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Source file src/debug/dwarf/type.go

Documentation: debug/dwarf

     1  // Copyright 2009 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  // DWARF type information structures.
     6  // The format is heavily biased toward C, but for simplicity
     7  // the String methods use a pseudo-Go syntax.
     8  
     9  package dwarf
    10  
    11  import "strconv"
    12  
    13  // A Type conventionally represents a pointer to any of the
    14  // specific Type structures (CharType, StructType, etc.).
    15  type Type interface {
    16  	Common() *CommonType
    17  	String() string
    18  	Size() int64
    19  }
    20  
    21  // A CommonType holds fields common to multiple types.
    22  // If a field is not known or not applicable for a given type,
    23  // the zero value is used.
    24  type CommonType struct {
    25  	ByteSize int64  // size of value of this type, in bytes
    26  	Name     string // name that can be used to refer to type
    27  }
    28  
    29  func (c *CommonType) Common() *CommonType { return c }
    30  
    31  func (c *CommonType) Size() int64 { return c.ByteSize }
    32  
    33  // Basic types
    34  
    35  // A BasicType holds fields common to all basic types.
    36  type BasicType struct {
    37  	CommonType
    38  	BitSize   int64
    39  	BitOffset int64
    40  }
    41  
    42  func (b *BasicType) Basic() *BasicType { return b }
    43  
    44  func (t *BasicType) String() string {
    45  	if t.Name != "" {
    46  		return t.Name
    47  	}
    48  	return "?"
    49  }
    50  
    51  // A CharType represents a signed character type.
    52  type CharType struct {
    53  	BasicType
    54  }
    55  
    56  // A UcharType represents an unsigned character type.
    57  type UcharType struct {
    58  	BasicType
    59  }
    60  
    61  // An IntType represents a signed integer type.
    62  type IntType struct {
    63  	BasicType
    64  }
    65  
    66  // A UintType represents an unsigned integer type.
    67  type UintType struct {
    68  	BasicType
    69  }
    70  
    71  // A FloatType represents a floating point type.
    72  type FloatType struct {
    73  	BasicType
    74  }
    75  
    76  // A ComplexType represents a complex floating point type.
    77  type ComplexType struct {
    78  	BasicType
    79  }
    80  
    81  // A BoolType represents a boolean type.
    82  type BoolType struct {
    83  	BasicType
    84  }
    85  
    86  // An AddrType represents a machine address type.
    87  type AddrType struct {
    88  	BasicType
    89  }
    90  
    91  // An UnspecifiedType represents an implicit, unknown, ambiguous or nonexistent type.
    92  type UnspecifiedType struct {
    93  	BasicType
    94  }
    95  
    96  // qualifiers
    97  
    98  // A QualType represents a type that has the C/C++ "const", "restrict", or "volatile" qualifier.
    99  type QualType struct {
   100  	CommonType
   101  	Qual string
   102  	Type Type
   103  }
   104  
   105  func (t *QualType) String() string { return t.Qual + " " + t.Type.String() }
   106  
   107  func (t *QualType) Size() int64 { return t.Type.Size() }
   108  
   109  // An ArrayType represents a fixed size array type.
   110  type ArrayType struct {
   111  	CommonType
   112  	Type          Type
   113  	StrideBitSize int64 // if > 0, number of bits to hold each element
   114  	Count         int64 // if == -1, an incomplete array, like char x[].
   115  }
   116  
   117  func (t *ArrayType) String() string {
   118  	return "[" + strconv.FormatInt(t.Count, 10) + "]" + t.Type.String()
   119  }
   120  
   121  func (t *ArrayType) Size() int64 {
   122  	if t.Count == -1 {
   123  		return 0
   124  	}
   125  	return t.Count * t.Type.Size()
   126  }
   127  
   128  // A VoidType represents the C void type.
   129  type VoidType struct {
   130  	CommonType
   131  }
   132  
   133  func (t *VoidType) String() string { return "void" }
   134  
   135  // A PtrType represents a pointer type.
   136  type PtrType struct {
   137  	CommonType
   138  	Type Type
   139  }
   140  
   141  func (t *PtrType) String() string { return "*" + t.Type.String() }
   142  
   143  // A StructType represents a struct, union, or C++ class type.
