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

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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.

Backports commit a40d4701bc9f6e6a3bbfb7b4fbe756a5b72b5df1 from qemu
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Peter Maydell 2018-03-01 08:40:28 -05:00 committed by Lioncash
parent f5a35908da
commit f9c5c1a604
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1 changed files with 30 additions and 8 deletions
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38
qemu/tcg/tcg.h
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@ -379,14 +379,36 @@ 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;