// Copyright 2014 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 amd64 arm64 mips64 mips64le ppc64 ppc64le s390x

package runtime

import "unsafe"

const (
	// addrBits is the number of bits needed to represent a virtual address.
	//
	// In Linux the user address space for each architecture is limited as
	// follows (taken from the processor.h file for the architecture):
	//
	// Architecture  Name              Maximum Value (exclusive)
	// ---------------------------------------------------------------------
	// amd64         TASK_SIZE_MAX     0x007ffffffff000 (47 bit addresses)
	// arm64         TASK_SIZE_64      0x01000000000000 (48 bit addresses)
	// ppc64{,le}    TASK_SIZE_USER64  0x00400000000000 (46 bit addresses)
	// mips64{,le}   TASK_SIZE64       0x00010000000000 (40 bit addresses)
	// s390x         TASK_SIZE         1<<64 (64 bit addresses)
	//
	// These values may increase over time. In particular, ppc64
	// and mips64 support arbitrary 64-bit addresses in hardware,
	// but Linux imposes the above limits. amd64 has hardware
	// support for 57 bit addresses as of 2017 (56 bits for user
	// space), but Linux only uses addresses above 1<<47 for
	// mappings that explicitly pass a high hint address.
	//
	// On AMD64, virtual addresses are 48-bit (or 57-bit) numbers sign extended to 64.
	// We shift the address left 16 to eliminate the sign extended part and make
	// room in the bottom for the count.
	//
	// On s390x, there's not much we can do, so we just hope that
	// the kernel doesn't get to really high addresses.
	addrBits = 48

	// In addition to the 16 bits taken from the top, we can take 3 from the
	// bottom, because node must be pointer-aligned, giving a total of 19 bits
	// of count.
	cntBits = 64 - addrBits + 3
)

func lfstackPack(node *lfnode, cnt uintptr) uint64 {
	return uint64(uintptr(unsafe.Pointer(node)))<<(64-addrBits) | uint64(cnt&(1<<cntBits-1))
}

func lfstackUnpack(val uint64) *lfnode {
	if GOARCH == "amd64" {
		// amd64 systems can place the stack above the VA hole, so we need to sign extend
		// val before unpacking.
		return (*lfnode)(unsafe.Pointer(uintptr(int64(val) >> cntBits << 3)))
	}
	return (*lfnode)(unsafe.Pointer(uintptr(val >> cntBits << 3)))
}