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Source file src/go/types/sizes.go

Documentation: go/types

     1  // Copyright 2013 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  // This file implements Sizes.
     6  
     7  package types
     8  
     9  // Sizes defines the sizing functions for package unsafe.
    10  type Sizes interface {
    11  	// Alignof returns the alignment of a variable of type T.
    12  	// Alignof must implement the alignment guarantees required by the spec.
    13  	Alignof(T Type) int64
    14  
    15  	// Offsetsof returns the offsets of the given struct fields, in bytes.
    16  	// Offsetsof must implement the offset guarantees required by the spec.
    17  	Offsetsof(fields []*Var) []int64
    18  
    19  	// Sizeof returns the size of a variable of type T.
    20  	// Sizeof must implement the size guarantees required by the spec.
    21  	Sizeof(T Type) int64
    22  }
    23  
    24  // StdSizes is a convenience type for creating commonly used Sizes.
    25  // It makes the following simplifying assumptions:
    26  //
    27  //	- The size of explicitly sized basic types (int16, etc.) is the
    28  //	  specified size.
    29  //	- The size of strings and interfaces is 2*WordSize.
    30  //	- The size of slices is 3*WordSize.
    31  //	- The size of an array of n elements corresponds to the size of
    32  //	  a struct of n consecutive fields of the array's element type.
    33  //      - The size of a struct is the offset of the last field plus that
    34  //	  field's size. As with all element types, if the struct is used
    35  //	  in an array its size must first be aligned to a multiple of the
    36  //	  struct's alignment.
    37  //	- All other types have size WordSize.
    38  //	- Arrays and structs are aligned per spec definition; all other
    39  //	  types are naturally aligned with a maximum alignment MaxAlign.
    40  //
    41  // *StdSizes implements Sizes.
    42  //
    43  type StdSizes struct {
    44  	WordSize int64 // word size in bytes - must be >= 4 (32bits)
    45  	MaxAlign int64 // maximum alignment in bytes - must be >= 1
    46  }
    47  
    48  func (s *StdSizes) Alignof(T Type) int64 {
    49  	// For arrays and structs, alignment is defined in terms
    50  	// of alignment of the elements and fields, respectively.
    51  	switch t := T.Underlying().(type) {
    52  	case *Array:
    53  		// spec: "For a variable x of array type: unsafe.Alignof(x)
    54  		// is the same as unsafe.Alignof(x[0]), but at least 1."
    55  		return s.Alignof(t.elem)
    56  	case *Struct:
    57  		// spec: "For a variable x of struct type: unsafe.Alignof(x)
    58  		// is the largest of the values unsafe.Alignof(x.f) for each
    59  		// field f of x, but at least 1."
    60  		max := int64(1)
    61  		for _, f := range t.fields {
    62  			if a := s.Alignof(f.typ); a > max {
    63  				max = a
    64  			}
    65  		}
    66  		return max
    67  	case *Slice, *Interface:
    68  		// Multiword data structures are effectively structs
    69  		// in which each element has size WordSize.
    70  		return s.WordSize
    71  	case *Basic:
    72  		// Strings are like slices and interfaces.
    73  		if t.Info()&IsString != 0 {
    74  			return s.WordSize
    75  		}
    76  	}
    77  	a := s.Sizeof(T) // may be 0
    78  	// spec: "For a variable x of any type: unsafe.Alignof(x) is at least 1."
    79  	if a < 1 {
    80  		return 1
    81  	}
    82  	// complex{64,128} are aligned like [2]float{32,64}.
    83  	if isComplex(T) {
    84  		a /= 2
    85  	}
    86  	if a > s.MaxAlign {
    87  		return s.MaxAlign
    88  	}
    89  	return a
    90  }
    91  
    92  func (s *StdSizes) Offsetsof(fields []*Var) []int64 {
    93  	offsets := make([]int64, len(fields))
    94  	var o int64
    95  	for i, f := range fields {
    96  		a := s.Alignof(f.typ)
    97  		o = align(o, a)
    98  		offsets[i] = o
    99  		o += s.Sizeof(f.typ)
   100  	}
   101  	return offsets
   102  }
   103  
   104  var basicSizes = [...]byte{
   105  	Bool:       1,
   106  	Int8:       1,
   107  	Int16:      2,
   108  	Int32:      4,
   109  	Int64:      8,
   110  	Uint8:      1,
   111  	Uint16:     2,
   112  	Uint32:     4,
   113  	Uint64:     8,
   114  	Float32:    4,
