cmd/compile: initial function value devirtualization

Today, PGO-based devirtualization only applies to interface calls. This
CL extends initial support to function values (i.e., function/closure
pointers passed as arguments or stored in a struct).

This CL is a minimal implementation with several limitations.

* Export data lookup of function value callees not implemented
  (equivalent of CL 497175; done in CL 540258).
* Callees must be standard static functions. Callees that are closures
  (requiring closure context) are not supported.

For #61577.

Change-Id: I7d328859035249e176294cd0d9885b2d08c853f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/539699
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

2 files changed, 478 insertions, 162 deletions

diff --git a/src/cmd/compile/internal/devirtualize/pgo.go b/src/cmd/compile/internal/devirtualize/pgo.go
index 9aed38dc95..0a34e7eb8d 100644
--- a/src/cmd/compile/internal/devirtualize/pgo.go
+++ b/src/cmd/compile/internal/devirtualize/pgo.go
@@ -12,6 +12,8 @@ import (
 	"cmd/compile/internal/pgo"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
+	"cmd/internal/obj"
+	"cmd/internal/src"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -53,8 +55,10 @@ type CallStat struct {
 // ProfileGuided performs call devirtualization of indirect calls based on
 // profile information.
 //
-// Specifically, it performs conditional devirtualization of interface calls
-// for the hottest callee. That is, it performs a transformation like:
+// Specifically, it performs conditional devirtualization of interface calls or
+// function value calls for the hottest callee.
+//
+// That is, for interface calls it performs a transformation like:
 //
 //	type Iface interface {
 //		Foo()
@@ -78,6 +82,24 @@ type CallStat struct {
 //		}
 //	}
 //
+// For function value calls it performs a transformation like:
+//
+//	func Concrete() {}
+//
+//	func foo(fn func()) {
+//		fn()
+//	}
+//
+// to:
+//
+//	func foo(fn func()) {
+//		if internal/abi.FuncPCABIInternal(fn) == internal/abi.FuncPCABIInternal(Concrete) {
+//			Concrete()
+//		} else {
+//			fn()
+//		}
+//	}
+//
 // The primary benefit of this transformation is enabling inlining of the
 // direct call.
 func ProfileGuided(fn *ir.Func, p *pgo.Profile) {
@@ -125,7 +147,8 @@ func ProfileGuided(fn *ir.Func, p *pgo.Profile) {
 			}
 		}
 
-		if call.Op() != ir.OCALLINTER {
+		op := call.Op()
+		if op != ir.OCALLFUNC && op != ir.OCALLINTER {
 			return n
 		}
 
@@ -140,23 +163,19 @@ func ProfileGuided(fn *ir.Func, p *pgo.Profile) {
 			return n
 		}
 
-		// Bail if we do not have a hot callee.
-		callee, weight := findHotConcreteCallee(p, fn, call)
-		if callee == nil {
-			return n
-		}
-		// Bail if we do not have a Type node for the hot callee.
-		ctyp := methodRecvType(callee)
-		if ctyp == nil {
-			return n
-		}
-		// Bail if we know for sure it won't inline.
-		if !shouldPGODevirt(callee) {
-			return n
+		var newNode ir.Node
+		var callee *ir.Func
+		var weight int64
+		switch op {
+		case ir.OCALLFUNC:
+			newNode, callee, weight = maybeDevirtualizeFunctionCall(p, fn, call)
+		case ir.OCALLINTER:
+			newNode, callee, weight = maybeDevirtualizeInterfaceCall(p, fn, call)
+		default:
+			panic("unreachable")
 		}
 
-		if !base.PGOHash.MatchPosWithInfo(n.Pos(), "devirt", nil) {
-			// De-selected by PGO Hash.
+		if newNode == nil {
 			return n
 		}
 
@@ -165,12 +184,109 @@ func ProfileGuided(fn *ir.Func, p *pgo.Profile) {
 			stat.DevirtualizedWeight = weight
 		}
 
