deadcode.go

Documentation: cmd/link/internal/ld

     1  // Copyright 2019 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 ld
     6  
     7  import (
     8  	"cmd/internal/goobj"
     9  	"cmd/internal/objabi"
    10  	"cmd/internal/sys"
    11  	"cmd/link/internal/loader"
    12  	"cmd/link/internal/sym"
    13  	"fmt"
    14  	"internal/buildcfg"
    15  	"strings"
    16  	"unicode"
    17  )
    18  
    19  var _ = fmt.Print
    20  
    21  type deadcodePass struct {
    22  	ctxt *Link
    23  	ldr  *loader.Loader
    24  	wq   heap // work queue, using min-heap for better locality
    25  
    26  	ifaceMethod        map[methodsig]bool // methods called from reached interface call sites
    27  	genericIfaceMethod map[string]bool    // names of methods called from reached generic interface call sites
    28  	markableMethods    []methodref        // methods of reached types
    29  	reflectSeen        bool               // whether we have seen a reflect method call
    30  	dynlink            bool
    31  
    32  	methodsigstmp []methodsig // scratch buffer for decoding method signatures
    33  	pkginits      []loader.Sym
    34  	mapinitnoop   loader.Sym
    35  }
    36  
    37  func (d *deadcodePass) init() {
    38  	d.ldr.InitReachable()
    39  	d.ifaceMethod = make(map[methodsig]bool)
    40  	d.genericIfaceMethod = make(map[string]bool)
    41  	if buildcfg.Experiment.FieldTrack {
    42  		d.ldr.Reachparent = make([]loader.Sym, d.ldr.NSym())
    43  	}
    44  	d.dynlink = d.ctxt.DynlinkingGo()
    45  
    46  	if d.ctxt.BuildMode == BuildModeShared {
    47  		// Mark all symbols defined in this library as reachable when
    48  		// building a shared library.
    49  		n := d.ldr.NDef()
    50  		for i := 1; i < n; i++ {
    51  			s := loader.Sym(i)
    52  			d.mark(s, 0)
    53  		}
    54  		d.mark(d.ctxt.mainInittasks, 0)
    55  		return
    56  	}
    57  
    58  	var names []string
    59  
    60  	// In a normal binary, start at main.main and the init
    61  	// functions and mark what is reachable from there.
    62  	if d.ctxt.linkShared && (d.ctxt.BuildMode == BuildModeExe || d.ctxt.BuildMode == BuildModePIE) {
    63  		names = append(names, "main.main", "main..inittask")
    64  	} else {
    65  		// The external linker refers main symbol directly.
    66  		if d.ctxt.LinkMode == LinkExternal && (d.ctxt.BuildMode == BuildModeExe || d.ctxt.BuildMode == BuildModePIE) {
    67  			if d.ctxt.HeadType == objabi.Hwindows && d.ctxt.Arch.Family == sys.I386 {
    68  				*flagEntrySymbol = "_main"
    69  			} else {
    70  				*flagEntrySymbol = "main"
    71  			}
    72  		}
    73  		names = append(names, *flagEntrySymbol)
    74  	}
    75  	// runtime.unreachableMethod is a function that will throw if called.
    76  	// We redirect unreachable methods to it.
    77  	names = append(names, "runtime.unreachableMethod")
    78  	if d.ctxt.BuildMode == BuildModePlugin {
    79  		names = append(names, objabi.PathToPrefix(*flagPluginPath)+"..inittask", objabi.PathToPrefix(*flagPluginPath)+".main", "go:plugin.tabs")
    80  
    81  		// We don't keep the go.plugin.exports symbol,
    82  		// but we do keep the symbols it refers to.
    83  		exportsIdx := d.ldr.Lookup("go:plugin.exports", 0)
    84  		if exportsIdx != 0 {
    85  			relocs := d.ldr.Relocs(exportsIdx)
    86  			for i := 0; i < relocs.Count(); i++ {
    87  				d.mark(relocs.At(i).Sym(), 0)
    88  			}
    89  		}
    90  	}
    91  
    92  	if d.ctxt.Debugvlog > 1 {
    93  		d.ctxt.Logf("deadcode start names: %v\n", names)
    94  	}
    95  
    96  	for _, name := range names {
    97  		// Mark symbol as a data/ABI0 symbol.
    98  		d.mark(d.ldr.Lookup(name, 0), 0)
    99  		if abiInternalVer != 0 {
   100  			// Also mark any Go functions (internal ABI).
