escape.go

     1  // Copyright 2018 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 escape
     6  
     7  import (
     8  	"fmt"
     9  	"go/constant"
    10  	"go/token"
    11  
    12  	"cmd/compile/internal/base"
    13  	"cmd/compile/internal/ir"
    14  	"cmd/compile/internal/logopt"
    15  	"cmd/compile/internal/typecheck"
    16  	"cmd/compile/internal/types"
    17  	"cmd/internal/src"
    18  )
    19  
    20  // Escape analysis.
    21  //
    22  // Here we analyze functions to determine which Go variables
    23  // (including implicit allocations such as calls to "new" or "make",
    24  // composite literals, etc.) can be allocated on the stack. The two
    25  // key invariants we have to ensure are: (1) pointers to stack objects
    26  // cannot be stored in the heap, and (2) pointers to a stack object
    27  // cannot outlive that object (e.g., because the declaring function
    28  // returned and destroyed the object's stack frame, or its space is
    29  // reused across loop iterations for logically distinct variables).
    30  //
    31  // We implement this with a static data-flow analysis of the AST.
    32  // First, we construct a directed weighted graph where vertices
    33  // (termed "locations") represent variables allocated by statements
    34  // and expressions, and edges represent assignments between variables
    35  // (with weights representing addressing/dereference counts).
    36  //
    37  // Next we walk the graph looking for assignment paths that might
    38  // violate the invariants stated above. If a variable v's address is
    39  // stored in the heap or elsewhere that may outlive it, then v is
    40  // marked as requiring heap allocation.
    41  //
    42  // To support interprocedural analysis, we also record data-flow from
    43  // each function's parameters to the heap and to its result
    44  // parameters. This information is summarized as "parameter tags",
    45  // which are used at static call sites to improve escape analysis of
    46  // function arguments.
    47  
    48  // Constructing the location graph.
    49  //
    50  // Every allocating statement (e.g., variable declaration) or
    51  // expression (e.g., "new" or "make") is first mapped to a unique
    52  // "location."
    53  //
    54  // We also model every Go assignment as a directed edges between
    55  // locations. The number of dereference operations minus the number of
    56  // addressing operations is recorded as the edge's weight (termed
    57  // "derefs"). For example:
    58  //
    59  //     p = &q    // -1
    60  //     p = q     //  0
    61  //     p = *q    //  1
    62  //     p = **q   //  2
    63  //
    64  //     p = **&**&q  // 2
    65  //
    66  // Note that the & operator can only be applied to addressable
    67  // expressions, and the expression &x itself is not addressable, so
    68  // derefs cannot go below -1.
    69  //
    70  // Every Go language construct is lowered into this representation,
    71  // generally without sensitivity to flow, path, or context; and
    72  // without distinguishing elements within a compound variable. For
    73  // example:
    74  //
    75  //     var x struct { f, g *int }
    76  //     var u []*int
    77  //
    78  //     x.f = u[0]
    79  //
    80  // is modeled simply as
    81  //
    82  //     x = *u
    83  //
    84  // That is, we don't distinguish x.f from x.g, or u[0] from u[1],
    85  // u[2], etc. However, we do record the implicit dereference involved
    86  // in indexing a slice.
    87  
    88  // A batch holds escape analysis state that's shared across an entire
    89  // batch of functions being analyzed at once.
    90  type batch struct {
    91  	allLocs         []*location
    92  	closures        []closure
    93  	reassignOracles map[*ir.Func]*ir.ReassignOracle
    94  
    95  	heapLoc    location
    96  	mutatorLoc location
    97  	calleeLoc  location
    98  	blankLoc   location
    99  }
   100  
   101  // A closure holds a closure expression and its spill hole (i.e.,
   102  // where the hole representing storing into its closure record).
   103  type closure struct {
   104  	k   hole
   105  	clo *ir.ClosureExpr
   106  }
   107  
   108  // An escape holds state specific to a single function being analyzed
   109  // within a batch.
   110  type escape struct {
   111  	*batch
   112  
   113  	curfn *ir.Func // function being analyzed
   114  
   115  	labels map[*types.Sym]labelState // known labels
   116  
   117  	// loopDepth counts the current loop nesting depth within
   118  	// curfn. It increments within each "for" loop and at each
   119  	// label with a corresponding backwards "goto" (i.e.,
   120  	// unstructured loop).
   121  	loopDepth int
   122  }
   123  
   124  func Funcs(all []*ir.Func) {
   125  	// Make a cache of ir.ReassignOracles. The cache is lazily populated.
