MHeap_MapBits(h);
 	}

 	return p;
 }

 // Runtime stubs.

 void*
 runtime·mal(uintptr n)
 {
 	return runtime·mallocgc(n, 0, 1, 1);
 }

 func new(n uint32) (ret *uint8) {
 	ret = runtime·mal(n);
 }

 // Stack allocator uses malloc/free most of the time,
 // but if we're in the middle of malloc and need stack,
 // we have to do something else to avoid deadlock.
 // In that case, we fall back on a fixed-size free-list
 // allocator, assuming that inside malloc all the stack
 // frames are small, so that all the stack allocations
 // will be a single size, the minimum (right now, 5k).
 static struct {
 	Lock;
 	FixAlloc;
 } stacks;

 enum {
 	FixedStack = StackMin,
 };

 void*
 runtime·stackalloc(uint32 n)
 {
 	void *v;

 	// Stackalloc must be called on scheduler stack, so that we
 	// never try to grow the stack during the code that stackalloc runs.
 	// Doing so would cause a deadlock (issue 1547).
 	if(g != m->g0)
 		runtime·throw("stackalloc not on scheduler stack");

 	if(m->mallocing || m->gcing || n == FixedStack) {
 		runtime·lock(&stacks);
 		if(stacks.size == 0)
 			runtime·FixAlloc_Init(&stacks, n, runtime·SysAlloc, nil, nil);
 		if(stacks.size != n) {
 			runtime·printf("stackalloc: in malloc, size=%D want %d", (uint64)stacks.size, n);
 			runtime·throw("stackalloc");
 		}
 		v = runtime·FixAlloc_Alloc(&stacks);
 		mstats.stacks_inuse = stacks.inuse;
 		mstats.stacks_sys = stacks.sys;
 		runtime·unlock(&stacks);
 		return v;
 	}
 	return runtime·mallocgc(n, FlagNoProfiling|FlagNoGC, 0, 0);
 }

 void
 runtime·stackfree(void *v, uintptr n)
 {
 	if(m->mallocing || m->gcing || n == FixedStack) {
 		runtime·lock(&stacks);
 		runtime·FixAlloc_Free(&stacks, v);
 		mstats.stacks_inuse = stacks.inuse;
 		mstats.stacks_sys = stacks.sys;
 		runtime·unlock(&stacks);
 		return;
 	}
 	runtime·free(v);
 }

 func Alloc(n uintptr) (p *byte) {
 	p = runtime·malloc(n);
 }

 func Free(p *byte) {
 	runtime·free(p);
 }

 func Lookup(p *byte) (base *byte, size uintptr) {
 	runtime·mlookup(p, &base, &size, nil);
 }

 func GC() {
 	runtime·gc(1);
 }

 func SetFinalizer(obj Eface, finalizer Eface) {
 	byte *base;
 	uintptr size;
 	FuncType *ft;
 	int32 i, nret;
 	Type *t;

 	if(obj.type == nil) {
 		runtime·printf("runtime.SetFinalizer: first argument is nil interface\n");
 	throw:
 		runtime·throw("runtime.SetFinalizer");
 	}
 	if(obj.type->kind != KindPtr) {
 		runtime·printf("runtime.SetFinalizer: first argument is %S, not pointer\n", *obj.type->string);
 		goto throw;
 	}
 	if(!runtime·mlookup(obj.data, &base, &size, nil) || obj.data != base) {
 		runtime·printf("runtime.SetFinalizer: pointer not at beginning of allocated block\n");
 		goto throw;
 	}
 	nret = 0;
 	if(finalizer.type != nil) {
 		if(finalizer.type->kind != KindFunc) {
 		badfunc:
 			runtime·printf("runtime.SetFinalizer: second argument is %S, not func(%S)\n", *finalizer.type->string, *obj.type->string);
 			goto throw;
 		}
 		ft = (FuncType*)finalizer.type;
 		if(ft->dotdotdot || ft->in.len != 1 || *(Type**)ft->in.array != obj.type)
 			goto badfunc;

 		// compute size needed for return parameters
 		for(i=0; i<ft->out.len; i++) {
 			t = ((Type**)ft->out.array)[i];
 			nret = (nret + t->align - 1) & ~(t->align - 1);
 			nret += t->size;
 		}
 		nret = (nret + sizeof(void*)-1) & ~(sizeof(void*)-1);

 		if(runtime·getfinalizer(obj.data, 0)) {
 			runtime·printf("runtime.SetFinalizer: finalizer already set\n");
 			goto throw;
 		}
 	}
 	runtime·addfinalizer(obj.data, finalizer.data, nret);
 }
	// Copyright 2009 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.

