irgraph.go

Documentation: cmd/compile/internal/pgo

     1  // Copyright 2022 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  // A note on line numbers: when working with line numbers, we always use the
     6  // binary-visible relative line number. i.e., the line number as adjusted by
     7  // //line directives (ctxt.InnermostPos(ir.Node.Pos()).RelLine()). Use
     8  // NodeLineOffset to compute line offsets.
     9  //
    10  // If you are thinking, "wait, doesn't that just make things more complex than
    11  // using the real line number?", then you are 100% correct. Unfortunately,
    12  // pprof profiles generated by the runtime always contain line numbers as
    13  // adjusted by //line directives (because that is what we put in pclntab). Thus
    14  // for the best behavior when attempting to match the source with the profile
    15  // it makes sense to use the same line number space.
    16  //
    17  // Some of the effects of this to keep in mind:
    18  //
    19  //  - For files without //line directives there is no impact, as RelLine() ==
    20  //    Line().
    21  //  - For functions entirely covered by the same //line directive (i.e., a
    22  //    directive before the function definition and no directives within the
    23  //    function), there should also be no impact, as line offsets within the
    24  //    function should be the same as the real line offsets.
    25  //  - Functions containing //line directives may be impacted. As fake line
    26  //    numbers need not be monotonic, we may compute negative line offsets. We
    27  //    should accept these and attempt to use them for best-effort matching, as
    28  //    these offsets should still match if the source is unchanged, and may
    29  //    continue to match with changed source depending on the impact of the
    30  //    changes on fake line numbers.
    31  //  - Functions containing //line directives may also contain duplicate lines,
    32  //    making it ambiguous which call the profile is referencing. This is a
    33  //    similar problem to multiple calls on a single real line, as we don't
    34  //    currently track column numbers.
    35  //
    36  // Long term it would be best to extend pprof profiles to include real line
    37  // numbers. Until then, we have to live with these complexities. Luckily,
    38  // //line directives that change line numbers in strange ways should be rare,
    39  // and failing PGO matching on these files is not too big of a loss.
    40  
    41  package pgo
    42  
    43  import (
    44  	"cmd/compile/internal/base"
    45  	"cmd/compile/internal/ir"
    46  	"cmd/compile/internal/pgo/internal/graph"
    47  	"cmd/compile/internal/typecheck"
    48  	"cmd/compile/internal/types"
    49  	"errors"
    50  	"fmt"
    51  	"internal/profile"
    52  	"os"
    53  	"sort"
    54  )
    55  
    56  // IRGraph is a call graph with nodes pointing to IRs of functions and edges
    57  // carrying weights and callsite information.
    58  //
    59  // Nodes for indirect calls may have missing IR (IRNode.AST == nil) if the node
    60  // is not visible from this package (e.g., not in the transitive deps). Keeping
    61  // these nodes allows determining the hottest edge from a call even if that
    62  // callee is not available.
    63  //
    64  // TODO(prattmic): Consider merging this data structure with Graph. This is
    65  // effectively a copy of Graph aggregated to line number and pointing to IR.
    66  type IRGraph struct {
    67  	// Nodes of the graph. Each node represents a function, keyed by linker
    68  	// symbol name.
    69  	IRNodes map[string]*IRNode
    70  }
    71  
    72  // IRNode represents a node (function) in the IRGraph.
    73  type IRNode struct {
    74  	// Pointer to the IR of the Function represented by this node.
    75  	AST *ir.Func
    76  	// Linker symbol name of the Function represented by this node.
    77  	// Populated only if AST == nil.
    78  	LinkerSymbolName string
    79  
    80  	// Set of out-edges in the callgraph. The map uniquely identifies each
    81  	// edge based on the callsite and callee, for fast lookup.
    82  	OutEdges map[NamedCallEdge]*IREdge
    83  }
    84  
    85  // Name returns the symbol name of this function.
    86  func (i *IRNode) Name() string {
    87  	if i.AST != nil {
    88  		return ir.LinkFuncName(i.AST)
    89  	}
    90  	return i.LinkerSymbolName
    91  }
    92  
    93  // IREdge represents a call edge in the IRGraph with source, destination,
    94  // weight, callsite, and line number information.
    95  type IREdge struct {
    96  	// Source and destination of the edge in IRNode.
