deadcode.go

     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/abi"
    15  	"internal/buildcfg"
    16  	"strings"
    17  	"unicode"
    18  )
    19  
    20  var _ = fmt.Print
    21  
    22  type deadcodePass struct {
    23  	ctxt *Link
    24  	ldr  *loader.Loader
    25  	wq   heap // work queue, using min-heap for better locality
    26  
    27  	ifaceMethod        map[methodsig]bool // methods called from reached interface call sites
    28  	genericIfaceMethod map[string]bool    // names of methods called from reached generic interface call sites
    29  	markableMethods    []methodref        // methods of reached types
    30  	reflectSeen        bool               // whether we have seen a reflect method call
    31  	dynlink            bool
    32  
    33  	methodsigstmp []methodsig // scratch buffer for decoding method signatures
    34  	pkginits      []loader.Sym
    35  	mapinitnoop   loader.Sym
    36  }
    37  
    38  func (d *deadcodePass) init() {
    39  	d.ldr.InitReachable()
    40  	d.ifaceMethod = make(map[methodsig]bool)
    41  	d.genericIfaceMethod = make(map[string]bool)
    42  	if buildcfg.Experiment.FieldTrack {
    43  		d.ldr.Reachparent = make([]loader.Sym, d.ldr.NSym())
    44  	}
    45  	d.dynlink = d.ctxt.DynlinkingGo()
    46  
    47  	if d.ctxt.BuildMode == BuildModeShared {
    48  		// Mark all symbols defined in this library as reachable when
    49  		// building a shared library.
    50  		n := d.ldr.NDef()
    51  		for i := 1; i < n; i++ {
    52  			s := loader.Sym(i)
    53  			if d.ldr.SymType(s).IsText() && d.ldr.SymSize(s) == 0 {
    54  				// Zero-sized text symbol is a function deadcoded by the
    55  				// compiler. It doesn't really get compiled, and its
    56  				// metadata may be missing.
    57  				continue
    58  			}
    59  			d.mark(s, 0)
    60  		}
    61  		d.mark(d.ctxt.mainInittasks, 0)
    62  		return
    63  	}
    64  
    65  	var names []string
    66  
    67  	// In a normal binary, start at main.main and the init
    68  	// functions and mark what is reachable from there.
    69  	if d.ctxt.linkShared && (d.ctxt.BuildMode == BuildModeExe || d.ctxt.BuildMode == BuildModePIE) {
    70  		names = append(names, "main.main", "main..inittask")
    71  	} else {
    72  		// The external linker refers main symbol directly.
    73  		if d.ctxt.LinkMode == LinkExternal && (d.ctxt.BuildMode == BuildModeExe || d.ctxt.BuildMode == BuildModePIE) {
    74  			if d.ctxt.HeadType == objabi.Hwindows && d.ctxt.Arch.Family == sys.I386 {
    75  				*flagEntrySymbol = "_main"
    76  			} else {
    77  				*flagEntrySymbol = "main"
    78  			}
    79  		}
    80  		names = append(names, *flagEntrySymbol)
    81  	}
    82  	// runtime.unreachableMethod is a function that will throw if called.
    83  	// We redirect unreachable methods to it.
    84  	names = append(names, "runtime.unreachableMethod")
    85  	if d.ctxt.BuildMode == BuildModePlugin {
    86  		names = append(names, objabi.PathToPrefix(*flagPluginPath)+"..inittask", objabi.PathToPrefix(*flagPluginPath)+".main", "go:plugin.tabs")
    87  
    88  		// We don't keep the go.plugin.exports symbol,
    89  		// but we do keep the symbols it refers to.
