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Source file src/runtime/lfstack_64bit.go

     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	}
    49	

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