	// See malloc.h for overview.
	//
	// TODO(rsc): double-check stats.

	package runtime
	#include "runtime.h"
	#include "stack.h"
	#include "malloc.h"
	#include "defs.h"
	#include "type.h"

	MHeap runtime·mheap;
	extern MStats mstats; // defined in extern.go

	extern volatile int32 runtime·MemProfileRate;

	// Same algorithm from chan.c, but a different
	// instance of the static uint32 x.
	// Not protected by a lock - let the threads use
	// the same random number if they like.
	static uint32
	fastrand1(void)
	{
	static uint32 x = 0x49f6428aUL;

	x += x;
	if(x & 0x80000000L)
	x ^= 0x88888eefUL;
	return x;
	}

	// Allocate an object of at least size bytes.
	// Small objects are allocated from the per-thread cache's free lists.
	// Large objects (> 32 kB) are allocated straight from the heap.
	void*
	runtime·mallocgc(uintptr size, uint32 flag, int32 dogc, int32 zeroed)
	{
	int32 sizeclass, rate;
	MCache *c;
	uintptr npages;
	MSpan *s;
	void *v;

	if(runtime·gcwaiting && g != m->g0 && m->locks == 0)
	runtime·gosched();
	if(m->mallocing)
	runtime·throw("malloc/free - deadlock");
	m->mallocing = 1;
	if(size == 0)
	size = 1;

	mstats.nmalloc++;
	if(size <= MaxSmallSize) {
	// Allocate from mcache free lists.
	sizeclass = runtime·SizeToClass(size);
	size = runtime·class_to_size[sizeclass];
	c = m->mcache;
	v = runtime·MCache_Alloc(c, sizeclass, size, zeroed);
	if(v == nil)
	runtime·throw("out of memory");
	mstats.alloc += size;
	mstats.total_alloc += size;
	mstats.by_size[sizeclass].nmalloc++;
	} else {
	// TODO(rsc): Report tracebacks for very large allocations.

	// Allocate directly from heap.
	npages = size >> PageShift;
	if((size & PageMask) != 0)
	npages++;
	s = runtime·MHeap_Alloc(&runtime·mheap, npages, 0, 1);
	if(s == nil)
	runtime·throw("out of memory");
	size = npages<<PageShift;
	mstats.alloc += size;
	mstats.total_alloc += size;
	v = (void*)(s->start << PageShift);

	// setup for mark sweep
	runtime·markspan(v, 0, 0, true);
	}
	if(!(flag & FlagNoGC))
	runtime·markallocated(v, size, (flag&FlagNoPointers) != 0);

	m->mallocing = 0;

	if(!(flag & FlagNoProfiling) && (rate = runtime·MemProfileRate) > 0) {
	if(size >= rate)
	goto profile;
	if(m->mcache->next_sample > size)
	m->mcache->next_sample -= size;
	else {
	// pick next profile time
	if(rate > 0x3fffffff) // make 2*rate not overflow
	rate = 0x3fffffff;
	m->mcache->next_sample = fastrand1() % (2*rate);
	profile:
	runtime·setblockspecial(v);
	runtime·MProf_Malloc(v, size);
	}
	}

	if(dogc && mstats.heap_alloc >= mstats.next_gc)
	runtime·gc(0);
	return v;
	}

	void*
	runtime·malloc(uintptr size)
	{
	return runtime·mallocgc(size, 0, 0, 1);
	}

	// Free the object whose base pointer is v.
	void
	runtime·free(void *v)
	{
	int32 sizeclass;
	MSpan *s;
	MCache *c;
	uint32 prof;
	uintptr size;

	if(v == nil)
	return;

	// If you change this also change mgc0.c:/^sweepspan,
	// which has a copy of the guts of free.

