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Source file src/runtime/lfstack_64bit.go

Documentation: runtime

  // 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)
  	// ---------------------------------------------------------------------
  	// arm64         TASK_SIZE_64      Depends on configuration.
  	// ppc64{,le}    TASK_SIZE_USER64  0x400000000000UL (46 bit addresses)
  	// mips64{,le}   TASK_SIZE64       0x010000000000UL (40 bit addresses)
  	// s390x         TASK_SIZE         0x020000000000UL (41 bit addresses)
  	//
  	// These values may increase over time.
  	//
  	// On AMD64, virtual addresses are 48-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.
  	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)))
  }
  

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