var Pkg = xdc.useModule('xdc.bld.PackageContents');
Pkg.attrs.copts = '--gcc'

@CustomHeader
module myModule
{
   metaonly struct Foo
   {
      Bits8 Bar;
      Bits16 Baz;
   }

   @Encoded typedef Foo Foo_s;

   //The rest of your module here
}

typedef struct
{
   xdc_Bits8 Bar;
   xdc_Bits16 Baz;
}__attribute__((packed)) company_whatever_myModule_Foo_s;

//Helpful description from Assert.xs
 
/* The three functions below must exist for each encoded type defined in this
 * module's spec. sizeof and alignof are invoked if the encoded type is used
 * in a structure, while encode is used when a value of the encoded type
 * needs to be represented on the target.
 * In this case, the encoded type is 'Id', which is why the function names
 * start with that prefix.
 */

function Foo_s$alignof()
{
   return 1;
}

//TODO: We might be able to / need to do more complicated things here
function Foo_s$encode(desc)
{
   return "{ (xdc_Bits8)" + desc.Bar + ", /* Bar */\n" +
  "          (xdc_Bits16)" + desc.Baz + ", /* Baz */\n" +
  "        }";
}


function Foo_s$sizeof()
{
   return 3;
}

var Log = xdc.useModule('xdc.runtime.Log');
print('meta-type - ' + Log.EventDesc);
print('c-type - ' + Log.Event);
print('sizeof - ' + Log.EventDesc.$sizeof());
print('sizeof - ' + Log.Event.$sizeof());

meta-type - <unknown>
c-type - <unknown>
js: "./Test.cfg", line 72: XDC runtime error: <unknown>: no property named '$sizeof'
    "./package/cfg/Test_x64P.cfg", line 747
    "./package/cfg/Test_x64P.cfg", line 802
    "./package/cfg/Test_x64P.cfg", line 734

print("sizeof(myModule.Foo_s) = " + xdc.loadCapsule("company/whatever/myModule.xs").Foo_s$sizeof());

config Foo myFoo = {Bar:0, Baz:0};

{(xdc_Bits8)25, (xdc_bits16)100,}