	if(m->mallocing)
	runtime·throw("malloc/free - deadlock");
	m->mallocing = 1;

	if(!runtime·mlookup(v, nil, nil, &s)) {
	runtime·printf("free %p: not an allocated block\n", v);
	runtime·throw("free runtime·mlookup");
	}
	prof = runtime·blockspecial(v);

	// Find size class for v.
	sizeclass = s->sizeclass;
	if(sizeclass == 0) {
	// Large object.
	size = s->npages<<PageShift;
	(uintptr)(s->start<<PageShift) = 1; // mark as "needs to be zeroed"
	// Must mark v freed before calling unmarkspan and MHeap_Free:
	// they might coalesce v into other spans and change the bitmap further.
	runtime·markfreed(v, size);
	runtime·unmarkspan(v, 1<<PageShift);
	runtime·MHeap_Free(&runtime·mheap, s, 1);
	} else {
	// Small object.
	c = m->mcache;
	size = runtime·class_to_size[sizeclass];
	if(size > sizeof(uintptr))
	((uintptr*)v)[1] = 1; // mark as "needs to be zeroed"
	// Must mark v freed before calling MCache_Free:
	// it might coalesce v and other blocks into a bigger span
	// and change the bitmap further.
	runtime·markfreed(v, size);
	mstats.by_size[sizeclass].nfree++;
	runtime·MCache_Free(c, v, sizeclass, size);
	}
	mstats.alloc -= size;
	if(prof)
	runtime·MProf_Free(v, size);
	m->mallocing = 0;
	}

	int32
	runtime·mlookup(void v, byte base, uintptr size, MSpan **sp)
	{
	uintptr n, i;
	byte *p;
	MSpan *s;

	mstats.nlookup++;
	s = runtime·MHeap_LookupMaybe(&runtime·mheap, v);
	if(sp)
	*sp = s;
	if(s == nil) {
	runtime·checkfreed(v, 1);
	if(base)
	*base = nil;
	if(size)
	*size = 0;
	return 0;
	}

	p = (byte*)((uintptr)s->start<<PageShift);
	if(s->sizeclass == 0) {
	// Large object.
	if(base)
	*base = p;
	if(size)
	*size = s->npages<<PageShift;
	return 1;
	}

	if((byte)v >= (byte)s->limit) {
	// pointers past the last block do not count as pointers.
	return 0;
	}

	n = runtime·class_to_size[s->sizeclass];
	i = ((byte*)v - p)/n;
	if(base)
	base = p + in;
	if(size)
	*size = n;

	return 1;
	}

	MCache*
	runtime·allocmcache(void)
	{
	MCache *c;

	runtime·lock(&runtime·mheap);
	c = runtime·FixAlloc_Alloc(&runtime·mheap.cachealloc);
	mstats.mcache_inuse = runtime·mheap.cachealloc.inuse;
	mstats.mcache_sys = runtime·mheap.cachealloc.sys;
	runtime·unlock(&runtime·mheap);
	return c;
	}

	uintptr runtime·sizeof_C_MStats = sizeof(MStats);

	#define MaxArena32 (2U<<30)

	void
	runtime·mallocinit(void)
	{
	byte *p;
	uintptr arena_size, bitmap_size;
	extern byte end[];

	runtime·InitSizes();

