Source file src/runtime/type.go

Documentation: runtime

     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  // Runtime type representation.
     6  
     7  package runtime
     8  
     9  import "unsafe"
    10  
    11  // tflag is documented in reflect/type.go.
    12  //
    13  // tflag values must be kept in sync with copies in:
    14  //	cmd/compile/internal/gc/reflect.go
    15  //	cmd/link/internal/ld/decodesym.go
    16  //	reflect/type.go
    17  //      internal/reflectlite/type.go
    18  type tflag uint8
    19  
    20  const (
    21  	tflagUncommon      tflag = 1 << 0
    22  	tflagExtraStar     tflag = 1 << 1
    23  	tflagNamed         tflag = 1 << 2
    24  	tflagRegularMemory tflag = 1 << 3 // equal and hash can treat values of this type as a single region of t.size bytes
    25  )
    26  
    27  // Needs to be in sync with ../cmd/link/internal/ld/decodesym.go:/^func.commonsize,
    28  // ../cmd/compile/internal/gc/reflect.go:/^func.dcommontype and
    29  // ../reflect/type.go:/^type.rtype.
    30  // ../internal/reflectlite/type.go:/^type.rtype.
    31  type _type struct {
    32  	size       uintptr
    33  	ptrdata    uintptr // size of memory prefix holding all pointers
    34  	hash       uint32
    35  	tflag      tflag
    36  	align      uint8
    37  	fieldAlign uint8
    38  	kind       uint8
    39  	// function for comparing objects of this type
    40  	// (ptr to object A, ptr to object B) -> ==?
    41  	equal func(unsafe.Pointer, unsafe.Pointer) bool
    42  	// gcdata stores the GC type data for the garbage collector.
    43  	// If the KindGCProg bit is set in kind, gcdata is a GC program.
    44  	// Otherwise it is a ptrmask bitmap. See mbitmap.go for details.
    45  	gcdata    *byte
    46  	str       nameOff
    47  	ptrToThis typeOff
    48  }
    49  
    50  func (t *_type) string() string {
    51  	s := t.nameOff(t.str).name()
    52  	if t.tflag&tflagExtraStar != 0 {
    53  		return s[1:]
    54  	}
    55  	return s
    56  }
    57  
    58  func (t *_type) uncommon() *uncommontype {
    59  	if t.tflag&tflagUncommon == 0 {
    60  		return nil
    61  	}
    62  	switch t.kind & kindMask {
    63  	case kindStruct:
    64  		type u struct {
    65  			structtype
    66  			u uncommontype
    67  		}
    68  		return &(*u)(unsafe.Pointer(t)).u
    69  	case kindPtr:
    70  		type u struct {
    71  			ptrtype
    72  			u uncommontype
    73  		}
    74  		return &(*u)(unsafe.Pointer(t)).u
    75  	case kindFunc:
    76  		type u struct {
    77  			functype
    78  			u uncommontype
    79  		}
    80  		return &(*u)(unsafe.Pointer(t)).u
    81  	case kindSlice:
    82  		type u struct {
    83  			slicetype
    84  			u uncommontype
    85  		}
    86  		return &(*u)(unsafe.Pointer(t)).u
    87  	case kindArray:
    88  		type u struct {
    89  			arraytype
    90  			u uncommontype
    91  		}
    92  		return &(*u)(unsafe.Pointer(t)).u
    93  	case kindChan:
    94  		type u struct {
    95  			chantype
    96  			u uncommontype
    97  		}
    98  		return &(*u)(unsafe.Pointer(t)).u
    99  	case kindMap:
   100  		type u struct {
   101  			maptype
   102  			u uncommontype
   103  		}
   104  		return &(*u)(unsafe.Pointer(t)).u
   105  	case kindInterface:
   106  		type u struct {
   107  			interfacetype
   108  			u uncommontype
   109  		}
   110  		return &(*u)(unsafe.Pointer(t)).u
   111  	default:
   112  		type u struct {
   113  			_type
   114  			u uncommontype
   115  		}
   116  		return &(*u)(unsafe.Pointer(t)).u
   117  	}
   118  }
   119  
   120  func (t *_type) name() string {
   121  	if t.tflag&tflagNamed == 0 {
   122  		return ""
   123  	}
   124  	s := t.string()
   125  	i := len(s) - 1
   126  	for i >= 0 && s[i] != '.' {
   127  		i--
   128  	}
   129  	return s[i+1:]
   130  }
   131  
   132  // pkgpath returns the path of the package where t was defined, if
   133  // available. This is not the same as the reflect package's PkgPath
   134  // method, in that it returns the package path for struct and interface
   135  // types, not just named types.
