sizes.go

Documentation: cmd/compile/internal/noder

     1  // Copyright 2021 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 noder
     6  
     7  import (
     8  	"fmt"
     9  
    10  	"cmd/compile/internal/types"
    11  	"cmd/compile/internal/types2"
    12  )
    13  
    14  // Code below based on go/types.StdSizes.
    15  // Intentional differences are marked with "gc:".
    16  
    17  type gcSizes struct{}
    18  
    19  func (s *gcSizes) Alignof(T types2.Type) int64 {
    20  	// For arrays and structs, alignment is defined in terms
    21  	// of alignment of the elements and fields, respectively.
    22  	switch t := T.Underlying().(type) {
    23  	case *types2.Array:
    24  		// spec: "For a variable x of array type: unsafe.Alignof(x)
    25  		// is the same as unsafe.Alignof(x[0]), but at least 1."
    26  		return s.Alignof(t.Elem())
    27  	case *types2.Struct:
    28  		// spec: "For a variable x of struct type: unsafe.Alignof(x)
    29  		// is the largest of the values unsafe.Alignof(x.f) for each
    30  		// field f of x, but at least 1."
    31  		max := int64(1)
    32  		for i, nf := 0, t.NumFields(); i < nf; i++ {
    33  			if a := s.Alignof(t.Field(i).Type()); a > max {
    34  				max = a
    35  			}
    36  		}
    37  		return max
    38  	case *types2.Slice, *types2.Interface:
    39  		// Multiword data structures are effectively structs
    40  		// in which each element has size PtrSize.
    41  		return int64(types.PtrSize)
    42  	case *types2.Basic:
    43  		// Strings are like slices and interfaces.
    44  		if t.Info()&types2.IsString != 0 {
    45  			return int64(types.PtrSize)
    46  		}
    47  	}
    48  	a := s.Sizeof(T) // may be 0
    49  	// spec: "For a variable x of any type: unsafe.Alignof(x) is at least 1."
    50  	if a < 1 {
    51  		return 1
    52  	}
    53  	// complex{64,128} are aligned like [2]float{32,64}.
    54  	if isComplex(T) {
    55  		a /= 2
    56  	}
    57  	if a > int64(types.RegSize) {
    58  		return int64(types.RegSize)
    59  	}
    60  	return a
    61  }
    62  
    63  func isComplex(T types2.Type) bool {
    64  	basic, ok := T.Underlying().(*types2.Basic)
    65  	return ok && basic.Info()&types2.IsComplex != 0
    66  }
    67  
    68  func (s *gcSizes) Offsetsof(fields []*types2.Var) []int64 {
    69  	offsets := make([]int64, len(fields))
    70  	var o int64
    71  	for i, f := range fields {
    72  		typ := f.Type()
    73  		a := s.Alignof(typ)
    74  		o = types.Rnd(o, a)
    75  		offsets[i] = o
    76  		o += s.Sizeof(typ)
    77  	}
    78  	return offsets
    79  }
    80  
    81  func (s *gcSizes) Sizeof(T types2.Type) int64 {
    82  	switch t := T.Underlying().(type) {
    83  	case *types2.Basic:
    84  		k := t.Kind()
    85  		if int(k) < len(basicSizes) {
    86  			if s := basicSizes[k]; s > 0 {
    87  				return int64(s)
    88  			}
    89  		}
    90  		switch k {
    91  		case types2.String:
    92  			return int64(types.PtrSize) * 2
    93  		case types2.Int, types2.Uint, types2.Uintptr, types2.UnsafePointer:
    94  			return int64(types.PtrSize)
    95  		}
    96  		panic(fmt.Sprintf("unimplemented basic: %v (kind %v)", T, k))
    97  	case *types2.Array:
    98  		n := t.Len()
    99  		if n <= 0 {
   100  			return 0
   101  		}
   102  		// n > 0
   103  		// gc: Size includes alignment padding.
   104  		return s.Sizeof(t.Elem()) * n
   105  	case *types2.Slice:
   106  		return int64(types.PtrSize) * 3
   107  	case *types2.Struct:
   108  		n := t.NumFields()
   109  		if n == 0 {
   110  			return 0
   111  		}
   112  		fields := make([]*types2.Var, n)
   113  		for i := range fields {
   114  			fields[i] = t.Field(i)
   115  		}
   116  		offsets := s.Offsetsof(fields)
   117  
   118  		// gc: The last field of a non-zero-sized struct is not allowed to
   119  		// have size 0.
   120  		last := s.Sizeof(fields[n-1].Type())
   121  		if last == 0 && offsets[n-1] > 0 {
   122  			last = 1
   123  		}
   124  
   125  		// gc: Size includes alignment padding.
   126  		return types.Rnd(offsets[n-1]+last, s.Alignof(t))
   127  	case *types2.Interface:
   128  		return int64(types.PtrSize) * 2
   129  	case *types2.Chan, *types2.Map, *types2.Pointer, *types2.Signature:
   130  		return int64(types.PtrSize)
   131  	default:
   132  		panic(fmt.Sprintf("unimplemented type: %T", t))
   133  	}
   134  }
   135  
   136  var basicSizes = [...]byte{
   137  	types2.Bool:       1,
   138  	types2.Int8:       1,
   139  	types2.Int16:      2,
   140  	types2.Int32:      4,
   141  	types2.Int64:      8,
   142  	types2.Uint8:      1,
   143  	types2.Uint16:     2,
   144  	types2.Uint32:     4,
   145  	types2.Uint64:     8,
   146  	types2.Float32:    4,
   147  	types2.Float64:    8,
   148  	types2.Complex64:  8,
   149  	types2.Complex128: 16,
   150  }
   151
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