    97  	Src, Dst       *IRNode
    98  	Weight         int64
    99  	CallSiteOffset int // Line offset from function start line.
   100  }
   101  
   102  // NamedCallEdge identifies a call edge by linker symbol names and call site
   103  // offset.
   104  type NamedCallEdge struct {
   105  	CallerName     string
   106  	CalleeName     string
   107  	CallSiteOffset int // Line offset from function start line.
   108  }
   109  
   110  // NamedEdgeMap contains all unique call edges in the profile and their
   111  // edge weight.
   112  type NamedEdgeMap struct {
   113  	Weight map[NamedCallEdge]int64
   114  
   115  	// ByWeight lists all keys in Weight, sorted by edge weight.
   116  	ByWeight []NamedCallEdge
   117  }
   118  
   119  // CallSiteInfo captures call-site information and its caller/callee.
   120  type CallSiteInfo struct {
   121  	LineOffset int // Line offset from function start line.
   122  	Caller     *ir.Func
   123  	Callee     *ir.Func
   124  }
   125  
   126  // Profile contains the processed PGO profile and weighted call graph used for
   127  // PGO optimizations.
   128  type Profile struct {
   129  	// Aggregated edge weights across the profile. This helps us determine
   130  	// the percentage threshold for hot/cold partitioning.
   131  	TotalWeight int64
   132  
   133  	// NamedEdgeMap contains all unique call edges in the profile and their
   134  	// edge weight.
   135  	NamedEdgeMap NamedEdgeMap
   136  
   137  	// WeightedCG represents the IRGraph built from profile, which we will
   138  	// update as part of inlining.
   139  	WeightedCG *IRGraph
   140  }
   141  
   142  // New generates a profile-graph from the profile.
   143  func New(profileFile string) (*Profile, error) {
   144  	f, err := os.Open(profileFile)
   145  	if err != nil {
   146  		return nil, fmt.Errorf("error opening profile: %w", err)
   147  	}
   148  	defer f.Close()
   149  	p, err := profile.Parse(f)
   150  	if errors.Is(err, profile.ErrNoData) {
   151  		// Treat a completely empty file the same as a profile with no
   152  		// samples: nothing to do.
   153  		return nil, nil
   154  	} else if err != nil {
   155  		return nil, fmt.Errorf("error parsing profile: %w", err)
   156  	}
   157  
   158  	if len(p.Sample) == 0 {
   159  		// We accept empty profiles, but there is nothing to do.
   160  		return nil, nil
   161  	}
   162  
   163  	valueIndex := -1
   164  	for i, s := range p.SampleType {
   165  		// Samples count is the raw data collected, and CPU nanoseconds is just
   166  		// a scaled version of it, so either one we can find is fine.
   167  		if (s.Type == "samples" && s.Unit == "count") ||
   168  			(s.Type == "cpu" && s.Unit == "nanoseconds") {
   169  			valueIndex = i
   170  			break
   171  		}
   172  	}
   173  
   174  	if valueIndex == -1 {
   175  		return nil, fmt.Errorf(`profile does not contain a sample index with value/type "samples/count" or cpu/nanoseconds"`)
   176  	}
   177  
   178  	g := graph.NewGraph(p, &graph.Options{
   179  		SampleValue: func(v []int64) int64 { return v[valueIndex] },
   180  	})
   181  
   182  	namedEdgeMap, totalWeight, err := createNamedEdgeMap(g)
   183  	if err != nil {
   184  		return nil, err
   185  	}
   186  
   187  	if totalWeight == 0 {
   188  		return nil, nil // accept but ignore profile with no samples.
   189  	}
   190  
   191  	// Create package-level call graph with weights from profile and IR.
   192  	wg := createIRGraph(namedEdgeMap)
   193  
   194  	return &Profile{
   195  		TotalWeight:  totalWeight,
   196  		NamedEdgeMap: namedEdgeMap,
   197  		WeightedCG:   wg,
   198  	}, nil
   199  }
   200  
   201  // createNamedEdgeMap builds a map of callsite-callee edge weights from the
   202  // profile-graph.
   203  //
   204  // Caller should ignore the profile if totalWeight == 0.
   205  func createNamedEdgeMap(g *graph.Graph) (edgeMap NamedEdgeMap, totalWeight int64, err error) {
   206  	seenStartLine := false
   207  
   208  	// Process graph and build various node and edge maps which will
   209  	// be consumed by AST walk.