-		return rewriteCondCall(call, fn, callee, ctyp)
+		return newNode
 	}
 
 	ir.EditChildren(fn, edit)
 }
 
+// Devirtualize interface call if possible and eligible. Returns the new
+// ir.Node if call was devirtualized, and if so also the callee and weight of
+// the devirtualized edge.
+func maybeDevirtualizeInterfaceCall(p *pgo.Profile, fn *ir.Func, call *ir.CallExpr) (ir.Node, *ir.Func, int64) {
+	// Bail if we do not have a hot callee.
+	callee, weight := findHotConcreteInterfaceCallee(p, fn, call)
+	if callee == nil {
+		return nil, nil, 0
+	}
+	// Bail if we do not have a Type node for the hot callee.
+	ctyp := methodRecvType(callee)
+	if ctyp == nil {
+		return nil, nil, 0
+	}
+	// Bail if we know for sure it won't inline.
+	if !shouldPGODevirt(callee) {
+		return nil, nil, 0
+	}
+	// Bail if de-selected by PGO Hash.
+	if !base.PGOHash.MatchPosWithInfo(call.Pos(), "devirt", nil) {
+		return nil, nil, 0
+	}
+
+	return rewriteInterfaceCall(call, fn, callee, ctyp), callee, weight
+}
+
+// Devirtualize an indirect function call if possible and eligible. Returns the new
+// ir.Node if call was devirtualized, and if so also the callee and weight of
+// the devirtualized edge.
+func maybeDevirtualizeFunctionCall(p *pgo.Profile, fn *ir.Func, call *ir.CallExpr) (ir.Node, *ir.Func, int64) {
+	// Bail if this is a direct call; no devirtualization necessary.
+	callee := pgo.DirectCallee(call.Fun)
+	if callee != nil {
+		return nil, nil, 0
+	}
+
+	// Bail if we do not have a hot callee.
+	callee, weight := findHotConcreteFunctionCallee(p, fn, call)
+	if callee == nil {
+		return nil, nil, 0
+	}
+
+	// TODO(go.dev/issue/61577): Closures need the closure context passed
+	// via the context register. That requires extra plumbing that we
+	// haven't done yet.
+	if callee.OClosure != nil {
+		if base.Debug.PGODebug >= 3 {
+			fmt.Printf("callee %s is a closure, skipping\n", ir.FuncName(callee))
+		}
+		return nil, nil, 0
+	}
+	// TODO(prattmic): We don't properly handle methods as callees in two
+	// different dimensions:
+	//
+	// 1. Method expressions. e.g.,
+	//
+	//      var fn func(*os.File, []byte) (int, error) = (*os.File).Read
+	//
+	// In this case, typ will report *os.File as the receiver while
+	// ctyp reports it as the first argument. types.Identical ignores
+	// receiver parameters, so it treats these as different, even though
+	// they are still call compatible.
+	//
+	// 2. Method values. e.g.,
+	//
+	//      var f *os.File
+	//      var fn func([]byte) (int, error) = f.Read
+	//
+	// types.Identical will treat these as compatible (since receiver
+	// parameters are ignored). However, in this case, we do not call
+	// (*os.File).Read directly. Instead, f is stored in closure context
+	// and we call the wrapper (*os.File).Read-fm. However, runtime/pprof
+	// hides wrappers from profiles, making it appear that there is a call
+	// directly to the method. We could recognize this pattern return the
+	// wrapper rather than the method.
+	//
+	// N.B. perf profiles will report wrapper symbols directly, so
+	// ideally we should support direct wrapper references as well.
+	if callee.Type().Recv() != nil {
+		if base.Debug.PGODebug >= 3 {
+			fmt.Printf("callee %s is a method, skipping\n", ir.FuncName(callee))
+		}
+		return nil, nil, 0
+	}
+
+	// Bail if we know for sure it won't inline.
+	if !shouldPGODevirt(callee) {
+		return nil, nil, 0
+	}
+	// Bail if de-selected by PGO Hash.
+	if !base.PGOHash.MatchPosWithInfo(call.Pos(), "devirt", nil) {
+		return nil, nil, 0
+	}
+
+	return rewriteFunctionCall(call, fn, callee), callee, weight
+}
+
 // shouldPGODevirt checks if we should perform PGO devirtualization to the
 // target function.
 //
@@ -279,11 +395,90 @@ func constructCallStat(p *pgo.Profile, fn *ir.Func, name string, call *ir.CallEx
 	return &stat
 }
 
