github.com/MangoDowner/go-gm@v0.0.0-20180818020936-8baa2bd4408c/src/runtime/lfstack_64bit.go (about)

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // +build amd64 arm64 mips64 mips64le ppc64 ppc64le s390x
     6  
     7  package runtime
     8  
     9  import "unsafe"
    10  
    11  const (
    12  	// addrBits is the number of bits needed to represent a virtual address.
    13  	//
    14  	// In Linux the user address space for each architecture is limited as
    15  	// follows (taken from the processor.h file for the architecture):
    16  	//
    17  	// Architecture  Name              Maximum Value (exclusive)
    18  	// ---------------------------------------------------------------------
    19  	// arm64         TASK_SIZE_64      Depends on configuration.
    20  	// ppc64{,le}    TASK_SIZE_USER64  0x400000000000UL (46 bit addresses)
    21  	// mips64{,le}   TASK_SIZE64       0x010000000000UL (40 bit addresses)
    22  	// s390x         TASK_SIZE         0x020000000000UL (41 bit addresses)
    23  	//
    24  	// These values may increase over time.
    25  	//
    26  	// On AMD64, virtual addresses are 48-bit numbers sign extended to 64.
    27  	// We shift the address left 16 to eliminate the sign extended part and make
    28  	// room in the bottom for the count.
    29  	addrBits = 48
    30  
    31  	// In addition to the 16 bits taken from the top, we can take 3 from the
    32  	// bottom, because node must be pointer-aligned, giving a total of 19 bits
    33  	// of count.
    34  	cntBits = 64 - addrBits + 3
    35  )
    36  
    37  func lfstackPack(node *lfnode, cnt uintptr) uint64 {
    38  	return uint64(uintptr(unsafe.Pointer(node)))<<(64-addrBits) | uint64(cnt&(1<<cntBits-1))
    39  }
    40  
    41  func lfstackUnpack(val uint64) *lfnode {
    42  	if GOARCH == "amd64" {
    43  		// amd64 systems can place the stack above the VA hole, so we need to sign extend
    44  		// val before unpacking.
    45  		return (*lfnode)(unsafe.Pointer(uintptr(int64(val) >> cntBits << 3)))
    46  	}
    47  	return (*lfnode)(unsafe.Pointer(uintptr(val >> cntBits << 3)))
    48  }