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// Copyright 2022 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// A note on line numbers: when working with line numbers, we always use the
// binary-visible relative line number. i.e., the line number as adjusted by
// //line directives (ctxt.InnermostPos(ir.Node.Pos()).RelLine()). Use
// NodeLineOffset to compute line offsets.
//
// If you are thinking, "wait, doesn't that just make things more complex than
// using the real line number?", then you are 100% correct. Unfortunately,
// pprof profiles generated by the runtime always contain line numbers as
// adjusted by //line directives (because that is what we put in pclntab). Thus
// for the best behavior when attempting to match the source with the profile
// it makes sense to use the same line number space.
//
// Some of the effects of this to keep in mind:
//
// - For files without //line directives there is no impact, as RelLine() ==
// Line().
// - For functions entirely covered by the same //line directive (i.e., a
// directive before the function definition and no directives within the
// function), there should also be no impact, as line offsets within the
// function should be the same as the real line offsets.
// - Functions containing //line directives may be impacted. As fake line
// numbers need not be monotonic, we may compute negative line offsets. We
// should accept these and attempt to use them for best-effort matching, as
// these offsets should still match if the source is unchanged, and may
// continue to match with changed source depending on the impact of the
// changes on fake line numbers.
// - Functions containing //line directives may also contain duplicate lines,
// making it ambiguous which call the profile is referencing. This is a
// similar problem to multiple calls on a single real line, as we don't
// currently track column numbers.
//
// Long term it would be best to extend pprof profiles to include real line
// numbers. Until then, we have to live with these complexities. Luckily,
// //line directives that change line numbers in strange ways should be rare,
// and failing PGO matching on these files is not too big of a loss.
package pgo
import (
"cmd/compile/internal/base"
"cmd/compile/internal/ir"
"cmd/compile/internal/pgo/internal/graph"
"cmd/compile/internal/typecheck"
"cmd/compile/internal/types"
"fmt"
"internal/profile"
"os"
)
// IRGraph is a call graph with nodes pointing to IRs of functions and edges
// carrying weights and callsite information.
//
// Nodes for indirect calls may have missing IR (IRNode.AST == nil) if the node
// is not visible from this package (e.g., not in the transitive deps). Keeping
// these nodes allows determining the hottest edge from a call even if that
// callee is not available.
//
// TODO(prattmic): Consider merging this data structure with Graph. This is
// effectively a copy of Graph aggregated to line number and pointing to IR.
type IRGraph struct {
// Nodes of the graph
IRNodes map[string]*IRNode
}
// IRNode represents a node (function) in the IRGraph.
type IRNode struct {
// Pointer to the IR of the Function represented by this node.
AST *ir.Func
// Linker symbol name of the Function represented by this node.
// Populated only if AST == nil.
LinkerSymbolName string
// Set of out-edges in the callgraph. The map uniquely identifies each
// edge based on the callsite and callee, for fast lookup.
OutEdges map[NodeMapKey]*IREdge
}
// Name returns the symbol name of this function.
func (i *IRNode) Name() string {
if i.AST != nil {
return ir.LinkFuncName(i.AST)
}
return i.LinkerSymbolName
}
// IREdge represents a call edge in the IRGraph with source, destination,
// weight, callsite, and line number information.
type IREdge struct {
// Source and destination of the edge in IRNode.
Src, Dst *IRNode
Weight int64
CallSiteOffset int // Line offset from function start line.
}
// NodeMapKey represents a hash key to identify unique call-edges in profile
// and in IR. Used for deduplication of call edges found in profile.
//
// TODO(prattmic): rename to something more descriptive.
type NodeMapKey struct {
CallerName string
CalleeName string
CallSiteOffset int // Line offset from function start line.
}
// Weights capture both node weight and edge weight.
type Weights struct {
NFlat int64
NCum int64
EWeight int64
}
// CallSiteInfo captures call-site information and its caller/callee.
type CallSiteInfo struct {
LineOffset int // Line offset from function start line.
Caller *ir.Func
Callee *ir.Func
}
// Profile contains the processed PGO profile and weighted call graph used for
// PGO optimizations.
type Profile struct {
// Aggregated NodeWeights and EdgeWeights across the profile. This
// helps us determine the percentage threshold for hot/cold
// partitioning.
TotalNodeWeight int64
TotalEdgeWeight int64
// NodeMap contains all unique call-edges in the profile and their
// aggregated weight.
NodeMap map[NodeMapKey]*Weights
// WeightedCG represents the IRGraph built from profile, which we will
// update as part of inlining.
