iface.go

     1  // Copyright 2023 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  package abi
     6  
     7  import (
     8  	"internal/goarch"
     9  	"unsafe"
    10  )
    11  
    12  // The first word of every non-empty interface type contains an *ITab.
    13  // It records the underlying concrete type (Type), the interface type it
    14  // is implementing (Inter), and some ancillary information.
    15  //
    16  // allocated in non-garbage-collected memory
    17  type ITab struct {
    18  	Inter *InterfaceType
    19  	Type  *Type
    20  	Hash  uint32     // copy of Type.Hash. Used for type switches.
    21  	Fun   [1]uintptr // variable sized. fun[0]==0 means Type does not implement Inter.
    22  }
    23  
    24  // Size returns the size of the itab in memory.
    25  func (it *ITab) Size() int {
    26  	size := int(unsafe.Sizeof(ITab{}))
    27  	if it.Fun[0] == 0 {
    28  		return size
    29  	}
    30  	return size + (len(it.Inter.Methods)-1)*goarch.PtrSize
    31  }
    32  
    33  // EmptyInterface describes the layout of a "interface{}" or a "any."
    34  // These are represented differently than non-empty interface, as the first
    35  // word always points to an abi.Type.
    36  type EmptyInterface struct {
    37  	Type *Type
    38  	Data unsafe.Pointer
    39  }
    40  
    41  // NonEmptyInterface describes the layout of an interface that contains any methods.
    42  type NonEmptyInterface struct {
    43  	ITab *ITab
    44  	Data unsafe.Pointer
    45  }
    46  
    47  // CommonInterface describes the layout of both [EmptyInterface] and [NonEmptyInterface].
    48  type CommonInterface struct {
    49  	// Either an *ITab or a *Type, unexported to avoid accidental use.
    50  	_ unsafe.Pointer
    51  
    52  	Data unsafe.Pointer
    53  }
    54
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