-// rewriteCondCall devirtualizes the given call using a direct method call to
-// concretetyp.
-func rewriteCondCall(call *ir.CallExpr, curfn, callee *ir.Func, concretetyp *types.Type) ir.Node {
+// copyInputs copies the inputs to a call: the receiver (for interface calls)
+// or function value (for function value calls) and the arguments. These
+// expressions are evaluated once and assigned to temporaries.
+//
+// The assignment statement is added to init and the copied receiver/fn
+// expression and copied arguments expressions are returned.
+func copyInputs(curfn *ir.Func, pos src.XPos, recvOrFn ir.Node, args []ir.Node, init *ir.Nodes) (ir.Node, []ir.Node) {
+	// Evaluate receiver/fn and argument expressions. The receiver/fn is
+	// used twice but we don't want to cause side effects twice. The
+	// arguments are used in two different calls and we can't trivially
+	// copy them.
+	//
+	// recvOrFn must be first in the assignment list as its side effects
+	// must be ordered before argument side effects.
+	var lhs, rhs []ir.Node
+	newRecvOrFn := typecheck.TempAt(pos, curfn, recvOrFn.Type())
+	lhs = append(lhs, newRecvOrFn)
+	rhs = append(rhs, recvOrFn)
+
+	for _, arg := range args {
+		argvar := typecheck.TempAt(pos, curfn, arg.Type())
+
+		lhs = append(lhs, argvar)
+		rhs = append(rhs, arg)
+	}
+
+	asList := ir.NewAssignListStmt(pos, ir.OAS2, lhs, rhs)
+	init.Append(typecheck.Stmt(asList))
+
+	return newRecvOrFn, lhs[1:]
+}
+
+// retTemps returns a slice of temporaries to be used for storing result values from call.
+func retTemps(curfn *ir.Func, pos src.XPos, call *ir.CallExpr) []ir.Node {
+	sig := call.Fun.Type()
+	var retvars []ir.Node
+	for _, ret := range sig.Results() {
+		retvars = append(retvars, typecheck.TempAt(pos, curfn, ret.Type))
+	}
+	return retvars
+}
+
+// condCall returns an ir.InlinedCallExpr that performs a call to thenCall if
+// cond is true and elseCall if cond is false. The return variables of the
+// InlinedCallExpr evaluate to the return values from the call.
+func condCall(curfn *ir.Func, pos src.XPos, cond ir.Node, thenCall, elseCall *ir.CallExpr, init ir.Nodes) *ir.InlinedCallExpr {
+	// Doesn't matter whether we use thenCall or elseCall, they must have
+	// the same return types.
+	retvars := retTemps(curfn, pos, thenCall)
+
+	var thenBlock, elseBlock ir.Nodes
+	if len(retvars) == 0 {
+		thenBlock.Append(thenCall)
+		elseBlock.Append(elseCall)
+	} else {
+		// Copy slice so edits in one location don't affect another.
+		thenRet := append([]ir.Node(nil), retvars...)
+		thenAsList := ir.NewAssignListStmt(pos, ir.OAS2, thenRet, []ir.Node{thenCall})
+		thenBlock.Append(typecheck.Stmt(thenAsList))
+
+		elseRet := append([]ir.Node(nil), retvars...)
+		elseAsList := ir.NewAssignListStmt(pos, ir.OAS2, elseRet, []ir.Node{elseCall})
+		elseBlock.Append(typecheck.Stmt(elseAsList))
+	}
+
+	nif := ir.NewIfStmt(pos, cond, thenBlock, elseBlock)
+	nif.SetInit(init)
+	nif.Likely = true
+
+	body := []ir.Node{typecheck.Stmt(nif)}
+
+	// This isn't really an inlined call of course, but InlinedCallExpr
+	// makes handling reassignment of return values easier.
+	res := ir.NewInlinedCallExpr(pos, body, retvars)
+	res.SetType(thenCall.Type())
+	res.SetTypecheck(1)
+	return res
+}
+
+// rewriteInterfaceCall devirtualizes the given interface call using a direct
+// method call to concretetyp.
+func rewriteInterfaceCall(call *ir.CallExpr, curfn, callee *ir.Func, concretetyp *types.Type) ir.Node {
 	if base.Flag.LowerM != 0 {
-		fmt.Printf("%v: PGO devirtualizing %v to %v\n", ir.Line(call), call.Fun, callee)
+		fmt.Printf("%v: PGO devirtualizing interface call %v to %v\n", ir.Line(call), call.Fun, callee)
 	}
 
 	// We generate an OINCALL of:
@@ -314,46 +509,15 @@ func rewriteCondCall(call *ir.CallExpr, curfn, callee *ir.Func, concretetyp *typ
 	// making it less like to inline. We may want to compensate for this
 	// somehow.
 
-	var retvars []ir.Node
-
-	sig := call.Fun.Type()
-
-	for _, ret := range sig.Results() {
-		retvars = append(retvars, typecheck.TempAt(base.Pos, curfn, ret.Type))
-	}
-
 	sel := call.Fun.(*ir.SelectorExpr)
 	method := sel.Sel
 	pos := call.Pos()
 	init := ir.TakeInit(call)
 
-	// Evaluate receiver and argument expressions. The receiver is used
-	// twice but we don't want to cause side effects twice. The arguments
-	// are used in two different calls and we can't trivially copy them.
-	//
-	// recv must be first in the assignment list as its side effects must
-	// be ordered before argument side effects.
-	var lhs, rhs []ir.Node
-	recv := typecheck.TempAt(base.Pos, curfn, sel.X.Type())
-	lhs = append(lhs, recv)
-	rhs = append(rhs, sel.X)
-
-	// Move arguments to assignments prior to the if statement. We cannot
-	// simply copy the args' IR, as some IR constructs cannot be copied,
-	// such as labels (possible in InlinedCall nodes).
-	args := call.Args.Take()
-	for _, arg := range args {
-		argvar := typecheck.TempAt(base.Pos, curfn, arg.Type())
-
-		lhs = append(lhs, argvar)
-		rhs = append(rhs, arg)
-	}
-
-	asList := ir.NewAssignListStmt(pos, ir.OAS2, lhs, rhs)
-	init.Append(typecheck.Stmt(asList))
+	recv, args := copyInputs(curfn, pos, sel.X, call.Args.Take(), &init)
 