   101  			d.mark(d.ldr.Lookup(name, abiInternalVer), 0)
   102  		}
   103  	}
   104  
   105  	// All dynamic exports are roots.
   106  	for _, s := range d.ctxt.dynexp {
   107  		if d.ctxt.Debugvlog > 1 {
   108  			d.ctxt.Logf("deadcode start dynexp: %s<%d>\n", d.ldr.SymName(s), d.ldr.SymVersion(s))
   109  		}
   110  		d.mark(s, 0)
   111  	}
   112  
   113  	d.mapinitnoop = d.ldr.Lookup("runtime.mapinitnoop", abiInternalVer)
   114  	if d.mapinitnoop == 0 {
   115  		panic("could not look up runtime.mapinitnoop")
   116  	}
   117  	if d.ctxt.mainInittasks != 0 {
   118  		d.mark(d.ctxt.mainInittasks, 0)
   119  	}
   120  }
   121  
   122  func (d *deadcodePass) flood() {
   123  	var methods []methodref
   124  	for !d.wq.empty() {
   125  		symIdx := d.wq.pop()
   126  
   127  		// Methods may be called via reflection. Give up on static analysis,
   128  		// and mark all exported methods of all reachable types as reachable.
   129  		d.reflectSeen = d.reflectSeen || d.ldr.IsReflectMethod(symIdx)
   130  
   131  		isgotype := d.ldr.IsGoType(symIdx)
   132  		relocs := d.ldr.Relocs(symIdx)
   133  		var usedInIface bool
   134  
   135  		if isgotype {
   136  			if d.dynlink {
   137  				// When dynamic linking, a type may be passed across DSO
   138  				// boundary and get converted to interface at the other side.
   139  				d.ldr.SetAttrUsedInIface(symIdx, true)
   140  			}
   141  			usedInIface = d.ldr.AttrUsedInIface(symIdx)
   142  		}
   143  
   144  		methods = methods[:0]
   145  		for i := 0; i < relocs.Count(); i++ {
   146  			r := relocs.At(i)
   147  			if r.Weak() {
   148  				convertWeakToStrong := false
   149  				// When build with "-linkshared", we can't tell if the
   150  				// interface method in itab will be used or not.
   151  				// Ignore the weak attribute.
   152  				if d.ctxt.linkShared && d.ldr.IsItab(symIdx) {
   153  					convertWeakToStrong = true
   154  				}
   155  				// If the program uses plugins, we can no longer treat
   156  				// relocs from pkg init functions to outlined map init
   157  				// fragments as weak, since doing so can cause package
   158  				// init clashes between the main program and the
   159  				// plugin. See #62430 for more details.
   160  				if d.ctxt.canUsePlugins && r.Type().IsDirectCall() {
   161  					convertWeakToStrong = true
   162  				}
   163  				if !convertWeakToStrong {
   164  					// skip this reloc
   165  					continue
   166  				}
   167  			}
   168  			t := r.Type()
   169  			switch t {
   170  			case objabi.R_METHODOFF:
   171  				if i+2 >= relocs.Count() {
   172  					panic("expect three consecutive R_METHODOFF relocs")
   173  				}
   174  				if usedInIface {
   175  					methods = append(methods, methodref{src: symIdx, r: i})
   176  					// The method descriptor is itself a type descriptor, and
   177  					// it can be used to reach other types, e.g. by using
   178  					// reflect.Type.Method(i).Type.In(j). We need to traverse
   179  					// its child types with UsedInIface set. (See also the
   180  					// comment below.)
   181  					rs := r.Sym()
   182  					if !d.ldr.AttrUsedInIface(rs) {
   183  						d.ldr.SetAttrUsedInIface(rs, true)
   184  						if d.ldr.AttrReachable(rs) {
   185  							d.ldr.SetAttrReachable(rs, false)
   186  							d.mark(rs, symIdx)
   187  						}
   188  					}
   189  				}
   190  				i += 2
   191  				continue
   192  			case objabi.R_USETYPE:
   193  				// type symbol used for DWARF. we need to load the symbol but it may not
   194  				// be otherwise reachable in the program.
   195  				// do nothing for now as we still load all type symbols.
   196  				continue
   197  			case objabi.R_USEIFACE:
   198  				// R_USEIFACE is a marker relocation that tells the linker the type is
   199  				// converted to an interface, i.e. should have UsedInIface set. See the
   200  				// comment below for why we need to unset the Reachable bit and re-mark it.