   144  type StructType struct {
   145  	CommonType
   146  	StructName string
   147  	Kind       string // "struct", "union", or "class".
   148  	Field      []*StructField
   149  	Incomplete bool // if true, struct, union, class is declared but not defined
   150  }
   151  
   152  // A StructField represents a field in a struct, union, or C++ class type.
   153  type StructField struct {
   154  	Name       string
   155  	Type       Type
   156  	ByteOffset int64
   157  	ByteSize   int64 // usually zero; use Type.Size() for normal fields
   158  	BitOffset  int64 // within the ByteSize bytes at ByteOffset
   159  	BitSize    int64 // zero if not a bit field
   160  }
   161  
   162  func (t *StructType) String() string {
   163  	if t.StructName != "" {
   164  		return t.Kind + " " + t.StructName
   165  	}
   166  	return t.Defn()
   167  }
   168  
   169  func (t *StructType) Defn() string {
   170  	s := t.Kind
   171  	if t.StructName != "" {
   172  		s += " " + t.StructName
   173  	}
   174  	if t.Incomplete {
   175  		s += " /*incomplete*/"
   176  		return s
   177  	}
   178  	s += " {"
   179  	for i, f := range t.Field {
   180  		if i > 0 {
   181  			s += "; "
   182  		}
   183  		s += f.Name + " " + f.Type.String()
   184  		s += "@" + strconv.FormatInt(f.ByteOffset, 10)
   185  		if f.BitSize > 0 {
   186  			s += " : " + strconv.FormatInt(f.BitSize, 10)
   187  			s += "@" + strconv.FormatInt(f.BitOffset, 10)
   188  		}
   189  	}
   190  	s += "}"
   191  	return s
   192  }
   193  
   194  // An EnumType represents an enumerated type.
   195  // The only indication of its native integer type is its ByteSize
   196  // (inside CommonType).
   197  type EnumType struct {
   198  	CommonType
   199  	EnumName string
   200  	Val      []*EnumValue
   201  }
   202  
   203  // An EnumValue represents a single enumeration value.
   204  type EnumValue struct {
   205  	Name string
   206  	Val  int64
   207  }
   208  
   209  func (t *EnumType) String() string {
   210  	s := "enum"
   211  	if t.EnumName != "" {
   212  		s += " " + t.EnumName
   213  	}
   214  	s += " {"
   215  	for i, v := range t.Val {
   216  		if i > 0 {
   217  			s += "; "
   218  		}
   219  		s += v.Name + "=" + strconv.FormatInt(v.Val, 10)
   220  	}
   221  	s += "}"
   222  	return s
   223  }
   224  
   225  // A FuncType represents a function type.
   226  type FuncType struct {
   227  	CommonType
   228  	ReturnType Type
   229  	ParamType  []Type
   230  }
   231  
   232  func (t *FuncType) String() string {
   233  	s := "func("
   234  	for i, t := range t.ParamType {
   235  		if i > 0 {
   236  			s += ", "
   237  		}
   238  		s += t.String()
   239  	}
   240  	s += ")"
   241  	if t.ReturnType != nil {
   242  		s += " " + t.ReturnType.String()
   243  	}
   244  	return s
   245  }
   246  
   247  // A DotDotDotType represents the variadic ... function parameter.
   248  type DotDotDotType struct {
   249  	CommonType
   250  }
   251  
   252  func (t *DotDotDotType) String() string { return "..." }
   253  
   254  // A TypedefType represents a named type.
   255  type TypedefType struct {
   256  	CommonType
   257  	Type Type
   258  }
   259  
   260  func (t *TypedefType) String() string { return t.Name }
   261  
   262  func (t *TypedefType) Size() int64 { return t.Type.Size() }
   263  
   264  // typeReader is used to read from either the info section or the
   265  // types section.
   266  type typeReader interface {
   267  	Seek(Offset)
   268  	Next() (*Entry, error)
   269  	clone() typeReader
   270  	offset() Offset
   271  	// AddressSize returns the size in bytes of addresses in the current
   272  	// compilation unit.
   273  	AddressSize() int
   274  }
   275  
   276  // Type reads the type at off in the DWARF ``info'' section.