   126  	// TODO(thepudds): consider adding a field on ir.Func instead. We might also be able
   127  	// to use that field elsewhere, like in walk. See discussion in https://go.dev/cl/688075.
   128  	reassignOracles := make(map[*ir.Func]*ir.ReassignOracle)
   129  
   130  	ir.VisitFuncsBottomUp(all, func(list []*ir.Func, recursive bool) {
   131  		Batch(list, reassignOracles)
   132  	})
   133  }
   134  
   135  // Batch performs escape analysis on a minimal batch of
   136  // functions.
   137  func Batch(fns []*ir.Func, reassignOracles map[*ir.Func]*ir.ReassignOracle) {
   138  	var b batch
   139  	b.heapLoc.attrs = attrEscapes | attrPersists | attrMutates | attrCalls
   140  	b.mutatorLoc.attrs = attrMutates
   141  	b.calleeLoc.attrs = attrCalls
   142  	b.reassignOracles = reassignOracles
   143  
   144  	// Construct data-flow graph from syntax trees.
   145  	for _, fn := range fns {
   146  		if base.Flag.W > 1 {
   147  			s := fmt.Sprintf("\nbefore escape %v", fn)
   148  			ir.Dump(s, fn)
   149  		}
   150  		b.initFunc(fn)
   151  	}
   152  	for _, fn := range fns {
   153  		if !fn.IsClosure() {
   154  			b.walkFunc(fn)
   155  		}
   156  	}
   157  
   158  	// We've walked the function bodies, so we've seen everywhere a
   159  	// variable might be reassigned or have its address taken. Now we
   160  	// can decide whether closures should capture their free variables
   161  	// by value or reference.
   162  	for _, closure := range b.closures {
   163  		b.flowClosure(closure.k, closure.clo)
   164  	}
   165  	b.closures = nil
   166  
   167  	for _, loc := range b.allLocs {
   168  		// Try to replace some non-constant expressions with literals.
   169  		b.rewriteWithLiterals(loc.n, loc.curfn)
   170  
   171  		// Check if the node must be heap allocated for certain reasons
   172  		// such as OMAKESLICE for a large slice.
   173  		if why := HeapAllocReason(loc.n); why != "" {
   174  			b.flow(b.heapHole().addr(loc.n, why), loc)
   175  		}
   176  	}
   177  
   178  	b.walkAll()
   179  	b.finish(fns)
   180  }
   181  
   182  func (b *batch) with(fn *ir.Func) *escape {
   183  	return &escape{
   184  		batch:     b,
   185  		curfn:     fn,
   186  		loopDepth: 1,
   187  	}
   188  }
   189  
   190  func (b *batch) initFunc(fn *ir.Func) {
   191  	e := b.with(fn)
   192  	if fn.Esc() != escFuncUnknown {
   193  		base.Fatalf("unexpected node: %v", fn)
   194  	}
   195  	fn.SetEsc(escFuncPlanned)
   196  	if base.Flag.LowerM > 3 {
   197  		ir.Dump("escAnalyze", fn)
   198  	}
   199  
   200  	// Allocate locations for local variables.
   201  	for _, n := range fn.Dcl {
   202  		e.newLoc(n, true)
   203  	}
   204  
   205  	// Also for hidden parameters (e.g., the ".this" parameter to a
   206  	// method value wrapper).
   207  	if fn.OClosure == nil {
   208  		for _, n := range fn.ClosureVars {
   209  			e.newLoc(n.Canonical(), true)
   210  		}
   211  	}
   212  
   213  	// Initialize resultIndex for result parameters.
   214  	for i, f := range fn.Type().Results() {
   215  		e.oldLoc(f.Nname.(*ir.Name)).resultIndex = 1 + i
   216  	}
   217  }
   218  
   219  func (b *batch) walkFunc(fn *ir.Func) {
   220  	e := b.with(fn)
   221  	fn.SetEsc(escFuncStarted)
   222  
   223  	// Identify labels that mark the head of an unstructured loop.
   224  	ir.Visit(fn, func(n ir.Node) {
   225  		switch n.Op() {
   226  		case ir.OLABEL:
   227  			n := n.(*ir.LabelStmt)
   228  			if n.Label.IsBlank() {
   229  				break
   230  			}
   231  			if e.labels == nil {
   232  				e.labels = make(map[*types.Sym]labelState)
   233  			}
   234  			e.labels[n.Label] = nonlooping
   235  
   236  		case ir.OGOTO:
   237  			// If we visited the label before the goto,
   238  			// then this is a looping label.