   201  				rs := r.Sym()
   202  				if d.ldr.IsItab(rs) {
   203  					// This relocation can also point at an itab, in which case it
   204  					// means "the _type field of that itab".
   205  					rs = decodeItabType(d.ldr, d.ctxt.Arch, rs)
   206  				}
   207  				if !d.ldr.IsGoType(rs) && !d.ctxt.linkShared {
   208  					panic(fmt.Sprintf("R_USEIFACE in %s references %s which is not a type or itab", d.ldr.SymName(symIdx), d.ldr.SymName(rs)))
   209  				}
   210  				if !d.ldr.AttrUsedInIface(rs) {
   211  					d.ldr.SetAttrUsedInIface(rs, true)
   212  					if d.ldr.AttrReachable(rs) {
   213  						d.ldr.SetAttrReachable(rs, false)
   214  						d.mark(rs, symIdx)
   215  					}
   216  				}
   217  				continue
   218  			case objabi.R_USEIFACEMETHOD:
   219  				// R_USEIFACEMETHOD is a marker relocation that marks an interface
   220  				// method as used.
   221  				rs := r.Sym()
   222  				if d.ctxt.linkShared && (d.ldr.SymType(rs) == sym.SDYNIMPORT || d.ldr.SymType(rs) == sym.Sxxx) {
   223  					// Don't decode symbol from shared library (we'll mark all exported methods anyway).
   224  					// We check for both SDYNIMPORT and Sxxx because name-mangled symbols haven't
   225  					// been resolved at this point.
   226  					continue
   227  				}
   228  				m := d.decodeIfaceMethod(d.ldr, d.ctxt.Arch, rs, r.Add())
   229  				if d.ctxt.Debugvlog > 1 {
   230  					d.ctxt.Logf("reached iface method: %v\n", m)
   231  				}
   232  				d.ifaceMethod[m] = true
   233  				continue
   234  			case objabi.R_USENAMEDMETHOD:
   235  				name := d.decodeGenericIfaceMethod(d.ldr, r.Sym())
   236  				if d.ctxt.Debugvlog > 1 {
   237  					d.ctxt.Logf("reached generic iface method: %s\n", name)
   238  				}
   239  				d.genericIfaceMethod[name] = true
   240  				continue // don't mark referenced symbol - it is not needed in the final binary.
   241  			case objabi.R_INITORDER:
   242  				// inittasks has already run, so any R_INITORDER links are now
   243  				// superfluous - the only live inittask records are those which are
   244  				// in a scheduled list somewhere (e.g. runtime.moduledata.inittasks).
   245  				continue
   246  			}
   247  			rs := r.Sym()
   248  			if isgotype && usedInIface && d.ldr.IsGoType(rs) && !d.ldr.AttrUsedInIface(rs) {
   249  				// If a type is converted to an interface, it is possible to obtain an
   250  				// interface with a "child" type of it using reflection (e.g. obtain an
   251  				// interface of T from []chan T). We need to traverse its "child" types
   252  				// with UsedInIface attribute set.
   253  				// When visiting the child type (chan T in the example above), it will
   254  				// have UsedInIface set, so it in turn will mark and (re)visit its children
   255  				// (e.g. T above).
   256  				// We unset the reachable bit here, so if the child type is already visited,
   257  				// it will be visited again.
   258  				// Note that a type symbol can be visited at most twice, one without
   259  				// UsedInIface and one with. So termination is still guaranteed.
   260  				d.ldr.SetAttrUsedInIface(rs, true)
   261  				d.ldr.SetAttrReachable(rs, false)
   262  			}
   263  			d.mark(rs, symIdx)
   264  		}
   265  		naux := d.ldr.NAux(symIdx)
   266  		for i := 0; i < naux; i++ {
   267  			a := d.ldr.Aux(symIdx, i)
   268  			if a.Type() == goobj.AuxGotype {
   269  				// A symbol being reachable doesn't imply we need its
   270  				// type descriptor. Don't mark it.
   271  				continue
   272  			}
   273  			d.mark(a.Sym(), symIdx)
   274  		}
   275  		// Record sym if package init func (here naux != 0 is a cheap way
   276  		// to check first if it is a function symbol).
   277  		if naux != 0 && d.ldr.IsPkgInit(symIdx) {
   278  
   279  			d.pkginits = append(d.pkginits, symIdx)
   280  		}
   281  		// Some host object symbols have an outer object, which acts like a
   282  		// "carrier" symbol, or it holds all the symbols for a particular
   283  		// section. We need to mark all "referenced" symbols from that carrier,
   284  		// so we make sure we're pulling in all outer symbols, and their sub
   285  		// symbols. This is not ideal, and these carrier/section symbols could
   286  		// be removed.