WeightedCG *IRGraph
}
// New generates a profile-graph from the profile.
func New(profileFile string) (*Profile, error) {
f, err := os.Open(profileFile)
if err != nil {
return nil, fmt.Errorf("error opening profile: %w", err)
}
defer f.Close()
profile, err := profile.Parse(f)
if err != nil {
return nil, fmt.Errorf("error parsing profile: %w", err)
}
if len(profile.Sample) == 0 {
// We accept empty profiles, but there is nothing to do.
return nil, nil
}
valueIndex := -1
for i, s := range profile.SampleType {
// Samples count is the raw data collected, and CPU nanoseconds is just
// a scaled version of it, so either one we can find is fine.
if (s.Type == "samples" && s.Unit == "count") ||
(s.Type == "cpu" && s.Unit == "nanoseconds") {
valueIndex = i
break
}
}
if valueIndex == -1 {
return nil, fmt.Errorf(`profile does not contain a sample index with value/type "samples/count" or cpu/nanoseconds"`)
}
g := graph.NewGraph(profile, &graph.Options{
SampleValue: func(v []int64) int64 { return v[valueIndex] },
})
p := &Profile{
NodeMap: make(map[NodeMapKey]*Weights),
WeightedCG: &IRGraph{
IRNodes: make(map[string]*IRNode),
},
}
// Build the node map and totals from the profile graph.
if err := p.processprofileGraph(g); err != nil {
return nil, err
}
if p.TotalNodeWeight == 0 || p.TotalEdgeWeight == 0 {
return nil, nil // accept but ignore profile with no samples.
}
// Create package-level call graph with weights from profile and IR.
p.initializeIRGraph()
return p, nil
}
// processprofileGraph builds various maps from the profile-graph.
//
// It initializes NodeMap and Total{Node,Edge}Weight based on the name and
// callsite to compute node and edge weights which will be used later on to
// create edges for WeightedCG.
//
// Caller should ignore the profile if p.TotalNodeWeight == 0 || p.TotalEdgeWeight == 0.
func (p *Profile) processprofileGraph(g *graph.Graph) error {
nFlat := make(map[string]int64)
nCum := make(map[string]int64)
seenStartLine := false
// Accummulate weights for the same node.
for _, n := range g.Nodes {
canonicalName := n.Info.Name
nFlat[canonicalName] += n.FlatValue()
nCum[canonicalName] += n.CumValue()
}
// Process graph and build various node and edge maps which will
// be consumed by AST walk.
for _, n := range g.Nodes {
seenStartLine = seenStartLine || n.Info.StartLine != 0
p.TotalNodeWeight += n.FlatValue()
canonicalName := n.Info.Name
// Create the key to the nodeMapKey.
nodeinfo := NodeMapKey{
CallerName: canonicalName,
CallSiteOffset: n.Info.Lineno - n.Info.StartLine,
}
for _, e := range n.Out {
p.TotalEdgeWeight += e.WeightValue()
nodeinfo.CalleeName = e.Dest.Info.Name
if w, ok := p.NodeMap[nodeinfo]; ok {
w.EWeight += e.WeightValue()
} else {
weights := new(Weights)
weights.NFlat = nFlat[canonicalName]
weights.NCum = nCum[canonicalName]
weights.EWeight = e.WeightValue()
p.NodeMap[nodeinfo] = weights
}
}
}
if p.TotalNodeWeight == 0 || p.TotalEdgeWeight == 0 {
return nil // accept but ignore profile with no samples.
}
if !seenStartLine {
// TODO(prattmic): If Function.start_line is missing we could
// fall back to using absolute line numbers, which is better
// than nothing.
return fmt.Errorf("profile missing Function.start_line data (Go version of profiled application too old? Go 1.20+ automatically adds this to profiles)")
}
return nil
}
// initializeIRGraph builds the IRGraph by visiting all the ir.Func in decl list
// of a package.
func (p *Profile) initializeIRGraph() {
// Bottomup walk over the function to create IRGraph.
ir.VisitFuncsBottomUp(typecheck.Target.Decls, func(list []*ir.Func, recursive bool) {
for _, fn := range list {
p.VisitIR(fn)
}
})
// Add additional edges for indirect calls. This must be done second so
// that IRNodes is fully populated (see the dummy node TODO in
// addIndirectEdges).