 	// Copy slice so edits in one location don't affect another.
-	argvars := append([]ir.Node(nil), lhs[1:]...)
+	argvars := append([]ir.Node(nil), args...)
 	call.Args = argvars
 
 	tmpnode := typecheck.TempAt(base.Pos, curfn, concretetyp)
@@ -367,38 +531,84 @@ func rewriteCondCall(call *ir.CallExpr, curfn, callee *ir.Func, concretetyp *typ
 	concreteCallee := typecheck.XDotMethod(pos, tmpnode, method, true)
 	// Copy slice so edits in one location don't affect another.
 	argvars = append([]ir.Node(nil), argvars...)
-	concreteCall := typecheck.Call(pos, concreteCallee, argvars, call.IsDDD)
+	concreteCall := typecheck.Call(pos, concreteCallee, argvars, call.IsDDD).(*ir.CallExpr)
 
-	var thenBlock, elseBlock ir.Nodes
-	if len(retvars) == 0 {
-		thenBlock.Append(concreteCall)
-		elseBlock.Append(call)
-	} else {
-		// Copy slice so edits in one location don't affect another.
-		thenRet := append([]ir.Node(nil), retvars...)
-		thenAsList := ir.NewAssignListStmt(pos, ir.OAS2, thenRet, []ir.Node{concreteCall})
-		thenBlock.Append(typecheck.Stmt(thenAsList))
+	res := condCall(curfn, pos, tmpok, concreteCall, call, init)
 
-		elseRet := append([]ir.Node(nil), retvars...)
-		elseAsList := ir.NewAssignListStmt(pos, ir.OAS2, elseRet, []ir.Node{call})
-		elseBlock.Append(typecheck.Stmt(elseAsList))
+	if base.Debug.PGODebug >= 3 {
+		fmt.Printf("PGO devirtualizing interface call to %+v. After: %+v\n", concretetyp, res)
 	}
 
-	cond := ir.NewIfStmt(pos, nil, nil, nil)
-	cond.SetInit(init)
-	cond.Cond = tmpok
-	cond.Body = thenBlock
-	cond.Else = elseBlock
-	cond.Likely = true
+	return res
+}
 
-	body := []ir.Node{typecheck.Stmt(cond)}
+// rewriteFunctionCall devirtualizes the given OCALLFUNC using a direct
+// function call to callee.
+func rewriteFunctionCall(call *ir.CallExpr, curfn, callee *ir.Func) ir.Node {
+	if base.Flag.LowerM != 0 {
+		fmt.Printf("%v: PGO devirtualizing function call %v to %v\n", ir.Line(call), call.Fun, callee)
+	}
 
-	res := ir.NewInlinedCallExpr(pos, body, retvars)
-	res.SetType(call.Type())
-	res.SetTypecheck(1)
+	// We generate an OINCALL of:
+	//
+	// var fn FuncType
+	//
+	// var arg1 A1
+	// var argN AN
+	//
+	// var ret1 R1
+	// var retN RN
+	//
+	// fn, arg1, argN = fn expr, arg1 expr, argN expr
+	//
+	// fnPC := internal/abi.FuncPCABIInternal(fn)
+	// concretePC := internal/abi.FuncPCABIInternal(concrete)
+	//
+	// if fnPC == concretePC {
+	//   ret1, retN = concrete(arg1, ... argN) // Same closure context passed (TODO)
+	// } else {
+	//   ret1, retN = fn(arg1, ... argN)
+	// }
+	//
+	// OINCALL retvars: ret1, ... retN
+	//
+	// This isn't really an inlined call of course, but InlinedCallExpr
+	// makes handling reassignment of return values easier.
+
+	pos := call.Pos()
+	init := ir.TakeInit(call)
+
+	fn, args := copyInputs(curfn, pos, call.Fun, call.Args.Take(), &init)
+
+	// Copy slice so edits in one location don't affect another.
+	argvars := append([]ir.Node(nil), args...)
+	call.Args = argvars
+
+	// FuncPCABIInternal takes an interface{}, emulate that. This is needed
+	// for to ensure we get the MAKEFACE we need for SSA.
+	fnIface := typecheck.Expr(ir.NewConvExpr(pos, ir.OCONV, types.Types[types.TINTER], fn))
+	calleeIface := typecheck.Expr(ir.NewConvExpr(pos, ir.OCONV, types.Types[types.TINTER], callee.Nname))
+
+	fnPC := ir.FuncPC(pos, fnIface, obj.ABIInternal)
+	concretePC := ir.FuncPC(pos, calleeIface, obj.ABIInternal)
+
+	pcEq := typecheck.Expr(ir.NewBinaryExpr(base.Pos, ir.OEQ, fnPC, concretePC))
+
+	// TODO(go.dev/issue/61577): Handle callees that a closures and need a
+	// copy of the closure context from call. For now, we skip callees that
+	// are closures in maybeDevirtualizeFunctionCall.
+	if callee.OClosure != nil {
+		base.Fatalf("Callee is a closure: %+v", callee)
+	}
+
+	// Copy slice so edits in one location don't affect another.
+	argvars = append([]ir.Node(nil), argvars...)
+	concreteCall := typecheck.Call(pos, callee.Nname, argvars, call.IsDDD).(*ir.CallExpr)
+
+	res := condCall(curfn, pos, pcEq, concreteCall, call, init)
 
