runtime: add new page allocator constants and description

This change is the first of a series of changes which replace the
current page allocator (which is based on the contents of mgclarge.go
and some of mheap.go) with one based on free/used bitmaps.

It adds in the key constants for the page allocator as well as a comment
describing the implementation.

Updates #35112.

Change-Id: I839d3a07f46842ad379701d27aa691885afdba63
Reviewed-on: https://go-review.googlesource.com/c/go/+/190619
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/src/runtime/mpagealloc_32bit.go b/src/runtime/mpagealloc_32bit.go
new file mode 100644
index 0000000000..c91b2bbe3f
--- /dev/null
+++ b/src/runtime/mpagealloc_32bit.go
@@ -0,0 +1,23 @@
+// Copyright 2019 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.
+
+// +build 386 arm mips mipsle wasm darwin,arm64
+
+// wasm is a treated as a 32-bit architecture for the purposes of the page
+// allocator, even though it has 64-bit pointers. This is because any wasm
+// pointer always has its top 32 bits as zero, so the effective heap address
+// space is only 2^32 bytes in size (see heapAddrBits).
+
+// darwin/arm64 is treated as a 32-bit architecture for the purposes of the
+// page allocator, even though it has 64-bit pointers and a 33-bit address
+// space (see heapAddrBits). The 33 bit address space cannot be rounded up
+// to 64 bits because there are too many summary levels to fit in just 33
+// bits.
+
+package runtime
+
+const (
+	// The number of levels in the radix tree.
+	summaryLevels = 4
+)