   287  		if d.ldr.IsExternal(symIdx) {
   288  			d.mark(d.ldr.OuterSym(symIdx), symIdx)
   289  			d.mark(d.ldr.SubSym(symIdx), symIdx)
   290  		}
   291  
   292  		if len(methods) != 0 {
   293  			if !isgotype {
   294  				panic("method found on non-type symbol")
   295  			}
   296  			// Decode runtime type information for type methods
   297  			// to help work out which methods can be called
   298  			// dynamically via interfaces.
   299  			methodsigs := d.decodetypeMethods(d.ldr, d.ctxt.Arch, symIdx, &relocs)
   300  			if len(methods) != len(methodsigs) {
   301  				panic(fmt.Sprintf("%q has %d method relocations for %d methods", d.ldr.SymName(symIdx), len(methods), len(methodsigs)))
   302  			}
   303  			for i, m := range methodsigs {
   304  				methods[i].m = m
   305  				if d.ctxt.Debugvlog > 1 {
   306  					d.ctxt.Logf("markable method: %v of sym %v %s\n", m, symIdx, d.ldr.SymName(symIdx))
   307  				}
   308  			}
   309  			d.markableMethods = append(d.markableMethods, methods...)
   310  		}
   311  	}
   312  }
   313  
   314  // mapinitcleanup walks all pkg init functions and looks for weak relocations
   315  // to mapinit symbols that are no longer reachable. It rewrites
   316  // the relocs to target a new no-op routine in the runtime.
   317  func (d *deadcodePass) mapinitcleanup() {
   318  	for _, idx := range d.pkginits {
   319  		relocs := d.ldr.Relocs(idx)
   320  		var su *loader.SymbolBuilder
   321  		for i := 0; i < relocs.Count(); i++ {
   322  			r := relocs.At(i)
   323  			rs := r.Sym()
   324  			if r.Weak() && r.Type().IsDirectCall() && !d.ldr.AttrReachable(rs) {
   325  				// double check to make sure target is indeed map.init
   326  				rsn := d.ldr.SymName(rs)
   327  				if !strings.Contains(rsn, "map.init") {
   328  					panic(fmt.Sprintf("internal error: expected map.init sym for weak call reloc, got %s -> %s", d.ldr.SymName(idx), rsn))
   329  				}
   330  				d.ldr.SetAttrReachable(d.mapinitnoop, true)
   331  				if d.ctxt.Debugvlog > 1 {
   332  					d.ctxt.Logf("deadcode: %s rewrite %s ref to %s\n",
   333  						d.ldr.SymName(idx), rsn,
   334  						d.ldr.SymName(d.mapinitnoop))
   335  				}
   336  				if su == nil {
   337  					su = d.ldr.MakeSymbolUpdater(idx)
   338  				}
   339  				su.SetRelocSym(i, d.mapinitnoop)
   340  			}
   341  		}
   342  	}
   343  }
   344  
   345  func (d *deadcodePass) mark(symIdx, parent loader.Sym) {
   346  	if symIdx != 0 && !d.ldr.AttrReachable(symIdx) {
   347  		d.wq.push(symIdx)
   348  		d.ldr.SetAttrReachable(symIdx, true)
   349  		if buildcfg.Experiment.FieldTrack && d.ldr.Reachparent[symIdx] == 0 {
   350  			d.ldr.Reachparent[symIdx] = parent
   351  		}
   352  		if *flagDumpDep {
   353  			to := d.ldr.SymName(symIdx)
   354  			if to != "" {
   355  				to = d.dumpDepAddFlags(to, symIdx)
   356  				from := "_"
   357  				if parent != 0 {
   358  					from = d.ldr.SymName(parent)
   359  					from = d.dumpDepAddFlags(from, parent)
   360  				}
   361  				fmt.Printf("%s -> %s\n", from, to)
   362  			}
   363  		}
   364  	}
   365  }
   366  
   367  func (d *deadcodePass) dumpDepAddFlags(name string, symIdx loader.Sym) string {
   368  	var flags strings.Builder
   369  	if d.ldr.AttrUsedInIface(symIdx) {
   370  		flags.WriteString("<UsedInIface>")
   371  	}
   372  	if d.ldr.IsReflectMethod(symIdx) {
   373  		flags.WriteString("<ReflectMethod>")
   374  	}
   375  	if flags.Len() > 0 {
   376  		return name + " " + flags.String()
   377  	}
   378  	return name
   379  }
   380  
   381  func (d *deadcodePass) markMethod(m methodref) {
   382  	relocs := d.ldr.Relocs(m.src)
   383  	d.mark(relocs.At(m.r).Sym(), m.src)
   384  	d.mark(relocs.At(m.r+1).Sym(), m.src)
   385  	d.mark(relocs.At(m.r+2).Sym(), m.src)
   386  }
   387  
   388  // deadcode marks all reachable symbols.
