cmd/compile: fixes for non-constant Sizeof/Alignof/Offsetof

Includes Robert's suggested fix in validate.go to not fail on
non-constant alignof/offsetof/sizeof calls. Further changes to wait on
transforming these calls until stenciling time, when we can call
EvalConst() to evaluate them once all the relevant types are known.

Added a bunch of new tests for non-constant Sizeof/Alignof/Offsetof.

Fixes #47716

Change-Id: I469af888eb9ce3a853124d919eda753971009b3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/344250
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>

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

@@ -138,10 +138,18 @@ func Call(pos src.XPos, typ *types.Type, fun ir.Node, args []ir.Node, dots bool)
 		//    until arg type known
 		// OAPPEND: transformAppend requires that the arg is a slice
 		// ODELETE: transformDelete requires that the arg is a map
+		// OALIGNOF, OSIZEOF: can be eval'ed to a constant until types known.
 		switch fun.BuiltinOp {
-		case ir.OMAKE, ir.OREAL, ir.OIMAG, ir.OAPPEND, ir.ODELETE:
+		case ir.OMAKE, ir.OREAL, ir.OIMAG, ir.OAPPEND, ir.ODELETE, ir.OALIGNOF, ir.OOFFSETOF, ir.OSIZEOF:
 			hasTParam := false
 			for _, arg := range args {
+				if fun.BuiltinOp == ir.OOFFSETOF {
+					// It's the type of left operand of the
+					// selection that matters, not the type of
+					// the field itself (which is irrelevant for
+					// offsetof).
+					arg = arg.(*ir.SelectorExpr).X
+				}
 				if arg.Type().HasTParam() {
 					hasTParam = true
 					break

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

@@ -1037,7 +1037,7 @@ func (subst *subster) node(n ir.Node) ir.Node {
 				name := call.X.Name()
 				if name.BuiltinOp != ir.OXXX {
 					switch name.BuiltinOp {
-					case ir.OMAKE, ir.OREAL, ir.OIMAG, ir.OAPPEND, ir.ODELETE:
+					case ir.OMAKE, ir.OREAL, ir.OIMAG, ir.OAPPEND, ir.ODELETE, ir.OALIGNOF, ir.OOFFSETOF, ir.OSIZEOF:
 						// Transform these builtins now that we
 						// know the type of the args.
 						m = transformBuiltin(call)

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

@@ -811,7 +811,10 @@ func transformBuiltin(n *ir.CallExpr) ir.Node {
 			return transformRealImag(u1.(*ir.UnaryExpr))
 		case ir.OPANIC:
 			return transformPanic(u1.(*ir.UnaryExpr))
-		case ir.OCLOSE, ir.ONEW, ir.OALIGNOF, ir.OOFFSETOF, ir.OSIZEOF:
+		case ir.OALIGNOF, ir.OOFFSETOF, ir.OSIZEOF:
+			// This corresponds to the EvalConst() call near end of typecheck().
+			return typecheck.EvalConst(u1)
+		case ir.OCLOSE, ir.ONEW:
 			// nothing more to do
 			return u1
 		}

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

@@ -81,7 +81,16 @@ func (g *irgen) validateBuiltin(name string, call *syntax.CallExpr) {
 		// Check that types2+gcSizes calculates sizes the same
 		// as cmd/compile does.
 
-		got, ok := constant.Int64Val(g.info.Types[call].Value)
+		tv := g.info.Types[call]
+		if !tv.IsValue() {
+			base.FatalfAt(g.pos(call), "expected a value")
+		}
+
+		if tv.Value == nil {
+			break // unsafe op is not a constant, so no further validation
+		}
+
+		got, ok := constant.Int64Val(tv.Value)
 		if !ok {
 			base.FatalfAt(g.pos(call), "expected int64 constant value")
 		}

src/cmd/compile/internal/typecheck/const.go

@@ -881,7 +881,9 @@ func evalunsafe(n ir.Node) int64 {
 	case ir.OOFFSETOF:
 		// must be a selector.
 		n := n.(*ir.UnaryExpr)
-		if n.X.Op() != ir.OXDOT {
+		// ODOT and ODOTPTR are allowed in case the OXDOT transformation has
+		// already happened (e.g. during -G=3 stenciling).
+		if n.X.Op() != ir.OXDOT && n.X.Op() != ir.ODOT && n.X.Op() != ir.ODOTPTR {
 			base.Errorf("invalid expression %v", n)
 			return 0
 		}

test/typeparam/issue47716.go

@@ -0,0 +1,68 @@
+// run -gcflags=-G=3
+
+// Copyright 2021 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
+
+import (
+	"fmt"
+	"unsafe"
+)
+
+// size returns the size of type T
+func size[T any](x T) uintptr {
+	return unsafe.Sizeof(x)
+}
+
+// size returns the alignment of type T
+func align[T any](x T) uintptr {
+	return unsafe.Alignof(x)
+}
+
+type Tstruct[T any] struct {
+	f1 T
+	f2 int
+}
+
+// offset returns the offset of field f2 in the generic type Tstruct
+func (r *Tstruct[T]) offset() uintptr {
+	return unsafe.Offsetof(r.f2)
+}
+
+func main() {
+	v1 := int(5)
+	if got, want := size(v1), unsafe.Sizeof(v1); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+	if got, want := align(v1), unsafe.Alignof(v1); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+
+	v2 := "abc"
+	if got, want := size(v2), unsafe.Sizeof(v2); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+	if got, want := align(v2), unsafe.Alignof(v2); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+
+	var v3 Tstruct[int]
+	if got, want := unsafe.Offsetof(v3.f2), unsafe.Sizeof(v1); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+
+	var v4 Tstruct[interface{}]
+	var v5 interface{}
+	if got, want := unsafe.Offsetof(v4.f2), unsafe.Sizeof(v5); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+
+	if got, want := v3.offset(), unsafe.Offsetof(v3.f2); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+	if got, want := v4.offset(), unsafe.Offsetof(v4.f2); got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+}