   277  func (d *Data) Type(off Offset) (Type, error) {
   278  	return d.readType("info", d.Reader(), off, d.typeCache, nil)
   279  }
   280  
   281  // readType reads a type from r at off of name. It adds types to the
   282  // type cache, appends new typedef types to typedefs, and computes the
   283  // sizes of types. Callers should pass nil for typedefs; this is used
   284  // for internal recursion.
   285  func (d *Data) readType(name string, r typeReader, off Offset, typeCache map[Offset]Type, typedefs *[]*TypedefType) (Type, error) {
   286  	if t, ok := typeCache[off]; ok {
   287  		return t, nil
   288  	}
   289  	r.Seek(off)
   290  	e, err := r.Next()
   291  	if err != nil {
   292  		return nil, err
   293  	}
   294  	addressSize := r.AddressSize()
   295  	if e == nil || e.Offset != off {
   296  		return nil, DecodeError{name, off, "no type at offset"}
   297  	}
   298  
   299  	// If this is the root of the recursion, prepare to resolve
   300  	// typedef sizes once the recursion is done. This must be done
   301  	// after the type graph is constructed because it may need to
   302  	// resolve cycles in a different order than readType
   303  	// encounters them.
   304  	if typedefs == nil {
   305  		var typedefList []*TypedefType
   306  		defer func() {
   307  			for _, t := range typedefList {
   308  				t.Common().ByteSize = t.Type.Size()
   309  			}
   310  		}()
   311  		typedefs = &typedefList
   312  	}
   313  
   314  	// Parse type from Entry.
   315  	// Must always set typeCache[off] before calling
   316  	// d.readType recursively, to handle circular types correctly.
   317  	var typ Type
   318  
   319  	nextDepth := 0
   320  
   321  	// Get next child; set err if error happens.
   322  	next := func() *Entry {
   323  		if !e.Children {
   324  			return nil
   325  		}
   326  		// Only return direct children.
   327  		// Skip over composite entries that happen to be nested
   328  		// inside this one. Most DWARF generators wouldn't generate
   329  		// such a thing, but clang does.
   330  		// See golang.org/issue/6472.
   331  		for {
   332  			kid, err1 := r.Next()
   333  			if err1 != nil {
   334  				err = err1
   335  				return nil
   336  			}
   337  			if kid == nil {
   338  				err = DecodeError{name, r.offset(), "unexpected end of DWARF entries"}
   339  				return nil
   340  			}
   341  			if kid.Tag == 0 {
   342  				if nextDepth > 0 {
   343  					nextDepth--
   344  					continue
   345  				}
   346  				return nil
   347  			}
   348  			if kid.Children {
   349  				nextDepth++
   350  			}
   351  			if nextDepth > 0 {
   352  				continue
   353  			}
   354  			return kid
   355  		}
   356  	}
   357  
   358  	// Get Type referred to by Entry's AttrType field.
   359  	// Set err if error happens. Not having a type is an error.
   360  	typeOf := func(e *Entry) Type {
   361  		tval := e.Val(AttrType)
   362  		var t Type
   363  		switch toff := tval.(type) {
   364  		case Offset:
   365  			if t, err = d.readType(name, r.clone(), toff, typeCache, typedefs); err != nil {
   366  				return nil
   367  			}
   368  		case uint64:
   369  			if t, err = d.sigToType(toff); err != nil {
   370  				return nil
   371  			}
   372  		default:
   373  			// It appears that no Type means "void".
   374  			return new(VoidType)
   375  		}
   376  		return t
   377  	}
   378  
   379  	switch e.Tag {
   380  	case TagArrayType:
   381  		// Multi-dimensional array.  (DWARF v2 §5.4)
   382  		// Attributes:
   383  		//	AttrType:subtype [required]
   384  		//	AttrStrideSize: size in bits of each element of the array
   385  		//	AttrByteSize: size of entire array
   386  		// Children:
   387  		//	TagSubrangeType or TagEnumerationType giving one dimension.
   388  		//	dimensions are in left to right order.
   389  		t := new(ArrayType)
   390  		typ = t
   391  		typeCache[off] = t
   392  		if t.Type = typeOf(e); err != nil {
   393  			goto Error
   394  		}
   395  		t.StrideBitSize, _ = e.Val(AttrStrideSize).(int64)
   396  
   397  		// Accumulate dimensions,
   398  		var dims []int64
   399  		for kid := next(); kid != nil; kid = next() {
   400  			// TODO(rsc): Can also be TagEnumerationType
   401  			// but haven't seen that in the wild yet.