    90  		exportsIdx := d.ldr.Lookup("go:plugin.exports", 0)
    91  		if exportsIdx != 0 {
    92  			relocs := d.ldr.Relocs(exportsIdx)
    93  			for i := 0; i < relocs.Count(); i++ {
    94  				d.mark(relocs.At(i).Sym(), 0)
    95  			}
    96  		}
    97  	}
    98  
    99  	if d.ctxt.Debugvlog > 1 {
   100  		d.ctxt.Logf("deadcode start names: %v\n", names)
   101  	}
   102  
   103  	for _, name := range names {
   104  		// Mark symbol as a data/ABI0 symbol.
   105  		d.mark(d.ldr.Lookup(name, 0), 0)
   106  		if abiInternalVer != 0 {
   107  			// Also mark any Go functions (internal ABI).
   108  			d.mark(d.ldr.Lookup(name, abiInternalVer), 0)
   109  		}
   110  	}
   111  
   112  	// All dynamic exports are roots.
   113  	for _, s := range d.ctxt.dynexp {
   114  		if d.ctxt.Debugvlog > 1 {
   115  			d.ctxt.Logf("deadcode start dynexp: %s<%d>\n", d.ldr.SymName(s), d.ldr.SymVersion(s))
   116  		}
   117  		d.mark(s, 0)
   118  	}
   119  	// So are wasmexports.
   120  	for _, s := range d.ldr.WasmExports {
   121  		if d.ctxt.Debugvlog > 1 {
   122  			d.ctxt.Logf("deadcode start wasmexport: %s<%d>\n", d.ldr.SymName(s), d.ldr.SymVersion(s))
   123  		}
   124  		d.mark(s, 0)
   125  	}
   126  
   127  	d.mapinitnoop = d.ldr.Lookup("runtime.mapinitnoop", abiInternalVer)
   128  	if d.mapinitnoop == 0 {
   129  		panic("could not look up runtime.mapinitnoop")
   130  	}
   131  	if d.ctxt.mainInittasks != 0 {
   132  		d.mark(d.ctxt.mainInittasks, 0)
   133  	}
   134  }
   135  
   136  func (d *deadcodePass) flood() {
   137  	var methods []methodref
   138  	for !d.wq.empty() {
   139  		symIdx := d.wq.pop()
   140  
   141  		// Methods may be called via reflection. Give up on static analysis,
   142  		// and mark all exported methods of all reachable types as reachable.
   143  		d.reflectSeen = d.reflectSeen || d.ldr.IsReflectMethod(symIdx)
   144  
   145  		isgotype := d.ldr.IsGoType(symIdx)
   146  		relocs := d.ldr.Relocs(symIdx)
   147  		var usedInIface bool
   148  
   149  		if isgotype {
   150  			if d.dynlink {
   151  				// When dynamic linking, a type may be passed across DSO
   152  				// boundary and get converted to interface at the other side.
   153  				d.ldr.SetAttrUsedInIface(symIdx, true)
   154  			}
   155  			usedInIface = d.ldr.AttrUsedInIface(symIdx)
   156  		}
   157  
   158  		methods = methods[:0]
   159  		for i := 0; i < relocs.Count(); i++ {
   160  			r := relocs.At(i)
   161  			if r.Weak() {
   162  				convertWeakToStrong := false
   163  				// When build with "-linkshared", we can't tell if the
   164  				// interface method in itab will be used or not.
   165  				// Ignore the weak attribute.
   166  				if d.ctxt.linkShared && d.ldr.IsItab(symIdx) {
   167  					convertWeakToStrong = true
   168  				}
   169  				// If the program uses plugins, we can no longer treat
   170  				// relocs from pkg init functions to outlined map init
   171  				// fragments as weak, since doing so can cause package
   172  				// init clashes between the main program and the
   173  				// plugin. See #62430 for more details.