   210  	weight := make(map[NamedCallEdge]int64)
   211  	for _, n := range g.Nodes {
   212  		seenStartLine = seenStartLine || n.Info.StartLine != 0
   213  
   214  		canonicalName := n.Info.Name
   215  		// Create the key to the nodeMapKey.
   216  		namedEdge := NamedCallEdge{
   217  			CallerName:     canonicalName,
   218  			CallSiteOffset: n.Info.Lineno - n.Info.StartLine,
   219  		}
   220  
   221  		for _, e := range n.Out {
   222  			totalWeight += e.WeightValue()
   223  			namedEdge.CalleeName = e.Dest.Info.Name
   224  			// Create new entry or increment existing entry.
   225  			weight[namedEdge] += e.WeightValue()
   226  		}
   227  	}
   228  
   229  	if totalWeight == 0 {
   230  		return NamedEdgeMap{}, 0, nil // accept but ignore profile with no samples.
   231  	}
   232  
   233  	if !seenStartLine {
   234  		// TODO(prattmic): If Function.start_line is missing we could
   235  		// fall back to using absolute line numbers, which is better
   236  		// than nothing.
   237  		return NamedEdgeMap{}, 0, fmt.Errorf("profile missing Function.start_line data (Go version of profiled application too old? Go 1.20+ automatically adds this to profiles)")
   238  	}
   239  
   240  	byWeight := make([]NamedCallEdge, 0, len(weight))
   241  	for namedEdge := range weight {
   242  		byWeight = append(byWeight, namedEdge)
   243  	}
   244  	sort.Slice(byWeight, func(i, j int) bool {
   245  		ei, ej := byWeight[i], byWeight[j]
   246  		if wi, wj := weight[ei], weight[ej]; wi != wj {
   247  			return wi > wj // want larger weight first
   248  		}
   249  		// same weight, order by name/line number
   250  		if ei.CallerName != ej.CallerName {
   251  			return ei.CallerName < ej.CallerName
   252  		}
   253  		if ei.CalleeName != ej.CalleeName {
   254  			return ei.CalleeName < ej.CalleeName
   255  		}
   256  		return ei.CallSiteOffset < ej.CallSiteOffset
   257  	})
   258  
   259  	edgeMap = NamedEdgeMap{
   260  		Weight:   weight,
   261  		ByWeight: byWeight,
   262  	}
   263  
   264  	return edgeMap, totalWeight, nil
   265  }
   266  
   267  // initializeIRGraph builds the IRGraph by visiting all the ir.Func in decl list
   268  // of a package.
   269  func createIRGraph(namedEdgeMap NamedEdgeMap) *IRGraph {
   270  	g := &IRGraph{
   271  		IRNodes: make(map[string]*IRNode),
   272  	}
   273  
   274  	// Bottomup walk over the function to create IRGraph.
   275  	ir.VisitFuncsBottomUp(typecheck.Target.Funcs, func(list []*ir.Func, recursive bool) {
   276  		for _, fn := range list {
   277  			visitIR(fn, namedEdgeMap, g)
   278  		}
   279  	})
   280  
   281  	// Add additional edges for indirect calls. This must be done second so
   282  	// that IRNodes is fully populated (see the dummy node TODO in
   283  	// addIndirectEdges).
   284  	//
   285  	// TODO(prattmic): visitIR above populates the graph via direct calls
   286  	// discovered via the IR. addIndirectEdges populates the graph via
   287  	// calls discovered via the profile. This combination of opposite
   288  	// approaches is a bit awkward, particularly because direct calls are
   289  	// discoverable via the profile as well. Unify these into a single
   290  	// approach.
   291  	addIndirectEdges(g, namedEdgeMap)
   292  
   293  	return g
   294  }
   295  
   296  // visitIR traverses the body of each ir.Func adds edges to g from ir.Func to
   297  // any called function in the body.
   298  func visitIR(fn *ir.Func, namedEdgeMap NamedEdgeMap, g *IRGraph) {
   299  	name := ir.LinkFuncName(fn)
   300  	node, ok := g.IRNodes[name]
   301  	if !ok {
   302  		node = &IRNode{
   303  			AST: fn,
   304  		}
   305  		g.IRNodes[name] = node
   306  	}
   307  
   308  	// Recursively walk over the body of the function to create IRGraph edges.
   309  	createIRGraphEdge(fn, node, name, namedEdgeMap, g)
   310  }
   311  
   312  // createIRGraphEdge traverses the nodes in the body of ir.Func and adds edges
   313  // between the callernode which points to the ir.Func and the nodes in the
   314  // body.
