cmd/compile: use edge weights to decide inlineability in PGO

Currently, with PGO, the inliner uses node weights to decide if a
function is inlineable (with a larger budget). But the actual
inlining is determined by the weight of the call edge. There is a
discrepancy that, if a callee node is hot but the call edge is not,
it would not inlined, and marking the callee inlineable would of
no use.

Instead of using two kinds of weights, we just use the edge
weights to decide inlineability. If a function is the callee of a
hot call edge, its inlineability is determined with a larger
threshold. For a function that exceeds the regular inlining budget,
it is still inlined only when the call edge is hot, as it would
exceed the regular inlining cost for non-hot call sites, even if
it is marked inlineable.

For #55022.

Change-Id: I93fa9919fc6bcbb394e6cfe54ec96a96eede08f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/447015
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 6 insertions, 19 deletions

diff --git a/src/cmd/compile/internal/inline/inl.go b/src/cmd/compile/internal/inline/inl.go
index 335bb23ecb..98bfb4e382 100644
--- a/src/cmd/compile/internal/inline/inl.go
+++ b/src/cmd/compile/internal/inline/inl.go
@@ -56,9 +56,9 @@ const (
 )
 
 var (
-	// List of all hot nodes.
+	// List of all hot callee nodes.
 	// TODO(prattmic): Make this non-global.
-	candHotNodeMap = make(map[*pgo.IRNode]struct{})
+	candHotCalleeMap = make(map[*pgo.IRNode]struct{})
 
 	// List of all hot call sites. CallSiteInfo.Callee is always nil.
 	// TODO(prattmic): Make this non-global.
@@ -68,9 +68,6 @@ var (
 	// TODO(prattmic): Make this non-global.
 	inlinedCallSites = make(map[pgo.CallSiteInfo]struct{})
 
-	// Threshold in percentage for hot function inlining.
-	inlineHotFuncThresholdPercent = float64(2)
-
 	// Threshold in percentage for hot callsite inlining.
 	inlineHotCallSiteThresholdPercent = float64(0.1)
 
@@ -80,13 +77,6 @@ var (
 
 // pgoInlinePrologue records the hot callsites from ir-graph.
 func pgoInlinePrologue(p *pgo.Profile) {
-	if s, err := strconv.ParseFloat(base.Debug.InlineHotFuncThreshold, 64); err == nil {
-		inlineHotFuncThresholdPercent = s
-		if base.Debug.PGOInline > 0 {
-			fmt.Printf("hot-node-thres=%v\n", inlineHotFuncThresholdPercent)
-		}
-	}
-
 	if s, err := strconv.ParseFloat(base.Debug.InlineHotCallSiteThreshold, 64); err == nil {
 		inlineHotCallSiteThresholdPercent = s
 		if base.Debug.PGOInline > 0 {
@@ -102,10 +92,6 @@ func pgoInlinePrologue(p *pgo.Profile) {
 		for _, f := range list {
 			name := ir.PkgFuncName(f)
 			if n, ok := p.WeightedCG.IRNodes[name]; ok {
-				nodeweight := pgo.WeightInPercentage(n.Flat, p.TotalNodeWeight)
-				if nodeweight > inlineHotFuncThresholdPercent {
-					candHotNodeMap[n] = struct{}{}
-				}
 				for _, e := range p.WeightedCG.OutEdges[n] {
 					if e.Weight != 0 {
 						edgeweightpercent := pgo.WeightInPercentage(e.Weight, p.TotalEdgeWeight)
@@ -113,6 +99,7 @@ func pgoInlinePrologue(p *pgo.Profile) {
 							csi := pgo.CallSiteInfo{Line: e.CallSite, Caller: n.AST}
 							if _, ok := candHotEdgeMap[csi]; !ok {
 								candHotEdgeMap[csi] = struct{}{}
+								candHotCalleeMap[e.Dst] = struct{}{}
 							}
 						}
 					}
@@ -122,7 +109,7 @@ func pgoInlinePrologue(p *pgo.Profile) {
 	})
 	if base.Debug.PGOInline > 0 {
 		fmt.Printf("hot-cg before inline in dot format:")
-		p.PrintWeightedCallGraphDOT(inlineHotFuncThresholdPercent, inlineHotCallSiteThresholdPercent)
+		p.PrintWeightedCallGraphDOT(inlineHotCallSiteThresholdPercent)
 	}
 }
 
@@ -139,7 +126,7 @@ func pgoInlineEpilogue(p *pgo.Profile) {
 		})
 		// Print the call-graph after inlining. This is a debugging feature.
 		fmt.Printf("hot-cg after inline in dot:")
-		p.PrintWeightedCallGraphDOT(inlineHotFuncThresholdPercent, inlineHotCallSiteThresholdPercent)
+		p.PrintWeightedCallGraphDOT(inlineHotCallSiteThresholdPercent)
 	}
 }
 
@@ -270,7 +257,7 @@ func CanInline(fn *ir.Func, profile *pgo.Profile) {
 	budget := int32(inlineMaxBudget)
 	if profile != nil {
 		if n, ok := profile.WeightedCG.IRNodes[ir.PkgFuncName(fn)]; ok {
-			if _, ok := candHotNodeMap[n]; ok {
+			if _, ok := candHotCalleeMap[n]; ok {
 				budget = int32(inlineHotMaxBudget)
 				if base.Debug.PGOInline > 0 {
 					fmt.Printf("hot-node enabled increased budget=%v for func=%v\n", budget, ir.PkgFuncName(fn))