 	if base.Debug.PGODebug >= 3 {
-		fmt.Printf("PGO devirtualizing call to %+v. After: %+v\n", concretetyp, res)
+		fmt.Printf("PGO devirtualizing function call to %+v. After: %+v\n", ir.FuncName(callee), res)
 	}
 
 	return res
@@ -429,15 +639,15 @@ func interfaceCallRecvTypeAndMethod(call *ir.CallExpr) (*types.Type, *types.Sym)
 	return sel.X.Type(), sel.Sel
 }
 
-// findHotConcreteCallee returns the *ir.Func of the hottest callee of an
-// indirect call, if available, and its edge weight.
-func findHotConcreteCallee(p *pgo.Profile, caller *ir.Func, call *ir.CallExpr) (*ir.Func, int64) {
+// findHotConcreteCallee returns the *ir.Func of the hottest callee of a call,
+// if available, and its edge weight. extraFn can perform additional
+// applicability checks on each candidate edge. If extraFn returns false,
+// candidate will not be considered a valid callee candidate.
+func findHotConcreteCallee(p *pgo.Profile, caller *ir.Func, call *ir.CallExpr, extraFn func(callerName string, callOffset int, candidate *pgo.IREdge) bool) (*ir.Func, int64) {
 	callerName := ir.LinkFuncName(caller)
 	callerNode := p.WeightedCG.IRNodes[callerName]
 	callOffset := pgo.NodeLineOffset(call, caller)
 
-	inter, method := interfaceCallRecvTypeAndMethod(call)
-
 	var hottest *pgo.IREdge
 
 	// Returns true if e is hotter than hottest.
@@ -504,6 +714,35 @@ func findHotConcreteCallee(p *pgo.Profile, caller *ir.Func, call *ir.CallExpr) (
 			continue
 		}
 
+		if extraFn != nil && !extraFn(callerName, callOffset, e) {
+			continue
+		}
+
+		if base.Debug.PGODebug >= 2 {
+			fmt.Printf("%v: edge %s:%d -> %s (weight %d): hottest so far\n", ir.Line(call), callerName, callOffset, e.Dst.Name(), e.Weight)
+		}
+		hottest = e
+	}
+
+	if hottest == nil {
+		if base.Debug.PGODebug >= 2 {
+			fmt.Printf("%v: call %s:%d: no hot callee\n", ir.Line(call), callerName, callOffset)
+		}
+		return nil, 0
+	}
+
+	if base.Debug.PGODebug >= 2 {
+		fmt.Printf("%v call %s:%d: hottest callee %s (weight %d)\n", ir.Line(call), callerName, callOffset, hottest.Dst.Name(), hottest.Weight)
+	}
+	return hottest.Dst.AST, hottest.Weight
+}
+
+// findHotConcreteInterfaceCallee returns the *ir.Func of the hottest callee of an
+// interface call, if available, and its edge weight.
+func findHotConcreteInterfaceCallee(p *pgo.Profile, caller *ir.Func, call *ir.CallExpr) (*ir.Func, int64) {
+	inter, method := interfaceCallRecvTypeAndMethod(call)
+
+	return findHotConcreteCallee(p, caller, call, func(callerName string, callOffset int, e *pgo.IREdge) bool {
 		ctyp := methodRecvType(e.Dst.AST)
 		if ctyp == nil {
 			// Not a method.
@@ -511,7 +750,7 @@ func findHotConcreteCallee(p *pgo.Profile, caller *ir.Func, call *ir.CallExpr) (
 			if base.Debug.PGODebug >= 2 {
 				fmt.Printf("%v: edge %s:%d -> %s (weight %d): callee not a method\n", ir.Line(call), callerName, callOffset, e.Dst.Name(), e.Weight)
 			}
-			continue
+			return false
 		}
 
 		// If ctyp doesn't implement inter it is most likely from a
@@ -530,7 +769,7 @@ func findHotConcreteCallee(p *pgo.Profile, caller *ir.Func, call *ir.CallExpr) (
 				why := typecheck.ImplementsExplain(ctyp, inter)
 				fmt.Printf("%v: edge %s:%d -> %s (weight %d): %v doesn't implement %v (%s)\n", ir.Line(call), callerName, callOffset, e.Dst.Name(), e.Weight, ctyp, inter, why)
 			}
-			continue
+			return false
 		}
 