   239  			n := n.(*ir.BranchStmt)
   240  			if e.labels[n.Label] == nonlooping {
   241  				e.labels[n.Label] = looping
   242  			}
   243  		}
   244  	})
   245  
   246  	e.block(fn.Body)
   247  
   248  	if len(e.labels) != 0 {
   249  		base.FatalfAt(fn.Pos(), "leftover labels after walkFunc")
   250  	}
   251  }
   252  
   253  func (b *batch) flowClosure(k hole, clo *ir.ClosureExpr) {
   254  	for _, cv := range clo.Func.ClosureVars {
   255  		n := cv.Canonical()
   256  		loc := b.oldLoc(cv)
   257  		if !loc.captured {
   258  			base.FatalfAt(cv.Pos(), "closure variable never captured: %v", cv)
   259  		}
   260  
   261  		// Capture by value for variables <= 128 bytes that are never reassigned.
   262  		n.SetByval(!loc.addrtaken && !loc.reassigned && n.Type().Size() <= 128)
   263  		if !n.Byval() {
   264  			n.SetAddrtaken(true)
   265  			if n.Sym().Name == typecheck.LocalDictName {
   266  				base.FatalfAt(n.Pos(), "dictionary variable not captured by value")
   267  			}
   268  		}
   269  
   270  		if base.Flag.LowerM > 1 {
   271  			how := "ref"
   272  			if n.Byval() {
   273  				how = "value"
   274  			}
   275  			base.WarnfAt(n.Pos(), "%v capturing by %s: %v (addr=%v assign=%v width=%d)", n.Curfn, how, n, loc.addrtaken, loc.reassigned, n.Type().Size())
   276  		}
   277  
   278  		// Flow captured variables to closure.
   279  		k := k
   280  		if !cv.Byval() {
   281  			k = k.addr(cv, "reference")
   282  		}
   283  		b.flow(k.note(cv, "captured by a closure"), loc)
   284  	}
   285  }
   286  
   287  func (b *batch) finish(fns []*ir.Func) {
   288  	// Record parameter tags for package export data.
   289  	for _, fn := range fns {
   290  		fn.SetEsc(escFuncTagged)
   291  
   292  		for i, param := range fn.Type().RecvParams() {
   293  			param.Note = b.paramTag(fn, 1+i, param)
   294  		}
   295  	}
   296  
   297  	for _, loc := range b.allLocs {
   298  		n := loc.n
   299  		if n == nil {
   300  			continue
   301  		}
   302  
   303  		if n.Op() == ir.ONAME {
   304  			n := n.(*ir.Name)
   305  			n.Opt = nil
   306  		}
   307  
   308  		// Update n.Esc based on escape analysis results.
   309  
   310  		// Omit escape diagnostics for go/defer wrappers, at least for now.
   311  		// Historically, we haven't printed them, and test cases don't expect them.
   312  		// TODO(mdempsky): Update tests to expect this.
   313  		goDeferWrapper := n.Op() == ir.OCLOSURE && n.(*ir.ClosureExpr).Func.Wrapper()
   314  
   315  		if loc.hasAttr(attrEscapes) {
   316  			if n.Op() == ir.ONAME {
   317  				if base.Flag.CompilingRuntime {
   318  					base.ErrorfAt(n.Pos(), 0, "%v escapes to heap, not allowed in runtime", n)
   319  				}
   320  				if base.Flag.LowerM != 0 {
   321  					base.WarnfAt(n.Pos(), "moved to heap: %v", n)
   322  				}
   323  			} else {
   324  				if base.Flag.LowerM != 0 && !goDeferWrapper {
   325  					if n.Op() == ir.OAPPEND {
   326  						base.WarnfAt(n.Pos(), "append escapes to heap")
   327  					} else {
   328  						base.WarnfAt(n.Pos(), "%v escapes to heap", n)
   329  					}
   330  				}
   331  				if logopt.Enabled() {
   332  					var e_curfn *ir.Func // TODO(mdempsky): Fix.