   136  func (t *_type) pkgpath() string {
   137  	if u := t.uncommon(); u != nil {
   138  		return t.nameOff(u.pkgpath).name()
   139  	}
   140  	switch t.kind & kindMask {
   141  	case kindStruct:
   142  		st := (*structtype)(unsafe.Pointer(t))
   143  		return st.pkgPath.name()
   144  	case kindInterface:
   145  		it := (*interfacetype)(unsafe.Pointer(t))
   146  		return it.pkgpath.name()
   147  	}
   148  	return ""
   149  }
   150  
   151  // reflectOffs holds type offsets defined at run time by the reflect package.
   152  //
   153  // When a type is defined at run time, its *rtype data lives on the heap.
   154  // There are a wide range of possible addresses the heap may use, that
   155  // may not be representable as a 32-bit offset. Moreover the GC may
   156  // one day start moving heap memory, in which case there is no stable
   157  // offset that can be defined.
   158  //
   159  // To provide stable offsets, we add pin *rtype objects in a global map
   160  // and treat the offset as an identifier. We use negative offsets that
   161  // do not overlap with any compile-time module offsets.
   162  //
   163  // Entries are created by reflect.addReflectOff.
   164  var reflectOffs struct {
   165  	lock mutex
   166  	next int32
   167  	m    map[int32]unsafe.Pointer
   168  	minv map[unsafe.Pointer]int32
   169  }
   170  
   171  func reflectOffsLock() {
   172  	lock(&reflectOffs.lock)
   173  	if raceenabled {
   174  		raceacquire(unsafe.Pointer(&reflectOffs.lock))
   175  	}
   176  }
   177  
   178  func reflectOffsUnlock() {
   179  	if raceenabled {
   180  		racerelease(unsafe.Pointer(&reflectOffs.lock))
   181  	}
   182  	unlock(&reflectOffs.lock)
   183  }
   184  
   185  func resolveNameOff(ptrInModule unsafe.Pointer, off nameOff) name {
   186  	if off == 0 {
   187  		return name{}
   188  	}
   189  	base := uintptr(ptrInModule)
   190  	for md := &firstmoduledata; md != nil; md = md.next {
   191  		if base >= md.types && base < md.etypes {
   192  			res := md.types + uintptr(off)
   193  			if res > md.etypes {
   194  				println("runtime: nameOff", hex(off), "out of range", hex(md.types), "-", hex(md.etypes))
   195  				throw("runtime: name offset out of range")
   196  			}
   197  			return name{(*byte)(unsafe.Pointer(res))}
   198  		}
   199  	}
   200  
   201  	// No module found. see if it is a run time name.
