From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52616) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1c1XrM-0007qG-D2 for qemu-devel@nongnu.org; Tue, 01 Nov 2016 08:08:09 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1c1XrG-0007jS-Sv for qemu-devel@nongnu.org; Tue, 01 Nov 2016 08:08:08 -0400 Received: from mail-oi0-x242.google.com ([2607:f8b0:4003:c06::242]:34539) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1c1XrG-0007j8-Nn for qemu-devel@nongnu.org; Tue, 01 Nov 2016 08:08:02 -0400 Received: by mail-oi0-x242.google.com with SMTP id 62so13568344oif.1 for ; Tue, 01 Nov 2016 05:08:02 -0700 (PDT) Sender: Richard Henderson From: Richard Henderson Date: Tue, 1 Nov 2016 06:07:49 -0600 Message-Id: <1478002070-26676-9-git-send-email-rth@twiddle.net> In-Reply-To: <1478002070-26676-1-git-send-email-rth@twiddle.net> References: <1478002070-26676-1-git-send-email-rth@twiddle.net> Subject: [Qemu-devel] [PULL v2 for-2.8 8/9] tcg/tcg.h: Improve documentation of TCGv_i32 etc types List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: peter.maydell@linaro.org From: Peter Maydell 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 Message-Id: <1477067922-26202-1-git-send-email-peter.maydell@linaro.org> Signed-off-by: Richard Henderson --- tcg/tcg.h | 38 ++++++++++++++++++++++++++++++-------- 1 file changed, 30 insertions(+), 8 deletions(-) diff --git a/tcg/tcg.h b/tcg/tcg.h index dc1281f..a35e4c4 100644 --- a/tcg/tcg.h +++ b/tcg/tcg.h @@ -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; -- 2.7.4