Commit a40d4701 authored by Peter Maydell's avatar Peter Maydell Committed by Richard Henderson
Browse files

tcg/tcg.h: Improve documentation of TCGv_i32 etc types 



The typedefs we use for the TCGv_i32, TCGv_i64 and TCGv_ptr
types are somewhat confusing, because we define them as
pointers to structs, but the structs themselves are never
defined. Explain in the comments a bit more clearly why
this is OK and what is going on under the hood.

Signed-off-by: default avatarPeter Maydell <peter.maydell@linaro.org>
Message-Id: <1477067922-26202-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: default avatarRichard Henderson <rth@twiddle.net>
parent 15610d42

tcg/tcg.h

+30 −8
Original line number Diff line number Diff line
@@ -376,14 +376,36 @@ static inline unsigned get_alignment_bits(TCGMemOp memop) 


typedef tcg_target_ulong TCGArg;



/* Define a type and accessor macros for variables.  Using pointer types

   is nice because it gives some level of type safely.  Converting to and

   from intptr_t rather than int reduces the number of sign-extension

   instructions that get implied on 64-bit hosts.  Users of tcg_gen_* don't

   need to know about any of this, and should treat TCGv as an opaque type.

   In addition we do typechecking for different types of variables.  TCGv_i32

   and TCGv_i64 are 32/64-bit variables respectively.  TCGv and TCGv_ptr

   are aliases for target_ulong and host pointer sized values respectively.  */

/* Define type and accessor macros for TCG variables.



   TCG variables are the inputs and outputs of TCG ops, as described

   in tcg/README. Target CPU front-end code uses these types to deal

   with TCG variables as it emits TCG code via the tcg_gen_* functions.

   They come in several flavours:

    * TCGv_i32 : 32 bit integer type

    * TCGv_i64 : 64 bit integer type

    * TCGv_ptr : a host pointer type

    * TCGv : an integer type the same size as target_ulong

             (an alias for either TCGv_i32 or TCGv_i64)

   The compiler's type checking will complain if you mix them

   up and pass the wrong sized TCGv to a function.



   Users of tcg_gen_* don't need to know about any of the internal

   details of these, and should treat them as opaque types.

   You won't be able to look inside them in a debugger either.



   Internal implementation details follow:



   Note that there is no definition of the structs TCGv_i32_d etc anywhere.

   This is deliberate, because the values we store in variables of type

   TCGv_i32 are not really pointers-to-structures. They're just small

   integers, but keeping them in pointer types like this means that the

   compiler will complain if you accidentally pass a TCGv_i32 to a

   function which takes a TCGv_i64, and so on. Only the internals of

   TCG need to care about the actual contents of the types, and they always

   box and unbox via the MAKE_TCGV_* and GET_TCGV_* functions.

   Converting to and from intptr_t rather than int reduces the number

   of sign-extension instructions that get implied on 64-bit hosts.  */



typedef struct TCGv_i32_d *TCGv_i32;

typedef struct TCGv_i64_d *TCGv_i64;