   333  					logopt.LogOpt(n.Pos(), "escape", "escape", ir.FuncName(e_curfn))
   334  				}
   335  			}
   336  			n.SetEsc(ir.EscHeap)
   337  		} else {
   338  			if base.Flag.LowerM != 0 && n.Op() != ir.ONAME && !goDeferWrapper {
   339  				if n.Op() == ir.OAPPEND {
   340  					base.WarnfAt(n.Pos(), "append does not escape")
   341  				} else {
   342  					base.WarnfAt(n.Pos(), "%v does not escape", n)
   343  				}
   344  			}
   345  			n.SetEsc(ir.EscNone)
   346  			if !loc.hasAttr(attrPersists) {
   347  				switch n.Op() {
   348  				case ir.OCLOSURE:
   349  					n := n.(*ir.ClosureExpr)
   350  					n.SetTransient(true)
   351  				case ir.OMETHVALUE:
   352  					n := n.(*ir.SelectorExpr)
   353  					n.SetTransient(true)
   354  				case ir.OSLICELIT:
   355  					n := n.(*ir.CompLitExpr)
   356  					n.SetTransient(true)
   357  				}
   358  			}
   359  		}
   360  
   361  		// If the result of a string->[]byte conversion is never mutated,
   362  		// then it can simply reuse the string's memory directly.
   363  		if base.Debug.ZeroCopy != 0 {
   364  			if n, ok := n.(*ir.ConvExpr); ok && n.Op() == ir.OSTR2BYTES && !loc.hasAttr(attrMutates) {
   365  				if base.Flag.LowerM >= 1 {
   366  					base.WarnfAt(n.Pos(), "zero-copy string->[]byte conversion")
   367  				}
   368  				n.SetOp(ir.OSTR2BYTESTMP)
   369  			}
   370  		}
   371  	}
   372  }
   373  
   374  // inMutualBatch reports whether function fn is in the batch of
   375  // mutually recursive functions being analyzed. When this is true,
   376  // fn has not yet been analyzed, so its parameters and results
   377  // should be incorporated directly into the flow graph instead of
   378  // relying on its escape analysis tagging.
   379  func (b *batch) inMutualBatch(fn *ir.Name) bool {
   380  	if fn.Defn != nil && fn.Defn.Esc() < escFuncTagged {
   381  		if fn.Defn.Esc() == escFuncUnknown {
   382  			base.FatalfAt(fn.Pos(), "graph inconsistency: %v", fn)
   383  		}
   384  		return true
   385  	}
   386  	return false
   387  }
   388  
   389  const (
   390  	escFuncUnknown = 0 + iota
   391  	escFuncPlanned
   392  	escFuncStarted
   393  	escFuncTagged
   394  )
   395  
   396  // Mark labels that have no backjumps to them as not increasing e.loopdepth.
   397  type labelState int
   398  
   399  const (
   400  	looping labelState = 1 + iota
   401  	nonlooping
   402  )
   403  
   404  func (b *batch) paramTag(fn *ir.Func, narg int, f *types.Field) string {
   405  	name := func() string {
   406  		if f.Nname != nil {
   407  			return f.Nname.Sym().Name
   408  		}
   409  		return fmt.Sprintf("arg#%d", narg)
   410  	}
   411  
   412  	// Only report diagnostics for user code;
   413  	// not for wrappers generated around them.
   414  	// TODO(mdempsky): Generalize this.
   415  	diagnose := base.Flag.LowerM != 0 && !(fn.Wrapper() || fn.Dupok())
   416  
   417  	if len(fn.Body) == 0 {
   418  		// Assume that uintptr arguments must be held live across the call.
   419  		// This is most important for syscall.Syscall.
   420  		// See golang.org/issue/13372.
   421  		// This really doesn't have much to do with escape analysis per se,
   422  		// but we are reusing the ability to annotate an individual function
   423  		// argument and pass those annotations along to importing code.
   424  		fn.Pragma |= ir.UintptrKeepAlive
   425  
   426  		if f.Type.IsUintptr() {
   427  			if diagnose {
   428  				base.WarnfAt(f.Pos, "assuming %v is unsafe uintptr", name())
   429  			}
   430  			return ""
   431  		}
   432  
   433  		if !f.Type.HasPointers() { // don't bother tagging for scalars
   434  			return ""
   435  		}
   436  
   437  		var esc leaks
   438  
   439  		// External functions are assumed unsafe, unless
   440  		// //go:noescape is given before the declaration.