   202  	reflectOffsLock()
   203  	res, found := reflectOffs.m[int32(off)]
   204  	reflectOffsUnlock()
   205  	if !found {
   206  		println("runtime: nameOff", hex(off), "base", hex(base), "not in ranges:")
   207  		for next := &firstmoduledata; next != nil; next = next.next {
   208  			println("\ttypes", hex(next.types), "etypes", hex(next.etypes))
   209  		}
   210  		throw("runtime: name offset base pointer out of range")
   211  	}
   212  	return name{(*byte)(res)}
   213  }
   214  
   215  func (t *_type) nameOff(off nameOff) name {
   216  	return resolveNameOff(unsafe.Pointer(t), off)
   217  }
   218  
   219  func resolveTypeOff(ptrInModule unsafe.Pointer, off typeOff) *_type {
   220  	if off == 0 {
   221  		return nil
   222  	}
   223  	base := uintptr(ptrInModule)
   224  	var md *moduledata
   225  	for next := &firstmoduledata; next != nil; next = next.next {
   226  		if base >= next.types && base < next.etypes {
   227  			md = next
   228  			break
   229  		}
   230  	}
   231  	if md == nil {
   232  		reflectOffsLock()
   233  		res := reflectOffs.m[int32(off)]
   234  		reflectOffsUnlock()
   235  		if res == nil {
   236  			println("runtime: typeOff", hex(off), "base", hex(base), "not in ranges:")
   237  			for next := &firstmoduledata; next != nil; next = next.next {
   238  				println("\ttypes", hex(next.types), "etypes", hex(next.etypes))
   239  			}
   240  			throw("runtime: type offset base pointer out of range")
   241  		}
   242  		return (*_type)(res)
   243  	}
   244  	if t := md.typemap[off]; t != nil {
   245  		return t
   246  	}
   247  	res := md.types + uintptr(off)
   248  	if res > md.etypes {
   249  		println("runtime: typeOff", hex(off), "out of range", hex(md.types), "-", hex(md.etypes))
   250  		throw("runtime: type offset out of range")
   251  	}
   252  	return (*_type)(unsafe.Pointer(res))
   253  }
   254  
   255  func (t *_type) typeOff(off typeOff) *_type {
   256  	return resolveTypeOff(unsafe.Pointer(t), off)
   257  }
   258  
   259  func (t *_type) textOff(off textOff) unsafe.Pointer {
   260  	base := uintptr(unsafe.Pointer(t))
   261  	var md *moduledata
   262  	for next := &firstmoduledata; next != nil; next = next.next {
   263  		if base >= next.types && base < next.etypes {
   264  			md = next
   265  			break
   266  		}
   267  	}
   268  	if md == nil {
   269  		reflectOffsLock()
   270  		res := reflectOffs.m[int32(off)]
   271  		reflectOffsUnlock()
   272  		if res == nil {
   273  			println("runtime: textOff", hex(off), "base", hex(base), "not in ranges:")
   274  			for next := &firstmoduledata; next != nil; next = next.next {
   275  				println("\ttypes", hex(next.types), "etypes", hex(next.etypes))
   276  			}
   277  			throw("runtime: text offset base pointer out of range")
   278  		}
   279  		return res
   280  	}
   281  	res := uintptr(0)
   282  
   283  	// The text, or instruction stream is generated as one large buffer.  The off (offset) for a method is
   284  	// its offset within this buffer.  If the total text size gets too large, there can be issues on platforms like ppc64 if
   285  	// the target of calls are too far for the call instruction.  To resolve the large text issue, the text is split
   286  	// into multiple text sections to allow the linker to generate long calls when necessary.  When this happens, the vaddr
   287  	// for each text section is set to its offset within the text.  Each method's offset is compared against the section
   288  	// vaddrs and sizes to determine the containing section.  Then the section relative offset is added to the section's
   289  	// relocated baseaddr to compute the method addess.
   290  
   291  	if len(md.textsectmap) > 1 {
   292  		for i := range md.textsectmap {
   293  			sectaddr := md.textsectmap[i].vaddr
   294  			sectlen := md.textsectmap[i].length
   295  			if uintptr(off) >= sectaddr && uintptr(off) < sectaddr+sectlen {
   296  				res = md.textsectmap[i].baseaddr + uintptr(off) - uintptr(md.textsectmap[i].vaddr)
   297  				break
   298  			}
   299  		}
   300  	} else {
   301  		// single text section
   302  		res = md.text + uintptr(off)
   303  	}
   304  
   305  	if res > md.etext && GOARCH != "wasm" { // on wasm, functions do not live in the same address space as the linear memory
   306  		println("runtime: textOff", hex(off), "out of range", hex(md.text), "-", hex(md.etext))
   307  		throw("runtime: text offset out of range")
   308  	}
   309  	return unsafe.Pointer(res)
   310  }
   311  
   312  func (t *functype) in() []*_type {
   313  	// See funcType in reflect/type.go for details on data layout.
