runtime: move some malloc constants to internal/runtime/gc

These constants are needed by some future generator programs.

Change-Id: I5dccd009cbb3b2f321523bc0d8eaeb4c82e5df81
Reviewed-on: https://go-review.googlesource.com/c/go/+/655276
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/src/internal/runtime/gc/malloc.go b/src/internal/runtime/gc/malloc.go
new file mode 100644
index 0000000000..5eb99e2f0d
--- /dev/null
+++ b/src/internal/runtime/gc/malloc.go
@@ -0,0 +1,47 @@
+// Copyright 2025 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.
+
+package gc
+
+import "internal/goarch"
+
+const (
+	ptrBits = 8 * goarch.PtrSize
+
+	// A malloc header is functionally a single type pointer, but
+	// we need to use 8 here to ensure 8-byte alignment of allocations
+	// on 32-bit platforms. It's wasteful, but a lot of code relies on
+	// 8-byte alignment for 8-byte atomics.
+	MallocHeaderSize = 8
+
+	// The minimum object size that has a malloc header, exclusive.
+	//
+	// The size of this value controls overheads from the malloc header.
+	// The minimum size is bound by writeHeapBitsSmall, which assumes that the
+	// pointer bitmap for objects of a size smaller than this doesn't cross
+	// more than one pointer-word boundary. This sets an upper-bound on this
+	// value at the number of bits in a uintptr, multiplied by the pointer
+	// size in bytes.
+	//
+	// We choose a value here that has a natural cutover point in terms of memory
+	// overheads. This value just happens to be the maximum possible value this
+	// can be.
+	//
+	// A span with heap bits in it will have 128 bytes of heap bits on 64-bit
+	// platforms, and 256 bytes of heap bits on 32-bit platforms. The first size
+	// class where malloc headers match this overhead for 64-bit platforms is
+	// 512 bytes (8 KiB / 512 bytes * 8 bytes-per-header = 128 bytes of overhead).
+	// On 32-bit platforms, this same point is the 256 byte size class
+	// (8 KiB / 256 bytes * 8 bytes-per-header = 256 bytes of overhead).
+	//
+	// Guaranteed to be exactly at a size class boundary. The reason this value is
+	// an exclusive minimum is subtle. Suppose we're allocating a 504-byte object
+	// and its rounded up to 512 bytes for the size class. If minSizeForMallocHeader
+	// is 512 and an inclusive minimum, then a comparison against minSizeForMallocHeader
+	// by the two values would produce different results. In other words, the comparison
+	// would not be invariant to size-class rounding. Eschewing this property means a
+	// more complex check or possibly storing additional state to determine whether a
+	// span has malloc headers.
+	MinSizeForMallocHeader = goarch.PtrSize * ptrBits
+)