//
// TODO(prattmic): VisitIR above populates the graph via direct calls
// discovered via the IR. addIndirectEdges populates the graph via
// calls discovered via the profile. This combination of opposite
// approaches is a bit awkward, particularly because direct calls are
// discoverable via the profile as well. Unify these into a single
// approach.
p.addIndirectEdges()
}
// VisitIR traverses the body of each ir.Func and use NodeMap to determine if
// we need to add an edge from ir.Func and any node in the ir.Func body.
func (p *Profile) VisitIR(fn *ir.Func) {
g := p.WeightedCG
if g.IRNodes == nil {
g.IRNodes = make(map[string]*IRNode)
}
name := ir.LinkFuncName(fn)
node, ok := g.IRNodes[name]
if !ok {
node = &IRNode{
AST: fn,
}
g.IRNodes[name] = node
}
// Recursively walk over the body of the function to create IRGraph edges.
p.createIRGraphEdge(fn, node, name)
}
// NodeLineOffset returns the line offset of n in fn.
func NodeLineOffset(n ir.Node, fn *ir.Func) int {
// See "A note on line numbers" at the top of the file.
line := int(base.Ctxt.InnermostPos(n.Pos()).RelLine())
startLine := int(base.Ctxt.InnermostPos(fn.Pos()).RelLine())
return line - startLine
}
// addIREdge adds an edge between caller and new node that points to `callee`
// based on the profile-graph and NodeMap.
func (p *Profile) addIREdge(callerNode *IRNode, callerName string, call ir.Node, callee *ir.Func) {
g := p.WeightedCG
calleeName := ir.LinkFuncName(callee)
calleeNode, ok := g.IRNodes[calleeName]
if !ok {
calleeNode = &IRNode{
AST: callee,
}
g.IRNodes[calleeName] = calleeNode
}
nodeinfo := NodeMapKey{
CallerName: callerName,
CalleeName: calleeName,
CallSiteOffset: NodeLineOffset(call, callerNode.AST),
}
var weight int64
if weights, ok := p.NodeMap[nodeinfo]; ok {
weight = weights.EWeight
}
// Add edge in the IRGraph from caller to callee.
edge := &IREdge{
Src: callerNode,
Dst: calleeNode,
Weight: weight,
CallSiteOffset: nodeinfo.CallSiteOffset,
}
if callerNode.OutEdges == nil {
callerNode.OutEdges = make(map[NodeMapKey]*IREdge)
}
callerNode.OutEdges[nodeinfo] = edge
}
// addIndirectEdges adds indirect call edges found in the profile to the graph,
// to be used for devirtualization.
//
// targetDeclFuncs is the set of functions in typecheck.Target.Decls. Only
// edges from these functions will be added.
//
// Devirtualization is only applied to typecheck.Target.Decls functions, so there
// is no need to add edges from other functions.
//
// N.B. despite the name, addIndirectEdges will add any edges discovered via
// the profile. We don't know for sure that they are indirect, but assume they
// are since direct calls would already be added. (e.g., direct calls that have
// been deleted from source since the profile was taken would be added here).
//
// TODO(prattmic): Devirtualization runs before inlining, so we can't devirtualize
// calls inside inlined call bodies. If we did add that, we'd need edges from
// inlined bodies as well.
func (p *Profile) addIndirectEdges() {
g := p.WeightedCG
// g.IRNodes is populated with the set of functions in the local
// package build by VisitIR. We want to filter for local functions
// below, but we also add unknown callees to IRNodes as we go. So make
// an initial copy of IRNodes to recall just the local functions.
localNodes := make(map[string]*IRNode, len(g.IRNodes))
for k, v := range g.IRNodes {
localNodes[k] = v
}
for key, weights := range p.NodeMap {
// All callers in the local package build were added to IRNodes
// in VisitIR. If a caller isn't in the local package build we
// can skip adding edges, since we won't be devirtualizing in
// them anyway. This keeps the graph smaller.
callerNode, ok := localNodes[key.CallerName]
if !ok {
continue
}
// Already handled this edge?
if _, ok := callerNode.OutEdges[key]; ok {
continue
}
calleeNode, ok := g.IRNodes[key.CalleeName]
if !ok {
// IR is missing for this callee. Most likely this is
// because the callee isn't in the transitive deps of
// this package.
//
// Record this call anyway. If this is the hottest,
// then we want to skip devirtualization rather than
// devirtualizing to the second most common callee.
//
// TODO(prattmic): VisitIR populates IRNodes with all
// of the functions discovered via local package
// function declarations and calls. Thus we could miss
// functions that are available in export data of
// transitive deps, but aren't directly reachable. We
// need to do a lookup directly from package export
// data to get complete coverage.
calleeNode = &IRNode{
LinkerSymbolName: key.CalleeName,
// TODO: weights? We don't need them.
