The Zither Assembly Backend

The Zither assembly backend gives defines C preprocessor macros meant to be used in assembly code

Output layout

Given a FIDL library by the name of id1.id2.....idn, one header <fidl/id1/id2/.../idn/data/asm/${filename}.h> is generated per original FIDL source file, containing the bindings for the declarations defined therein.

GN integration

${fidl_target}_zither.asm gives a C library target with the generated headers as public dependencies.

Bindings

Any declaration type not mentioned below is ignored.

Constants

library example.lib;

const INT_CONST uint32 = 10;  // Or any integral type.
const STR_CONST string = "string constant";

yields

#define EXAMPLE_LIB_INT_CONST (10)
#define EXAMPLE_LIB_STR_CONST ("string constant")

Enums

library example.lib;

type MyEnum = enum : int8 {  // Or any valid integral type
    ZERO = 0;
    ONE = 1;
};

yields

#define EXAMPLE_LIB_MY_ENUM_ZERO (0)
#define EXAMPLE_LIB_MY_ENUM_ONE (1)

Bits

library example.lib;

type MyBits = bits : uint8 {  // Or any valid integral type
    ONE = 1;
    TWO = 2;
    FOUR = 4;
};

yields

#define EXAMPLE_LIB_MY_BITS_ONE (1)
#define EXAMPLE_LIB_MY_BITS_ONE_SHIFT (0)

#define EXAMPLE_LIB_MY_BITS_TWO (2)
#define EXAMPLE_LIB_MY_BITS_TWO_SHIFT (1)

#define EXAMPLE_LIB_MY_BITS_FOUR (4)
#define EXAMPLE_LIB_MY_BITS_FOUR_SHIFT (2)

Where the *_SHIFT macros give the base-2 logarithm of the associated bits values.

Structs

library example.lib;

type MyStruct = struct {
    member_a uint64;
    member_b bool;
};

yields

#define EXAMPLE_LIB_MY_STRUCT_SIZEOF (16)
#define EXAMPLE_LIB_MY_STRUCT_MEMBER_A (0)
#define EXAMPLE_LIB_MY_STRUCT_MEMBER_B (8)

where the *_SIZEOF macro gives the size of the struct on the wire (that is, including padding), and where the macros associated with members give the field's offset into the wire layout.

Overlays

type MyOverlay = strict overlay {
    1: a MyOverlayStructVariant;
    2: b uint32;
};

type MyOverlayStructVariant = struct {
  value uint64;
};

yields

#define EXAMPLE_MY_OVERLAY_SIZEOF (24)
#define EXAMPLE_MY_OVERLAY_DISCRIMINANT (0)
#define EXAMPLE_MY_OVERLAY_VALUE (8)

#define EXAMPLE_MY_OVERLAY_A (1)
#define EXAMPLE_MY_OVERLAY_B (2)