   174  				if d.ctxt.canUsePlugins && r.Type().IsDirectCall() {
   175  					convertWeakToStrong = true
   176  				}
   177  				if !convertWeakToStrong {
   178  					// skip this reloc
   179  					continue
   180  				}
   181  			}
   182  			t := r.Type()
   183  			switch t {
   184  			case objabi.R_METHODOFF:
   185  				if i+2 >= relocs.Count() {
   186  					panic("expect three consecutive R_METHODOFF relocs")
   187  				}
   188  				if usedInIface {
   189  					methods = append(methods, methodref{src: symIdx, r: i})
   190  					// The method descriptor is itself a type descriptor, and
   191  					// it can be used to reach other types, e.g. by using
   192  					// reflect.Type.Method(i).Type.In(j). We need to traverse
   193  					// its child types with UsedInIface set. (See also the
   194  					// comment below.)
   195  					rs := r.Sym()
   196  					if !d.ldr.AttrUsedInIface(rs) {
   197  						d.ldr.SetAttrUsedInIface(rs, true)
   198  						if d.ldr.AttrReachable(rs) {
   199  							d.ldr.SetAttrReachable(rs, false)
   200  							d.mark(rs, symIdx)
   201  						}
   202  					}
   203  				}
   204  				i += 2
   205  				continue
   206  			case objabi.R_USETYPE:
   207  				// type symbol used for DWARF. we need to load the symbol but it may not
   208  				// be otherwise reachable in the program.
   209  				// do nothing for now as we still load all type symbols.
   210  				continue
   211  			case objabi.R_USEIFACE:
   212  				// R_USEIFACE is a marker relocation that tells the linker the type is
   213  				// converted to an interface, i.e. should have UsedInIface set. See the
   214  				// comment below for why we need to unset the Reachable bit and re-mark it.
   215  				rs := r.Sym()
   216  				if d.ldr.IsItab(rs) {
   217  					// This relocation can also point at an itab, in which case it
   218  					// means "the Type field of that itab".
   219  					rs = decodeItabType(d.ldr, d.ctxt.Arch, rs)
   220  				}
   221  				if !d.ldr.IsGoType(rs) && !d.ctxt.linkShared {
   222  					panic(fmt.Sprintf("R_USEIFACE in %s references %s which is not a type or itab", d.ldr.SymName(symIdx), d.ldr.SymName(rs)))
   223  				}
   224  				if !d.ldr.AttrUsedInIface(rs) {
   225  					d.ldr.SetAttrUsedInIface(rs, true)
   226  					if d.ldr.AttrReachable(rs) {
   227  						d.ldr.SetAttrReachable(rs, false)
   228  						d.mark(rs, symIdx)
   229  					}
   230  				}
   231  				continue
   232  			case objabi.R_USEIFACEMETHOD:
   233  				// R_USEIFACEMETHOD is a marker relocation that marks an interface
   234  				// method as used.
   235  				rs := r.Sym()
   236  				if d.ctxt.linkShared && (d.ldr.SymType(rs) == sym.SDYNIMPORT || d.ldr.SymType(rs) == sym.Sxxx) {
   237  					// Don't decode symbol from shared library (we'll mark all exported methods anyway).
   238  					// We check for both SDYNIMPORT and Sxxx because name-mangled symbols haven't
   239  					// been resolved at this point.
   240  					continue
   241  				}
   242  				m := d.decodeIfaceMethod(d.ldr, d.ctxt.Arch, rs, r.Add())
   243  				if d.ctxt.Debugvlog > 1 {
   244  					d.ctxt.Logf("reached iface method: %v\n", m)
   245  				}
   246  				d.ifaceMethod[m] = true
   247  				continue
   248  			case objabi.R_USENAMEDMETHOD:
   249  				name := d.decodeGenericIfaceMethod(d.ldr, r.Sym())
   250  				if d.ctxt.Debugvlog > 1 {
   251  					d.ctxt.Logf("reached generic iface method: %s\n", name)
   252  				}
   253  				d.genericIfaceMethod[name] = true
   254  				continue // don't mark referenced symbol - it is not needed in the final binary.
   255  			case objabi.R_INITORDER:
   256  				// inittasks has already run, so any R_INITORDER links are now
   257  				// superfluous - the only live inittask records are those which are
   258  				// in a scheduled list somewhere (e.g. runtime.moduledata.inittasks).