 		// If the method name is different it is most likely from a
@@ -539,24 +778,35 @@ func findHotConcreteCallee(p *pgo.Profile, caller *ir.Func, call *ir.CallExpr) (
 			if base.Debug.PGODebug >= 2 {
 				fmt.Printf("%v: edge %s:%d -> %s (weight %d): callee is a different method\n", ir.Line(call), callerName, callOffset, e.Dst.Name(), e.Weight)
 			}
-			continue
+			return false
 		}
 
-		if base.Debug.PGODebug >= 2 {
-			fmt.Printf("%v: edge %s:%d -> %s (weight %d): hottest so far\n", ir.Line(call), callerName, callOffset, e.Dst.Name(), e.Weight)
-		}
-		hottest = e
-	}
+		return true
+	})
+}
 
-	if hottest == nil {
-		if base.Debug.PGODebug >= 2 {
-			fmt.Printf("%v: call %s:%d: no hot callee\n", ir.Line(call), callerName, callOffset)
+// findHotConcreteFunctionCallee returns the *ir.Func of the hottest callee of an
+// indirect function call, if available, and its edge weight.
+func findHotConcreteFunctionCallee(p *pgo.Profile, caller *ir.Func, call *ir.CallExpr) (*ir.Func, int64) {
+	typ := call.Fun.Type().Underlying()
+
+	return findHotConcreteCallee(p, caller, call, func(callerName string, callOffset int, e *pgo.IREdge) bool {
+		ctyp := e.Dst.AST.Type().Underlying()
+
+		// If ctyp doesn't match typ it is most likely from a different
+		// call on the same line.
+		//
+		// Note that we are comparing underlying types, as different
+		// defined types are OK. e.g., a call to a value of type
+		// net/http.HandlerFunc can be devirtualized to a function with
+		// the same underlying type.
+		if !types.Identical(typ, ctyp) {
+			if base.Debug.PGODebug >= 2 {
+				fmt.Printf("%v: edge %s:%d -> %s (weight %d): %v doesn't match %v\n", ir.Line(call), callerName, callOffset, e.Dst.Name(), e.Weight, ctyp, typ)
+			}
+			return false
 		}
-		return nil, 0
-	}
 
-	if base.Debug.PGODebug >= 2 {
-		fmt.Printf("%v call %s:%d: hottest callee %s (weight %d)\n", ir.Line(call), callerName, callOffset, hottest.Dst.Name(), hottest.Weight)
-	}
-	return hottest.Dst.AST, hottest.Weight
+		return true
+	})
 }
diff --git a/src/cmd/compile/internal/devirtualize/pgo_test.go b/src/cmd/compile/internal/devirtualize/pgo_test.go
index 8383da56cb..84c96df122 100644
--- a/src/cmd/compile/internal/devirtualize/pgo_test.go
+++ b/src/cmd/compile/internal/devirtualize/pgo_test.go
@@ -8,8 +8,8 @@ import (
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/pgo"
-	"cmd/compile/internal/types"
 	"cmd/compile/internal/typecheck"
+	"cmd/compile/internal/types"
 	"cmd/internal/obj"
 	"cmd/internal/src"
 	"testing"
@@ -31,66 +31,81 @@ func makePos(b *src.PosBase, line, col uint) src.XPos {
 	return base.Ctxt.PosTable.XPos(src.MakePos(b, line, col))
 }
 
-func TestFindHotConcreteCallee(t *testing.T) {
+type profileBuilder struct {
+	p *pgo.Profile
+}
+
+func newProfileBuilder() *profileBuilder {
 	// findHotConcreteCallee only uses pgo.Profile.WeightedCG, so we're
 	// going to take a shortcut and only construct that.
-	p := &pgo.Profile{
-		WeightedCG: &pgo.IRGraph{
-			IRNodes: make(map[string]*pgo.IRNode),
+	return &profileBuilder{
+		p: &pgo.Profile{
+			WeightedCG: &pgo.IRGraph{
+				IRNodes: make(map[string]*pgo.IRNode),
+			},
 		},
 	}
+}
 
-	// Create a new IRNode and add it to p.
-	//
-	// fn may be nil, in which case the node will set LinkerSymbolName.
-	newNode := func(name string, fn *ir.Func) *pgo.IRNode {
-		n := &pgo.IRNode{
-			OutEdges: make(map[pgo.NamedCallEdge]*pgo.IREdge),
-		}
-		if fn != nil {
-			n.AST = fn
-		} else {
-			n.LinkerSymbolName = name
-		}
-		p.WeightedCG.IRNodes[name] = n
-		return n
+// Profile returns the constructed profile.
+func (p *profileBuilder) Profile() *pgo.Profile {
+	return p.p
+}
+
+// NewNode creates a new IRNode and adds it to the profile.
+//
+// fn may be nil, in which case the node will set LinkerSymbolName.
+func (p *profileBuilder) NewNode(name string, fn *ir.Func) *pgo.IRNode {
+	n := &pgo.IRNode{
+		OutEdges: make(map[pgo.NamedCallEdge]*pgo.IREdge),
+	}
+	if fn != nil {
+		n.AST = fn
+	} else {
+		n.LinkerSymbolName = name
 	}
+	p.p.WeightedCG.IRNodes[name] = n
+	return n
+}
 