   441  		if fn.Pragma&ir.Noescape != 0 {
   442  			if diagnose && f.Sym != nil {
   443  				base.WarnfAt(f.Pos, "%v does not escape", name())
   444  			}
   445  			esc.AddMutator(0)
   446  			esc.AddCallee(0)
   447  		} else {
   448  			if diagnose && f.Sym != nil {
   449  				base.WarnfAt(f.Pos, "leaking param: %v", name())
   450  			}
   451  			esc.AddHeap(0)
   452  		}
   453  
   454  		return esc.Encode()
   455  	}
   456  
   457  	if fn.Pragma&ir.UintptrEscapes != 0 {
   458  		if f.Type.IsUintptr() {
   459  			if diagnose {
   460  				base.WarnfAt(f.Pos, "marking %v as escaping uintptr", name())
   461  			}
   462  			return ""
   463  		}
   464  		if f.IsDDD() && f.Type.Elem().IsUintptr() {
   465  			// final argument is ...uintptr.
   466  			if diagnose {
   467  				base.WarnfAt(f.Pos, "marking %v as escaping ...uintptr", name())
   468  			}
   469  			return ""
   470  		}
   471  	}
   472  
   473  	if !f.Type.HasPointers() { // don't bother tagging for scalars
   474  		return ""
   475  	}
   476  
   477  	// Unnamed parameters are unused and therefore do not escape.
   478  	if f.Sym == nil || f.Sym.IsBlank() {
   479  		var esc leaks
   480  		return esc.Encode()
   481  	}
   482  
   483  	n := f.Nname.(*ir.Name)
   484  	loc := b.oldLoc(n)
   485  	esc := loc.paramEsc
   486  	esc.Optimize()
   487  
   488  	if diagnose && !loc.hasAttr(attrEscapes) {
   489  		b.reportLeaks(f.Pos, name(), esc, fn.Type())
   490  	}
   491  
   492  	return esc.Encode()
   493  }
   494  
   495  func (b *batch) reportLeaks(pos src.XPos, name string, esc leaks, sig *types.Type) {
   496  	warned := false
   497  	if x := esc.Heap(); x >= 0 {
   498  		if x == 0 {
   499  			base.WarnfAt(pos, "leaking param: %v", name)
   500  		} else {
   501  			// TODO(mdempsky): Mention level=x like below?
   502  			base.WarnfAt(pos, "leaking param content: %v", name)
   503  		}
   504  		warned = true
   505  	}
   506  	for i := 0; i < numEscResults; i++ {
   507  		if x := esc.Result(i); x >= 0 {
   508  			res := sig.Result(i).Nname.Sym().Name
   509  			base.WarnfAt(pos, "leaking param: %v to result %v level=%d", name, res, x)
   510  			warned = true
   511  		}
   512  	}
   513  
   514  	if base.Debug.EscapeMutationsCalls <= 0 {
   515  		if !warned {
   516  			base.WarnfAt(pos, "%v does not escape", name)
   517  		}
   518  		return
   519  	}
   520  
   521  	if x := esc.Mutator(); x >= 0 {
   522  		base.WarnfAt(pos, "mutates param: %v derefs=%v", name, x)
   523  		warned = true
   524  	}
   525  	if x := esc.Callee(); x >= 0 {
   526  		base.WarnfAt(pos, "calls param: %v derefs=%v", name, x)
   527  		warned = true
   528  	}
   529  
   530  	if !warned {
   531  		base.WarnfAt(pos, "%v does not escape, mutate, or call", name)
   532  	}
   533  }
   534  
   535  // rewriteWithLiterals attempts to replace certain non-constant expressions
   536  // within n with a literal if possible.
   537  func (b *batch) rewriteWithLiterals(n ir.Node, fn *ir.Func) {
   538  	if n == nil || fn == nil {
   539  		return
   540  	}
   541  
   542  	assignTemp := func(pos src.XPos, n ir.Node, init *ir.Nodes) {
   543  		// Preserve any side effects of n by assigning it to an otherwise unused temp.
   544  		tmp := typecheck.TempAt(pos, fn, n.Type())
   545  		init.Append(typecheck.Stmt(ir.NewDecl(pos, ir.ODCL, tmp)))
   546  		init.Append(typecheck.Stmt(ir.NewAssignStmt(pos, tmp, n)))
   547  	}
   548  
   549  	switch n.Op() {
   550  	case ir.OMAKESLICE:
   551  		// Check if we can replace a non-constant argument to make with
   552  		// a literal to allow for this slice to be stack allocated if otherwise allowed.
   553  		n := n.(*ir.MakeExpr)
   554  
   555  		r := &n.Cap
   556  		if n.Cap == nil {
   557  			r = &n.Len
   558  		}
   559  
   560  		if (*r).Op() != ir.OLITERAL {
   561  			// Look up a cached ReassignOracle for the function, lazily computing one if needed.