   402  			switch kid.Tag {
   403  			case TagSubrangeType:
   404  				count, ok := kid.Val(AttrCount).(int64)
   405  				if !ok {
   406  					// Old binaries may have an upper bound instead.
   407  					count, ok = kid.Val(AttrUpperBound).(int64)
   408  					if ok {
   409  						count++ // Length is one more than upper bound.
   410  					} else if len(dims) == 0 {
   411  						count = -1 // As in x[].
   412  					}
   413  				}
   414  				dims = append(dims, count)
   415  			case TagEnumerationType:
   416  				err = DecodeError{name, kid.Offset, "cannot handle enumeration type as array bound"}
   417  				goto Error
   418  			}
   419  		}
   420  		if len(dims) == 0 {
   421  			// LLVM generates this for x[].
   422  			dims = []int64{-1}
   423  		}
   424  
   425  		t.Count = dims[0]
   426  		for i := len(dims) - 1; i >= 1; i-- {
   427  			t.Type = &ArrayType{Type: t.Type, Count: dims[i]}
   428  		}
   429  
   430  	case TagBaseType:
   431  		// Basic type.  (DWARF v2 §5.1)
   432  		// Attributes:
   433  		//	AttrName: name of base type in programming language of the compilation unit [required]
   434  		//	AttrEncoding: encoding value for type (encFloat etc) [required]
   435  		//	AttrByteSize: size of type in bytes [required]
   436  		//	AttrBitOffset: for sub-byte types, size in bits
   437  		//	AttrBitSize: for sub-byte types, bit offset of high order bit in the AttrByteSize bytes
   438  		name, _ := e.Val(AttrName).(string)
   439  		enc, ok := e.Val(AttrEncoding).(int64)
   440  		if !ok {
   441  			err = DecodeError{name, e.Offset, "missing encoding attribute for " + name}
   442  			goto Error
   443  		}
   444  		switch enc {
   445  		default:
   446  			err = DecodeError{name, e.Offset, "unrecognized encoding attribute value"}
   447  			goto Error
   448  
   449  		case encAddress:
   450  			typ = new(AddrType)
   451  		case encBoolean:
   452  			typ = new(BoolType)
   453  		case encComplexFloat:
   454  			typ = new(ComplexType)
   455  			if name == "complex" {
   456  				// clang writes out 'complex' instead of 'complex float' or 'complex double'.
   457  				// clang also writes out a byte size that we can use to distinguish.
   458  				// See issue 8694.
   459  				switch byteSize, _ := e.Val(AttrByteSize).(int64); byteSize {
   460  				case 8:
   461  					name = "complex float"
   462  				case 16:
   463  					name = "complex double"
   464  				}
   465  			}
   466  		case encFloat:
   467  			typ = new(FloatType)
   468  		case encSigned:
   469  			typ = new(IntType)
   470  		case encUnsigned:
   471  			typ = new(UintType)
   472  		case encSignedChar:
   473  			typ = new(CharType)
   474  		case encUnsignedChar:
   475  			typ = new(UcharType)
   476  		}
   477  		typeCache[off] = typ
   478  		t := typ.(interface {
   479  			Basic() *BasicType
   480  		}).Basic()
   481  		t.Name = name
   482  		t.BitSize, _ = e.Val(AttrBitSize).(int64)
   483  		t.BitOffset, _ = e.Val(AttrBitOffset).(int64)
   484  
   485  	case TagClassType, TagStructType, TagUnionType:
   486  		// Structure, union, or class type.  (DWARF v2 §5.5)
   487  		// Attributes:
   488  		//	AttrName: name of struct, union, or class
   489  		//	AttrByteSize: byte size [required]
   490  		//	AttrDeclaration: if true, struct/union/class is incomplete
   491  		// Children:
   492  		//	TagMember to describe one member.
   493  		//		AttrName: name of member [required]
   494  		//		AttrType: type of member [required]
   495  		//		AttrByteSize: size in bytes
   496  		//		AttrBitOffset: bit offset within bytes for bit fields
   497  		//		AttrBitSize: bit size for bit fields
   498  		//		AttrDataMemberLoc: location within struct [required for struct, class]
   499  		// There is much more to handle C++, all ignored for now.
