runtime: replace _MaxMem with maxAlloc

Now that we have memLimit, also having _MaxMem is a bit confusing.

Replace it with maxAlloc, which better conveys what it limits. We also
define maxAlloc slightly differently: since it's now clear that it
limits allocation size, we can account for a subtle difference between
32-bit and 64-bit.

Change-Id: Iac39048018cc0dae7f0919e25185fee4b3eed529
Reviewed-on: https://go-review.googlesource.com/85890
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

4 files changed, 24 insertions, 18 deletions

diff --git a/src/runtime/chan.go b/src/runtime/chan.go
index 678128b4ce..10ee97d924 100644
--- a/src/runtime/chan.go
+++ b/src/runtime/chan.go
@@ -78,7 +78,7 @@ func makechan(t *chantype, size int) *hchan {
 		throw("makechan: bad alignment")
 	}
 
-	if size < 0 || uintptr(size) > maxSliceCap(elem.size) || uintptr(size)*elem.size > _MaxMem-hchanSize {
+	if size < 0 || uintptr(size) > maxSliceCap(elem.size) || uintptr(size)*elem.size > maxAlloc-hchanSize {
 		panic(plainError("makechan: size out of range"))
 	}
 
diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index f3e738116c..299add4b35 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -196,7 +196,13 @@ const (
 	//
 	// This is also the maximum heap pointer value.
 	memLimit = 1 << memLimitBits
-	_MaxMem  = memLimit - 1
+
+	// maxAlloc is the maximum size of an allocation. On 64-bit,
+	// it's theoretically possible to allocate memLimit bytes. On
+	// 32-bit, however, this is one less than memLimit because the
+	// number of bytes in the address space doesn't actually fit
+	// in a uintptr.
+	maxAlloc = memLimit - (1-_64bit)*1
 
 	// heapArenaBytes is the size of a heap arena. The heap
 	// consists of mappings of size heapArenaBytes, aligned to
diff --git a/src/runtime/slice.go b/src/runtime/slice.go
index 351fec067d..9f35a89400 100644
--- a/src/runtime/slice.go
+++ b/src/runtime/slice.go
@@ -25,14 +25,14 @@ type notInHeapSlice struct {
 // The index is the size of the slice element.
 var maxElems = [...]uintptr{
 	^uintptr(0),
-	_MaxMem / 1, _MaxMem / 2, _MaxMem / 3, _MaxMem / 4,
-	_MaxMem / 5, _MaxMem / 6, _MaxMem / 7, _MaxMem / 8,
-	_MaxMem / 9, _MaxMem / 10, _MaxMem / 11, _MaxMem / 12,
-	_MaxMem / 13, _MaxMem / 14, _MaxMem / 15, _MaxMem / 16,
-	_MaxMem / 17, _MaxMem / 18, _MaxMem / 19, _MaxMem / 20,
-	_MaxMem / 21, _MaxMem / 22, _MaxMem / 23, _MaxMem / 24,
-	_MaxMem / 25, _MaxMem / 26, _MaxMem / 27, _MaxMem / 28,
-	_MaxMem / 29, _MaxMem / 30, _MaxMem / 31, _MaxMem / 32,
+	maxAlloc / 1, maxAlloc / 2, maxAlloc / 3, maxAlloc / 4,
+	maxAlloc / 5, maxAlloc / 6, maxAlloc / 7, maxAlloc / 8,
+	maxAlloc / 9, maxAlloc / 10, maxAlloc / 11, maxAlloc / 12,
+	maxAlloc / 13, maxAlloc / 14, maxAlloc / 15, maxAlloc / 16,
+	maxAlloc / 17, maxAlloc / 18, maxAlloc / 19, maxAlloc / 20,
+	maxAlloc / 21, maxAlloc / 22, maxAlloc / 23, maxAlloc / 24,
+	maxAlloc / 25, maxAlloc / 26, maxAlloc / 27, maxAlloc / 28,
+	maxAlloc / 29, maxAlloc / 30, maxAlloc / 31, maxAlloc / 32,
 }
 
 // maxSliceCap returns the maximum capacity for a slice.
@@ -40,7 +40,7 @@ func maxSliceCap(elemsize uintptr) uintptr {
 	if elemsize < uintptr(len(maxElems)) {
 		return maxElems[elemsize]
 	}
-	return _MaxMem / elemsize
+	return maxAlloc / elemsize
 }
 
 func makeslice(et *_type, len, cap int) slice {
@@ -133,13 +133,13 @@ func growslice(et *_type, old slice, cap int) slice {
 		lenmem = uintptr(old.len)
 		newlenmem = uintptr(cap)
 		capmem = roundupsize(uintptr(newcap))
-		overflow = uintptr(newcap) > _MaxMem
+		overflow = uintptr(newcap) > maxAlloc
 		newcap = int(capmem)
 	case ptrSize:
 		lenmem = uintptr(old.len) * ptrSize
 		newlenmem = uintptr(cap) * ptrSize
 		capmem = roundupsize(uintptr(newcap) * ptrSize)
-		overflow = uintptr(newcap) > _MaxMem/ptrSize
+		overflow = uintptr(newcap) > maxAlloc/ptrSize
 		newcap = int(capmem / ptrSize)
 	default:
 		lenmem = uintptr(old.len) * et.size
@@ -163,7 +163,7 @@ func growslice(et *_type, old slice, cap int) slice {
 	//   s = append(s, d, d, d, d)
 	//   print(len(s), "\n")
 	// }
-	if cap < old.cap || overflow || capmem > _MaxMem {
+	if cap < old.cap || overflow || capmem > maxAlloc {
 		panic(errorString("growslice: cap out of range"))
 	}
 
diff --git a/src/runtime/string.go b/src/runtime/string.go
index 22be091375..97909196e9 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -266,7 +266,7 @@ func rawbyteslice(size int) (b []byte) {
 
 // rawruneslice allocates a new rune slice. The rune slice is not zeroed.
 func rawruneslice(size int) (b []rune) {
-	if uintptr(size) > _MaxMem/4 {
+	if uintptr(size) > maxAlloc/4 {
 		throw("out of memory")
 	}
 	mem := roundupsize(uintptr(size) * 4)
@@ -395,7 +395,7 @@ func findnull(s *byte) int {
 	if s == nil {
 		return 0
 	}
-	p := (*[_MaxMem/2 - 1]byte)(unsafe.Pointer(s))
+	p := (*[maxAlloc/2 - 1]byte)(unsafe.Pointer(s))
 	l := 0
 	for p[l] != 0 {
 		l++
@@ -407,7 +407,7 @@ func findnullw(s *uint16) int {
 	if s == nil {
 		return 0
 	}
-	p := (*[_MaxMem/2/2 - 1]uint16)(unsafe.Pointer(s))
+	p := (*[maxAlloc/2/2 - 1]uint16)(unsafe.Pointer(s))
 	l := 0
 	for p[l] != 0 {
 		l++
@@ -424,7 +424,7 @@ func gostringnocopy(str *byte) string {
 
 func gostringw(strw *uint16) string {
 	var buf [8]byte
-	str := (*[_MaxMem/2/2 - 1]uint16)(unsafe.Pointer(strw))
+	str := (*[maxAlloc/2/2 - 1]uint16)(unsafe.Pointer(strw))
 	n1 := 0
 	for i := 0; str[i] != 0; i++ {
 		n1 += encoderune(buf[:], rune(str[i]))