   314  	uadd := uintptr(unsafe.Sizeof(functype{}))
   315  	if t.typ.tflag&tflagUncommon != 0 {
   316  		uadd += unsafe.Sizeof(uncommontype{})
   317  	}
   318  	return (*[1 << 20]*_type)(add(unsafe.Pointer(t), uadd))[:t.inCount]
   319  }
   320  
   321  func (t *functype) out() []*_type {
   322  	// See funcType in reflect/type.go for details on data layout.
   323  	uadd := uintptr(unsafe.Sizeof(functype{}))
   324  	if t.typ.tflag&tflagUncommon != 0 {
   325  		uadd += unsafe.Sizeof(uncommontype{})
   326  	}
   327  	outCount := t.outCount & (1<<15 - 1)
   328  	return (*[1 << 20]*_type)(add(unsafe.Pointer(t), uadd))[t.inCount : t.inCount+outCount]
   329  }
   330  
   331  func (t *functype) dotdotdot() bool {
   332  	return t.outCount&(1<<15) != 0
   333  }
   334  
   335  type nameOff int32
   336  type typeOff int32
   337  type textOff int32
   338  
   339  type method struct {
   340  	name nameOff
   341  	mtyp typeOff
   342  	ifn  textOff
   343  	tfn  textOff
   344  }
   345  
   346  type uncommontype struct {
   347  	pkgpath nameOff
   348  	mcount  uint16 // number of methods
   349  	xcount  uint16 // number of exported methods
   350  	moff    uint32 // offset from this uncommontype to [mcount]method
   351  	_       uint32 // unused
   352  }
   353  
   354  type imethod struct {
   355  	name nameOff
   356  	ityp typeOff
   357  }
   358  
   359  type interfacetype struct {
   360  	typ     _type
   361  	pkgpath name
   362  	mhdr    []imethod
   363  }
   364  
   365  type maptype struct {
   366  	typ    _type
   367  	key    *_type
   368  	elem   *_type
   369  	bucket *_type // internal type representing a hash bucket
   370  	// function for hashing keys (ptr to key, seed) -> hash
   371  	hasher     func(unsafe.Pointer, uintptr) uintptr
   372  	keysize    uint8  // size of key slot
   373  	elemsize   uint8  // size of elem slot
   374  	bucketsize uint16 // size of bucket
   375  	flags      uint32
   376  }
   377  
   378  // Note: flag values must match those used in the TMAP case
   379  // in ../cmd/compile/internal/gc/reflect.go:dtypesym.
   380  func (mt *maptype) indirectkey() bool { // store ptr to key instead of key itself
   381  	return mt.flags&1 != 0
   382  }
   383  func (mt *maptype) indirectelem() bool { // store ptr to elem instead of elem itself
   384  	return mt.flags&2 != 0
   385  }
   386  func (mt *maptype) reflexivekey() bool { // true if k==k for all keys
   387  	return mt.flags&4 != 0
   388  }
   389  func (mt *maptype) needkeyupdate() bool { // true if we need to update key on an overwrite
   390  	return mt.flags&8 != 0
   391  }
   392  func (mt *maptype) hashMightPanic() bool { // true if hash function might panic
   393  	return mt.flags&16 != 0
   394  }
   395  
   396  type arraytype struct {
   397  	typ   _type
   398  	elem  *_type
   399  	slice *_type
   400  	len   uintptr
   401  }
   402  
   403  type chantype struct {
   404  	typ  _type
   405  	elem *_type
   406  	dir  uintptr
   407  }
   408  
   409  type slicetype struct {
   410  	typ  _type
   411  	elem *_type
   412  }
   413  
   414  type functype struct {
   415  	typ      _type
   416  	inCount  uint16
   417  	outCount uint16
   418  }
   419  
   420  type ptrtype struct {
   421  	typ  _type
   422  	elem *_type
   423  }
   424  
   425  type structfield struct {
   426  	name       name
   427  	typ        *_type
   428  	offsetAnon uintptr
   429  }
   430  
   431  func (f *structfield) offset() uintptr {
   432  	return f.offsetAnon >> 1
   433  }
   434  
   435  type structtype struct {
   436  	typ     _type
   437  	pkgPath name
   438  	fields  []structfield
   439  }
   440  
   441  // name is an encoded type name with optional extra data.