   500  		t := new(StructType)
   501  		typ = t
   502  		typeCache[off] = t
   503  		switch e.Tag {
   504  		case TagClassType:
   505  			t.Kind = "class"
   506  		case TagStructType:
   507  			t.Kind = "struct"
   508  		case TagUnionType:
   509  			t.Kind = "union"
   510  		}
   511  		t.StructName, _ = e.Val(AttrName).(string)
   512  		t.Incomplete = e.Val(AttrDeclaration) != nil
   513  		t.Field = make([]*StructField, 0, 8)
   514  		var lastFieldType *Type
   515  		var lastFieldBitOffset int64
   516  		for kid := next(); kid != nil; kid = next() {
   517  			if kid.Tag != TagMember {
   518  				continue
   519  			}
   520  			f := new(StructField)
   521  			if f.Type = typeOf(kid); err != nil {
   522  				goto Error
   523  			}
   524  			switch loc := kid.Val(AttrDataMemberLoc).(type) {
   525  			case []byte:
   526  				// TODO: Should have original compilation
   527  				// unit here, not unknownFormat.
   528  				b := makeBuf(d, unknownFormat{}, "location", 0, loc)
   529  				if b.uint8() != opPlusUconst {
   530  					err = DecodeError{name, kid.Offset, "unexpected opcode"}
   531  					goto Error
   532  				}
   533  				f.ByteOffset = int64(b.uint())
   534  				if b.err != nil {
   535  					err = b.err
   536  					goto Error
   537  				}
   538  			case int64:
   539  				f.ByteOffset = loc
   540  			}
   541  
   542  			haveBitOffset := false
   543  			f.Name, _ = kid.Val(AttrName).(string)
   544  			f.ByteSize, _ = kid.Val(AttrByteSize).(int64)
   545  			f.BitOffset, haveBitOffset = kid.Val(AttrBitOffset).(int64)
   546  			f.BitSize, _ = kid.Val(AttrBitSize).(int64)
   547  			t.Field = append(t.Field, f)
   548  
   549  			bito := f.BitOffset
   550  			if !haveBitOffset {
   551  				bito = f.ByteOffset * 8
   552  			}
   553  			if bito == lastFieldBitOffset && t.Kind != "union" {
   554  				// Last field was zero width. Fix array length.
   555  				// (DWARF writes out 0-length arrays as if they were 1-length arrays.)
   556  				zeroArray(lastFieldType)
   557  			}
   558  			lastFieldType = &f.Type
   559  			lastFieldBitOffset = bito
   560  		}
   561  		if t.Kind != "union" {
   562  			b, ok := e.Val(AttrByteSize).(int64)
   563  			if ok && b*8 == lastFieldBitOffset {
   564  				// Final field must be zero width. Fix array length.
   565  				zeroArray(lastFieldType)
   566  			}
   567  		}
   568  
   569  	case TagConstType, TagVolatileType, TagRestrictType:
   570  		// Type modifier (DWARF v2 §5.2)
   571  		// Attributes:
   572  		//	AttrType: subtype
   573  		t := new(QualType)
   574  		typ = t
   575  		typeCache[off] = t
   576  		if t.Type = typeOf(e); err != nil {
   577  			goto Error
   578  		}
   579  		switch e.Tag {
   580  		case TagConstType:
   581  			t.Qual = "const"
   582  		case TagRestrictType:
   583  			t.Qual = "restrict"
   584  		case TagVolatileType:
   585  			t.Qual = "volatile"
   586  		}
   587  
   588  	case TagEnumerationType:
   589  		// Enumeration type (DWARF v2 §5.6)
   590  		// Attributes:
   591  		//	AttrName: enum name if any
   592  		//	AttrByteSize: bytes required to represent largest value
   593  		// Children:
   594  		//	TagEnumerator:
   595  		//		AttrName: name of constant
   596  		//		AttrConstValue: value of constant
   597  		t := new(EnumType)
   598  		typ = t
   599  		typeCache[off] = t
   600  		t.EnumName, _ = e.Val(AttrName).(string)
   601  		t.Val = make([]*EnumValue, 0, 8)
   602  		for kid := next(); kid != nil; kid = next() {
   603  			if kid.Tag == TagEnumerator {
   604  				f := new(EnumValue)
   605  				f.Name, _ = kid.Val(AttrName).(string)