-	// Add a new call edge from caller to callee.
-	addEdge := func(caller, callee *pgo.IRNode, offset int, weight int64) {
-		namedEdge := pgo.NamedCallEdge{
-			CallerName:     caller.Name(),
-			CalleeName:     callee.Name(),
-			CallSiteOffset: offset,
-		}
-		irEdge := &pgo.IREdge{
-			Src:            caller,
-			Dst:            callee,
-			CallSiteOffset: offset,
-			Weight:         weight,
-		}
-		caller.OutEdges[namedEdge] = irEdge
+// Add a new call edge from caller to callee.
+func addEdge(caller, callee *pgo.IRNode, offset int, weight int64) {
+	namedEdge := pgo.NamedCallEdge{
+		CallerName:     caller.Name(),
+		CalleeName:     callee.Name(),
+		CallSiteOffset: offset,
+	}
+	irEdge := &pgo.IREdge{
+		Src:            caller,
+		Dst:            callee,
+		CallSiteOffset: offset,
+		Weight:         weight,
 	}
+	caller.OutEdges[namedEdge] = irEdge
+}
 
-	pkgFoo := types.NewPkg("example.com/foo", "foo")
-	basePos := src.NewFileBase("foo.go", "/foo.go")
+// Create a new struct type named structName with a method named methName and
+// return the method.
+func makeStructWithMethod(pkg *types.Pkg, structName, methName string) *ir.Func {
+	// type structName struct{}
+	structType := types.NewStruct(nil)
+
+	// func (structName) methodName()
+	recv := types.NewField(src.NoXPos, typecheck.Lookup(structName), structType)
+	sig := types.NewSignature(recv, nil, nil)
+	fn := ir.NewFunc(src.NoXPos, src.NoXPos, pkg.Lookup(structName+"."+methName), sig)
 
-	// Create a new struct type named structName with a method named methName and
-	// return the method.
-	makeStructWithMethod := func(structName, methName string) *ir.Func {
-		// type structName struct{}
-		structType := types.NewStruct(nil)
+	// Add the method to the struct.
+	structType.SetMethods([]*types.Field{types.NewField(src.NoXPos, typecheck.Lookup(methName), sig)})
 
-		// func (structName) methodName()
-		recv := types.NewField(src.NoXPos, typecheck.Lookup(structName), structType)
-		sig := types.NewSignature(recv, nil, nil)
-		fn := ir.NewFunc(src.NoXPos, src.NoXPos, pkgFoo.Lookup(structName + "." + methName), sig)
+	return fn
+}
 
-		// Add the method to the struct.
-		structType.SetMethods([]*types.Field{types.NewField(src.NoXPos, typecheck.Lookup(methName), sig)})
+func TestFindHotConcreteInterfaceCallee(t *testing.T) {
+	p := newProfileBuilder()
 
-		return fn
-	}
+	pkgFoo := types.NewPkg("example.com/foo", "foo")
+	basePos := src.NewFileBase("foo.go", "/foo.go")
 
 	const (
 		// Caller start line.
@@ -112,21 +127,21 @@ func TestFindHotConcreteCallee(t *testing.T) {
 
 	callerFn := ir.NewFunc(makePos(basePos, callerStart, 1), src.NoXPos, pkgFoo.Lookup("Caller"), types.NewSignature(nil, nil, nil))
 
-	hotCalleeFn := makeStructWithMethod("HotCallee", "Foo")
-	coldCalleeFn := makeStructWithMethod("ColdCallee", "Foo")
-	wrongLineCalleeFn := makeStructWithMethod("WrongLineCallee", "Foo")
-	wrongMethodCalleeFn := makeStructWithMethod("WrongMethodCallee", "Bar")
+	hotCalleeFn := makeStructWithMethod(pkgFoo, "HotCallee", "Foo")
+	coldCalleeFn := makeStructWithMethod(pkgFoo, "ColdCallee", "Foo")
+	wrongLineCalleeFn := makeStructWithMethod(pkgFoo, "WrongLineCallee", "Foo")
+	wrongMethodCalleeFn := makeStructWithMethod(pkgFoo, "WrongMethodCallee", "Bar")
 
