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// Copyright 2020 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.
// Package devirtualize implements two "devirtualization" optimization passes:
//
// - "Static" devirtualization which replaces interface method calls with
// direct concrete-type method calls where possible.
// - "Profile-guided" devirtualization which replaces indirect calls with a
// conditional direct call to the hottest concrete callee from a profile, as
// well as a fallback using the original indirect call.
package devirtualize
import (
"cmd/compile/internal/base"
"cmd/compile/internal/ir"
"cmd/compile/internal/typecheck"
"cmd/compile/internal/types"
)
// Static devirtualizes calls within fn where possible when the concrete callee
// is available statically.
func Static(fn *ir.Func) {
ir.CurFunc = fn
// For promoted methods (including value-receiver methods promoted to pointer-receivers),
// the interface method wrapper may contain expressions that can panic (e.g., ODEREF, ODOTPTR, ODOTINTER).
// Devirtualization involves inlining these expressions (and possible panics) to the call site.
// This normally isn't a problem, but for go/defer statements it can move the panic from when/where
// the call executes to the go/defer statement itself, which is a visible change in semantics (e.g., #52072).
// To prevent this, we skip devirtualizing calls within go/defer statements altogether.
goDeferCall := make(map[*ir.CallExpr]bool)
ir.VisitList(fn.Body, func(n ir.Node) {
switch n := n.(type) {
case *ir.GoDeferStmt:
if call, ok := n.Call.(*ir.CallExpr); ok {
goDeferCall[call] = true
}
return
case *ir.CallExpr:
if !goDeferCall[n] {
staticCall(n)
}
}
})
}
// staticCall devirtualizes the given call if possible when the concrete callee
// is available statically.
func staticCall(call *ir.CallExpr) {
if call.Op() != ir.OCALLINTER {
return
}
sel := call.X.(*ir.SelectorExpr)
r := ir.StaticValue(sel.X)
if r.Op() != ir.OCONVIFACE {
return
}
recv := r.(*ir.ConvExpr)
typ := recv.X.Type()
if typ.IsInterface() {
return
}
// If typ is a shape type, then it was a type argument originally
// and we'd need an indirect call through the dictionary anyway.
// We're unable to devirtualize this call.
if typ.IsShape() {
return
}
// If typ *has* a shape type, then it's an shaped, instantiated
// type like T[go.shape.int], and its methods (may) have an extra
// dictionary parameter. We could devirtualize this call if we
// could derive an appropriate dictionary argument.
//
// TODO(mdempsky): If typ has has a promoted non-generic method,
// then that method won't require a dictionary argument. We could
// still devirtualize those calls.
//
// TODO(mdempsky): We have the *runtime.itab in recv.TypeWord. It
// should be possible to compute the represented type's runtime
// dictionary from this (e.g., by adding a pointer from T[int]'s
// *runtime._type to .dict.T[int]; or by recognizing static
// references to go:itab.T[int],iface and constructing a direct
// reference to .dict.T[int]).
if typ.HasShape() {
if base.Flag.LowerM != 0 {
base.WarnfAt(call.Pos(), "cannot devirtualize %v: shaped receiver %v", call, typ)
}
return
}
// Further, if sel.X's type has a shape type, then it's a shaped
// interface type. In this case, the (non-dynamic) TypeAssertExpr
// we construct below would attempt to create an itab
// corresponding to this shaped interface type; but the actual
// itab pointer in the interface value will correspond to the
// original (non-shaped) interface type instead. These are
// functionally equivalent, but they have distinct pointer
// identities, which leads to the type assertion failing.
//
// TODO(mdempsky): We know the type assertion here is safe, so we
// could instead set a flag so that walk skips the itab check. For
// now, punting is easy and safe.
if sel.X.Type().HasShape() {
if base.Flag.LowerM != 0 {
base.WarnfAt(call.Pos(), "cannot devirtualize %v: shaped interface %v", call, sel.X.Type())
}
return
}
dt := ir.NewTypeAssertExpr(sel.Pos(), sel.X, nil)
dt.SetType(typ)
x := typecheck.Callee(ir.NewSelectorExpr(sel.Pos(), ir.OXDOT, dt, sel.Sel))
switch x.Op() {
case ir.ODOTMETH:
x := x.(*ir.SelectorExpr)
if base.Flag.LowerM != 0 {
base.WarnfAt(call.Pos(), "devirtualizing %v to %v", sel, typ)
}
call.SetOp(ir.OCALLMETH)
call.X = x
case ir.ODOTINTER:
// Promoted method from embedded interface-typed field (#42279).
x := x.(*ir.SelectorExpr)
if base.Flag.LowerM != 0 {
base.WarnfAt(call.Pos(), "partially devirtualizing %v to %v", sel, typ)
}
call.SetOp(ir.OCALLINTER)
call.X = x
default:
// TODO(mdempsky): Turn back into Fatalf after more testing.
if base.Flag.LowerM != 0 {
base.WarnfAt(call.Pos(), "failed to devirtualize %v (%v)", x, x.Op())
}
return
}
// Duplicated logic from typecheck for function call return
// value types.
//
// Receiver parameter size may have changed; need to update
// call.Type to get correct stack offsets for result
// parameters.
types.CheckSize(x.Type())
switch ft := x.Type(); ft.NumResults() {
case 0:
case 1:
call.SetType(ft.Results().Field(0).Type)
default:
call.SetType(ft.Results())
}
// Desugar OCALLMETH, if we created one (#57309).
typecheck.FixMethodCall(call)
}