   259  				continue
   260  			}
   261  			rs := r.Sym()
   262  			if isgotype && usedInIface && d.ldr.IsGoType(rs) && !d.ldr.AttrUsedInIface(rs) {
   263  				// If a type is converted to an interface, it is possible to obtain an
   264  				// interface with a "child" type of it using reflection (e.g. obtain an
   265  				// interface of T from []chan T). We need to traverse its "child" types
   266  				// with UsedInIface attribute set.
   267  				// When visiting the child type (chan T in the example above), it will
   268  				// have UsedInIface set, so it in turn will mark and (re)visit its children
   269  				// (e.g. T above).
   270  				// We unset the reachable bit here, so if the child type is already visited,
   271  				// it will be visited again.
   272  				// Note that a type symbol can be visited at most twice, one without
   273  				// UsedInIface and one with. So termination is still guaranteed.
   274  				d.ldr.SetAttrUsedInIface(rs, true)
   275  				d.ldr.SetAttrReachable(rs, false)
   276  			}
   277  			d.mark(rs, symIdx)
   278  		}
   279  		naux := d.ldr.NAux(symIdx)
   280  		for i := 0; i < naux; i++ {
   281  			a := d.ldr.Aux(symIdx, i)
   282  			if a.Type() == goobj.AuxGotype {
   283  				// A symbol being reachable doesn't imply we need its
   284  				// type descriptor. Don't mark it.
   285  				continue
   286  			}
   287  			d.mark(a.Sym(), symIdx)
   288  		}
   289  		// Record sym if package init func (here naux != 0 is a cheap way
   290  		// to check first if it is a function symbol).
   291  		if naux != 0 && d.ldr.IsPkgInit(symIdx) {
   292  
   293  			d.pkginits = append(d.pkginits, symIdx)
   294  		}
   295  		// Some host object symbols have an outer object, which acts like a
   296  		// "carrier" symbol, or it holds all the symbols for a particular
   297  		// section. We need to mark all "referenced" symbols from that carrier,
   298  		// so we make sure we're pulling in all outer symbols, and their sub
   299  		// symbols. This is not ideal, and these carrier/section symbols could
   300  		// be removed.
   301  		if d.ldr.IsExternal(symIdx) {
   302  			d.mark(d.ldr.OuterSym(symIdx), symIdx)
   303  			d.mark(d.ldr.SubSym(symIdx), symIdx)
   304  		}
   305  
   306  		if len(methods) != 0 {
   307  			if !isgotype {
   308  				panic("method found on non-type symbol")
   309  			}
   310  			// Decode runtime type information for type methods
   311  			// to help work out which methods can be called
   312  			// dynamically via interfaces.
   313  			methodsigs := d.decodetypeMethods(d.ldr, d.ctxt.Arch, symIdx, &relocs)
   314  			if len(methods) != len(methodsigs) {
   315  				panic(fmt.Sprintf("%q has %d method relocations for %d methods", d.ldr.SymName(symIdx), len(methods), len(methodsigs)))
   316  			}
   317  			for i, m := range methodsigs {
   318  				methods[i].m = m
   319  				if d.ctxt.Debugvlog > 1 {
   320  					d.ctxt.Logf("markable method: %v of sym %v %s\n", m, symIdx, d.ldr.SymName(symIdx))
   321  				}
   322  			}
   323  			d.markableMethods = append(d.markableMethods, methods...)
   324  		}
   325  	}
   326  }
   327  
   328  // mapinitcleanup walks all pkg init functions and looks for weak relocations
   329  // to mapinit symbols that are no longer reachable. It rewrites
   330  // the relocs to target a new no-op routine in the runtime.