-	callerNode := newNode("example.com/foo.Caller", callerFn)
-	hotCalleeNode := newNode("example.com/foo.HotCallee.Foo", hotCalleeFn)
-	coldCalleeNode := newNode("example.com/foo.ColdCallee.Foo", coldCalleeFn)
-	wrongLineCalleeNode := newNode("example.com/foo.WrongCalleeLine.Foo", wrongLineCalleeFn)
-	wrongMethodCalleeNode := newNode("example.com/foo.WrongCalleeMethod.Foo", wrongMethodCalleeFn)
+	callerNode := p.NewNode("example.com/foo.Caller", callerFn)
+	hotCalleeNode := p.NewNode("example.com/foo.HotCallee.Foo", hotCalleeFn)
+	coldCalleeNode := p.NewNode("example.com/foo.ColdCallee.Foo", coldCalleeFn)
+	wrongLineCalleeNode := p.NewNode("example.com/foo.WrongCalleeLine.Foo", wrongLineCalleeFn)
+	wrongMethodCalleeNode := p.NewNode("example.com/foo.WrongCalleeMethod.Foo", wrongMethodCalleeFn)
 
-	hotMissingCalleeNode := newNode("example.com/bar.HotMissingCallee.Foo", nil)
+	hotMissingCalleeNode := p.NewNode("example.com/bar.HotMissingCallee.Foo", nil)
 
 	addEdge(callerNode, wrongLineCalleeNode, wrongCallOffset, 100) // Really hot, but wrong line.
-	addEdge(callerNode, wrongMethodCalleeNode, callOffset, 100) // Really hot, but wrong method type.
+	addEdge(callerNode, wrongMethodCalleeNode, callOffset, 100)    // Really hot, but wrong method type.
 	addEdge(callerNode, hotCalleeNode, callOffset, 10)
 	addEdge(callerNode, coldCalleeNode, callOffset, 1)
 
@@ -141,7 +156,7 @@ func TestFindHotConcreteCallee(t *testing.T) {
 	sel := typecheck.NewMethodExpr(src.NoXPos, iface, typecheck.Lookup("Foo"))
 	call := ir.NewCallExpr(makePos(basePos, callerStart+callOffset, 1), ir.OCALLINTER, sel, nil)
 
-	gotFn, gotWeight := findHotConcreteCallee(p, callerFn, call)
+	gotFn, gotWeight := findHotConcreteInterfaceCallee(p.Profile(), callerFn, call)
 	if gotFn != hotCalleeFn {
 		t.Errorf("findHotConcreteInterfaceCallee func got %v want %v", gotFn, hotCalleeFn)
 	}
@@ -149,3 +164,54 @@ func TestFindHotConcreteCallee(t *testing.T) {
 		t.Errorf("findHotConcreteInterfaceCallee weight got %v want 10", gotWeight)
 	}
 }
+
+func TestFindHotConcreteFunctionCallee(t *testing.T) {
+	// TestFindHotConcreteInterfaceCallee already covered basic weight
+	// comparisons, which is shared logic. Here we just test type signature
+	// disambiguation.
+
+	p := newProfileBuilder()
+
+	pkgFoo := types.NewPkg("example.com/foo", "foo")
+	basePos := src.NewFileBase("foo.go", "/foo.go")
+
+	const (
+		// Caller start line.
+		callerStart = 42
+
+		// The line offset of the call we care about.
+		callOffset = 1
+	)
+
+	callerFn := ir.NewFunc(makePos(basePos, callerStart, 1), src.NoXPos, pkgFoo.Lookup("Caller"), types.NewSignature(nil, nil, nil))
+
+	// func HotCallee()
+	hotCalleeFn := ir.NewFunc(src.NoXPos, src.NoXPos, pkgFoo.Lookup("HotCallee"), types.NewSignature(nil, nil, nil))
+
+	// func WrongCallee() bool
+	wrongCalleeFn := ir.NewFunc(src.NoXPos, src.NoXPos, pkgFoo.Lookup("WrongCallee"), types.NewSignature(nil, nil,
+		[]*types.Field{
+			types.NewField(src.NoXPos, nil, types.Types[types.TBOOL]),
+		},
+	))
+
+	callerNode := p.NewNode("example.com/foo.Caller", callerFn)
+	hotCalleeNode := p.NewNode("example.com/foo.HotCallee", hotCalleeFn)
+	wrongCalleeNode := p.NewNode("example.com/foo.WrongCallee", wrongCalleeFn)
+
+	addEdge(callerNode, wrongCalleeNode, callOffset, 100) // Really hot, but wrong function type.
+	addEdge(callerNode, hotCalleeNode, callOffset, 10)
+
+	// var fn func()
+	name := ir.NewNameAt(src.NoXPos, typecheck.Lookup("fn"), types.NewSignature(nil, nil, nil))
+	// fn()
+	call := ir.NewCallExpr(makePos(basePos, callerStart+callOffset, 1), ir.OCALL, name, nil)
+
+	gotFn, gotWeight := findHotConcreteFunctionCallee(p.Profile(), callerFn, call)
+	if gotFn != hotCalleeFn {
+		t.Errorf("findHotConcreteFunctionCallee func got %v want %v", gotFn, hotCalleeFn)
+	}
+	if gotWeight != 10 {
+		t.Errorf("findHotConcreteFunctionCallee weight got %v want 10", gotWeight)
+	}
+}