   562  			ro := b.reassignOracle(fn)
   563  			if ro == nil {
   564  				base.Fatalf("no ReassignOracle for function %v with closure parent %v", fn, fn.ClosureParent)
   565  			}
   566  			if s := ro.StaticValue(*r); s.Op() == ir.OLITERAL {
   567  				lit, ok := s.(*ir.BasicLit)
   568  				if !ok || lit.Val().Kind() != constant.Int {
   569  					base.Fatalf("unexpected BasicLit Kind")
   570  				}
   571  				if constant.Compare(lit.Val(), token.GEQ, constant.MakeInt64(0)) {
   572  					if !base.LiteralAllocHash.MatchPos(n.Pos(), nil) {
   573  						// De-selected by literal alloc optimizations debug hash.
   574  						return
   575  					}
   576  					// Preserve any side effects of the original expression, then replace it.
   577  					assignTemp(n.Pos(), *r, n.PtrInit())
   578  					*r = ir.NewBasicLit(n.Pos(), (*r).Type(), lit.Val())
   579  				}
   580  			}
   581  		}
   582  	case ir.OCONVIFACE:
   583  		// Check if we can replace a non-constant expression in an interface conversion with
   584  		// a literal to avoid heap allocating the underlying interface value.
   585  		conv := n.(*ir.ConvExpr)
   586  		if conv.X.Op() != ir.OLITERAL && !conv.X.Type().IsInterface() {
   587  			// TODO(thepudds): likely could avoid some work by tightening the check of conv.X's type.
   588  			// Look up a cached ReassignOracle for the function, lazily computing one if needed.
   589  			ro := b.reassignOracle(fn)
   590  			if ro == nil {
   591  				base.Fatalf("no ReassignOracle for function %v with closure parent %v", fn, fn.ClosureParent)
   592  			}
   593  			v := ro.StaticValue(conv.X)
   594  			if v != nil && v.Op() == ir.OLITERAL && ir.ValidTypeForConst(conv.X.Type(), v.Val()) {
   595  				if !base.LiteralAllocHash.MatchPos(n.Pos(), nil) {
   596  					// De-selected by literal alloc optimizations debug hash.
   597  					return
   598  				}
   599  				if base.Debug.EscapeDebug >= 3 {
   600  					base.WarnfAt(n.Pos(), "rewriting OCONVIFACE value from %v (%v) to %v (%v)", conv.X, conv.X.Type(), v, v.Type())
   601  				}
   602  				// Preserve any side effects of the original expression, then replace it.
   603  				assignTemp(conv.Pos(), conv.X, conv.PtrInit())
   604  				v := v.(*ir.BasicLit)
   605  				conv.X = ir.NewBasicLit(conv.Pos(), conv.X.Type(), v.Val())
   606  				typecheck.Expr(conv)
   607  			}
   608  		}
   609  	}
   610  }
   611  
   612  // reassignOracle returns an initialized *ir.ReassignOracle for fn.
   613  // If fn is a closure, it returns the ReassignOracle for the ultimate parent.
   614  //
   615  // A new ReassignOracle is initialized lazily if needed, and the result
   616  // is cached to reduce duplicative work of preparing a ReassignOracle.
   617  func (b *batch) reassignOracle(fn *ir.Func) *ir.ReassignOracle {
   618  	if ro, ok := b.reassignOracles[fn]; ok {
   619  		return ro // Hit.
   620  	}
   621  
   622  	// For closures, we want the ultimate parent's ReassignOracle,
   623  	// so walk up the parent chain, if any.
   624  	f := fn
   625  	for f.ClosureParent != nil && !f.ClosureParent.IsPackageInit() {
   626  		f = f.ClosureParent
   627  	}
   628  
   629  	if f != fn {
   630  		// We found a parent.
   631  		ro := b.reassignOracles[f]
   632  		if ro != nil {
   633  			// Hit, via a parent. Before returning, store this ro for the original fn as well.
   634  			b.reassignOracles[fn] = ro
   635  			return ro
   636  		}
   637  	}
   638  
   639  	// Miss. We did not find a ReassignOracle for fn or a parent, so lazily create one.
   640  	ro := &ir.ReassignOracle{}
   641  	ro.Init(f)
   642  
   643  	// Cache the answer for the original fn.
   644  	b.reassignOracles[fn] = ro
   645  	if f != fn {
   646  		// Cache for the parent as well.
   647  		b.reassignOracles[f] = ro
   648  	}
   649  	return ro
   650  }
   651
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