// Copyright 2009 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 gc

import (
	"cmd/compile/internal/base"
	"cmd/compile/internal/ir"
)

// evalunsafe evaluates a package unsafe operation and returns the result.
func evalunsafe(n ir.Node) int64 {
	switch n.Op() {
	case ir.OALIGNOF, ir.OSIZEOF:
		n.SetLeft(typecheck(n.Left(), ctxExpr))
		n.SetLeft(defaultlit(n.Left(), nil))
		tr := n.Left().Type()
		if tr == nil {
			return 0
		}
		dowidth(tr)
		if n.Op() == ir.OALIGNOF {
			return int64(tr.Align)
		}
		return tr.Width

	case ir.OOFFSETOF:
		// must be a selector.
		if n.Left().Op() != ir.OXDOT {
			base.Errorf("invalid expression %v", n)
			return 0
		}

		// Remember base of selector to find it back after dot insertion.
		// Since r->left may be mutated by typechecking, check it explicitly
		// first to track it correctly.
		n.Left().SetLeft(typecheck(n.Left().Left(), ctxExpr))
		sbase := n.Left().Left()

		n.SetLeft(typecheck(n.Left(), ctxExpr))
		if n.Left().Type() == nil {
			return 0
		}
		switch n.Left().Op() {
		case ir.ODOT, ir.ODOTPTR:
			break
		case ir.OCALLPART:
			base.Errorf("invalid expression %v: argument is a method value", n)
			return 0
		default:
			base.Errorf("invalid expression %v", n)
			return 0
		}

		// Sum offsets for dots until we reach sbase.
		var v int64
		for r := n.Left(); r != sbase; r = r.Left() {
			switch r.Op() {
			case ir.ODOTPTR:
				// For Offsetof(s.f), s may itself be a pointer,
				// but accessing f must not otherwise involve
				// indirection via embedded pointer types.
				if r.Left() != sbase {
					base.Errorf("invalid expression %v: selector implies indirection of embedded %v", n, r.Left())
					return 0
				}
				fallthrough
			case ir.ODOT:
				v += r.Offset()
			default:
				ir.Dump("unsafenmagic", n.Left())
				base.Fatalf("impossible %#v node after dot insertion", r.Op())
			}
		}
		return v
	}

	base.Fatalf("unexpected op %v", n.Op())
	return 0
}