pkg/compiler: add offsetof type

Similar to C offsetof gives offset of a field
from the beginning of the parent struct.
We have several TODOs in descriptions asking for this.

4 files changed, 16 insertions, 2 deletions

diff --git a/pkg/compiler/check.go b/pkg/compiler/check.go
index 0b296e2ea..24acfe50f 100644
--- a/pkg/compiler/check.go
+++ b/pkg/compiler/check.go
@@ -415,7 +415,12 @@ func (comp *compiler) checkLenTargetRec(t0, t *ast.Type, targets []*ast.Type,
 		parent := parents[pi]
 		if parent.name != "" && (parent.name == target.Ident || target.Ident == prog.ParentRef) ||
 			parent.name == "" && target.Ident == prog.SyscallRef {
-			if len(targets) != 0 {
+			if len(targets) == 0 {
+				if t.Ident == "offsetof" {
+					comp.error(target.Pos, "%v must refer to fields", t.Ident)
+					return
+				}
+			} else {
 				parents1 := make([]parentDesc, pi+1)
 				copy(parents1, parents[:pi+1])
 				comp.checkLenTargetRec(t0, t, targets, parents1, warned)
diff --git a/pkg/compiler/testdata/all.txt b/pkg/compiler/testdata/all.txt
index 25461ff9a..68629c685 100644
--- a/pkg/compiler/testdata/all.txt
+++ b/pkg/compiler/testdata/all.txt
@@ -90,6 +90,8 @@ len_expr3 {
 	f34	bytesize[len_expr1:f11:f22:f41, int32]
 	f35	bytesize[len_expr2:f23:f41, int32]
 	f36	bytesize[len_expr2:f24:f41, int32]
+	f37	offsetof[f32, int32]
+	f38	offsetof[len_expr2:f21, int32]
 }
 
 len_expr4 {
diff --git a/pkg/compiler/testdata/errors2.txt b/pkg/compiler/testdata/errors2.txt
index 311be3ea9..db8f87e25 100644
--- a/pkg/compiler/testdata/errors2.txt
+++ b/pkg/compiler/testdata/errors2.txt
@@ -147,6 +147,8 @@ slen1 {
 	f5 len[slen22:f, int32]		### len path slen22 does not refer to a struct
 	f6 len[syscall, int32]		### no argument name after syscall reference
 	f7 len[syscall:b, int32]	### len target b does not exist
+	f8 offsetof[parent, int32]	### offsetof must refer to fields
+	f9 offsetof[slen1, int32]	### offsetof must refer to fields
 	slen2 ptr[in, array[slen2]]
 	slen21 slen2
 	slen22 array[slen2]
diff --git a/pkg/compiler/types.go b/pkg/compiler/types.go
index 6d9211686..18027615d 100644
--- a/pkg/compiler/types.go
+++ b/pkg/compiler/types.go
@@ -204,13 +204,14 @@ var typeArray = &typeDesc{
 }
 
 var typeLen = &typeDesc{
-	Names:       []string{"len", "bytesize", "bytesize2", "bytesize4", "bytesize8", "bitsize"},
+	Names:       []string{"len", "bytesize", "bytesize2", "bytesize4", "bytesize8", "bitsize", "offsetof"},
 	CanBeArgRet: canBeArg,
 	CantBeOpt:   true,
 	NeedBase:    true,
 	Args:        []namedArg{{Name: "len target", Type: typeArgLenTarget}},
 	Gen: func(comp *compiler, t *ast.Type, args []*ast.Type, base prog.IntTypeCommon) prog.Type {
 		var bitSize uint64
+		var offset bool
 		switch t.Ident {
 		case "bytesize":
 			bitSize = 8
@@ -219,6 +220,9 @@ var typeLen = &typeDesc{
 			bitSize = byteSize * 8
 		case "bitsize":
 			bitSize = 1
+		case "offsetof":
+			bitSize = 8
+			offset = true
 		}
 		path := []string{args[0].Ident}
 		for _, col := range args[0].Colon {
@@ -228,6 +232,7 @@ var typeLen = &typeDesc{
 			IntTypeCommon: base,
 			Path:          path,
 			BitSize:       bitSize,
+			Offset:        offset,
 		}
 	},
 }