   389  //
   390  // The basis of the dead code elimination is a flood fill of symbols,
   391  // following their relocations, beginning at *flagEntrySymbol.
   392  //
   393  // This flood fill is wrapped in logic for pruning unused methods.
   394  // All methods are mentioned by relocations on their receiver's *rtype.
   395  // These relocations are specially defined as R_METHODOFF by the compiler
   396  // so we can detect and manipulated them here.
   397  //
   398  // There are three ways a method of a reachable type can be invoked:
   399  //
   400  //  1. direct call
   401  //  2. through a reachable interface type
   402  //  3. reflect.Value.Method (or MethodByName), or reflect.Type.Method
   403  //     (or MethodByName)
   404  //
   405  // The first case is handled by the flood fill, a directly called method
   406  // is marked as reachable.
   407  //
   408  // The second case is handled by decomposing all reachable interface
   409  // types into method signatures. Each encountered method is compared
   410  // against the interface method signatures, if it matches it is marked
   411  // as reachable. This is extremely conservative, but easy and correct.
   412  //
   413  // The third case is handled by looking for functions that compiler flagged
   414  // as REFLECTMETHOD. REFLECTMETHOD on a function F means that F does a method
   415  // lookup with reflection, but the compiler was not able to statically determine
   416  // the method name.
   417  //
   418  // All functions that call reflect.Value.Method or reflect.Type.Method are REFLECTMETHODs.
   419  // Functions that call reflect.Value.MethodByName or reflect.Type.MethodByName with
   420  // a non-constant argument are REFLECTMETHODs, too. If we find a REFLECTMETHOD,
   421  // we give up on static analysis, and mark all exported methods of all reachable
   422  // types as reachable.
   423  //
   424  // If the argument to MethodByName is a compile-time constant, the compiler
   425  // emits a relocation with the method name. Matching methods are kept in all
   426  // reachable types.
   427  //
   428  // Any unreached text symbols are removed from ctxt.Textp.
   429  func deadcode(ctxt *Link) {
   430  	ldr := ctxt.loader
   431  	d := deadcodePass{ctxt: ctxt, ldr: ldr}
   432  	d.init()
   433  	d.flood()
   434  
   435  	if ctxt.DynlinkingGo() {
   436  		// Exported methods may satisfy interfaces we don't know
   437  		// about yet when dynamically linking.
   438  		d.reflectSeen = true
   439  	}
   440  
   441  	for {
   442  		// Mark all methods that could satisfy a discovered
   443  		// interface as reachable. We recheck old marked interfaces
   444  		// as new types (with new methods) may have been discovered
   445  		// in the last pass.
   446  		rem := d.markableMethods[:0]
   447  		for _, m := range d.markableMethods {
   448  			if (d.reflectSeen && (m.isExported() || d.dynlink)) || d.ifaceMethod[m.m] || d.genericIfaceMethod[m.m.name] {
   449  				d.markMethod(m)
   450  			} else {
   451  				rem = append(rem, m)
   452  			}
   453  		}
   454  		d.markableMethods = rem
   455  
   456  		if d.wq.empty() {
   457  			// No new work was discovered. Done.
   458  			break
   459  		}
   460  		d.flood()
   461  	}
   462  	if *flagPruneWeakMap {
   463  		d.mapinitcleanup()
   464  	}
   465  }
   466  
   467  // methodsig is a typed method signature (name + type).
   468  type methodsig struct {
   469  	name string
   470  	typ  loader.Sym // type descriptor symbol of the function
   471  }
   472  
   473  // methodref holds the relocations from a receiver type symbol to its
   474  // method. There are three relocations, one for each of the fields in
   475  // the reflect.method struct: mtyp, ifn, and tfn.