   315  func createIRGraphEdge(fn *ir.Func, callernode *IRNode, name string, namedEdgeMap NamedEdgeMap, g *IRGraph) {
   316  	ir.VisitList(fn.Body, func(n ir.Node) {
   317  		switch n.Op() {
   318  		case ir.OCALLFUNC:
   319  			call := n.(*ir.CallExpr)
   320  			// Find the callee function from the call site and add the edge.
   321  			callee := DirectCallee(call.Fun)
   322  			if callee != nil {
   323  				addIREdge(callernode, name, n, callee, namedEdgeMap, g)
   324  			}
   325  		case ir.OCALLMETH:
   326  			call := n.(*ir.CallExpr)
   327  			// Find the callee method from the call site and add the edge.
   328  			callee := ir.MethodExprName(call.Fun).Func
   329  			addIREdge(callernode, name, n, callee, namedEdgeMap, g)
   330  		}
   331  	})
   332  }
   333  
   334  // NodeLineOffset returns the line offset of n in fn.
   335  func NodeLineOffset(n ir.Node, fn *ir.Func) int {
   336  	// See "A note on line numbers" at the top of the file.
   337  	line := int(base.Ctxt.InnermostPos(n.Pos()).RelLine())
   338  	startLine := int(base.Ctxt.InnermostPos(fn.Pos()).RelLine())
   339  	return line - startLine
   340  }
   341  
   342  // addIREdge adds an edge between caller and new node that points to `callee`
   343  // based on the profile-graph and NodeMap.
   344  func addIREdge(callerNode *IRNode, callerName string, call ir.Node, callee *ir.Func, namedEdgeMap NamedEdgeMap, g *IRGraph) {
   345  	calleeName := ir.LinkFuncName(callee)
   346  	calleeNode, ok := g.IRNodes[calleeName]
   347  	if !ok {
   348  		calleeNode = &IRNode{
   349  			AST: callee,
   350  		}
   351  		g.IRNodes[calleeName] = calleeNode
   352  	}
   353  
   354  	namedEdge := NamedCallEdge{
   355  		CallerName:     callerName,
   356  		CalleeName:     calleeName,
   357  		CallSiteOffset: NodeLineOffset(call, callerNode.AST),
   358  	}
   359  
   360  	// Add edge in the IRGraph from caller to callee.
   361  	edge := &IREdge{
   362  		Src:            callerNode,
   363  		Dst:            calleeNode,
   364  		Weight:         namedEdgeMap.Weight[namedEdge],
   365  		CallSiteOffset: namedEdge.CallSiteOffset,
   366  	}
   367  
   368  	if callerNode.OutEdges == nil {
   369  		callerNode.OutEdges = make(map[NamedCallEdge]*IREdge)
   370  	}
   371  	callerNode.OutEdges[namedEdge] = edge
   372  }
   373  
   374  // LookupFunc looks up a function or method in export data. It is expected to
   375  // be overridden by package noder, to break a dependency cycle.
   376  var LookupFunc = func(fullName string) (*ir.Func, error) {
   377  	base.Fatalf("pgo.LookupMethodFunc not overridden")
   378  	panic("unreachable")
   379  }
   380  
   381  // addIndirectEdges adds indirect call edges found in the profile to the graph,
   382  // to be used for devirtualization.
   383  //
   384  // N.B. despite the name, addIndirectEdges will add any edges discovered via
   385  // the profile. We don't know for sure that they are indirect, but assume they
   386  // are since direct calls would already be added. (e.g., direct calls that have
   387  // been deleted from source since the profile was taken would be added here).
   388  //
   389  // TODO(prattmic): Devirtualization runs before inlining, so we can't devirtualize
   390  // calls inside inlined call bodies. If we did add that, we'd need edges from
   391  // inlined bodies as well.
   392  func addIndirectEdges(g *IRGraph, namedEdgeMap NamedEdgeMap) {
   393  	// g.IRNodes is populated with the set of functions in the local
   394  	// package build by VisitIR. We want to filter for local functions
   395  	// below, but we also add unknown callees to IRNodes as we go. So make
   396  	// an initial copy of IRNodes to recall just the local functions.