   331  func (d *deadcodePass) mapinitcleanup() {
   332  	for _, idx := range d.pkginits {
   333  		relocs := d.ldr.Relocs(idx)
   334  		var su *loader.SymbolBuilder
   335  		for i := 0; i < relocs.Count(); i++ {
   336  			r := relocs.At(i)
   337  			rs := r.Sym()
   338  			if r.Weak() && r.Type().IsDirectCall() && !d.ldr.AttrReachable(rs) {
   339  				// double check to make sure target is indeed map.init
   340  				rsn := d.ldr.SymName(rs)
   341  				if !strings.Contains(rsn, "map.init") {
   342  					panic(fmt.Sprintf("internal error: expected map.init sym for weak call reloc, got %s -> %s", d.ldr.SymName(idx), rsn))
   343  				}
   344  				d.ldr.SetAttrReachable(d.mapinitnoop, true)
   345  				if d.ctxt.Debugvlog > 1 {
   346  					d.ctxt.Logf("deadcode: %s rewrite %s ref to %s\n",
   347  						d.ldr.SymName(idx), rsn,
   348  						d.ldr.SymName(d.mapinitnoop))
   349  				}
   350  				if su == nil {
   351  					su = d.ldr.MakeSymbolUpdater(idx)
   352  				}
   353  				su.SetRelocSym(i, d.mapinitnoop)
   354  			}
   355  		}
   356  	}
   357  }
   358  
   359  func (d *deadcodePass) mark(symIdx, parent loader.Sym) {
   360  	if symIdx != 0 && !d.ldr.AttrReachable(symIdx) {
   361  		d.wq.push(symIdx)
   362  		d.ldr.SetAttrReachable(symIdx, true)
   363  		if buildcfg.Experiment.FieldTrack && d.ldr.Reachparent[symIdx] == 0 {
   364  			d.ldr.Reachparent[symIdx] = parent
   365  		}
   366  		if *flagDumpDep {
   367  			to := d.ldr.SymName(symIdx)
   368  			if to != "" {
   369  				to = d.dumpDepAddFlags(to, symIdx)
   370  				from := "_"
   371  				if parent != 0 {
   372  					from = d.ldr.SymName(parent)
   373  					from = d.dumpDepAddFlags(from, parent)
   374  				}
   375  				fmt.Printf("%s -> %s\n", from, to)
   376  			}
   377  		}
   378  	}
   379  }
   380  
   381  func (d *deadcodePass) dumpDepAddFlags(name string, symIdx loader.Sym) string {
   382  	var flags strings.Builder
   383  	if d.ldr.AttrUsedInIface(symIdx) {
   384  		flags.WriteString("<UsedInIface>")
   385  	}
   386  	if d.ldr.IsReflectMethod(symIdx) {
   387  		flags.WriteString("<ReflectMethod>")
   388  	}
   389  	if flags.Len() > 0 {
   390  		return name + " " + flags.String()
   391  	}
   392  	return name
   393  }
   394  
   395  func (d *deadcodePass) markMethod(m methodref) {
   396  	relocs := d.ldr.Relocs(m.src)
   397  	d.mark(relocs.At(m.r).Sym(), m.src)
   398  	d.mark(relocs.At(m.r+1).Sym(), m.src)
   399  	d.mark(relocs.At(m.r+2).Sym(), m.src)
   400  }
   401  
   402  // deadcode marks all reachable symbols.
   403  //
   404  // The basis of the dead code elimination is a flood fill of symbols,
   405  // following their relocations, beginning at *flagEntrySymbol.
   406  //
   407  // This flood fill is wrapped in logic for pruning unused methods.
   408  // All methods are mentioned by relocations on their receiver's *rtype.
   409  // These relocations are specially defined as R_METHODOFF by the compiler
   410  // so we can detect and manipulated them here.
   411  //
   412  // There are three ways a method of a reachable type can be invoked:
   413  //
   414  //  1. direct call
   415  //  2. through a reachable interface type
   416  //  3. reflect.Value.Method (or MethodByName), or reflect.Type.Method
   417  //     (or MethodByName)
   418  //
   419  // The first case is handled by the flood fill, a directly called method
   420  // is marked as reachable.