	// Set up the allocation arena, a contiguous area of memory where
	// allocated data will be found. The arena begins with a bitmap large
	// enough to hold 4 bits per allocated word.
	if(sizeof(void*) == 8) {
	// On a 64-bit machine, allocate from a single contiguous reservation.
	// 16 GB should be big enough for now.
	//
	// The code will work with the reservation at any address, but ask
	// SysReserve to use 0x000000f800000000 if possible.
	// Allocating a 16 GB region takes away 36 bits, and the amd64
	// doesn't let us choose the top 17 bits, so that leaves the 11 bits
	// in the middle of 0x00f8 for us to choose. Choosing 0x00f8 means
	// that the valid memory addresses will begin 0x00f8, 0x00f9, 0x00fa, 0x00fb.
	// None of the bytes f8 f9 fa fb can appear in valid UTF-8, and
	// they are otherwise as far from ff (likely a common byte) as possible.
	// Choosing 0x00 for the leading 6 bits was more arbitrary, but it
	// is not a common ASCII code point either. Using 0x11f8 instead
	// caused out of memory errors on OS X during thread allocations.
	// These choices are both for debuggability and to reduce the
	// odds of the conservative garbage collector not collecting memory
	// because some non-pointer block of memory had a bit pattern
	// that matched a memory address.
	//
	// Actually we reserve 17 GB (because the bitmap ends up being 1 GB)
	// but it hardly matters: fc is not valid UTF-8 either, and we have to
	// allocate 15 GB before we get that far.
	arena_size = 16LL<<30;
	bitmap_size = arena_size / (sizeof(void)8/4);
	p = runtime·SysReserve((void*)(0x00f8ULL<<32), bitmap_size + arena_size);
	if(p == nil)
	runtime·throw("runtime: cannot reserve arena virtual address space");
	} else {
	// On a 32-bit machine, we can't typically get away
	// with a giant virtual address space reservation.
	// Instead we map the memory information bitmap
	// immediately after the data segment, large enough
	// to handle another 2GB of mappings (256 MB),
	// along with a reservation for another 512 MB of memory.
	// When that gets used up, we'll start asking the kernel
	// for any memory anywhere and hope it's in the 2GB
	// following the bitmap (presumably the executable begins
	// near the bottom of memory, so we'll have to use up
	// most of memory before the kernel resorts to giving out
	// memory before the beginning of the text segment).
	//
	// Alternatively we could reserve 512 MB bitmap, enough
	// for 4GB of mappings, and then accept any memory the
	// kernel threw at us, but normally that's a waste of 512 MB
	// of address space, which is probably too much in a 32-bit world.
	bitmap_size = MaxArena32 / (sizeof(void)8/4);
	arena_size = 512<<20;

	// SysReserve treats the address we ask for, end, as a hint,
	// not as an absolute requirement. If we ask for the end
	// of the data segment but the operating system requires
	// a little more space before we can start allocating, it will
	// give out a slightly higher pointer. That's fine.
	// Run with what we get back.
	p = runtime·SysReserve(end, bitmap_size + arena_size);
	if(p == nil)
	runtime·throw("runtime: cannot reserve arena virtual address space");
	}
	if((uintptr)p & (((uintptr)1<<PageShift)-1))
	runtime·throw("runtime: SysReserve returned unaligned address");

	runtime·mheap.bitmap = p;
	runtime·mheap.arena_start = p + bitmap_size;
	runtime·mheap.arena_used = runtime·mheap.arena_start;
	runtime·mheap.arena_end = runtime·mheap.arena_start + arena_size;

	// Initialize the rest of the allocator.
	runtime·MHeap_Init(&runtime·mheap, runtime·SysAlloc);
	m->mcache = runtime·allocmcache();

	// See if it works.
	runtime·free(runtime·malloc(1));
	}

	void*
	runtime·MHeap_SysAlloc(MHeap *h, uintptr n)
	{
	byte *p;

	if(n <= h->arena_end - h->arena_used) {
	// Keep taking from our reservation.
	p = h->arena_used;
	runtime·SysMap(p, n);
	h->arena_used += n;
	runtime·MHeap_MapBits(h);
	return p;
	}

	// On 64-bit, our reservation is all we have.
	if(sizeof(void*) == 8)
	return nil;

	// On 32-bit, once the reservation is gone we can
	// try to get memory at a location chosen by the OS
	// and hope that it is in the range we allocated bitmap for.
	p = runtime·SysAlloc(n);
	if(p == nil)
	return nil;

	if(p < h->arena_start \|\| p+n - h->arena_start >= MaxArena32) {
	runtime·printf("runtime: memory allocated by OS not in usable range\n");
	runtime·SysFree(p, n);
	return nil;
	}

	if(p+n > h->arena_used) {
	h->arena_used = p+n;
	if(h->arena_used > h->arena_end)
	h->arena_end = h->arena_used;
	runtime·MHeap_MapBits(h);
	}

	return p;
	}

	// Runtime stubs.