   606  				f.Val, _ = kid.Val(AttrConstValue).(int64)
   607  				n := len(t.Val)
   608  				if n >= cap(t.Val) {
   609  					val := make([]*EnumValue, n, n*2)
   610  					copy(val, t.Val)
   611  					t.Val = val
   612  				}
   613  				t.Val = t.Val[0 : n+1]
   614  				t.Val[n] = f
   615  			}
   616  		}
   617  
   618  	case TagPointerType:
   619  		// Type modifier (DWARF v2 §5.2)
   620  		// Attributes:
   621  		//	AttrType: subtype [not required!  void* has no AttrType]
   622  		//	AttrAddrClass: address class [ignored]
   623  		t := new(PtrType)
   624  		typ = t
   625  		typeCache[off] = t
   626  		if e.Val(AttrType) == nil {
   627  			t.Type = &VoidType{}
   628  			break
   629  		}
   630  		t.Type = typeOf(e)
   631  
   632  	case TagSubroutineType:
   633  		// Subroutine type.  (DWARF v2 §5.7)
   634  		// Attributes:
   635  		//	AttrType: type of return value if any
   636  		//	AttrName: possible name of type [ignored]
   637  		//	AttrPrototyped: whether used ANSI C prototype [ignored]
   638  		// Children:
   639  		//	TagFormalParameter: typed parameter
   640  		//		AttrType: type of parameter
   641  		//	TagUnspecifiedParameter: final ...
   642  		t := new(FuncType)
   643  		typ = t
   644  		typeCache[off] = t
   645  		if t.ReturnType = typeOf(e); err != nil {
   646  			goto Error
   647  		}
   648  		t.ParamType = make([]Type, 0, 8)
   649  		for kid := next(); kid != nil; kid = next() {
   650  			var tkid Type
   651  			switch kid.Tag {
   652  			default:
   653  				continue
   654  			case TagFormalParameter:
   655  				if tkid = typeOf(kid); err != nil {
   656  					goto Error
   657  				}
   658  			case TagUnspecifiedParameters:
   659  				tkid = &DotDotDotType{}
   660  			}
   661  			t.ParamType = append(t.ParamType, tkid)
   662  		}
   663  
   664  	case TagTypedef:
   665  		// Typedef (DWARF v2 §5.3)
   666  		// Attributes:
   667  		//	AttrName: name [required]
   668  		//	AttrType: type definition [required]
   669  		t := new(TypedefType)
   670  		typ = t
   671  		typeCache[off] = t
   672  		t.Name, _ = e.Val(AttrName).(string)
   673  		t.Type = typeOf(e)
   674  
   675  	case TagUnspecifiedType:
   676  		// Unspecified type (DWARF v3 §5.2)
   677  		// Attributes:
   678  		//	AttrName: name
   679  		t := new(UnspecifiedType)
   680  		typ = t
   681  		typeCache[off] = t
   682  		t.Name, _ = e.Val(AttrName).(string)
   683  	}
   684  
   685  	if err != nil {
   686  		goto Error
   687  	}
   688  
   689  	{
   690  		b, ok := e.Val(AttrByteSize).(int64)
   691  		if !ok {
   692  			b = -1
   693  			switch t := typ.(type) {
   694  			case *TypedefType:
   695  				// Record that we need to resolve this
   696  				// type's size once the type graph is
   697  				// constructed.
   698  				*typedefs = append(*typedefs, t)
   699  			case *PtrType:
   700  				b = int64(addressSize)
   701  			}
   702  		}
   703  		typ.Common().ByteSize = b
   704  	}
   705  	return typ, nil
   706  
   707  Error:
   708  	// If the parse fails, take the type out of the cache
   709  	// so that the next call with this offset doesn't hit
   710  	// the cache and return success.
   711  	delete(typeCache, off)
   712  	return nil, err
   713  }
   714  
   715  func zeroArray(t *Type) {
   716  	if t == nil {
   717  		return
   718  	}
   719  	at, ok := (*t).(*ArrayType)
   720  	if !ok || at.Type.Size() == 0 {
   721  		return
   722  	}
   723  	// Make a copy to avoid invalidating typeCache.
   724  	tt := *at
   725  	tt.Count = 0
   726  	*t = &tt
   727  }
   728  

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