   397  	localNodes := make(map[string]*IRNode, len(g.IRNodes))
   398  	for k, v := range g.IRNodes {
   399  		localNodes[k] = v
   400  	}
   401  
   402  	// N.B. We must consider edges in a stable order because export data
   403  	// lookup order (LookupMethodFunc, below) can impact the export data of
   404  	// this package, which must be stable across different invocations for
   405  	// reproducibility.
   406  	//
   407  	// The weight ordering of ByWeight is irrelevant, it just happens to be
   408  	// an ordered list of edges that is already available.
   409  	for _, key := range namedEdgeMap.ByWeight {
   410  		weight := namedEdgeMap.Weight[key]
   411  		// All callers in the local package build were added to IRNodes
   412  		// in VisitIR. If a caller isn't in the local package build we
   413  		// can skip adding edges, since we won't be devirtualizing in
   414  		// them anyway. This keeps the graph smaller.
   415  		callerNode, ok := localNodes[key.CallerName]
   416  		if !ok {
   417  			continue
   418  		}
   419  
   420  		// Already handled this edge?
   421  		if _, ok := callerNode.OutEdges[key]; ok {
   422  			continue
   423  		}
   424  
   425  		calleeNode, ok := g.IRNodes[key.CalleeName]
   426  		if !ok {
   427  			// IR is missing for this callee. VisitIR populates
   428  			// IRNodes with all functions discovered via local
   429  			// package function declarations and calls. This
   430  			// function may still be available from export data of
   431  			// a transitive dependency.
   432  			//
   433  			// TODO(prattmic): Parameterized types/functions are
   434  			// not supported.
   435  			//
   436  			// TODO(prattmic): This eager lookup during graph load
   437  			// is simple, but wasteful. We are likely to load many
   438  			// functions that we never need. We could delay load
   439  			// until we actually need the method in
   440  			// devirtualization. Instantiation of generic functions
   441  			// will likely need to be done at the devirtualization
   442  			// site, if at all.
   443  			fn, err := LookupFunc(key.CalleeName)
   444  			if err == nil {
   445  				if base.Debug.PGODebug >= 3 {
   446  					fmt.Printf("addIndirectEdges: %s found in export data\n", key.CalleeName)
   447  				}
   448  				calleeNode = &IRNode{AST: fn}
   449  
   450  				// N.B. we could call createIRGraphEdge to add
   451  				// direct calls in this newly-imported
   452  				// function's body to the graph. Similarly, we
   453  				// could add to this function's queue to add
   454  				// indirect calls. However, those would be
   455  				// useless given the visit order of inlining,
   456  				// and the ordering of PGO devirtualization and
   457  				// inlining. This function can only be used as
   458  				// an inlined body. We will never do PGO
   459  				// devirtualization inside an inlined call. Nor
   460  				// will we perform inlining inside an inlined
   461  				// call.
   462  			} else {
   463  				// Still not found. Most likely this is because
   464  				// the callee isn't in the transitive deps of
   465  				// this package.
   466  				//
   467  				// Record this call anyway. If this is the hottest,
   468  				// then we want to skip devirtualization rather than
   469  				// devirtualizing to the second most common callee.
   470  				if base.Debug.PGODebug >= 3 {
   471  					fmt.Printf("addIndirectEdges: %s not found in export data: %v\n", key.CalleeName, err)
   472  				}
   473  				calleeNode = &IRNode{LinkerSymbolName: key.CalleeName}
   474  			}
   475  
   476  			// Add dummy node back to IRNodes. We don't need this
   477  			// directly, but PrintWeightedCallGraphDOT uses these
   478  			// to print nodes.
   479  			g.IRNodes[key.CalleeName] = calleeNode
   480  		}
   481  		edge := &IREdge{
   482  			Src:            callerNode,
   483  			Dst:            calleeNode,
   484  			Weight:         weight,
   485  			CallSiteOffset: key.CallSiteOffset,
   486  		}
   487  
   488  		if callerNode.OutEdges == nil {
   489  			callerNode.OutEdges = make(map[NamedCallEdge]*IREdge)
   490  		}
   491  		callerNode.OutEdges[key] = edge
   492  	}
   493  }
   494  
   495  // WeightInPercentage converts profile weights to a percentage.
   496  func WeightInPercentage(value int64, total int64) float64 {
   497  	return (float64(value) / float64(total)) * 100
   498  }
   499  
   500  // PrintWeightedCallGraphDOT prints IRGraph in DOT format.