	void*
	runtime·mal(uintptr n)
	{
	return runtime·mallocgc(n, 0, 1, 1);
	}

	func new(n uint32) (ret *uint8) {
	ret = runtime·mal(n);
	}

	// Stack allocator uses malloc/free most of the time,
	// but if we're in the middle of malloc and need stack,
	// we have to do something else to avoid deadlock.
	// In that case, we fall back on a fixed-size free-list
	// allocator, assuming that inside malloc all the stack
	// frames are small, so that all the stack allocations
	// will be a single size, the minimum (right now, 5k).
	static struct {
	Lock;
	FixAlloc;
	} stacks;

	enum {
	FixedStack = StackMin,
	};

	void*
	runtime·stackalloc(uint32 n)
	{
	void *v;

	// Stackalloc must be called on scheduler stack, so that we
	// never try to grow the stack during the code that stackalloc runs.
	// Doing so would cause a deadlock (issue 1547).
	if(g != m->g0)
	runtime·throw("stackalloc not on scheduler stack");

	if(m->mallocing \|\| m->gcing \|\| n == FixedStack) {
	runtime·lock(&stacks);
	if(stacks.size == 0)
	runtime·FixAlloc_Init(&stacks, n, runtime·SysAlloc, nil, nil);
	if(stacks.size != n) {
	runtime·printf("stackalloc: in malloc, size=%D want %d", (uint64)stacks.size, n);
	runtime·throw("stackalloc");
	}
	v = runtime·FixAlloc_Alloc(&stacks);
	mstats.stacks_inuse = stacks.inuse;
	mstats.stacks_sys = stacks.sys;
	runtime·unlock(&stacks);
	return v;
	}
	return runtime·mallocgc(n, FlagNoProfiling\|FlagNoGC, 0, 0);
	}

	void
	runtime·stackfree(void *v, uintptr n)
	{
	if(m->mallocing \|\| m->gcing \|\| n == FixedStack) {
	runtime·lock(&stacks);
	runtime·FixAlloc_Free(&stacks, v);
	mstats.stacks_inuse = stacks.inuse;
	mstats.stacks_sys = stacks.sys;
	runtime·unlock(&stacks);
	return;
	}
	runtime·free(v);
	}

	func Alloc(n uintptr) (p *byte) {
	p = runtime·malloc(n);
	}

	func Free(p *byte) {
	runtime·free(p);
	}

	func Lookup(p byte) (base byte, size uintptr) {
	runtime·mlookup(p, &base, &size, nil);
	}

	func GC() {
	runtime·gc(1);
	}

	func SetFinalizer(obj Eface, finalizer Eface) {
	byte *base;
	uintptr size;
	FuncType *ft;
	int32 i, nret;
	Type *t;

	if(obj.type == nil) {
	runtime·printf("runtime.SetFinalizer: first argument is nil interface\n");
	throw:
	runtime·throw("runtime.SetFinalizer");
	}
	if(obj.type->kind != KindPtr) {
	runtime·printf("runtime.SetFinalizer: first argument is %S, not pointer\n", *obj.type->string);
	goto throw;
	}
	if(!runtime·mlookup(obj.data, &base, &size, nil) \|\| obj.data != base) {
	runtime·printf("runtime.SetFinalizer: pointer not at beginning of allocated block\n");
	goto throw;
	}
	nret = 0;
	if(finalizer.type != nil) {
	if(finalizer.type->kind != KindFunc) {
	badfunc:
	runtime·printf("runtime.SetFinalizer: second argument is %S, not func(%S)\n", finalizer.type->string, obj.type->string);
	goto throw;
	}
	ft = (FuncType*)finalizer.type;
	if(ft->dotdotdot \|\| ft->in.len != 1 \|\| (Type*)ft->in.array != obj.type)
	goto badfunc;

	// compute size needed for return parameters
	for(i=0; i<ft->out.len; i++) {
	t = ((Type**)ft->out.array)[i];
	nret = (nret + t->align - 1) & ~(t->align - 1);
	nret += t->size;
	}
	nret = (nret + sizeof(void)-1) & ~(sizeof(void)-1);

	if(runtime·getfinalizer(obj.data, 0)) {
	runtime·printf("runtime.SetFinalizer: finalizer already set\n");
	goto throw;
	}
	}
	runtime·addfinalizer(obj.data, finalizer.data, nret);
	}