   421  //
   422  // The second case is handled by decomposing all reachable interface
   423  // types into method signatures. Each encountered method is compared
   424  // against the interface method signatures, if it matches it is marked
   425  // as reachable. This is extremely conservative, but easy and correct.
   426  //
   427  // The third case is handled by looking for functions that compiler flagged
   428  // as REFLECTMETHOD. REFLECTMETHOD on a function F means that F does a method
   429  // lookup with reflection, but the compiler was not able to statically determine
   430  // the method name.
   431  //
   432  // All functions that call reflect.Value.Method or reflect.Type.Method are REFLECTMETHODs.
   433  // Functions that call reflect.Value.MethodByName or reflect.Type.MethodByName with
   434  // a non-constant argument are REFLECTMETHODs, too. If we find a REFLECTMETHOD,
   435  // we give up on static analysis, and mark all exported methods of all reachable
   436  // types as reachable.
   437  //
   438  // If the argument to MethodByName is a compile-time constant, the compiler
   439  // emits a relocation with the method name. Matching methods are kept in all
   440  // reachable types.
   441  //
   442  // Any unreached text symbols are removed from ctxt.Textp.
   443  func deadcode(ctxt *Link) {
   444  	ldr := ctxt.loader
   445  	d := deadcodePass{ctxt: ctxt, ldr: ldr}
   446  	d.init()
   447  	d.flood()
   448  
   449  	if ctxt.DynlinkingGo() {
   450  		// Exported methods may satisfy interfaces we don't know
   451  		// about yet when dynamically linking.
   452  		d.reflectSeen = true
   453  	}
   454  
   455  	for {
   456  		// Mark all methods that could satisfy a discovered
   457  		// interface as reachable. We recheck old marked interfaces
   458  		// as new types (with new methods) may have been discovered
   459  		// in the last pass.
   460  		rem := d.markableMethods[:0]
   461  		for _, m := range d.markableMethods {
   462  			if (d.reflectSeen && (m.isExported() || d.dynlink)) || d.ifaceMethod[m.m] || d.genericIfaceMethod[m.m.name] {
   463  				d.markMethod(m)
   464  			} else {
   465  				rem = append(rem, m)
   466  			}
   467  		}
   468  		d.markableMethods = rem
   469  
   470  		if d.wq.empty() {
   471  			// No new work was discovered. Done.
   472  			break
   473  		}
   474  		d.flood()
   475  	}
   476  	if *flagPruneWeakMap {
   477  		d.mapinitcleanup()
   478  	}
   479  }
   480  
   481  // methodsig is a typed method signature (name + type).
   482  type methodsig struct {
   483  	name string
   484  	typ  loader.Sym // type descriptor symbol of the function
   485  }
   486  
   487  // methodref holds the relocations from a receiver type symbol to its
   488  // method. There are three relocations, one for each of the fields in
   489  // the reflect.method struct: mtyp, ifn, and tfn.
   490  type methodref struct {
   491  	m   methodsig
   492  	src loader.Sym // receiver type symbol
   493  	r   int        // the index of R_METHODOFF relocations
   494  }
   495  
   496  func (m methodref) isExported() bool {
   497  	for _, r := range m.m.name {
   498  		return unicode.IsUpper(r)
   499  	}
   500  	panic("methodref has no signature")
   501  }
   502  
   503  // decodeMethodSig decodes an array of method signature information.
   504  // Each element of the array is size bytes. The first 4 bytes is a
   505  // nameOff for the method name, and the next 4 bytes is a typeOff for
   506  // the function type.
   507  //
   508  // Conveniently this is the layout of both runtime.method and runtime.imethod.
   509  func (d *deadcodePass) decodeMethodSig(ldr *loader.Loader, arch *sys.Arch, symIdx loader.Sym, relocs *loader.Relocs, off, size, count int) []methodsig {
   510  	if cap(d.methodsigstmp) < count {
   511  		d.methodsigstmp = append(d.methodsigstmp[:0], make([]methodsig, count)...)