   442  // See reflect/type.go for details.
   443  type name struct {
   444  	bytes *byte
   445  }
   446  
   447  func (n name) data(off int) *byte {
   448  	return (*byte)(add(unsafe.Pointer(n.bytes), uintptr(off)))
   449  }
   450  
   451  func (n name) isExported() bool {
   452  	return (*n.bytes)&(1<<0) != 0
   453  }
   454  
   455  func (n name) nameLen() int {
   456  	return int(uint16(*n.data(1))<<8 | uint16(*n.data(2)))
   457  }
   458  
   459  func (n name) tagLen() int {
   460  	if *n.data(0)&(1<<1) == 0 {
   461  		return 0
   462  	}
   463  	off := 3 + n.nameLen()
   464  	return int(uint16(*n.data(off))<<8 | uint16(*n.data(off + 1)))
   465  }
   466  
   467  func (n name) name() (s string) {
   468  	if n.bytes == nil {
   469  		return ""
   470  	}
   471  	nl := n.nameLen()
   472  	if nl == 0 {
   473  		return ""
   474  	}
   475  	hdr := (*stringStruct)(unsafe.Pointer(&s))
   476  	hdr.str = unsafe.Pointer(n.data(3))
   477  	hdr.len = nl
   478  	return s
   479  }
   480  
   481  func (n name) tag() (s string) {
   482  	tl := n.tagLen()
   483  	if tl == 0 {
   484  		return ""
   485  	}
   486  	nl := n.nameLen()
   487  	hdr := (*stringStruct)(unsafe.Pointer(&s))
   488  	hdr.str = unsafe.Pointer(n.data(3 + nl + 2))
   489  	hdr.len = tl
   490  	return s
   491  }
   492  
   493  func (n name) pkgPath() string {
   494  	if n.bytes == nil || *n.data(0)&(1<<2) == 0 {
   495  		return ""
   496  	}
   497  	off := 3 + n.nameLen()
   498  	if tl := n.tagLen(); tl > 0 {
   499  		off += 2 + tl
   500  	}
   501  	var nameOff nameOff
   502  	copy((*[4]byte)(unsafe.Pointer(&nameOff))[:], (*[4]byte)(unsafe.Pointer(n.data(off)))[:])
   503  	pkgPathName := resolveNameOff(unsafe.Pointer(n.bytes), nameOff)
   504  	return pkgPathName.name()
   505  }
   506  
   507  func (n name) isBlank() bool {
   508  	if n.bytes == nil {
   509  		return false
   510  	}
   511  	if n.nameLen() != 1 {
   512  		return false
   513  	}
   514  	return *n.data(3) == '_'
   515  }
   516  
   517  // typelinksinit scans the types from extra modules and builds the
   518  // moduledata typemap used to de-duplicate type pointers.
   519  func typelinksinit() {
   520  	if firstmoduledata.next == nil {
   521  		return
   522  	}
   523  	typehash := make(map[uint32][]*_type, len(firstmoduledata.typelinks))
   524  
   525  	modules := activeModules()
   526  	prev := modules[0]
   527  	for _, md := range modules[1:] {
   528  		// Collect types from the previous module into typehash.
