src/runtime/mem_windows.go

// Copyright 2010 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 runtime

import (
	"unsafe"
)

const (
	_MEM_COMMIT   = 0x1000
	_MEM_RESERVE  = 0x2000
	_MEM_DECOMMIT = 0x4000
	_MEM_RELEASE  = 0x8000

	_PAGE_READWRITE = 0x0004
	_PAGE_NOACCESS  = 0x0001
)

//go:cgo_import_dynamic runtime._VirtualAlloc VirtualAlloc "kernel32.dll"
//go:cgo_import_dynamic runtime._VirtualFree VirtualFree "kernel32.dll"
//go:cgo_import_dynamic runtime._VirtualProtect VirtualProtect "kernel32.dll"

var (
	_VirtualAlloc,
	_VirtualFree,
	_VirtualProtect stdFunction
)

//go:nosplit
func sysAlloc(n uintptr, stat *uint64) unsafe.Pointer {
	xadd64(stat, int64(n))
	return unsafe.Pointer(stdcall4(_VirtualAlloc, 0, n, _MEM_COMMIT|_MEM_RESERVE, _PAGE_READWRITE))
}

func sysUnused(v unsafe.Pointer, n uintptr) {
	r := stdcall3(_VirtualFree, uintptr(v), n, _MEM_DECOMMIT)
	if r != 0 {
		return
	}

	// Decommit failed. Usual reason is that we've merged memory from two different
	// VirtualAlloc calls, and Windows will only let each VirtualFree handle pages from
	// a single VirtualAlloc. It is okay to specify a subset of the pages from a single alloc,
	// just not pages from multiple allocs. This is a rare case, arising only when we're
	// trying to give memory back to the operating system, which happens on a time
	// scale of minutes. It doesn't have to be terribly fast. Instead of extra bookkeeping
	// on all our VirtualAlloc calls, try freeing successively smaller pieces until
	// we manage to free something, and then repeat. This ends up being O(n log n)
	// in the worst case, but that's fast enough.
	for n > 0 {
		small := n
		for small >= 4096 && stdcall3(_VirtualFree, uintptr(v), small, _MEM_DECOMMIT) == 0 {
			small /= 2
			small &^= 4096 - 1
		}
		if small < 4096 {
			throw("runtime: failed to decommit pages")
		}
		v = add(v, small)
		n -= small
	}
}

func sysUsed(v unsafe.Pointer, n uintptr) {
	r := stdcall4(_VirtualAlloc, uintptr(v), n, _MEM_COMMIT, _PAGE_READWRITE)
	if r != uintptr(v) {
		throw("runtime: failed to commit pages")
	}

	// Commit failed. See SysUnused.
	for n > 0 {
		small := n
		for small >= 4096 && stdcall4(_VirtualAlloc, uintptr(v), small, _MEM_COMMIT, _PAGE_READWRITE) == 0 {
			small /= 2
			small &^= 4096 - 1
		}
		if small < 4096 {
			throw("runtime: failed to decommit pages")
		}
		v = add(v, small)
		n -= small
	}
}

func sysFree(v unsafe.Pointer, n uintptr, stat *uint64) {
	xadd64(stat, -int64(n))
	r := stdcall3(_VirtualFree, uintptr(v), 0, _MEM_RELEASE)
	if r == 0 {
		throw("runtime: failed to release pages")
	}
}

func sysFault(v unsafe.Pointer, n uintptr) {
	// SysUnused makes the memory inaccessible and prevents its reuse
	sysUnused(v, n)
}

func sysReserve(v unsafe.Pointer, n uintptr, reserved *bool) unsafe.Pointer {
	*reserved = true
	// v is just a hint.
	// First try at v.
	v = unsafe.Pointer(stdcall4(_VirtualAlloc, uintptr(v), n, _MEM_RESERVE, _PAGE_READWRITE))
	if v != nil {
		return v
	}

	// Next let the kernel choose the address.
	return unsafe.Pointer(stdcall4(_VirtualAlloc, 0, n, _MEM_RESERVE, _PAGE_READWRITE))
}

func sysMap(v unsafe.Pointer, n uintptr, reserved bool, stat *uint64) {
	xadd64(stat, int64(n))
	p := stdcall4(_VirtualAlloc, uintptr(v), n, _MEM_COMMIT, _PAGE_READWRITE)
	if p != uintptr(v) {
		throw("runtime: cannot map pages in arena address space")
	}
}