   512  	}
   513  	var methods = d.methodsigstmp[:count]
   514  	for i := 0; i < count; i++ {
   515  		methods[i].name = decodetypeName(ldr, symIdx, relocs, off)
   516  		methods[i].typ = decodeRelocSym(ldr, symIdx, relocs, int32(off+4))
   517  		off += size
   518  	}
   519  	return methods
   520  }
   521  
   522  // Decode the method of interface type symbol symIdx at offset off.
   523  func (d *deadcodePass) decodeIfaceMethod(ldr *loader.Loader, arch *sys.Arch, symIdx loader.Sym, off int64) methodsig {
   524  	p := ldr.Data(symIdx)
   525  	if p == nil {
   526  		panic(fmt.Sprintf("missing symbol %q", ldr.SymName(symIdx)))
   527  	}
   528  	if decodetypeKind(arch, p) != abi.Interface {
   529  		panic(fmt.Sprintf("symbol %q is not an interface", ldr.SymName(symIdx)))
   530  	}
   531  	relocs := ldr.Relocs(symIdx)
   532  	var m methodsig
   533  	m.name = decodetypeName(ldr, symIdx, &relocs, int(off))
   534  	m.typ = decodeRelocSym(ldr, symIdx, &relocs, int32(off+4))
   535  	return m
   536  }
   537  
   538  // Decode the method name stored in symbol symIdx. The symbol should contain just the bytes of a method name.
   539  func (d *deadcodePass) decodeGenericIfaceMethod(ldr *loader.Loader, symIdx loader.Sym) string {
   540  	return ldr.DataString(symIdx)
   541  }
   542  
   543  func (d *deadcodePass) decodetypeMethods(ldr *loader.Loader, arch *sys.Arch, symIdx loader.Sym, relocs *loader.Relocs) []methodsig {
   544  	p := ldr.Data(symIdx)
   545  	if !decodetypeHasUncommon(arch, p) {
   546  		panic(fmt.Sprintf("no methods on %q", ldr.SymName(symIdx)))
   547  	}
   548  	off := commonsize(arch) // reflect.rtype
   549  	switch decodetypeKind(arch, p) {
   550  	case abi.Struct: // reflect.structType
   551  		off += 4 * arch.PtrSize
   552  	case abi.Pointer: // reflect.ptrType
   553  		off += arch.PtrSize
   554  	case abi.Func: // reflect.funcType
   555  		off += arch.PtrSize // 4 bytes, pointer aligned
   556  	case abi.Slice: // reflect.sliceType
   557  		off += arch.PtrSize
   558  	case abi.Array: // reflect.arrayType
   559  		off += 3 * arch.PtrSize
   560  	case abi.Chan: // reflect.chanType
   561  		off += 2 * arch.PtrSize
   562  	case abi.Map:
   563  		if buildcfg.Experiment.SwissMap {
   564  			off += 7*arch.PtrSize + 4 // internal/abi.SwissMapType
   565  			if arch.PtrSize == 8 {
   566  				off += 4 // padding for final uint32 field (Flags).
   567  			}
   568  		} else {
   569  			off += 4*arch.PtrSize + 8 // internal/abi.OldMapType
   570  		}
   571  	case abi.Interface: // reflect.interfaceType
   572  		off += 3 * arch.PtrSize
   573  	default:
   574  		// just Sizeof(rtype)
   575  	}
   576  
   577  	mcount := int(decodeInuxi(arch, p[off+4:], 2))
   578  	moff := int(decodeInuxi(arch, p[off+4+2+2:], 4))
   579  	off += moff                // offset to array of reflect.method values
   580  	const sizeofMethod = 4 * 4 // sizeof reflect.method in program
   581  	return d.decodeMethodSig(ldr, arch, symIdx, relocs, off, sizeofMethod, mcount)
   582  }
   583
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