   476  type methodref struct {
   477  	m   methodsig
   478  	src loader.Sym // receiver type symbol
   479  	r   int        // the index of R_METHODOFF relocations
   480  }
   481  
   482  func (m methodref) isExported() bool {
   483  	for _, r := range m.m.name {
   484  		return unicode.IsUpper(r)
   485  	}
   486  	panic("methodref has no signature")
   487  }
   488  
   489  // decodeMethodSig decodes an array of method signature information.
   490  // Each element of the array is size bytes. The first 4 bytes is a
   491  // nameOff for the method name, and the next 4 bytes is a typeOff for
   492  // the function type.
   493  //
   494  // Conveniently this is the layout of both runtime.method and runtime.imethod.
   495  func (d *deadcodePass) decodeMethodSig(ldr *loader.Loader, arch *sys.Arch, symIdx loader.Sym, relocs *loader.Relocs, off, size, count int) []methodsig {
   496  	if cap(d.methodsigstmp) < count {
   497  		d.methodsigstmp = append(d.methodsigstmp[:0], make([]methodsig, count)...)
   498  	}
   499  	var methods = d.methodsigstmp[:count]
   500  	for i := 0; i < count; i++ {
   501  		methods[i].name = decodetypeName(ldr, symIdx, relocs, off)
   502  		methods[i].typ = decodeRelocSym(ldr, symIdx, relocs, int32(off+4))
   503  		off += size
   504  	}
   505  	return methods
   506  }
   507  
   508  // Decode the method of interface type symbol symIdx at offset off.
   509  func (d *deadcodePass) decodeIfaceMethod(ldr *loader.Loader, arch *sys.Arch, symIdx loader.Sym, off int64) methodsig {
   510  	p := ldr.Data(symIdx)
   511  	if p == nil {
   512  		panic(fmt.Sprintf("missing symbol %q", ldr.SymName(symIdx)))
   513  	}
   514  	if decodetypeKind(arch, p)&kindMask != kindInterface {
   515  		panic(fmt.Sprintf("symbol %q is not an interface", ldr.SymName(symIdx)))
   516  	}
   517  	relocs := ldr.Relocs(symIdx)
   518  	var m methodsig
   519  	m.name = decodetypeName(ldr, symIdx, &relocs, int(off))
   520  	m.typ = decodeRelocSym(ldr, symIdx, &relocs, int32(off+4))
   521  	return m
   522  }
   523  
   524  // Decode the method name stored in symbol symIdx. The symbol should contain just the bytes of a method name.
   525  func (d *deadcodePass) decodeGenericIfaceMethod(ldr *loader.Loader, symIdx loader.Sym) string {
   526  	return ldr.DataString(symIdx)
   527  }
   528  
   529  func (d *deadcodePass) decodetypeMethods(ldr *loader.Loader, arch *sys.Arch, symIdx loader.Sym, relocs *loader.Relocs) []methodsig {
   530  	p := ldr.Data(symIdx)
   531  	if !decodetypeHasUncommon(arch, p) {
   532  		panic(fmt.Sprintf("no methods on %q", ldr.SymName(symIdx)))
   533  	}
   534  	off := commonsize(arch) // reflect.rtype
   535  	switch decodetypeKind(arch, p) & kindMask {
   536  	case kindStruct: // reflect.structType
   537  		off += 4 * arch.PtrSize
   538  	case kindPtr: // reflect.ptrType
   539  		off += arch.PtrSize
   540  	case kindFunc: // reflect.funcType
   541  		off += arch.PtrSize // 4 bytes, pointer aligned
   542  	case kindSlice: // reflect.sliceType
   543  		off += arch.PtrSize
   544  	case kindArray: // reflect.arrayType
   545  		off += 3 * arch.PtrSize
   546  	case kindChan: // reflect.chanType
   547  		off += 2 * arch.PtrSize
   548  	case kindMap: // reflect.mapType
   549  		off += 4*arch.PtrSize + 8
   550  	case kindInterface: // reflect.interfaceType
   551  		off += 3 * arch.PtrSize
   552  	default:
   553  		// just Sizeof(rtype)
   554  	}
   555  
   556  	mcount := int(decodeInuxi(arch, p[off+4:], 2))
   557  	moff := int(decodeInuxi(arch, p[off+4+2+2:], 4))
   558  	off += moff                // offset to array of reflect.method values
   559  	const sizeofMethod = 4 * 4 // sizeof reflect.method in program
   560  	return d.decodeMethodSig(ldr, arch, symIdx, relocs, off, sizeofMethod, mcount)
   561  }
   562
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