}
// Add dummy node back to IRNodes. We don't need this
// directly, but PrintWeightedCallGraphDOT uses these
// to print nodes.
g.IRNodes[key.CalleeName] = calleeNode
}
edge := &IREdge{
Src: callerNode,
Dst: calleeNode,
Weight: weights.EWeight,
CallSiteOffset: key.CallSiteOffset,
}
if callerNode.OutEdges == nil {
callerNode.OutEdges = make(map[NodeMapKey]*IREdge)
}
callerNode.OutEdges[key] = edge
}
}
// createIRGraphEdge traverses the nodes in the body of ir.Func and adds edges
// between the callernode which points to the ir.Func and the nodes in the
// body.
func (p *Profile) createIRGraphEdge(fn *ir.Func, callernode *IRNode, name string) {
ir.VisitList(fn.Body, func(n ir.Node) {
switch n.Op() {
case ir.OCALLFUNC:
call := n.(*ir.CallExpr)
// Find the callee function from the call site and add the edge.
callee := DirectCallee(call.X)
if callee != nil {
p.addIREdge(callernode, name, n, callee)
}
case ir.OCALLMETH:
call := n.(*ir.CallExpr)
// Find the callee method from the call site and add the edge.
callee := ir.MethodExprName(call.X).Func
p.addIREdge(callernode, name, n, callee)
}
})
}
// WeightInPercentage converts profile weights to a percentage.
func WeightInPercentage(value int64, total int64) float64 {
return (float64(value) / float64(total)) * 100
}
// PrintWeightedCallGraphDOT prints IRGraph in DOT format.
func (p *Profile) PrintWeightedCallGraphDOT(edgeThreshold float64) {
fmt.Printf("\ndigraph G {\n")
fmt.Printf("forcelabels=true;\n")
// List of functions in this package.
funcs := make(map[string]struct{})
ir.VisitFuncsBottomUp(typecheck.Target.Decls, func(list []*ir.Func, recursive bool) {
for _, f := range list {
name := ir.LinkFuncName(f)
funcs[name] = struct{}{}
}
})
// Determine nodes of DOT.
//
// Note that ir.Func may be nil for functions not visible from this
// package.
nodes := make(map[string]*ir.Func)
for name := range funcs {
if n, ok := p.WeightedCG.IRNodes[name]; ok {
for _, e := range n.OutEdges {
if _, ok := nodes[e.Src.Name()]; !ok {
nodes[e.Src.Name()] = e.Src.AST
}
if _, ok := nodes[e.Dst.Name()]; !ok {
nodes[e.Dst.Name()] = e.Dst.AST
}
}
if _, ok := nodes[n.Name()]; !ok {
nodes[n.Name()] = n.AST
}
}
}
// Print nodes.
for name, ast := range nodes {
if _, ok := p.WeightedCG.IRNodes[name]; ok {
style := "solid"
if ast == nil {
style = "dashed"
}
if ast != nil && ast.Inl != nil {
fmt.Printf("\"%v\" [color=black, style=%s, label=\"%v,inl_cost=%d\"];\n", name, style, name, ast.Inl.Cost)
} else {
fmt.Printf("\"%v\" [color=black, style=%s, label=\"%v\"];\n", name, style, name)
}
}
}
// Print edges.
ir.VisitFuncsBottomUp(typecheck.Target.Decls, func(list []*ir.Func, recursive bool) {
for _, f := range list {
name := ir.LinkFuncName(f)
if n, ok := p.WeightedCG.IRNodes[name]; ok {
for _, e := range n.OutEdges {
style := "solid"
if e.Dst.AST == nil {
style = "dashed"
}
color := "black"
edgepercent := WeightInPercentage(e.Weight, p.TotalEdgeWeight)
if edgepercent > edgeThreshold {
color = "red"
}
fmt.Printf("edge [color=%s, style=%s];\n", color, style)
fmt.Printf("\"%v\" -> \"%v\" [label=\"%.2f\"];\n", n.Name(), e.Dst.Name(), edgepercent)
}
}
}
})
fmt.Printf("}\n")
}
// DirectCallee takes a function-typed expression and returns the underlying
// function that it refers to if statically known. Otherwise, it returns nil.
//
// Equivalent to inline.inlCallee without calling CanInline on closures.
func DirectCallee(fn ir.Node) *ir.Func {
fn = ir.StaticValue(fn)
switch fn.Op() {
case ir.OMETHEXPR:
fn := fn.(*ir.SelectorExpr)
n := ir.MethodExprName(fn)
// Check that receiver type matches fn.X.
// TODO(mdempsky): Handle implicit dereference
// of pointer receiver argument?
if n == nil || !types.Identical(n.Type().Recv().Type, fn.X.Type()) {
return nil
}
return n.Func
case ir.ONAME:
fn := fn.(*ir.Name)
if fn.Class == ir.PFUNC {
return fn.Func
}
case ir.OCLOSURE:
fn := fn.(*ir.ClosureExpr)
c := fn.Func
return c
}
return nil
}