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[Qemu-trivial] [PATCH] tcg/tcg.h: Improve documentation of TCGv_i32 etc
[Qemu-trivial] [PATCH] tcg/tcg.h: Improve documentation of TCGv_i32 etc types
Fri, 21 Oct 2016 17:38:42 +0100
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: Peter Maydell <address@hidden>
This confused at least two people who cared enough to post to
qemu-devel about it, so it seems worth rewriting and expanding
the comment in an attempt to avoid having to explain it on the
mailing list for a third time :-)
tcg/tcg.h | 38 ++++++++++++++++++++++++++++++--------
1 file changed, 30 insertions(+), 8 deletions(-)
diff --git a/tcg/tcg.h b/tcg/tcg.h
index c9949aa..622ace8 100644
@@ -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;
- [Qemu-trivial] [PATCH] tcg/tcg.h: Improve documentation of TCGv_i32 etc types,
Peter Maydell <=