runtime: set iOS addr space to 40 bits with incremental pagealloc

In iOS <14, the address space is strictly limited to 8 GiB, or 33 bits.
As a result, the page allocator also assumes all heap memory lives in
this region. This is especially necessary because the page allocator has
a PROT_NONE mapping proportional to the size of the usable address
space, so this keeps that mapping very small.

However starting with iOS 14, this restriction is relaxed, and mmap may
start returning addresses outside of the <14 range. Today this means
that in iOS 14 and later, users experience an error in the page
allocator when a heap arena is mapped outside of the old range.

This change increases the ios/arm64 heapAddrBits to 40 while
simultaneously making ios/arm64 use the 64-bit pagealloc implementation
(with reservations and incremental mapping) to accommodate both iOS
versions <14 and 14+.

Once iOS <14 is deprecated, we can remove these exceptions and treat
ios/arm64 like any other arm64 platform.

This change also makes the BaseChunkIdx expression a little bit easier
to read, while we're here.

Fixes #46860.

Change-Id: I13865f799777739109585f14f1cc49d6d57e096b
Reviewed-on: https://go-review.googlesource.com/c/go/+/344401
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 1 insertions, 7 deletions

diff --git a/src/runtime/mpagealloc_32bit.go b/src/runtime/mpagealloc_32bit.go
index 1d863f2fda..8c83b93412 100644
--- a/src/runtime/mpagealloc_32bit.go
+++ b/src/runtime/mpagealloc_32bit.go
@@ -2,19 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build 386 || arm || mips || mipsle || wasm || (ios && arm64)
+//go:build 386 || arm || mips || mipsle || wasm
 
 // 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).
 
-// ios/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
 
 import "unsafe"