   501  func (p *Profile) PrintWeightedCallGraphDOT(edgeThreshold float64) {
   502  	fmt.Printf("\ndigraph G {\n")
   503  	fmt.Printf("forcelabels=true;\n")
   504  
   505  	// List of functions in this package.
   506  	funcs := make(map[string]struct{})
   507  	ir.VisitFuncsBottomUp(typecheck.Target.Funcs, func(list []*ir.Func, recursive bool) {
   508  		for _, f := range list {
   509  			name := ir.LinkFuncName(f)
   510  			funcs[name] = struct{}{}
   511  		}
   512  	})
   513  
   514  	// Determine nodes of DOT.
   515  	//
   516  	// Note that ir.Func may be nil for functions not visible from this
   517  	// package.
   518  	nodes := make(map[string]*ir.Func)
   519  	for name := range funcs {
   520  		if n, ok := p.WeightedCG.IRNodes[name]; ok {
   521  			for _, e := range n.OutEdges {
   522  				if _, ok := nodes[e.Src.Name()]; !ok {
   523  					nodes[e.Src.Name()] = e.Src.AST
   524  				}
   525  				if _, ok := nodes[e.Dst.Name()]; !ok {
   526  					nodes[e.Dst.Name()] = e.Dst.AST
   527  				}
   528  			}
   529  			if _, ok := nodes[n.Name()]; !ok {
   530  				nodes[n.Name()] = n.AST
   531  			}
   532  		}
   533  	}
   534  
   535  	// Print nodes.
   536  	for name, ast := range nodes {
   537  		if _, ok := p.WeightedCG.IRNodes[name]; ok {
   538  			style := "solid"
   539  			if ast == nil {
   540  				style = "dashed"
   541  			}
   542  
   543  			if ast != nil && ast.Inl != nil {
   544  				fmt.Printf("\"%v\" [color=black, style=%s, label=\"%v,inl_cost=%d\"];\n", name, style, name, ast.Inl.Cost)
   545  			} else {
   546  				fmt.Printf("\"%v\" [color=black, style=%s, label=\"%v\"];\n", name, style, name)
   547  			}
   548  		}
   549  	}
   550  	// Print edges.
   551  	ir.VisitFuncsBottomUp(typecheck.Target.Funcs, func(list []*ir.Func, recursive bool) {
   552  		for _, f := range list {
   553  			name := ir.LinkFuncName(f)
   554  			if n, ok := p.WeightedCG.IRNodes[name]; ok {
   555  				for _, e := range n.OutEdges {
   556  					style := "solid"
   557  					if e.Dst.AST == nil {
   558  						style = "dashed"
   559  					}
   560  					color := "black"
   561  					edgepercent := WeightInPercentage(e.Weight, p.TotalWeight)
   562  					if edgepercent > edgeThreshold {
   563  						color = "red"
   564  					}
   565  
   566  					fmt.Printf("edge [color=%s, style=%s];\n", color, style)
   567  					fmt.Printf("\"%v\" -> \"%v\" [label=\"%.2f\"];\n", n.Name(), e.Dst.Name(), edgepercent)
   568  				}
   569  			}
   570  		}
   571  	})
   572  	fmt.Printf("}\n")
   573  }
   574  
   575  // DirectCallee takes a function-typed expression and returns the underlying
   576  // function that it refers to if statically known. Otherwise, it returns nil.
   577  //
   578  // Equivalent to inline.inlCallee without calling CanInline on closures.
   579  func DirectCallee(fn ir.Node) *ir.Func {
   580  	fn = ir.StaticValue(fn)
   581  	switch fn.Op() {
   582  	case ir.OMETHEXPR:
   583  		fn := fn.(*ir.SelectorExpr)
   584  		n := ir.MethodExprName(fn)
   585  		// Check that receiver type matches fn.X.
   586  		// TODO(mdempsky): Handle implicit dereference
   587  		// of pointer receiver argument?
   588  		if n == nil || !types.Identical(n.Type().Recv().Type, fn.X.Type()) {
   589  			return nil
   590  		}
   591  		return n.Func
   592  	case ir.ONAME:
   593  		fn := fn.(*ir.Name)
   594  		if fn.Class == ir.PFUNC {
   595  			return fn.Func
   596  		}
   597  	case ir.OCLOSURE:
   598  		fn := fn.(*ir.ClosureExpr)
   599  		c := fn.Func
   600  		return c
   601  	}
   602  	return nil
   603  }
   604
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