17.5.8 Field Labels

In structs, each field is associated with an offset, which is relative to the beginning of the struct. This is the offset used when reading and writing the field from/to the IO space when mapping.

The offset is reset to zero bits at the beginning of the struct type, and it is increased by the size of the fields:

struct
{
                /* Offset: 0#b */
    uint<1> f1; /* Offset: 1#b */
    sint<7> f2; /* Offset: 1#B */
    int     f3; /* Offset: 5#B */
    Bar     f4; /* Offset: 23#B */
}

It is possible to specify an alternative offset for a field using a field label.

Consider for example an entry in an ELF symbol table. Each entry has a st_info field which is 64-bits long, that in turn can be interpreted as two fields st_bind and st_type.

The obvious solution is to encode st_info as a sub-struct, like this:

struct
{
  elf32_word st_name;
  struct
  {
    uint<60> st_bind;
    uint<4> st_type;
  } st_info;
}

However, that makes it inconvenient to access the value of st_info as an integral value, using the bit-concatenation operator symbol.st_info.st_bind::symbol.st_info.st_type. We can use labels instead:

struct
{
  elf32_word st_name;
  elf64_word st_info;
  uint<60> st_bind @ 4#B;
  uint<4> st_type @ 4#B + 60#b;
}

The resulting struct has fields st_info, st_bind and st_type, with the last two overlapping the first.