cmd/gc: fix Offsetof computation.

The offset of an embedded field s.X must be relative to s
and not to the implicit s.Field of which X is a direct field.
Moreover, no indirections may happen on the path.

Fixes #4909.

R=nigeltao, ality, daniel.morsing, iant, gri, r
CC=golang-dev
https://golang.org/cl/8287043

1 files changed, 135 insertions, 2 deletions

diff --git a/test/sizeof.go b/test/sizeof.go
index a6abdd5c65..9aa95677d4 100644
--- a/test/sizeof.go
+++ b/test/sizeof.go
@@ -4,8 +4,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Test unsafe.Sizeof, unsafe.Alignof, and unsafe.Offsetof all return uintptr.
-
 package main
 
 import "unsafe"
@@ -18,8 +16,143 @@ var t T
 
 func isUintptr(uintptr) {}
 
+type T2 struct {
+	A int32
+	U2
+}
+
+type U2 struct {
+	B int32
+	C int32
+}
+
+var t2 T2
+var p2 *T2
+
 func main() {
+	// Test unsafe.Sizeof, unsafe.Alignof, and unsafe.Offsetof all return uintptr.
 	isUintptr(unsafe.Sizeof(t))
 	isUintptr(unsafe.Alignof(t))
 	isUintptr(unsafe.Offsetof(t.X))
+
+	// Test correctness of Offsetof with respect to embedded fields (issue 4909).
+	if unsafe.Offsetof(t2.C) != 8 {
+		println(unsafe.Offsetof(t2.C), "!= 8")
+		panic("unsafe.Offsetof(t2.C) != 8")
+	}
+	if unsafe.Offsetof(p2.C) != 8 {
+		println(unsafe.Offsetof(p2.C), "!= 8")
+		panic("unsafe.Offsetof(p2.C) != 8")
+	}
+	if unsafe.Offsetof(t2.U2.C) != 4 {
+		println(unsafe.Offsetof(t2.U2.C), "!= 4")
+		panic("unsafe.Offsetof(t2.U2.C) != 4")
+	}
+	if unsafe.Offsetof(p2.U2.C) != 4 {
+		println(unsafe.Offsetof(p2.U2.C), "!= 4")
+		panic("unsafe.Offsetof(p2.U2.C) != 4")
+	}
+	testDeep()
+	testNotEmbedded()
+}
+
+type (
+	S1 struct {
+		A int32
+		S2
+	}
+	S2 struct {
+		B int32
+		S3
+	}
+	S3 struct {
+		C int32
+		S4
+	}
+	S4 struct {
+		D int32
+		S5
+	}
+	S5 struct {
+		E int32
+		S6
+	}
+	S6 struct {
+		F int32
+		S7
+	}
+	S7 struct {
+		G int32
+		S8
+	}
+	S8 struct {
+		H int32
+		*S1
+	}
+)
+
+func testDeep() {
+	var s1 S1
+	switch {
+	case unsafe.Offsetof(s1.A) != 0:
+		panic("unsafe.Offsetof(s1.A) != 0")
+	case unsafe.Offsetof(s1.B) != 4:
+		panic("unsafe.Offsetof(s1.B) != 4")
+	case unsafe.Offsetof(s1.C) != 8:
+		panic("unsafe.Offsetof(s1.C) != 8")
+	case unsafe.Offsetof(s1.D) != 12:
+		panic("unsafe.Offsetof(s1.D) != 12")
+	case unsafe.Offsetof(s1.E) != 16:
+		panic("unsafe.Offsetof(s1.E) != 16")
+	case unsafe.Offsetof(s1.F) != 20:
+		panic("unsafe.Offsetof(s1.F) != 20")
+	case unsafe.Offsetof(s1.G) != 24:
+		panic("unsafe.Offsetof(s1.G) != 24")
+	case unsafe.Offsetof(s1.H) != 28:
+		panic("unsafe.Offsetof(s1.H) != 28")
+	case unsafe.Offsetof(s1.S1) != 32:
+		panic("unsafe.Offsetof(s1.S1) != 32")
+	case unsafe.Offsetof(s1.S1.S2.S3.S4.S5.S6.S7.S8.S1.S2) != 4:
+		panic("unsafe.Offsetof(s1.S1.S2.S3.S4.S5.S6.S7.S8.S1.S2) != 4")
+	}
+}
+
+func testNotEmbedded() {
+	type T2 struct {
+		B int32
+		C int32
+	}
+	type T1 struct {
+		A int32
+		T2
+	}
+	type T struct {
+		Dummy int32
+		F     T1
+		P     *T1
+	}
+
+	var t T
+	var p *T
+	switch {
+	case unsafe.Offsetof(t.F.B) != 4:
+		panic("unsafe.Offsetof(t.F.B) != 4")
+	case unsafe.Offsetof(t.F.C) != 8:
+		panic("unsafe.Offsetof(t.F.C) != 8")
+
+	case unsafe.Offsetof(t.P.B) != 4:
+		panic("unsafe.Offsetof(t.P.B) != 4")
+	case unsafe.Offsetof(t.P.C) != 8:
+		panic("unsafe.Offsetof(t.P.C) != 8")
+
+	case unsafe.Offsetof(p.F.B) != 4:
+		panic("unsafe.Offsetof(p.F.B) != 4")
+	case unsafe.Offsetof(p.F.C) != 8:
+		panic("unsafe.Offsetof(p.F.C) != 8")
+
+	case unsafe.Offsetof(p.P.B) != 4:
+		panic("unsafe.Offsetof(p.P.B) != 4")
+	case unsafe.Offsetof(p.P.C) != 8:
+		panic("unsafe.Offsetof(p.P.C) != 8")
+	}
 }