From patchwork Fri Oct 21 16:38:42 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Maydell X-Patchwork-Id: 78694 Delivered-To: patches@linaro.org Received: by 10.140.97.247 with SMTP id m110csp1391879qge; Fri, 21 Oct 2016 09:38:44 -0700 (PDT) X-Received: by 10.194.72.197 with SMTP id f5mr1513795wjv.166.1477067924652; Fri, 21 Oct 2016 09:38:44 -0700 (PDT) Return-Path: Received: from orth.archaic.org.uk (orth.archaic.org.uk. [2001:8b0:1d0::2]) by mx.google.com with ESMTPS id wt1si3327712wjc.140.2016.10.21.09.38.44 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 21 Oct 2016 09:38:44 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of pm215@archaic.org.uk designates 2001:8b0:1d0::2 as permitted sender) client-ip=2001:8b0:1d0::2; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of pm215@archaic.org.uk designates 2001:8b0:1d0::2 as permitted sender) smtp.mailfrom=pm215@archaic.org.uk; dmarc=fail (p=NONE dis=NONE) header.from=linaro.org Received: from pm215 by orth.archaic.org.uk with local (Exim 4.84_2) (envelope-from ) id 1bxcqB-0007Xi-80; Fri, 21 Oct 2016 17:38:43 +0100 From: Peter Maydell To: qemu-devel@nongnu.org, qemu-trivial@nongnu.org Cc: patches@linaro.org, Richard Henderson Subject: [PATCH] tcg/tcg.h: Improve documentation of TCGv_i32 etc types Date: Fri, 21 Oct 2016 17:38:42 +0100 Message-Id: <1477067922-26202-1-git-send-email-peter.maydell@linaro.org> X-Mailer: git-send-email 2.7.4 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 --- 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(-) -- 2.7.4 diff --git a/tcg/tcg.h b/tcg/tcg.h index c9949aa..622ace8 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;