   529  	collect:
   530  		for _, tl := range prev.typelinks {
   531  			var t *_type
   532  			if prev.typemap == nil {
   533  				t = (*_type)(unsafe.Pointer(prev.types + uintptr(tl)))
   534  			} else {
   535  				t = prev.typemap[typeOff(tl)]
   536  			}
   537  			// Add to typehash if not seen before.
   538  			tlist := typehash[t.hash]
   539  			for _, tcur := range tlist {
   540  				if tcur == t {
   541  					continue collect
   542  				}
   543  			}
   544  			typehash[t.hash] = append(tlist, t)
   545  		}
   546  
   547  		if md.typemap == nil {
   548  			// If any of this module's typelinks match a type from a
   549  			// prior module, prefer that prior type by adding the offset
   550  			// to this module's typemap.
   551  			tm := make(map[typeOff]*_type, len(md.typelinks))
   552  			pinnedTypemaps = append(pinnedTypemaps, tm)
   553  			md.typemap = tm
   554  			for _, tl := range md.typelinks {
   555  				t := (*_type)(unsafe.Pointer(md.types + uintptr(tl)))
   556  				for _, candidate := range typehash[t.hash] {
   557  					seen := map[_typePair]struct{}{}
   558  					if typesEqual(t, candidate, seen) {
   559  						t = candidate
   560  						break
   561  					}
   562  				}
   563  				md.typemap[typeOff(tl)] = t
   564  			}
   565  		}
   566  
   567  		prev = md
   568  	}
   569  }
   570  
   571  type _typePair struct {
   572  	t1 *_type
   573  	t2 *_type
   574  }
   575  
   576  // typesEqual reports whether two types are equal.
   577  //
   578  // Everywhere in the runtime and reflect packages, it is assumed that
   579  // there is exactly one *_type per Go type, so that pointer equality
   580  // can be used to test if types are equal. There is one place that
   581  // breaks this assumption: buildmode=shared. In this case a type can
   582  // appear as two different pieces of memory. This is hidden from the
   583  // runtime and reflect package by the per-module typemap built in
   584  // typelinksinit. It uses typesEqual to map types from later modules
   585  // back into earlier ones.
   586  //
   587  // Only typelinksinit needs this function.
   588  func typesEqual(t, v *_type, seen map[_typePair]struct{}) bool {
   589  	tp := _typePair{t, v}
   590  	if _, ok := seen[tp]; ok {
   591  		return true
   592  	}
   593  
   594  	// mark these types as seen, and thus equivalent which prevents an infinite loop if
   595  	// the two types are identical, but recursively defined and loaded from
   596  	// different modules
   597  	seen[tp] = struct{}{}
   598  
   599  	if t == v {
   600  		return true
   601  	}
   602  	kind := t.kind & kindMask
   603  	if kind != v.kind&kindMask {
   604  		return false
   605  	}
   606  	if t.string() != v.string() {
   607  		return false
   608  	}
   609  	ut := t.uncommon()
   610  	uv := v.uncommon()
   611  	if ut != nil || uv != nil {
   612  		if ut == nil || uv == nil {
   613  			return false
   614  		}
   615  		pkgpatht := t.nameOff(ut.pkgpath).name()
   616  		pkgpathv := v.nameOff(uv.pkgpath).name()
   617  		if pkgpatht != pkgpathv {
   618  			return false
   619  		}
   620  	}
   621  	if kindBool <= kind && kind <= kindComplex128 {
   622  		return true
   623  	}
   624  	switch kind {
   625  	case kindString, kindUnsafePointer:
   626  		return true
   627  	case kindArray:
   628  		at := (*arraytype)(unsafe.Pointer(t))
   629  		av := (*arraytype)(unsafe.Pointer(v))