   115  	Float64:    8,
   116  	Complex64:  8,
   117  	Complex128: 16,
   118  }
   119  
   120  func (s *StdSizes) Sizeof(T Type) int64 {
   121  	switch t := T.Underlying().(type) {
   122  	case *Basic:
   123  		assert(isTyped(T))
   124  		k := t.kind
   125  		if int(k) < len(basicSizes) {
   126  			if s := basicSizes[k]; s > 0 {
   127  				return int64(s)
   128  			}
   129  		}
   130  		if k == String {
   131  			return s.WordSize * 2
   132  		}
   133  	case *Array:
   134  		n := t.len
   135  		if n == 0 {
   136  			return 0
   137  		}
   138  		a := s.Alignof(t.elem)
   139  		z := s.Sizeof(t.elem)
   140  		return align(z, a)*(n-1) + z
   141  	case *Slice:
   142  		return s.WordSize * 3
   143  	case *Struct:
   144  		n := t.NumFields()
   145  		if n == 0 {
   146  			return 0
   147  		}
   148  		offsets := s.Offsetsof(t.fields)
   149  		return offsets[n-1] + s.Sizeof(t.fields[n-1].typ)
   150  	case *Interface:
   151  		return s.WordSize * 2
   152  	}
   153  	return s.WordSize // catch-all
   154  }
   155  
   156  // common architecture word sizes and alignments
   157  var gcArchSizes = map[string]*StdSizes{
   158  	"386":      {4, 4},
   159  	"arm":      {4, 4},
   160  	"arm64":    {8, 8},
   161  	"amd64":    {8, 8},
   162  	"amd64p32": {4, 8},
   163  	"mips":     {4, 4},
   164  	"mipsle":   {4, 4},
   165  	"mips64":   {8, 8},
   166  	"mips64le": {8, 8},
   167  	"ppc64":    {8, 8},
   168  	"ppc64le":  {8, 8},
   169  	"s390x":    {8, 8},
   170  	// When adding more architectures here,
   171  	// update the doc string of SizesFor below.
   172  }
   173  
   174  // SizesFor returns the Sizes used by a compiler for an architecture.
   175  // The result is nil if a compiler/architecture pair is not known.
   176  //
   177  // Supported architectures for compiler "gc":
   178  // "386", "arm", "arm64", "amd64", "amd64p32", "mips", "mipsle",
   179  // "mips64", "mips64le", "ppc64", "ppc64le", "s390x".
   180  func SizesFor(compiler, arch string) Sizes {
   181  	if compiler != "gc" {
   182  		return nil
   183  	}
   184  	s, ok := gcArchSizes[arch]
   185  	if !ok {
   186  		return nil
   187  	}
   188  	return s
   189  }
   190  
   191  // stdSizes is used if Config.Sizes == nil.
   192  var stdSizes = SizesFor("gc", "amd64")
   193  
   194  func (conf *Config) alignof(T Type) int64 {
   195  	if s := conf.Sizes; s != nil {
   196  		if a := s.Alignof(T); a >= 1 {
   197  			return a
   198  		}
   199  		panic("Config.Sizes.Alignof returned an alignment < 1")
   200  	}
   201  	return stdSizes.Alignof(T)
   202  }
   203  
   204  func (conf *Config) offsetsof(T *Struct) []int64 {
   205  	var offsets []int64
   206  	if T.NumFields() > 0 {
   207  		// compute offsets on demand
   208  		if s := conf.Sizes; s != nil {
   209  			offsets = s.Offsetsof(T.fields)
   210  			// sanity checks
   211  			if len(offsets) != T.NumFields() {
   212  				panic("Config.Sizes.Offsetsof returned the wrong number of offsets")
   213  			}
   214  			for _, o := range offsets {
   215  				if o < 0 {
   216  					panic("Config.Sizes.Offsetsof returned an offset < 0")
   217  				}
   218  			}
   219  		} else {
   220  			offsets = stdSizes.Offsetsof(T.fields)
   221  		}
   222  	}
   223  	return offsets
   224  }
   225  
   226  // offsetof returns the offset of the field specified via
   227  // the index sequence relative to typ. All embedded fields
   228  // must be structs (rather than pointer to structs).
   229  func (conf *Config) offsetof(typ Type, index []int) int64 {
   230  	var o int64
   231  	for _, i := range index {
   232  		s := typ.Underlying().(*Struct)
   233  		o += conf.offsetsof(s)[i]
   234  		typ = s.fields[i].typ
   235  	}
   236  	return o
   237  }
   238  
   239  func (conf *Config) sizeof(T Type) int64 {
   240  	if s := conf.Sizes; s != nil {
   241  		if z := s.Sizeof(T); z >= 0 {
   242  			return z
   243  		}
   244  		panic("Config.Sizes.Sizeof returned a size < 0")
   245  	}
   246  	return stdSizes.Sizeof(T)
   247  }
   248  
   249  // align returns the smallest y >= x such that y % a == 0.
   250  func align(x, a int64) int64 {
   251  	y := x + a - 1
   252  	return y - y%a
   253  }
   254  

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