Adding upstream version 1.21.8.upstream/1.21.8

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 158 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/devirtualize/devirtualize.go b/src/cmd/compile/internal/devirtualize/devirtualize.go
new file mode 100644
index 0000000..b156b66
--- /dev/null
+++ b/src/cmd/compile/internal/devirtualize/devirtualize.go
@@ -0,0 +1,158 @@
+// 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)
+}