   630  		return typesEqual(at.elem, av.elem, seen) && at.len == av.len
   631  	case kindChan:
   632  		ct := (*chantype)(unsafe.Pointer(t))
   633  		cv := (*chantype)(unsafe.Pointer(v))
   634  		return ct.dir == cv.dir && typesEqual(ct.elem, cv.elem, seen)
   635  	case kindFunc:
   636  		ft := (*functype)(unsafe.Pointer(t))
   637  		fv := (*functype)(unsafe.Pointer(v))
   638  		if ft.outCount != fv.outCount || ft.inCount != fv.inCount {
   639  			return false
   640  		}
   641  		tin, vin := ft.in(), fv.in()
   642  		for i := 0; i < len(tin); i++ {
   643  			if !typesEqual(tin[i], vin[i], seen) {
   644  				return false
   645  			}
   646  		}
   647  		tout, vout := ft.out(), fv.out()
   648  		for i := 0; i < len(tout); i++ {
   649  			if !typesEqual(tout[i], vout[i], seen) {
   650  				return false
   651  			}
   652  		}
   653  		return true
   654  	case kindInterface:
   655  		it := (*interfacetype)(unsafe.Pointer(t))
   656  		iv := (*interfacetype)(unsafe.Pointer(v))
   657  		if it.pkgpath.name() != iv.pkgpath.name() {
   658  			return false
   659  		}
   660  		if len(it.mhdr) != len(iv.mhdr) {
   661  			return false
   662  		}
   663  		for i := range it.mhdr {
   664  			tm := &it.mhdr[i]
   665  			vm := &iv.mhdr[i]
   666  			// Note the mhdr array can be relocated from
   667  			// another module. See #17724.
   668  			tname := resolveNameOff(unsafe.Pointer(tm), tm.name)
   669  			vname := resolveNameOff(unsafe.Pointer(vm), vm.name)
   670  			if tname.name() != vname.name() {
   671  				return false
   672  			}
   673  			if tname.pkgPath() != vname.pkgPath() {
   674  				return false
   675  			}
   676  			tityp := resolveTypeOff(unsafe.Pointer(tm), tm.ityp)
   677  			vityp := resolveTypeOff(unsafe.Pointer(vm), vm.ityp)
   678  			if !typesEqual(tityp, vityp, seen) {
   679  				return false
   680  			}
   681  		}
   682  		return true
   683  	case kindMap:
   684  		mt := (*maptype)(unsafe.Pointer(t))
   685  		mv := (*maptype)(unsafe.Pointer(v))
   686  		return typesEqual(mt.key, mv.key, seen) && typesEqual(mt.elem, mv.elem, seen)
   687  	case kindPtr:
   688  		pt := (*ptrtype)(unsafe.Pointer(t))
   689  		pv := (*ptrtype)(unsafe.Pointer(v))
   690  		return typesEqual(pt.elem, pv.elem, seen)
   691  	case kindSlice:
   692  		st := (*slicetype)(unsafe.Pointer(t))
   693  		sv := (*slicetype)(unsafe.Pointer(v))
   694  		return typesEqual(st.elem, sv.elem, seen)
   695  	case kindStruct:
   696  		st := (*structtype)(unsafe.Pointer(t))
   697  		sv := (*structtype)(unsafe.Pointer(v))
   698  		if len(st.fields) != len(sv.fields) {
   699  			return false
   700  		}
   701  		if st.pkgPath.name() != sv.pkgPath.name() {
   702  			return false
   703  		}
   704  		for i := range st.fields {
   705  			tf := &st.fields[i]
   706  			vf := &sv.fields[i]
   707  			if tf.name.name() != vf.name.name() {
   708  				return false
   709  			}
   710  			if !typesEqual(tf.typ, vf.typ, seen) {
   711  				return false
   712  			}
   713  			if tf.name.tag() != vf.name.tag() {
   714  				return false
   715  			}
   716  			if tf.offsetAnon != vf.offsetAnon {
   717  				return false
   718  			}
   719  		}
   720  		return true
   721  	default:
   722  		println("runtime: impossible type kind", kind)
   723  		throw("runtime: impossible type kind")
   724  		return false
   725  	}
   726  }
   727  

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