ssa: impl builtin alignof offsetof

ssa/expr.go

@@ -1257,7 +1257,23 @@ func (b Builder) BuiltinCall(fn string, args ...Expr) (ret Expr) {
 	case "Add":
 		return b.Advance(args[0], args[1])
 	case "Sizeof":
+		// instance of generic function
 		return b.Prog.Val(int(b.Prog.SizeOf(args[0].Type)))
+	case "Alignof":
+		// instance of generic function
+		return b.Prog.Val(int(b.Prog.td.ABITypeAlignment(args[0].ll)))
+	case "Offsetof":
+		// instance of generic function
+		if load := args[0].impl.IsALoadInst(); !load.IsNil() {
+			if gep := load.Operand(0).IsAGetElementPtrInst(); !gep.IsNil() {
+				typ := gep.GEPSourceElementType()
+				offset := gep.Operand(2).IsAConstantInt()
+				if typ.TypeKind() == llvm.StructTypeKind && !offset.IsNil() {
+					return b.Prog.Val(int(b.Prog.td.ElementOffset(typ, int(offset.SExtValue()))))
+				}
+			}
+		}
+		panic("invalid argument for unsafe.Offsetof: must be a selector expression")
 	}
 	panic("todo: " + fn)
 }