cmd/compile: fix unified IR's pointer-shaping

In CL 424734, I implemented pointer shaping for unified IR. Evidently
though, we didn't have any test cases that check that uses of
pointer-shaped expressions were handled correctly.

In the reported test case, the struct field "children items[*node[T]]"
gets shaped to "children items[go.shape.*uint8]" (underlying type
"[]go.shape.*uint8"); and so the expression "n.children[i]" has type
"go.shape.*uint8" and the ".items" field selection expression fails.

The fix implemented in this CL is that any expression of derived type
now gets an explicit "reshape" operation applied to it, to ensure it
has the appropriate type for its context. E.g., the "n.children[i]"
OINDEX expression above gets "reshaped" from "go.shape.*uint8" to
"*node[go.shape.int]", allowing the field selection to succeed.

This CL also adds a "-d=reshape" compiler debugging flag, because I
anticipate debugging reshaping operations will be something to come up
again in the future.

Fixes #54535.

Change-Id: Id847bd8f51300d2491d679505ee4d2e974ca972a
Reviewed-on: https://go-review.googlesource.com/c/go/+/424936
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: hopehook <hopehook@qq.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/base/debug.go

@@ -34,6 +34,7 @@ type DebugFlags struct {
 	NoRefName            int    `help:"do not include referenced symbol names in object file"`
 	PCTab                string `help:"print named pc-value table\nOne of: pctospadj, pctofile, pctoline, pctoinline, pctopcdata"`
 	Panic                int    `help:"show all compiler panics"`
+	Reshape              int    `help:"print information about expression reshaping"`
 	Slice                int    `help:"print information about slice compilation"`
 	SoftFloat            int    `help:"force compiler to emit soft-float code"`
 	SyncFrames           int    `help:"how many writer stack frames to include at sync points in unified export data"`

src/cmd/compile/internal/noder/codes.go

@@ -58,6 +58,7 @@ const (
 	exprNil
 	exprFuncInst
 	exprRecv
+	exprReshape
 )
 
 type codeAssign int

src/cmd/compile/internal/noder/reader.go

@@ -89,7 +89,7 @@ func (pr *pkgReader) newReader(k pkgbits.RelocKind, idx pkgbits.Index, marker pk
 	}
 }
 
-// A writer provides APIs for reading an individual element.
+// A reader provides APIs for reading an individual element.
 type reader struct {
 	pkgbits.Decoder
 
@@ -2367,6 +2367,41 @@ func (r *reader) expr() (res ir.Node) {
 		typ := r.exprType()
 		return typecheck.Expr(ir.NewUnaryExpr(pos, ir.ONEW, typ))
 
+	case exprReshape:
+		typ := r.typ()
+		x := r.expr()
+
+		if types.IdenticalStrict(x.Type(), typ) {
+			return x
+		}
+
+		// Comparison expressions are constructed as "untyped bool" still.
+		//
+		// TODO(mdempsky): It should be safe to reshape them here too, but
+		// maybe it's better to construct them with the proper type
+		// instead.
+		if x.Type() == types.UntypedBool && typ.IsBoolean() {
+			return x
+		}
+
+		base.AssertfAt(x.Type().HasShape() || typ.HasShape(), x.Pos(), "%L and %v are not shape types", x, typ)
+		base.AssertfAt(types.Identical(x.Type(), typ), x.Pos(), "%L is not shape-identical to %v", x, typ)
+
+		// We use ir.HasUniquePos here as a check that x only appears once
+		// in the AST, so it's okay for us to call SetType without
+		// breaking any other uses of it.
+		//
+		// Notably, any ONAMEs should already have the exactly right shape
+		// type and been caught by types.IdenticalStrict above.
+		base.AssertfAt(ir.HasUniquePos(x), x.Pos(), "cannot call SetType(%v) on %L", typ, x)
+
+		if base.Debug.Reshape != 0 {
+			base.WarnfAt(x.Pos(), "reshaping %L to %v", x, typ)
+		}
+
+		x.SetType(typ)
+		return x
+
 	case exprConvert:
 		implicit := r.Bool()
 		typ := r.typ()

src/cmd/compile/internal/noder/writer.go

@@ -1629,6 +1629,16 @@ func (w *writer) expr(expr syntax.Expr) {
 			w.typ(tv.Type)
 			return
 		}
+
+		// With shape types (and particular pointer shaping), we may have
+		// an expression of type "go.shape.*uint8", but need to reshape it
+		// to another shape-identical type to allow use in field
+		// selection, indexing, etc.
+		if typ := tv.Type; !tv.IsBuiltin() && !isTuple(typ) && !isUntyped(typ) {
+			w.Code(exprReshape)
+			w.typ(typ)
+			// fallthrough
+		}
 	}
 
 	if obj != nil {
@@ -2199,6 +2209,11 @@ func isUntyped(typ types2.Type) bool {
 	return ok && basic.Info()&types2.IsUntyped != 0
 }
 
+func isTuple(typ types2.Type) bool {
+	_, ok := typ.(*types2.Tuple)
+	return ok
+}
+
 func (w *writer) itab(typ, iface types2.Type) {
 	typ = types2.Default(typ)
 	iface = types2.Default(iface)

test/typeparam/issue54535.go

@@ -0,0 +1,37 @@
+// run
+
+// 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.
+
+package main
+
+type node[T any] struct {
+	items    items[T]
+	children items[*node[T]]
+}
+
+func (n *node[T]) f(i int, j int) bool {
+	if len(n.children[i].items) < j {
+		return false
+	}
+	return true
+}
+
+type items[T any] []T
+
+func main() {
+	_ = node[int]{}
+	_ = f[int]
+}
+
+type s[T, U any] struct {
+	a T
+	c U
+}
+
+func f[T any]() {
+	var x s[*struct{ b T }, *struct{ d int }]
+	_ = x.a.b
+	_ = x.c.d
+}