runtime: add the allocation headers GOEXPERIMENT and fork files

This change adds the allocation headers GOEXPERIMENT which is a no-op.
It forks two runtime files temporarily to make the GOEXPERIMENT easier
to maintain. The forked files are mbitmap.go and msize.go.

Change-Id: I60202c00e614e4517de7dd000029cf80dd0121ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/537980
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 27 insertions, 0 deletions

diff --git a/src/runtime/msize_allocheaders.go b/src/runtime/msize_allocheaders.go
new file mode 100644
index 0000000000..11b6c2ff6d
--- /dev/null
+++ b/src/runtime/msize_allocheaders.go
@@ -0,0 +1,27 @@
+// 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.
+
+//go:build goexperiment.allocheaders
+
+// Malloc small size classes.
+//
+// See malloc.go for overview.
+// See also mksizeclasses.go for how we decide what size classes to use.
+
+package runtime
+
+// Returns size of the memory block that mallocgc will allocate if you ask for the size.
+func roundupsize(size uintptr) uintptr {
+	if size < _MaxSmallSize {
+		if size <= smallSizeMax-8 {
+			return uintptr(class_to_size[size_to_class8[divRoundUp(size, smallSizeDiv)]])
+		} else {
+			return uintptr(class_to_size[size_to_class128[divRoundUp(size-smallSizeMax, largeSizeDiv)]])
+		}
+	}
+	if size+_PageSize < size {
+		return size
+	}
+	return alignUp(size, _PageSize)
+}